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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..c353ca0 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #69362 (https://www.gutenberg.org/ebooks/69362) diff --git a/old/69362-0.txt b/old/69362-0.txt deleted file mode 100644 index 3b0f7c0..0000000 --- a/old/69362-0.txt +++ /dev/null @@ -1,7749 +0,0 @@ -The Project Gutenberg eBook of Mendel's principles of heredity, by -William Bateson - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: Mendel's principles of heredity - A defence - -Author: William Bateson - -Release Date: November 15, 2022 [eBook #69362] - -Language: English - -Produced by: Thiers Halliwell, ellinora, Bryan Ness and the Online - Distributed Proofreading Team at https://www.pgdp.net (This - file was produced from images generously made available by - The Internet Archive/American Libraries.) - -*** START OF THE PROJECT GUTENBERG EBOOK MENDEL'S PRINCIPLES OF -HEREDITY *** - -Transcriber’s notes: - -The text of this e-book has mostly been preserved in its original form. -One spelling error was corrected (considertion → consideration) and a -few missing full stops inserted, but inconsistent hyphenation was left -unchanged. Italic text is denoted by _underscores_. Superscripted text -is indicated by a preceding caret mark, e.g. 2^{n} and subscripted -text by a downward arrow, e.g. A↓{2}. Footnotes have been numbered and -positioned below the relevant paragraphs. - - - - -MENDEL’S PRINCIPLES OF HEREDITY - - - - -London: C. J. CLAY AND SONS, -CAMBRIDGE UNIVERSITY PRESS WAREHOUSE, -AVE MARIA LANE, -AND -H. K. LEWIS, 136, GOWER STREET, W.C. - -[Illustration] - -Glasgow: 50, WELLINGTON STREET. -Leipzig: F. A. BROCKHAUS. -New York: THE MACMILLAN COMPANY. -Bombay and Calcutta: MACMILLAN AND CO., LTD. - -[_All Rights reserved._] - -[Illustration: - -GREGOR MENDEL -Abbot of Brünn -Born 1822. Died 1884. - -_From a photograph kindly supplied by the Very Rev. Dr Janeischek, the -present Abbot._] - - - - - MENDEL’S - - PRINCIPLES OF HEREDITY - - A DEFENCE - - BY - - W. BATESON, M.A., F.R.S. - - _WITH A TRANSLATION OF MENDEL’S ORIGINAL - PAPERS ON HYBRIDISATION._ - - CAMBRIDGE: - AT THE UNIVERSITY PRESS. - 1902 - - - - - Cambridge: - PRINTED BY J. AND C. F. CLAY, - AT THE UNIVERSITY PRESS. - - - - -PREFACE. - - -In the Study of Evolution progress had well-nigh stopped. The more -vigorous, perhaps also the more prudent, had left this field of science -to labour in others where the harvest is less precarious or the yield -more immediate. Of those who remained some still struggled to push -towards truth through the jungle of phenomena: most were content -supinely to rest on the great clearing Darwin made long since. - -Such was our state when two years ago it was suddenly discovered that -an unknown man, Gregor Johann Mendel, had, alone, and unheeded, broken -off from the rest--in the moment that Darwin was at work--and cut a way -through. - -This is no mere metaphor, it is simple fact. Each of us who now looks -at his own patch of work sees Mendel’s clue running through it: whither -that clue will lead, we dare not yet surmise. - -It was a moment of rejoicing, and they who had heard the news hastened -to spread them and take the instant way. In this work I am proud to -have borne my little part. - -But every gospel must be preached to all alike. It will be heard by the -Scribes, by the Pharisees, by Demetrius the Silversmith, and the rest. -Not lightly do men let their occupation go; small, then, would be our -wonder, did we find the established prophet unconvinced. Yet, is it -from misgiving that Mendel had the truth, or merely from indifference, -that no naturalist of repute, save Professor Weldon, has risen against -him? - -In the world of knowledge we are accustomed to look for some strenuous -effort to understand a new truth even in those who are indisposed to -believe. It was therefore with a regret approaching to indignation that -I read Professor Weldon’s criticism[1]. Were such a piece from the hand -of a junior it might safely be neglected; but coming from Professor -Weldon there was the danger--almost the certainty--that the small band -of younger men who are thinking of research in this field would take it -they had learnt the gist of Mendel, would imagine his teaching exposed -by Professor Weldon, and look elsewhere for lines of work. - - [1] _Biometrika_, I., 1902, Pt. II. - -In evolutionary studies we have no Areopagus. With us it is not--as -happily it is with Chemistry, Physics, Physiology, Pathology, and -other well-followed sciences--that an open court is always sitting, -composed of men themselves workers, keenly interested in every new -thing, skilled and well versed in the facts. Where this is the case, -doctrine is soon tried and the false trodden down. But in our sparse -and apathetic community error mostly grows unheeded, choking truth. -That fate must not befall Mendel now. - -It seemed imperative that Mendel’s own work should be immediately put -into the hands of all who will read it, and I therefore sought and -obtained the kind permission of the Royal Horticultural Society to -reprint and modify the translation they had already caused to be made -and published in their Journal. To this I add a translation of Mendel’s -minor paper of later date. As introduction to the subject, the same -Society has authorized me to reprint with alterations a lecture on -heredity delivered before them in 1900. For these privileges my warm -thanks are due. The introduction thus supplied, composed originally for -an audience not strictly scientific, is far too slight for the present -purpose. A few pages are added, but I have no time to make it what it -should be, and I must wait for another chance of treating the whole -subject on a more extended scale. It will perhaps serve to give the -beginner the slight assistance which will prepare him to get the most -from Mendel’s own memoir. - - * * * * * - -The next step was at once to defend Mendel from Professor Weldon. That -could only be done by following this critic from statement to statement -in detail, pointing out exactly where he has gone wrong, what he has -misunderstood, what omitted, what introduced in error. With such -matters it is easy to deal, and they would be as nothing could we find -in his treatment some word of allusion to the future; some hint to the -ignorant that this is a very big thing; some suggestion of what it all -_may_ mean if it _be_ true. - -Both to expose each error and to supply effectively what is wanting, -within the limits of a brief article, written with the running pen, -is difficult. For simplicity I have kept almost clear of reference to -facts not directly connected with the text, and have foregone recital -of the now long list of cases, both of plants and animals, where the -Mendelian principles have already been perceived. These subjects are -dealt with in a joint Report to the Evolution Committee of the Royal -Society, made by Miss E. R. Saunders and myself, now in the Press. To -Miss Saunders who has been associated with me in this work for several -years I wish to express my great indebtedness. Much of the present -article has indeed been written in consultation with her. The reader -who seeks fuller statement of facts and conceptions is referred to the -writings of other naturalists who have studied the phenomena at first -hand (of which a bibliography is appended) and to our own Report. - -I take this opportunity of acknowledging the unique facilities -generously granted me, as representative of the Evolution Committee, -by Messrs Sutton and Sons of Reading, to watch some of the many -experiments they have in progress, to inspect their admirable records, -and to utilise these facts for the advancement of the science of -heredity. My studies at Reading have been for the most part confined to -plants other than those immediately the subject of this discussion, but -some time ago I availed myself of a kind permission to examine their -stock of peas, thus obtaining information which, with other facts since -supplied, has greatly assisted me in treating this subject. - - * * * * * - -I venture to express the conviction, that if the facts now before us -are carefully studied, it will become evident that the experimental -study of heredity, pursued on the lines Mendel has made possible, is -second to no branch of science in the certainty and magnitude of the -results it offers. This study has one advantage which no other line of -scientific inquiry possesses, in that the special training necessary -for such work is easily learnt in the practice of it, and can be learnt -in no other way. All that is needed is the faithful resolve to scamp -nothing. - -If a tenth part of the labour and cost now devoted by leisured persons, -in this country alone, to the collection and maintenance of species of -animals and plants which have been collected a hundred times before, -were applied to statistical experiments in heredity, the result in a -few years would make a revolution not only in the industrial art of the -breeder but in our views of heredity, species and variation. We have at -last a brilliant method, and a solid basis from which to attack these -problems, offering an opportunity to the pioneer such as occurs but -seldom even in the history of modern science. - -We have been told of late, more than once, that Biology must become an -_exact_ science. The same is my own fervent hope. But exactness is not -always attainable by numerical precision: there have been students of -Nature, untrained in statistical nicety, whose instinct for truth yet -saved them from perverse inference, from slovenly argument, and from -misuse of authorities, reiterated and grotesque. - -The study of variation and heredity, in our ignorance of the causation -of those phenomena, _must_ be built of statistical data, as Mendel -knew long ago; but, as he also perceived, the ground must be prepared -by specific experiment. The phenomena of heredity and variation are -specific, and give loose and deceptive answers to any but specific -questions. That is where our _exact_ science will begin. Otherwise we -may one day see those huge foundations of “biometry” in ruins. - -But Professor Weldon, by coincidence a vehement preacher of precision, -in his haste to annul this first positive achievement of the precise -method, dispenses for the moment even with those unpretending forms of -precision which conventional naturalists have usefully practised. His -essay is a strange symptom of our present state. The facts of variation -and heredity are known to so few that anything passes for evidence; and -if only a statement, or especially a conclusion, be negative, neither -surprise nor suspicion are aroused. An author dealing in this fashion -with subjects commonly studied, of which the literature is familiar and -frequently verified, would meet with scant respect. The reader who has -the patience to examine Professor Weldon’s array of objections will -find that almost all are dispelled by no more elaborate process than a -reference to the original records. - -With sorrow I find such an article sent out to the world by a Journal -bearing, in any association, the revered name of Francis Galton, -or under the high sponsorship of Karl Pearson. I yield to no one in -admiration of the genius of these men. Never can we sufficiently regret -that those great intellects were not trained in the profession of the -naturalist. - -Mr Galton suggested that the new scientific firm should have a -mathematician and a biologist as partners, and--soundest advice--a -logician retained as consultant[2]. Biologist surely must one partner -be, but it will never do to have him sleeping. In many well-regulated -occupations there are persons known as “knockers-up,” whose thankless -task it is to rouse others from their slumber, and tell them work-time -is come round again. That part I am venturing to play this morning, and -if I have knocked a trifle loud, it is because there is need. - - _March, 1902._ - - [2] _Biometrika_, I. Pt. I. p. 5. - - - - -CONTENTS. - - -INTRODUCTION. - -THE PROBLEMS OF HEREDITY AND THEIR SOLUTION, pp. 1–39. - -Preliminary statement of Mendel’s principles, 8. Relation of Mendel’s -discovery to the law of Ancestral Heredity, 19. _Heterozygote_ and -_Homozygote_, 23. New conceptions necessitated by Mendel’s discovery, -26. Simple alternative characters, or _allelomorphs_, 27. _Compound -allelomorphs_ and their components, 29. Analytical Variations, 29. -Relation of Mendel’s principle to continuous variation, 32. Dominance, -32. Non-Mendelian phenomena, 33. False hybrids of Millardet, 34. Brief -historical notice, 36. - - -MENDEL’S EXPERIMENTS IN PLANT HYBRIDISATION, pp. 40–95. - -Introductory Remarks, 40. Selection of Experimental Plants, 42. -Division and Arrangement of Experiments, 44. Characters selected, 45. -Number of first crosses, 47. Possible sources of error, 47. Forms of -the Hybrids, 49. Dominant and recessive, 49. - -First generation bred from the Hybrids, 51. Numbers of each form in -offspring, 52. Second generation bred from the Hybrids, 55. Subsequent -generations bred from the Hybrids, 57. - -Offspring of Hybrids in which several differentiating characters are -associated, 59. The reproductive cells of the Hybrids, 66. Statement -of Mendel’s essential deductions, 67. Experiments to determine -constitution of germ-cells, 68. Statement of purity of germ-cells, 72. - -Experiments with _Phaseolus_, 76. Compound characters, 80. Concluding -Remarks, 84. - - -MENDEL’S EXPERIMENTS WITH HIERACIUM, 96–103. - - -A DEFENCE OF MENDEL’S PRINCIPLES OF HEREDITY, 104–208. - -_Introductory_, 104. - -I. The Mendelian Principle of Purity of Germ-cells and the Laws of -Heredity based on Ancestry, 108. - -II. Mendel and the critic’s version of him. - -The Law of Dominance, 117. - -III. The facts in regard to Dominance of Characters in Peas, 119. - -The normal characters: colours of cotyledons and seed-coats, 120. -Shape, 122. Stability and variability, 124. Results of crossing in -regard to seed-characters: normal and exceptional, 129. Analysis of -exceptions, 132. The “mule” or heterozygote, 133. - -IV. Professor Weldon’s collection of “Other evidence concerning -Dominance in Peas.” - -A. In regard to cotyledon colour: Preliminary, 137. Xenia, 139. -(1) Gärtner’s cases, 141. (2) Seton’s case, 143. (3) Tschermak’s -exceptions, 145. (3_a_) _Buchsbaum_ case, 145. (3_b_) _Telephone_ -cases, 146. (3_c_) _Couturier_ cases, 147. - -B. Seed-coats and Shapes. 1. Seed-coats, 148. 2. Seed-shapes: (_a_) -Rimpau’s cases, 150. (_b_) Tschermak’s cases, 152. 3. Other phenomena, -especially regarding seed-shapes, in the case of “grey” peas. Modern -evidence, 153. - -C. Evidence of Knight and Laxton, 158. - -D. Miscellaneous cases in other plants and animals: - -1. Stocks (_Matthiola_). Hoariness, 169. Flower-colour, 170. - -2. _Datura_, 172. - -3. Colours of Rats and Mice, 173. - -V. Professor Weldon’s quotations from Laxton, 178. - -Illustration from _Primula sinensis_, 182. - -VI. The Argument built on exceptions, 183. - -Ancestry and Dominance, 185. - -Ancestry and purity of germ-cells, 193. - -The value of the appeal to Ancestry, 197. - -VII. The question of absolute purity of germ-cells, 201. - -Conclusion, 208. - - - - -ERRATA. - - - p. 22, par. 3, line 2, for “falls” read “fall.” - p. 63, line 12, for “_AabbC_” read “_AaBbc_.” - p. 66, in heading, for “OF HYBRIDS” read “OF THE HYBRIDS.” - - -_Note to_ p. 125. None of the yellow seeds produced by _Laxton’s Alpha_ -germinated, though almost all the green seeds sown gave healthy plants. -The same was found in the case of _Express_, another variety which -bore some yellow seeds. In the case of _Blue Peter_, on the contrary, -the yellow seeds have grown as well as the green ones. Few however -were _wholly_ yellow. Of nine yellow seeds produced by crossing green -varieties together (p. 131), six did not germinate, and three which -did gave weak and very backward plants. Taken together, this evidence -makes it scarcely doubtful that the yellow colour in these cases was -pathological, and almost certainly due to exposure after ripening. - - - - -THE PROBLEMS OF HEREDITY AND THEIR SOLUTION[3]. - - [3] The first half of this paper is reprinted with additions and - modifications from the _Journal of the Royal Horticultural Society_, - 1900, vol. XXV., parts 1 and 2. Written almost immediately after the - rediscovery of Mendel, it will be seen to be already in some measure - out of date, but it may thus serve to show the relation of the new - conceptions to the old. - - -An exact determination of the laws of heredity will probably work more -change in man’s outlook on the world, and in his power over nature, -than any other advance in natural knowledge that can be clearly -foreseen. - -There is no doubt whatever that these laws can be determined. In -comparison with the labour that has been needed for other great -discoveries we may even expect that the necessary effort will be small. -It is rather remarkable that while in other branches of physiology such -great progress has of late been made, our knowledge of the phenomena -of heredity has increased but little; though that these phenomena -constitute the basis of all evolutionary science and the very central -problem of natural history is admitted by all. Nor is this due to the -special difficulty of such inquiries so much as to general neglect of -the subject. - -It is in the hope of inducing others to follow these lines of -investigation that I take the problems of heredity as the subject of -this lecture to the Royal Horticultural Society. - -No one has better opportunities of pursuing such work than -horticulturists and stock breeders. They are daily witnesses of -the phenomena of heredity. Their success also depends largely on a -knowledge of its laws, and obviously every increase in that knowledge -is of direct and special importance to them. - -The want of systematic study of heredity is due chiefly to -misapprehension. It is supposed that such work requires a lifetime. But -though for adequate study of the complex phenomena of inheritance long -periods of time must be necessary, yet in our present state of deep -ignorance almost of the outline of the facts, observations carefully -planned and faithfully carried out for even a few years may produce -results of great value. In fact, by far the most appreciable and -definite additions to our knowledge of these matters have been thus -obtained. - -There is besides some misapprehension as to the kind of knowledge which -is especially wanted at this time, and as to the modes by which we may -expect to obtain it. The present paper is written in the hope that it -may in some degree help to clear the ground of these difficulties by a -preliminary consideration of the question, How far have we got towards -an exact knowledge of heredity, and how can we get further? - -Now this is pre-eminently a subject in which we must distinguish what -we _can_ do from what we want to do. We _want_ to know the whole -truth of the matter; we want to know the physical basis, the inward -and essential nature, “the causes,” as they are sometimes called, -of heredity: but we want also to know the laws which the outward and -visible phenomena obey. - -Let us recognise from the outset that as to the essential nature of -these phenomena we still know absolutely nothing. We have no glimmering -of an idea as to what constitutes the essential process by which the -likeness of the parent is transmitted to the offspring. We can study -the processes of fertilisation and development in the finest detail -which the microscope manifests to us, and we may fairly say that we -have now a considerable grasp of the visible phenomena; but of the -nature of the physical basis of heredity we have no conception at all. -No one has yet any suggestion, working hypothesis, or mental picture -that has thus far helped in the slightest degree to penetrate beyond -what we see. The process is as utterly mysterious to us as a flash of -lightning is to a savage. We do not know what is the essential agent -in the transmission of parental characters, not even whether it is a -material agent or not. Not only is our ignorance complete, but no one -has the remotest idea how to set to work on that part of the problem. -We are in the state in which the students of physical science were, -in the period when it was open to anyone to believe that heat was a -material substance or not, as he chose. - -But apart from any conception of the essential modes of transmission of -characters, we _can_ study the outward facts of the transmission. Here, -if our knowledge is still very vague, we are at least beginning to see -how we ought to go to work. Formerly naturalists were content with -the collection of numbers of isolated instances of transmission--more -especially, striking and peculiar cases--the sudden appearance of -highly prepotent forms, and the like. We are now passing out of that -stage. It is not that the interest of particular cases has in any way -diminished--for such records will always have their value--but it -has become likely that general expressions will be found capable of -sufficiently wide application to be justly called “laws” of heredity. -That this is so was till recently due almost entirely to the work of Mr -F. Galton, to whom we are indebted for the first systematic attempt to -enuntiate such a law. - -All laws of heredity so far propounded are of a statistical character -and have been obtained by statistical methods. If we consider for a -moment what is actually meant by a “law of heredity” we shall see at -once why these investigations must follow statistical methods. For -a “law” of heredity is simply an attempt to declare the course of -heredity under given conditions. But if we attempt to predicate the -course of heredity we have to deal with conditions and groups of causes -wholly unknown to us, whose presence we cannot recognize, and whose -magnitude we cannot estimate in any particular case. The course of -heredity in particular cases therefore cannot be foreseen. - -Of the many factors which determine the degree to which a given -character shall be present in a given individual only one is usually -known to us, namely, the degree to which that character is present -in the parents. It is common knowledge that there is not that close -correspondence between parent and offspring which would result were -this factor the only one operating; but that, on the contrary, the -resemblance between the two is only an uncertain one. - -In dealing with phenomena of this class the study of single instances -reveals no regularity. It is only by collection of facts in great -numbers, and by statistical treatment of the mass, that any order or -law can be perceived. In the case of a chemical reaction, for instance, -by suitable means the conditions can be accurately reproduced, so that -in every individual case we can predict with certainty that the same -result will occur. But with heredity it is somewhat as it is in the -case of the rainfall. No one can say how much rain will fall to-morrow -in a given place, but we can predict with moderate accuracy how much -will fall next year, and for a period of years a prediction can be made -which accords very closely with the truth. - -Similar predictions can from statistical data be made as to the -duration of life and a great variety of events, the conditioning causes -of which are very imperfectly understood. It is predictions of this -kind that the study of heredity is beginning to make possible, and in -that sense laws of heredity can be perceived. - -We are as far as ever from knowing _why_ some characters are -transmitted, while others are not; nor can anyone yet foretell which -individual parent will transmit characters to the offspring, and which -will not; nevertheless the progress made is distinct. - -As yet investigations of this kind have been made in only a few -instances, the most notable being those of Galton on human stature, and -on the transmission of colours in Basset hounds. In each of these cases -he has shown that the expectation of inheritance is such that a simple -arithmetical rule is approximately followed. The rule thus arrived at -is that of the whole heritage of the offspring the two parents together -on an average contribute one half, the four grandparents one-quarter, -the eight great-grandparents one-eighth, and so on, the remainder -being contributed by the remoter ancestors. - -Such a law is obviously of practical importance. In any case to which -it applies we ought thus to be able to predict the degree with which -the purity of a strain may be increased by selection in each successive -generation. - -To take a perhaps impossibly crude example, if a seedling show any -particular character which it is desired to fix, on the assumption that -successive self-fertilisations are possible, according to Galton’s -law the expectation of purity should be in the first generation of -self-fertilisation 1 in 2, in the second generation 3 in 4, in the -third 7 in 8, and so on[4]. - - [4] See later. Galton gave a simple diagrammatic representation of - his law in _Nature_, 1898, vol. LVII. p. 293. - -But already many cases are known to which the rule in any simple -form will not apply. Galton points out that it takes no account of -individual prepotencies. There are, besides, numerous cases in which -on crossing two varieties the character of one variety almost always -appears in each member of the first cross-bred generation. Examples of -these will be familiar to those who have experience in such matters. -The offspring of the Polled Angus cow and the Shorthorn bull is almost -invariably polled or with very small loose “scurs.” Seedlings raised -by crossing _Atropa belladonna_ with the yellow-fruited variety have -without exception the blackish-purple fruits of the type. In several -hairy species when a cross with a glabrous variety is made, the first -cross-bred generation is altogether hairy[5]. - - [5] These we now recognize as examples of Mendelian ‘dominance.’ - -Still more numerous are examples in which the characters of one variety -very largely, though not exclusively, predominate in the offspring. - -These large classes of exceptions--to go no further--indicate that, -as we might in any case expect, the principle is not of universal -application, and will need various modifications if it is to be -extended to more complex cases of inheritance of varietal characters. -No more useful work can be imagined than a systematic determination of -the precise “law of heredity” in numbers of particular cases. - -Until lately the work which Galton accomplished stood almost alone in -this field, but quite recently remarkable additions to our knowledge -of these questions have been made. In the year 1900 Professor de Vries -published a brief account[6] of experiments which he has for several -years been carrying on, giving results of the highest value. - - [6] _Comptes Rendus_, March 26, 1900, and _Ber. d. Deutsch. Bot. - Ges._ xviii. 1900, p. 83. - -The description is very short, and there are several points as to which -more precise information is necessary both as to details of procedure -and as to statement of results. Nevertheless it is impossible to doubt -that the work as a whole constitutes a marked step forward, and the -full publication which is promised will be awaited with great interest. - -The work relates to the course of heredity in cases where definite -varieties differing from each other in some _one_ definite character -are crossed together. The cases are all examples of discontinuous -variation: that is to say, cases in which actual intermediates between -the parent forms are not usually produced on crossing[7]. It is shown -that the subsequent posterity obtained by self-fertilising these -cross-breds or hybrids, or by breeding them with each other, break up -into the original parent forms according to fixed numerical rule. - - [7] This conception of discontinuity is of course pre-Mendelian. - -Professor de Vries begins by reference to a remarkable memoir by Gregor -Mendel[8], giving the results of his experiments in crossing varieties -of _Pisum sativum_. These experiments of Mendel’s were carried out on -a large scale, his account of them is excellent and complete, and the -principles which he was able to deduce from them will certainly play a -conspicuous part in all future discussions of evolutionary problems. -It is not a little remarkable that Mendel’s work should have escaped -notice, and been so long forgotten. - - [8] ‘Versuche üb. Pflanzenhybriden’ in the _Verh. d. Naturf. Ver. - Brünn_, iv. 1865. - -For the purposes of his experiments Mendel selected seven pairs of -characters as follows:-- - -1. Shape of ripe seed, whether round; or angular and wrinkled. - -2. Colour of “endosperm” (cotyledons), whether some shade of yellow; or -a more or less intense green. - -3. Colour of the seed-skin, whether various shades of grey and -grey-brown; or white. - -4. Shape of seed-pod, whether simply inflated; or deeply constricted -between the seeds. - -5. Colour of unripe pod, whether a shade of green; or bright yellow. - -6. Nature of inflorescence, whether the flowers are arranged along the -axis of the plant; or are terminal and form a kind of umbel. - -7. Length of stem, whether about 6 or 7 ft. long, or about 3/4 to -1-1/2 ft. - -Large numbers of crosses were made between Peas differing in respect of -_one_ of each of these pairs of characters. It was found that in each -case the offspring of the cross exhibited the character of one of the -parents in almost undiminished intensity, and intermediates which could -not be at once referred to one or other of the parental forms were not -found. - -In the case of each pair of characters there is thus one which in the -first cross prevails to the exclusion of the other. This prevailing -character Mendel calls the _dominant_ character, the other being the -_recessive_ character[9]. - - [9] Note that by these novel terms the complications involved by use - of the expression “prepotent” are avoided. - -That the existence of such “dominant” and “recessive” characters is a -frequent phenomenon in cross-breeding, is well known to all who have -attended to these subjects. - -By letting the cross-breds fertilise themselves Mendel next raised -another generation. In this generation were individuals which showed -the dominant character, but also individuals which presented the -recessive character. Such a fact also was known in a good many -instances. But Mendel discovered that in this generation the numerical -proportion of dominants to recessives is on an average of cases -approximately constant, being in fact _as three to one_. With very -considerable regularity these numbers were approached in the case of -each of his pairs of characters. - -There are thus in the first generation raised from the cross-breds 75 -per cent. dominants and 25 per cent. recessives. - -These plants were again self-fertilised, and the offspring of each -plant separately sown. It next appeared that the offspring of the -recessives _remained pure recessive_, and in subsequent generations -never produced the dominant again. - -But when the seeds obtained by self-fertilising the dominants were -examined and sown it was found that the dominants were not all alike, -but consisted of two classes, (1) those which gave rise to pure -dominants, and (2) others which gave a mixed offspring, composed partly -of recessives, partly of dominants. Here also it was found that the -average numerical proportions were constant, those with pure dominant -offspring being to those with mixed offspring as one to two. Hence -it is seen that the 75 per cent. dominants are not really of similar -constitution, but consist of twenty-five which are pure dominants and -fifty which are really cross-breds, though, like the cross-breds raised -by crossing the two original varieties, they only exhibit the dominant -character. - -To resume, then, it was found that by self-fertilising the original -cross-breds the same proportion was always approached, namely-- - - 25 dominants, 50 cross-breds, 25 recessives, - or 1_D_ : 2_DR_ : 1_R_. - -Like the pure recessives, the pure dominants are thenceforth pure, and -only give rise to dominants in all succeeding generations studied. - -On the contrary the fifty cross-breds, as stated above, have mixed -offspring. But these offspring, again, in their numerical proportions, -follow the same law, namely, that there are three dominants to one -recessive. The recessives are pure like those of the last generation, -but the dominants can, by further self-fertilisation, and examination -or cultivation of the seeds produced, be again shown to be made up of -pure dominants and cross-breds in the same proportion of one dominant -to two cross-breds. - -The process of breaking up into the parent forms is thus continued in -each successive generation, the same numerical law being followed so -far as has yet been observed. - -Mendel made further experiments with _Pisum sativum_, crossing pairs -of varieties which differed from each other in _two_ characters, and -the results, though necessarily much more complex, showed that the law -exhibited in the simpler case of pairs differing in respect of one -character operated here also. - -In the case of the union of varieties _AB_ and _ab_ differing in two -distinct pairs of characters, _A_ and _a_, _B_ and _b_, of which _A_ -and _B_ are dominant, _a_ and _b_ recessive, Mendel found that in the -first cross-bred generation there was only _one_ class of offspring, -really _AaBb_. - -But by reason of the dominance of one character of each pair these -first crosses were hardly if at all distinguishable from _AB_. - -By letting these _AaBb_’s fertilise themselves, only _four_ classes of -offspring seemed to be produced, namely, - - _AB_ showing both dominant characters. - _Ab_ " dominant _A_ and recessive _b_. - _aB_ " recessive _a_ and dominant _B_. - _ab_ " both recessive characters _a_ and _b_. - -The numerical ratio in which these classes appeared were also regular -and approached the ratio - - 9_AB_ : 3_Ab_ : 3_aB_ : 1_ab_. - -But on cultivating these plants and allowing them to fertilise -themselves it was found that the members of the - -RATIOS - - 1 _ab_ class produce only _ab_’s. - - 3 {1 _aB_ class may produce either all _aB_’s, - {2 _or_ both _aB_’s and _ab_’s. - - -RATIOS - -3 { 1 _Ab_ class may produce either all _Ab_’s, - { 2 _or_ both _Ab_’s and _ab_’s. - - { 1 _AB_ class may produce either all _AB_’s, - { 2 _or_ both _AB_’s and _Ab_’s, -9 { 2 _or_ both _AB_’s and _aB_’s, - { 4 _or_ all four possible classes again, namely, - { _AB_’s, _Ab_’s, _aB_’s, and _ab_’s, - -and the average number of members of each class will approach the ratio -1 : 3 : 3 : 9 as indicated above. - -The details of these experiments and of others like them made with -_three_ pairs of differentiating characters are all set out in Mendel’s -memoir. - -Professor de Vries has worked at the same problem in some dozen species -belonging to several genera, using pairs of varieties characterised by -a great number of characters: for instance, colour of flowers, stems, -or fruits, hairiness, length of style, and so forth. He states that in -all these cases Mendel’s principles are followed. - -The numbers with which Mendel worked, though large, were not large -enough to give really smooth results[10]; but with a few rather -marked exceptions the observations are remarkably consistent, and -the approximation to the numbers demanded by the law is greatest in -those cases where the largest numbers were used. When we consider, -besides, that Tschermak and Correns announce definite confirmation in -the case of _Pisum_, and de Vries adds the evidence of his long series -of observations on other species and orders, there can be no doubt -that Mendel’s law is a substantial reality; though whether some of -the cases that depart most widely from it can be brought within the -terms of the same principle or not, can only be decided by further -experiments. - - [10] Professor Weldon (p. 232) takes great exception to this - statement, which he considerately attributes to “some writers.” - After examining the conclusions he obtained by algebraical study of - Mendel’s figures I am disposed to think my statement not very far out. - -One may naturally ask, How can these results be brought into harmony -with the facts of hybridisation hitherto known; and, if all this is -true, how is it that others who have carefully studied the phenomena -of hybridisation have not long ago perceived this law? The answer to -this question is given by Mendel at some length, and it is, I think, -satisfactory. He admits from the first that there are undoubtedly cases -of hybrids and cross-breds which maintain themselves pure and do not -break up. Such examples are plainly outside the scope of his law. Next -he points out, what to anyone who has rightly comprehended the nature -of discontinuity in variation is well known, that the variations in -_each_ character must be _separately_ regarded. In most experiments in -crossing, forms are taken which differ from each other in a multitude -of characters--some continuous, others discontinuous, some capable of -blending with their contraries, while others are not. The observer on -attempting to perceive any regularity is confused by the complications -thus introduced. Mendel’s law, as he fairly says, could only appear in -such cases by the use of overwhelming numbers, which are beyond the -possibilities of practical experiment. Lastly, no previous observer had -applied a strict statistical method. - -Both these answers should be acceptable to those who have studied the -facts of variation and have appreciated the nature of Species in the -light of those facts. That different species should follow different -laws, and that the same law should not apply to all characters alike, -is exactly what we have every right to expect. It will also be -remembered that the principle is only explicitly declared to apply to -discontinuous characters[11]. As stated also it can only be true where -reciprocal crossings lead to the same result. Moreover, it can only be -tested when there is no sensible diminution in fertility on crossing. - - [11] See later. - -Upon the appearance of de Vries’ paper announcing the “rediscovery” -and confirmation of Mendel’s law and its extension to a great number -of cases two other observers came forward almost simultaneously and -independently described series of experiments fully confirming Mendel’s -work. Of these papers the first is that of Correns, who repeated -Mendel’s original experiment with Peas having seeds of different -colours. The second is a long and very valuable memoir of Tschermak, -which gives an account of elaborate researches into the results of -crossing a number of varieties of _Pisum sativum_. These experiments -were in many cases carried out on a large scale, and prove the main -fact enuntiated by Mendel beyond any possibility of contradiction. -The more exhaustive of these researches are those of Tschermak on -Peas and Correns on several varieties of Maize. Both these elaborate -investigations have abundantly proved the general applicability of -Mendel’s law to the character of the plants studied, though both -indicate some few exceptions. The details of de Vries’ experiments are -promised in the second volume of his most valuable _Mutationstheorie_. -Correns in regard to Maize and Tschermak in the case of _P. sativum_ -have obtained further proof that Mendel’s law holds as well in the case -of varieties differing from each other in _two_ pairs of characters, -one of each pair being dominant, though of course a more complicated -expression is needed in such cases[12]. - - [12] Tschermak’s investigations were besides directed to a - re-examination of the question of the absence of beneficial - results on cross-fertilising _P. sativum_, a subject already much - investigated by Darwin, and upon this matter also important further - evidence is given in great detail. - -That we are in the presence of a new principle of the highest -importance is manifest. To what further conclusions it may lead us -cannot yet be foretold. But both Mendel and the authors who have -followed him lay stress on one conclusion, which will at once suggest -itself to anyone who reflects on the facts. For it will be seen that -the results are such as we might expect if it be imagined that the -cross-bred plant produced pollen grains and egg-cells, each of which -bears only _one_ of the alternative varietal characters and not both. -If this were so, and if on an average the same number of pollen grains -and egg-cells transmit each of the two characters, it is clear that on -a random assortment of pollen grains and egg-cells Mendel’s law would -be obeyed. For 25 per cent. of “dominant” pollen grains would unite -with 25 per cent. “dominant” egg-cells; 25 per cent. “recessive” pollen -grains would similarly unite with 25 per cent. “recessive” egg-cells; -while the remaining 50 per cent. of each kind would unite together. -It is this consideration which leads both Mendel and those who have -followed him to assert that these facts of crossing prove that each -egg-cell and each pollen grain is pure in respect of each character -to which the law applies. It is highly desirable that varieties -differing in the form of their pollen should be made the subject of -these experiments, for it is quite possible that in such a case strong -confirmation of this deduction might be obtained. [Preliminary trials -made with reference to this point have so far given negative results. -Remembering that a pollen grain is not a germ-cell, but only a bearer -of a germ-cell, the hope of seeing pollen grains differentiated -according to the characters they bear is probably remote. Better hopes -may perhaps be entertained in regard to spermatozoa, or possibly female -cells.] - -As an objection to the deduction of purity of germ-cells, however, it -is to be noted that though true intermediates did not generally occur, -yet the intensity in which the characters appeared did vary in degree, -and it is not easy to see how the hypothesis of _perfect_ purity in the -reproductive cells can be supported in such cases. Be this, however, as -it may, there is no doubt we are beginning to get new lights of a most -valuable kind on the nature of heredity and the laws which it obeys. It -is to be hoped that these indications will be at once followed up by -independent workers. Enough has been said to show how necessary it is -that the subjects of experiment should be chosen in such a way as to -bring the laws of heredity to a real test. For this purpose the first -essential is that the differentiating characters should be few, and -that all avoidable complications should be got rid of. Each experiment -should be reduced to its simplest possible limits. The results obtained -by Galton, and also the new ones especially described in this paper, -have each been reached by restricting the range of observation to one -character or group of characters, and it is certain that by similar -treatment our knowledge of heredity may be rapidly extended. - - * * * * * - -To the above popular presentation of the essential facts, made for -an audience not strictly scientific, some addition, however brief, -is called for. First, in regard to the law of Ancestry, spoken of on -p. 5. Those who are acquainted with Pearson’s _Grammar of Science_, -2nd ed. published early in 1900, the same author’s paper in _Proc. -R. S._ vol. 66, 1900, p. 140, or the extensive memoir (pubd. Oct. -1900), on the inheritance of coat-colour in horses and eye-colour in -man (_Phil. Trans._ 195, A, 1900, p. 79), will not need to be told that -the few words I have given above constitute a most imperfect diagram of -the operations of that law as now developed. Until the appearance of -these treatises it was, I believe, generally considered that the law -of Ancestral Heredity was to be taken as applying to phenomena like -these (coat-colour, eye-colour, &c.) where the inheritance is generally -_alternative_, as well as to the phenomena of _blended_ inheritance. - -Pearson, in the writings referred to, besides withdrawing other large -categories of phenomena from the scope of its operations, points out -that the law of Ancestral Heredity does not satisfactorily express the -cases of alternative inheritance. He urges, and with reason, that these -classes of phenomena should be separately dealt with. - - * * * * * - -The whole issue as regards the various possibilities of heredity now -recognized will be made clearer by a very brief exposition of the -several conceptions involved. - -If an organism producing germ-cells of a given constitution, uniform in -respect of the characters they bear, breeds with another organism[13] -bearing _precisely similar_ germ-cells, the offspring resulting will, -if the conditions are identical, be uniform. - - [13] For simplicity the case of self-fertilisation is omitted from - this consideration. - -In practice such a phenomenon is seen in _pure_-breeding. It is true -that we know no case in nature where all the germ-cells are thus -identical, and where no variation takes place beyond what we can -attribute to conditions, but we know many cases where such a result -is approached, and very many where all the essential features which we -regard as constituting the characters of the breed are reproduced with -approximate certainty in every member of the pure-bred race, which thus -closely approach to uniformity. - -But if two germ-cells of dissimilar constitution unite in -fertilisation, what offspring are we to expect[14]? First let us -premise that the answer to this question is known experimentally to -differ for many organisms and for many classes of characters, and may -almost certainly be in part determined by external circumstances. But -omitting the last qualification, certain principles are now clearly -detected, though what principle will apply in any given case can only -be determined by direct experiment made with that case. - - [14] In all the cases discussed it is assumed that the gametes are - similar except in regard to the “heritage” they bear, and that no - _original_ variation is taking place. The case of mosaics is also - left wholly out of account (see later). - -This is the phenomenon of _cross_-breeding. As generally used, this -term means the union of members of dissimilar varieties, or species: -though when dissimilar gametes[15] produced by two individuals -of the same variety unite in fertilisation, we have essentially -_cross_-breeding in respect of the character or characters in which -those gametes differ. We will suppose, as before, that these two -gametes bearing properties unlike in respect of a given character, are -borne by different individuals. - - [15] The term “gamete” is now generally used as the equivalent of - “germ-cell,” whether male or female, and the term “zygote” is here - used for brevity to denote the organism resulting from fertilisation. - -In the simplest case, suppose a gamete from an individual presenting -any character in intensity _A_ unite in fertilisation with another -from an individual presenting the same character in intensity _a_. For -brevity’s sake we may call the parent individuals _A_ and _a_, and the -resulting zygote _Aa_. What will the structure of _Aa_ be in regard to -the character we are considering? - -Up to Mendel no one proposed to answer this question in any other way -than by reference to the intensity of the character in the progenitors, -and _primarily_ in the parents, _A_ and _a_, in whose bodies the -gametes had been developed. It was well known that such a reference -gave a very poor indication of what _Aa_ would be. Both _A_ and _a_ -may come from a population consisting of individuals manifesting the -same character in various intensities. In the pedigree of either _A_ -or _a_ these various intensities may have occurred few or many times. -Common experience leads us to expect the probability in regard to _Aa_ -to be influenced by this history. The next step is that which Galton -took. He extended the reference beyond the immediate parents of _Aa_, -to its grandparents, great-grandparents, and so on, and in the cases he -studied he found that from a knowledge of the intensity in which the -given character was manifested in each progenitor, even for some few -generations back, a fairly accurate prediction could be made, not as to -the character of any individual _Aa_, but as to the average character -of _Aa_’s of similar parentage, in general. - -But suppose that instead of individuals presenting one character in -differing intensities, two individuals breed together distinguished by -characters which we know to be mutually exclusive, such as _A_ and _B_. -Here again we may speak of the individuals producing the gametes as _A_ -and _B_, and the resulting zygote as _AB_. What will _AB_ be like? The -population here again may consist of many like _A_ and like _B_. These -two forms may have been breeding together indiscriminately, and there -may have been many or few of either type in the pedigree of either _A_ -or _B_. - -Here again Galton applied his method with remarkable success. Referring -to the progenitors of _A_ and _B_, determining how many of each type -there were in the direct pedigree of _A_ and of _B_, he arrived at the -same formula as before, with the simple difference that instead of -expressing the probable average intensity of one character in several -individuals, the prediction is given in terms of the probable number of -_A_’s and _B_’s that would result on an average when particular _A_’s -and _B_’s of known pedigree breed together. - -The law as Galton gives it is as follows:-- - -“It is that the two parents contribute between them on the average -one-half, or (0·5) of the total heritage of the offspring; the four -grandparents, one-quarter, or (0·5)^2; the eight great-grandparents, -one-eighth, or (0·5)^3, and so on. Then the sum of the ancestral -contributions is expressed by the series - - {(0·5) + (0·5)^2 + (0·5)^3, &c.}, - -which, being equal to 1, accounts for the whole heritage.” - -In the former case where _A_ and _a_ are characters which can be -denoted by reference to a common scale, the law assumes of course that -the inheritance will be, to use Galton’s term, _blended_, namely that -the zygote resulting from the union of _A_ with _a_ will on the average -be more like _a_ than if _A_ had been united with _A_; and conversely -that an _Aa_ zygote will on the average _be more like A than an aa -zygote would be_. - -But in the case of _A_’s and _B_’s, which are assumed to be mutually -exclusive characters, we cannot speak of blending, but rather, to use -Galton’s term, of _alternative_ inheritance. - -Pearson, finding that the law whether formulated thus, or in the -modified form in which he restated it[16], did not express the -phenomena of alternative inheritance known to him with sufficient -accuracy to justify its strict application to them, and also on general -grounds, proposed that the phenomena of blended and alternative -inheritance should be treated apart--a suggestion[17] the wisdom of -which can scarcely be questioned. - - [16] In Pearson’s modification the parents contribute 0·3, the - grandparents 0·15, the great-grandparents ·075. - - [17] See the works referred to above. - -Now the law thus imperfectly set forth and every modification of it is -incomplete in one respect. It deals only with the characters of the -resulting zygotes and predicates nothing in regard to the gametes which -go to form them. A good prediction may be made as to any given group of -zygotes, but the various possible constitutions of the gametes are not -explicitly treated. - -Nevertheless a definite assumption is implicitly made regarding the -gametes. It is not in question that differences between these gametes -may occur in respect of the heritage they bear; yet it is assumed -that these differences will be distributed among the gametes of any -individual zygote in such a way that each gamete remains capable, -on fertilisation, of transmitting _all_ the characters (both of the -parent-zygote and of its progenitors) to the zygote which it then -contributes to form (and to the posterity of that zygote) in the -intensity indicated by the law. Hence the gametes of any individual -are taken as collectively a fair sample of all the racial characters -in their appropriate intensities, and this theory demands that there -shall have been no qualitative redistribution of characters among the -gametes of any zygote in such a way that some gametes shall be finally -excluded from partaking of and transmitting any specific part of -the heritage. The theory further demands--and by the analogy of what -we know otherwise not only of animals and plants, but of physical or -chemical laws, perhaps this is the most serious assumption of all--that -the structure of the gametes shall admit of their being capable of -transmitting any character in any intensity varying from zero to -totality with equal ease; and that gametes of each intensity are all -equally likely to occur, given a pedigree of appropriate arithmetical -composition. - -Such an assumption appears so improbable that even in cases where -the facts seem as yet to point to this conclusion with exceptional -clearness, as in the case of human stature, I cannot but feel there is -still room for reserve of judgment. - -However this may be, the Law of Ancestral Heredity, and all -modifications of it yet proposed, falls short in the respect specified -above, that _it does not directly attempt to give any account of the -distribution of the heritage among the gametes_ of any one individual. - -Mendel’s conception differs fundamentally from that involved in the Law -of Ancestral Heredity. The relation of his hypothesis to the foregoing -may be most easily shown if we consider it first in application to the -phenomena resulting from the cross-breeding of two pure varieties. - -Let us again consider the case of two varieties each displaying the -same character, but in the respective intensities _A_ and _a_. Each -gamete of the _A_ variety bears _A_, and each gamete of the _a_ variety -bears _a_. When they unite in fertilisation they form the zygote _Aa_. -What will be its characters? The Mendelian teaching would reply that -this can only be known by direct experiment with the two forms _A_ and -_a_, and that the characters _A_ and _a_ perceived in those two forms -or varieties need not give any indication as to the character of the -zygote _Aa_. It may display the character _A_, or _a_, or a character -half way between the two, or a character beyond _A_ or below _a_. The -character of _Aa_ is not regarded as a _heritage_ transmitted to it by -_A_ and by _a_, but as a character special and peculiar to _Aa_, just -as NaCl is not a body half way between sodium and chlorine, or such -that its properties can be predicted from or easily stated in terms of -theirs. - -If a concrete case may help, a tall pea _A_ crossed with a dwarf _a_ -often produces, not a plant having the height of either _A_ or _a_, but -something _taller_ than the pure tall variety _A_. - -But if the case obeys the Mendelian principles--as does that here -quoted--then it can be declared _first_ that the gametes of _Aa_ -will not be bearers of the character proper to _Aa_; but, generally -speaking, each gamete will either bear the pure _A_ character or the -pure _a_ character. There will in fact be a redistribution of the -characters brought in by the gametes which united to form the zygote -_Aa_, such that each gamete of _Aa_ is pure, as the parental gametes -were. _Secondly_ this redistribution will occur in such a way that, of -the gametes produced by such _Aa_’s, on an average there will be equal -numbers of _A_ gametes and of _a_ gametes. - -Consequently if _Aa_’s breed together, the new _A_ gametes may meet -each other in fertilisation, forming a zygote _AA_, namely, the pure -_A_ variety again; similarly two _a_ gametes may meet and form _aa_, -or the pure _a_ variety again. But if an _A_ gamete meets an _a_ it -will once more form _Aa_, with its special character. This _Aa_ is -the hybrid, or “mule” form, or as I have elsewhere called it, the -_heterozygote_, as distinguished from _AA_ or _aa_ the _homozygotes_. - -Similarly if the two gametes of two varieties distinguished by -characters, _A_ and _B_, which cannot be described in terms of any -common scale (such as for example the “rose” and “single” combs of -fowls) unite in fertilisation, again the character of the mule form -cannot be predicted. Before the experiment is made the “mule” may -present _any_ form. Its character or properties can as yet be no more -predicted than could those of the compounds of unknown elements before -the discovery of the periodic law. - -But again--if the case be Mendelian--the gametes borne by _AB_ will be -either _A_’s or _B_’s[18], and the cross-bred _AB_’s breeding together -will form _AA_’s, _AB_’s and _ BB_’s. Moreover, if as in the normal -Mendelian case, _AB_’s bear on an average equal numbers of _A_ gametes -and _B_ gametes, the numerical ratio of these resulting zygotes to each -other will be - - 1 _AA_ : 2 _AB_ : 1 _BB_. - - [18] This conception was clearly formed by Naudin simultaneously - with Mendel, but it was not worked out by him and remained a mere - suggestion. In one place also Focke came very near to the same idea - (see Bibliography). - -We have seen that Mendel makes no prediction as to the outward and -visible characters of _AB_, but only as to the essential constitution -and statistical condition of its gametes in regard to the characters -_A_ and _B_. Nevertheless in a large number of cases the character of -_AB_ is known to fall into one of three categories (omitting mosaics). - - (1) The cross-bred may almost always resemble one of its pure - parents so closely as to be practically indistinguishable from that - pure form, as in the case of the yellow cotyledon-colour of certain - varieties of peas when crossed with green-cotyledoned varieties; in - which case the parental character, yellow, thus manifested by the - cross-bred is called “dominant” and the parental character, green, - not manifested, is called recessive. - - (2) The cross-bred may present some condition intermediate between - the two parental forms, in which case we may still retain the term - “blend” as applied to the zygote. - - Such an “intermediate” may be the apparent mean between the two - parental forms or be nearer to one or other in any degree. Such - a case is that of a cross between a rich crimson Magenta Chinese - Primrose and a clear White, giving a flower of a colour appropriately - described as a “washy” magenta. - - (3) The cross-bred may present some form quite different from that - of either pure parent. Though, as has been stated, nothing can be - predicted of an unknown case, we already know a considerable number - of examples of this nature in which the mule-form _approaches - sometimes with great accuracy to that of a putative ancestor, near - or remote_. It is scarcely possible to doubt that several--though - perhaps not all--of Darwin’s “reversions on crossing” were of this - nature. - - Such a case is that of the “wild grey mouse” produced by the union - of an albino tame mouse and a piebald Japanese mouse[19]. These - “reversionary” mice bred together produce the parental tame types, - some other types, and “reversionary” mice again. - - [19] See von Guaita, _Ber. naturf. Ges. Freiburg_ X. 1898 and XI. - 1899, quoted by Professor Weldon (see later). - -From what has been said it will now be clear that the applicability of -the Mendelian hypothesis has, intrinsically, nothing whatever to do -with the question of the inheritance being _blended_ or _alternative_. -In fact, as soon as the relation of zygote characters to gamete -characters is appreciated, it is difficult to see any reason for -supposing that the manifestation of characters seen in the zygotes -should give any indication as to their mode of allotment among the -gametes. - -On a previous occasion I pointed out that the terms “Heredity” and -“Inheritance” are founded on a misapplication of metaphor, and in the -light of our present knowledge it is becoming clearer that the ideas of -“transmission” of a character by parent to offspring, or of there being -any “contribution” made by an ancestor to its posterity, must only be -admitted under the strictest reserve, and merely as descriptive terms. - - * * * * * - -We are now presented with some entirely new conceptions:-- - - (1) The purity of the gametes in regard to certain characters. - - (2) The distinction of all zygotes according as they are or are not - formed by the union of like or unlike gametes. In the former case, - apart from Variation, they breed true when mated with their like; in - the latter case their offspring, collectively, will be heterogeneous. - - (3) If the zygote be formed by the union of dissimilar gametes, we - may meet the phenomenon of (_a_) dominant and recessive characters; - (_b_) a blend form; (_c_) a form distinct from either parent, often - reversionary[20]. - - [20] This fact sufficiently indicates the difficulties involved in a - superficial treatment of the phenomenon of reversion. To call such - reversions as those named above “returns to ancestral type” would be, - if more than a descriptive phrase were intended, quite misleading. It - is not the ancestral _type_ that has come back, but something else - has come in its guise, as the offspring presently prove. For the - first time we thus begin to get a rationale of “reversion.” - -But there are additional and even more significant deductions from the -facts. We have seen that the gametes are differentiated in respect -of pure characters. Of these pure characters there may _conceivably_ -be any number associated together in one organism. In the pea Mendel -detected at least seven--not all seen by him combined in the same -plant, but there is every likelihood that they are all capable of being -thus combined. - -Each such character, which is capable of being dissociated or replaced -by its contrary, must henceforth be conceived of as a distinct -_unit-character_; and as we know that the several unit-characters are -of such a nature that any one of them is capable of independently -displacing or being displaced by one or more alternative characters -taken singly, we may recognize this fact by naming such unit-characters -_allelomorphs_. So far, we know very little of any allelomorphs -existing otherwise than as _pairs_ of contraries, but this is probably -merely due to experimental limitations and the rudimentary state of our -knowledge. - -In one case (combs of fowls) we know three characters, _pea_ comb, -_rose_ comb and _single_ comb; of which _pea_ and _single_, or _rose_ -and _single_, behave towards each other as a pair of allelomorphs, but -of the behaviour of _pea_ and _rose_ towards each other we know as yet -nothing. - -We have no reason as yet for affirming that any phenomenon properly -described as _displacement_ of one allelomorph by another occurs, -though the metaphor may be a useful one. In all cases where _dominance_ -has been perceived, we can affirm that the members of the allelomorphic -pair stand to each other in a relation the nature of which we are as -yet wholly unable to apprehend or illustrate. - -To the new conceptions already enumerated we may therefore add - - (4) _Unit-characters_ of which some, _when once arisen by Variation_, - are alternative to each other in the constitution of the gametes, - according to a definite system. - -From the relations subsisting between these characters, it follows that -as each zygotic union of allelomorphs is _resolved_ on the formation -of the gametes, no zygote can give rise to gametes collectively -representing more than _two_ characters allelomorphic to each other, -apart from new variation. - -From the fact of the existence of the interchangeable characters we -must, for purposes of treatment, and to complete the possibilities, -necessarily form the conception of an _irresoluble base_, though -whether such a conception has any objective reality we have no means as -yet of determining. - -We have now seen that when the varieties _A_ and _B_ are crossed -together, the heterozygote, _AB_, produces gametes bearing the pure -_A_ character and the pure _B_ character. In such a case we speak of -such characters as _simple_ allelomorphs. In many cases however a more -complex phenomenon happens. The character brought in on fertilisation -by one or other parent may be of such a nature that when the zygote, -_AB_, forms its gametes, these are not individually bearers merely -of _A_ and _B_, _but of a number of characters themselves again -integral_, which in, say _A_, behaved as one character so long as its -gametes united in fertilisation with others like themselves, but on -cross-fertilisation are resolved and redistributed among the gametes -produced by the cross-bred zygote. - -In such a case we call the character _A_ a _compound_ allelomorph, -and we can speak of the integral characters which constitute it as -_hypallelomorphs_. We ought to write the heterozygote (_A A′ A″_ ...) -_B_ and the gametes produced by it may be of the form _A_, _A′_, _A″_, -_A‴_,... _B_. Or the resolution may be incomplete in various degrees, -as we already suspect from certain instances; in which case we may have -gametes _A_, _A′ A″_, _A‴ A″″_, _A′ A″ A^v_,... _B_, and so on. Each -of these may meet a similar or a dissimilar gamete in fertilisation, -forming either a homozygote, or a heterozygote with its distinct -properties. - -In the case of compound allelomorphs we know as yet nothing of the -statistical relations of the several gametes. - -Thus we have the conception - - (5) _of a Compound character_, borne by one gamete, transmitted - entire as a single character so long as fertilisation only occurs - between like gametes, or is, in other words, “symmetrical,” but if - fertilisation take place with a dissimilar gamete (or possibly by - other causes), resolved into integral constituent characters, each - separately transmissible. - -Next, as, by the union of the gametes bearing the various -hypallelomorphs with other such gametes, or with gametes bearing -simple allelomorphs, in fertilisation, a number of new zygotes will -be formed, such as may not have been seen before in the breed: these -will inevitably be spoken of as _varieties_; and it is difficult not to -extend the idea of variation to them. To distinguish these from other -variations--which there must surely be--we may call them - - (6) _Analytical_ variations in contradistinction to - - (7) _Synthetical_ variations, occurring not by the separation of - pre-existing constituent-characters but by the addition of new - characters. - -Lastly, it is impossible to be presented with the fact that in -Mendelian cases the cross-bred produces on an average _equal_ numbers -of gametes of each kind, that is to say, a symmetrical result, without -suspecting that this fact must correspond with some symmetrical figure -of distribution of those gametes in the cell-divisions by which they -are produced. - - * * * * * - -At the present time these are the main conceptions--though by no means -all--arising directly from Mendel’s work. The first six are all more -or less clearly embodied by him, though not in every case developed -in accordance with modern knowledge. The seventh is not a Mendelian -conception, but the facts before us justify its inclusion in the above -list though for the present it is little more than a mere surmise. - - * * * * * - -In Mendelian cases it will now be perceived that all the zygotes -composing the population consist of a limited number of possible types, -each of definite constitution, bearing gametes also of a limited and -definite number of types, and definite constitution in respect of -pre-existing characters. It is now evident that in such cases each -several progenitor need not be brought to account in reckoning the -probable characters of each descendant; for the gametes of cross-breds -are differentiated at each successive generation, some parental -(Mendelian) characters being left out in the composition of each gamete -produced by a zygote arising by the union of bearers of opposite -allelomorphs. - -When from these considerations we return to the phenomena comprised in -the Law of Ancestral Heredity, what certainty have we that the same -conceptions are not applicable there also? - -It has now been shown that the question whether in the cross-bred -zygotes in general the characters blend or are mutually exclusive is an -entirely subordinate one, and distinctions with regard to the essential -nature of heredity based on these circumstances become irrelevant. - -In the case of a population presenting continuous variation in -regard to say, stature, it is easy to see how purity of the gametes -in respect of any intensities of that character might not in -ordinary circumstances be capable of detection. There are doubtless -more than two pure gametic forms of this character, but there may -quite conceivably be six or eight. When it is remembered that each -heterozygous combination of any two may have its own appropriate -stature, and that such a character is distinctly dependent on external -conditions, the mere fact that the observed curves of stature give -“chance distributions” is not surprising and may still be compatible -with purity of gametes in respect of certain pure types. In peas (_P. -sativum_), for example, from Mendel’s work we know that the tall forms -and the extreme dwarf forms exhibit gametic purity. I have seen at -Messrs Sutton’s strong evidence of the same nature in the case of -the tall Sweet Pea (_Lathyrus odoratus_) and the dwarf or procumbent -“Cupid” form. - -But in the case of the Sweet Pea we know at least one pure form of -definitely intermediate height, and in the case of _P. sativum_ there -are many. When the _extreme_ types breed together it will be remembered -the heterozygote commonly exceeds the taller in height. In the next -generation, since there is, in the case of extremes, so much margin -between the types of the two pure forms, the return of the offspring -to the three forms of which two are homozygous and one heterozygous is -clearly perceptible. - -If however instead of pure extreme varieties we were to take a pair of -varieties differing normally by only a foot or two, we might, owing -to the masking effects of conditions, &c., have great difficulty in -distinguishing the three forms in the second generation. There would -besides be twice as many heterozygous individuals as homozygous -individuals of each kind, giving a symmetrical distribution of -heights, and who might not--in pre-Mendelian days--have accepted such -evidence--made still less clear by influence of conditions--as proof of -Continuous Variation both of zygotes and gametes? - -Suppose, then, that instead of two pure types, we had six or eight -breeding together, each pair forming their own heterozygote, there -would be a very remote chance of such purity or fixity of type whether -of gamete or zygote being detected. - -_Dominance_, as we have seen, is merely a phenomenon incidental to -specific cases, between which no other common property has yet been -perceived. In the phenomena of _blended_ inheritance we clearly have no -dominance. In the cases of _alternative_ inheritance studied by Galton -and Pearson there is evidently no _universal_ dominance. From the -tables of Basset hound pedigrees there is clearly no definite dominance -of either of the coat-colours. In the case of eye-colour the published -tables do not, so far as I have discovered, furnish the material for a -decision, though it is scarcely possible the phenomenon, even if only -occasional, could have been overlooked. We must take it, then, there is -no sensible dominance in these cases; but whether there is or is not -sensible gametic purity is an altogether different question, which, -so far as I can judge, is as yet untouched. It may perfectly well be -that we shall be compelled to recognize that in many cases there is no -such purity, and that the characters may be carried by the gametes -in any proportion from zero to totality, just as some substances may -be carried in a solution in any proportion from zero to saturation -without discontinuous change of properties. That this will be found -true in _some_ cases is, on any hypothesis, certain; but to prove the -fact for any given case will be an exceedingly difficult operation, and -I scarcely think it has been yet carried through in such a way as to -leave no room for doubt. - -Conversely, the _absolute_ and _universal_ purity of the gametes has -certainly not yet been determined for any case; not even in those -cases where it looks most likely that such universal purity exists. -Impairment of such purity we may conceive either to occur in the form -of mosaic gametes, or of gametes with blended properties. On analogy -and from direct evidence we have every right to believe that gametes -of both these classes may occur in rare and exceptional cases, of as -yet unexplored nature[21], but such a phenomenon will not diminish the -significance of observed purity. - - [21] It will be understood from what follows, that the existence of - mosaic zygotes is no _proof_ that either component gamete was mosaic. - - * * * * * - -We have now seen the essential nature of the Mendelian principles and -are able to appreciate the exact relation in which they stand to the -group of cases included in the Law of Ancestral Heredity. In seeking -any general indication as to the common properties of the phenomena -which are already known to obey Mendelian principles we can as yet -point to none, and whether some such common features exist or not is -unknown. - - * * * * * - -There is however one group of cases, definite though as yet not -numerous, where we know that the Mendelian principles do not apply. -These are the phenomena upon which Mendel touches in his brief paper -on _Hieracium_. As he there states, the hybrids, if they are fertile -at all, produce offspring like themselves, not like their parents. In -further illustration of this phenomenon he cites Wichura’s _Salix_ -hybrids. Perhaps some dozen other such illustrations could be given -which rest on good evidence. To these cases the Mendelian principle -will in nowise apply, nor is it easy to conceive any modification of -the law of ancestral heredity which can express them. There the matter -at present rests. Among these cases, however, we perceive several more -or less common features. They are often, though not always, hybrids -between forms differing in many characters. The first cross frequently -is not the exact intermediate between the two parental types, but -may as in the few _Hieracium_ cases be irregular in this respect. -There is often some degree of sterility. In the absence of fuller and -statistical knowledge of such cases further discussion is impossible. - - * * * * * - -Another class of cases, untouched by any hypothesis of heredity yet -propounded, is that of the false hybrids of Millardet, where we -have fertilisation without transmission of one or several parental -characters. In these not only does the first cross show, in some -respect, the character or characters of _one parent only_, but in -its posterity _no reappearance of the lost character or characters -is observed_. The nature of such cases is still quite obscure, but -we have to suppose that the allelomorph of one gamete only developes -after fertilisation to the exclusion of the corresponding allelomorph -of the other gamete, much--if the crudity of the comparison may be -pardoned--as occurs on the female side in parthenogenesis without -fertilisation at all. - -To these as yet altogether unconformable cases we can scarcely doubt -that further experiment will add many more. Indeed we already have -tolerably clear evidence that many phenomena of inheritance are of a -much higher order of complexity. When the paper on _Pisum_ was written -Mendel apparently inclined to the view that with modifications his -law might be found to include all the phenomena of hybridisation, but -in the brief subsequent paper on _Hieracium_ he clearly recognized -the existence of cases of a different nature. Those who read that -contribution will be interested to see that he lays down a principle -which may be extended from hybridisation to heredity in general, that -the laws of each new case must be determined by separate experiment. - - * * * * * - -As regards the Mendelian principles, which it is the chief aim of -this introduction to present clearly before the reader, a professed -student of variation will easily be able to fill in the outline now -indicated, and to illustrate the various conceptions from phenomena -already familiar. To do this is beyond the scope of this short sketch. -But enough perhaps has now been said to show that by the application of -those principles we are enabled to reach and deal in a comprehensive -manner with phenomena of a fundamental nature, lying at the very root -of all conceptions not merely of the physiology of reproduction and -heredity, but even of the essential nature of living organisms; and I -think that I used no extravagant words when, in introducing Mendel’s -work to the notice of readers of the Royal Horticultural Society’s -Journal, I ventured to declare that his experiments are worthy to rank -with those which laid the foundation of the Atomic laws of Chemistry. - -As some biographical particulars of this remarkable investigator will -be welcome, I give the following brief notice, first published by Dr -Correns on the authority of Dr von Schanz: Gregor Johann Mendel was -born on July 22, 1822, at Heinzendorf bei Odrau, in Austrian Silesia. -He was the son of well-to-do peasants. In 1843 he entered as a novice -the “Königinkloster,” an Augustinian foundation in Altbrünn. In 1847 he -was ordained priest. From 1851 to 1853 he studied physics and natural -science at Vienna. Thence he returned to his cloister and became a -teacher in the Realschule at Brünn. Subsequently he was made Abbot, -and died January 6, 1884. The experiments described in his papers were -carried out in the garden of his Cloister. Besides the two papers on -hybridisation, dealing respectively with _Pisum_ and _Hieracium_, -Mendel contributed two brief notes to the _Verh. Zool. bot. Verein_, -Wien, on _Scopolia margaritalis_ (1853, III., p. 116) and on _Bruchus -pisi_ (_ibid._ 1854, IV., p. 27). In these papers he speaks of himself -as a pupil of Kollar. - -Mendel published in the Brünn journal statistical observations of a -meteorological character, but, so far as I am aware, no others relating -to natural history. Dr Correns tells me that in the latter part of his -life he engaged in the Ultramontane Controversy. He was for a time -President of the Brünn Society[22]. - - [22] A few additional particulars are given in Tschermak’s edition. - -For the photograph of Mendel which forms the frontispiece to this work, -I am indebted to the Very Rev. Dr Janeischek, the present Abbot of -Brünn, who most kindly supplied it for this purpose. - -So far as I have discovered there was, up to 1900, only one reference -to Mendel’s observations in scientific literature, namely that of -Focke, _Pflanzenmischlinge_, 1881, p. 109, where it is simply stated -that Mendel’s numerous experiments on _Pisum_ gave results similar to -those obtained by Knight, but that he believed he had found constant -numerical ratios among the types produced by hybridisation. In the same -work a similar brief reference is made to the paper on _Hieracium_. - -It may seem surprising that a work of such importance should so long -have failed to find recognition and to become current in the world of -science. It is true that the journal in which it appeared is scarce, -but this circumstance has seldom long delayed general recognition. The -cause is unquestionably to be found in that neglect of the experimental -study of the problem of Species which supervened on the general -acceptance of the Darwinian doctrines. The problem of Species, as -Kölreuter, Gärtner, Naudin, Wichura, and the other hybridists of the -middle of the nineteenth century conceived it, attracted thenceforth -no workers. The question, it was imagined, had been answered and the -debate ended. No one felt much interest in the matter. A host of other -lines of work were suddenly opened up, and in 1865 the more original -investigators naturally found those new methods of research more -attractive than the tedious observations of the hybridisers, whose -inquiries were supposed, moreover, to have led to no definite result. - -Nevertheless the total neglect of such a discovery is not easy to -account for. Those who are acquainted with the literature of this -branch of inquiry will know that the French Academy offered a prize -in 1861 to be awarded in 1862 on the subject “_Étudier les Hybrides -végétaux au point de vue de leur fécondité et de la perpétuité de -leurs caractères_.” This subject was doubtless chosen with reference -to the experiments of Godron of Nancy and Naudin, then of Paris. Both -these naturalists competed, and the accounts of the work of Godron on -_Datura_ and of Naudin on a number of species were published in the -years 1864 and 1865 respectively. Both, especially the latter, are -works of high consequence in the history of the science of heredity. -In the latter paper Naudin clearly enuntiated what we shall henceforth -know as the Mendelian conception of the dissociation of characters of -cross-breds in the formation of the germ-cells, though apparently he -never developed this conception. - -In the year 1864, George Bentham, then President of the Linnean -Society, took these treatises as the subject of his address to the -Anniversary meeting on the 24 May, Naudin’s work being known to him -from an abstract, the full paper having not yet appeared. Referring -to the hypothesis of dissociation which he fully described, he said -that it appeared to be new and well supported, but required much more -confirmation before it could be held as proven. (_J. Linn. Soc., Bot._, -VIII., _Proc._, p. XIV.) - -In 1865, the year of Mendel’s communication to the Brünn Society, -appeared Wichura’s famous treatise on his experiments with _Salix_ -to which Mendel refers. There are passages in this memoir which come -very near Mendel’s principles, but it is evident from the plan of his -experiments that Mendel had conceived the whole of his ideas before -that date. - -In 1868 appeared the first edition of Darwin’s _Animals and Plants_, -marking the very zenith of these studies, and thenceforth the decline -in the experimental investigation of Evolution and the problem of -Species has been steady. With the rediscovery and confirmation of -Mendel’s work by de Vries, Correns and Tschermak in 1900 a new era -begins. - -That Mendel’s work, appearing as it did, at a moment when several -naturalists of the first rank were still occupied with these problems, -should have passed wholly unnoticed, will always remain inexplicable, -the more so as the Brünn Society exchanged its publications with most -of the Academies of Europe, including both the Royal and Linnean -Societies. - -Naudin’s views were well known to Darwin and are discussed in _Animals -and Plants_ (ed. 1885, II., p. 23); but, put forward as they were -without full proof, they could not command universal credence. Gärtner, -too, had adopted opposite views; and Wichura, working with cases of -another order, had proved the fact that some hybrids breed true. -Consequently it is not to be wondered at that Darwin was sceptical. -Moreover, the Mendelian idea of the “hybrid-character,” or heterozygous -form, was unknown to him, a conception without which the hypothesis of -dissociation of characters is quite imperfect. - -Had Mendel’s work come into the hands of Darwin, it is not too much -to say that the history of the development of evolutionary philosophy -would have been very different from that which we have witnessed. - - - - -EXPERIMENTS IN PLANT-HYBRIDISATION[23]. - -By Gregor Mendel. - -(_Read at the Meetings of the 8th February and 8th March, 1865._) - - [23] [This translation was made by the Royal Horticultural Society, - and is reprinted with modifications and corrections, by permission. - The original paper was published in the _Verh. naturf. Ver. in Brünn, - Abhandlungen_, IV. 1865, which appeared in 1866.] - - -INTRODUCTORY REMARKS. - -Experience of artificial fertilisation, such as is effected with -ornamental plants in order to obtain new variations in colour, has -led to the experiments which will here be discussed. The striking -regularity with which the same hybrid forms always reappeared whenever -fertilisation took place between the same species induced further -experiments to be undertaken, the object of which was to follow up the -developments of the hybrids in their progeny. - -To this object numerous careful observers, such as Kölreuter, Gärtner, -Herbert, Lecoq, Wichura and others, have devoted a part of their lives -with inexhaustible perseverance. Gärtner especially, in his work “Die -Bastarderzeugung im Pflanzenreiche” (The Production of Hybrids in the -Vegetable Kingdom), has recorded very valuable observations; and quite -recently Wichura published the results of some profound investigations -into the hybrids of the Willow. That, so far, no generally applicable -law governing the formation and development of hybrids has been -successfully formulated can hardly be wondered at by anyone who -is acquainted with the extent of the task, and can appreciate the -difficulties with which experiments of this class have to contend. A -final decision can only be arrived at when we shall have before us the -results of detailed experiments made on plants belonging to the most -diverse orders. - -Those who survey the work done in this department will arrive at the -conviction that among all the numerous experiments made, not one has -been carried out to such an extent and in such a way as to make it -possible to determine the number of different forms under which the -offspring of hybrids appear, or to arrange these forms with certainty -according to their separate generations, or to definitely ascertain -their statistical relations[24]. - - [24] [It is to the clear conception of these three primary - necessities that the whole success of Mendel’s work is due. So far as - I know this conception was absolutely new in his day.] - -It requires indeed some courage to undertake a labour of such -far-reaching extent; it appears, however, to be the only right way by -which we can finally reach the solution of a question the importance of -which cannot be over-estimated in connection with the history of the -evolution of organic forms. - -The paper now presented records the results of such a detailed -experiment. This experiment was practically confined to a small plant -group, and is now, after eight years’ pursuit, concluded in all -essentials. Whether the plan upon which the separate experiments were -conducted and carried out was the best suited to attain the desired end -is left to the friendly decision of the reader. - - -SELECTION OF THE EXPERIMENTAL PLANTS. - -The value and utility of any experiment are determined by the fitness -of the material to the purpose for which it is used, and thus in the -case before us it cannot be immaterial what plants are subjected to -experiment and in what manner such experiments are conducted. - -The selection of the plant group which shall serve for experiments of -this kind must be made with all possible care if it be desired to avoid -from the outset every risk of questionable results. - -The experimental plants must necessarily-- - -1. Possess constant differentiating characters. - -2. The hybrids of such plants must, during the flowering period, be -protected from the influence of all foreign pollen, or be easily -capable of such protection. - -The hybrids and their offspring should suffer no marked disturbance in -their fertility in the successive generations. - -Accidental impregnation by foreign pollen, if it occurred during the -experiments and were not recognized, would lead to entirely erroneous -conclusions. Reduced fertility or entire sterility of certain forms, -such as occurs in the offspring of many hybrids, would render the -experiments very difficult or entirely frustrate them. In order to -discover the relations in which the hybrid forms stand towards each -other and also towards their progenitors it appears to be necessary -that all members of the series developed in each successive generation -should be, _without exception_, subjected to observation. - -At the very outset special attention was devoted to the _Leguminosæ_ -on account of their peculiar floral structure. Experiments which were -made with several members of this family led to the result that the -genus _Pisum_ was found to possess the necessary conditions. - -Some thoroughly distinct forms of this genus possess characters which -are constant, and easily and certainly recognisable, and when their -hybrids are mutually crossed they yield perfectly fertile progeny. -Furthermore, a disturbance through foreign pollen cannot easily occur, -since the fertilising organs are closely packed inside the keel and -the anther bursts within the bud, so that the stigma becomes covered -with pollen even before the flower opens. This circumstance is of -especial importance. As additional advantages worth mentioning, there -may be cited the easy culture of these plants in the open ground and -in pots, and also their relatively short period of growth. Artificial -fertilisation is certainly a somewhat elaborate process, but nearly -always succeeds. For this purpose the bud is opened before it is -perfectly developed, the keel is removed, and each stamen carefully -extracted by means of forceps, after which the stigma can at once be -dusted over with the foreign pollen. - -In all, thirty-four more or less distinct varieties of Peas were -obtained from several seedsmen and subjected to a two years’ trial. In -the case of one variety there were remarked, among a larger number of -plants all alike, a few forms which were markedly different. These, -however, did not vary in the following year, and agreed entirely -with another variety obtained from the same seedsmen; the seeds -were therefore doubtless merely accidentally mixed. All the other -varieties yielded perfectly constant and similar offspring; at any -rate, no essential difference was observed during two trial years. For -fertilisation twenty-two of these were selected and cultivated during -the whole period of the experiments. They remained constant without -any exception. - -Their systematic classification is difficult and uncertain. If we -adopt the strictest definition of a species, according to which only -those individuals belong to a species which under precisely the same -circumstances display precisely similar characters, no two of these -varieties could be referred to one species. According to the opinion of -experts, however, the majority belong to the species _Pisum sativum_; -while the rest are regarded and classed, some as sub-species of _P. -sativum_, and some as independent species, such as _P. quadratum_, _P. -saccharatum_, and _P. umbellatum_. The positions, however, which may be -assigned to them in a classificatory system are quite immaterial for -the purposes of the experiments in question. It has so far been found -to be just as impossible to draw a sharp line between the hybrids of -species and varieties as between species and varieties themselves. - - -DIVISION AND ARRANGEMENT OF THE EXPERIMENTS. - -If two plants which differ constantly in one or several characters -be crossed, numerous experiments have demonstrated that the common -characters are transmitted unchanged to the hybrids and their progeny; -but each pair of differentiating characters, on the other hand, -unite in the hybrid to form a new character, which in the progeny of -the hybrid is usually variable. The object of the experiment was to -observe these variations in the case of each pair of differentiating -characters, and to deduce the law according to which they appear in -the successive generations. The experiment resolves itself therefore -into just as many separate experiments as there are constantly -differentiating characters presented in the experimental plants. - -The various forms of Peas selected for crossing showed differences in -the length and colour of the stem; in the size and form of the leaves; -in the position, colour, and size of the flowers; in the length of -the flower stalk; in the colour, form, and size of the pods; in the -form and size of the seeds; and in the colour of the seed-coats and -the albumen [cotyledons]. Some of the characters noted do not permit -of a sharp and certain separation, since the difference is of a “more -or less” nature, which is often difficult to define. Such characters -could not be utilised for the separate experiments; these could only be -confined to characters which stand out clearly and definitely in the -plants. Lastly, the result must show whether they, in their entirety, -observe a regular behaviour in their hybrid unions, and whether from -these facts any conclusion can be come to regarding those characters -which possess a subordinate significance in the type. - -The characters which were selected for experiment relate: - -1. To the _difference in the form of the ripe seeds_. These are either -round or roundish, the wrinkling, when such occurs on the surface, -being always only shallow; or they are irregularly angular and deeply -wrinkled (_P. quadratum_). - -2. To the _difference in the colour of the seed albumen_ -(endosperm)[25]. The albumen of the ripe seeds is either pale yellow, -bright yellow and orange coloured, or it possesses a more or less -intense green tint. This difference of colour is easily seen in the -seeds as their coats are transparent. - - [25] [Mendel uses the terms “albumen” and “endosperm” somewhat - loosely to denote the cotyledons, containing food-material, within - the seed.] - -3. To the _difference in the colour of the seed-coat_. This is either -white, with which character white flowers are constantly correlated; or -it is grey, grey-brown, leather-brown, with or without violet spotting, -in which case the colour of the standards is violet, that of the wings -purple, and the stem in the axils of the leaves is of a reddish tint. -The grey seed-coats become dark brown in boiling water. - -4. To the _difference in the form of the ripe pods_. These are -either simply inflated, never contracted in places; or they are -deeply constricted between the seeds and more or less wrinkled (_P. -saccharatum_). - -5. To the _difference in the colour of the unripe pods_. They are -either light to dark green, or vividly yellow, in which colouring the -stalks, leaf-veins, and calyx participate[26]. - - [26] One species possesses a beautifully brownish-red coloured pod, - which when ripening turns to violet and blue. Trials with this - character were only begun last year. [Of these further experiments it - seems no account was published. Correns has since worked with such a - variety.] - -6. To the _difference in the position of the flowers_. They are either -axial, that is, distributed along the main stem; or they are terminal, -that is, bunched at the top of the stem and arranged almost in a false -umbel; in this case the upper part of the stem is more or less widened -in section (_P. umbellatum_)[27]. - - [27] [This is often called the Mummy Pea. It shows slight fasciation. - The form I know has white standard and salmon-red wings.] - -7. To the _difference in the length of the stem_. The length of the -stem[28] is very various in some forms; it is, however, a constant -character for each, in so far that healthy plants, grown in the same -soil, are only subject to unimportant variations in this character. - - [28] [In my account of these experiments (_R.H.S. Journal_, vol. XXV. - p. 54) I misunderstood this paragraph and took “axis” to mean the - _floral_ axis, instead of the main axis of the plant. The unit - of measurement, being indicated in the original by a dash (′), I - carelessly took to have been an _inch_, but the translation here - given is evidently correct.] - -In experiments with this character, in order to be able to discriminate -with certainty, the long axis of 6–7 ft. was always crossed with the -short one of 3/4 ft. to 1-1/2 ft. - -Each two of the differentiating characters enumerated above were united -by cross-fertilisation. There were made for the - - 1st trial 60 fertilisations on 15 plants. - 2nd " 58 " " 10 " - 3rd " 35 " " 10 " - 4th " 40 " " 10 " - 5th " 23 " " 5 " - 6th " 34 " " 10 " - 7th " 37 " " 10 " - -From a larger number of plants of the same variety only the most -vigorous were chosen for fertilisation. Weakly plants always afford -uncertain results, because even in the first generation of hybrids, -and still more so in the subsequent ones, many of the offspring either -entirely fail to flower or only form a few and inferior seeds. - -Furthermore, in all the experiments reciprocal crossings were effected -in such a way that each of the two varieties which in one set of -fertilisations served as seed-bearers in the other set were used as -pollen plants. - -The plants were grown in garden beds, a few also in pots, and were -maintained in their naturally upright position by means of sticks, -branches of trees, and strings stretched between. For each experiment -a number of pot plants were placed during the blooming period in a -greenhouse, to serve as control plants for the main experiment in the -open as regards possible disturbance by insects. Among the insects[29] -which visit Peas the beetle _Bruchus pisi_ might be detrimental to the -experiments should it appear in numbers. The female of this species is -known to lay the eggs in the flower, and in so doing opens the keel; -upon the tarsi of one specimen, which was caught in a flower, some -pollen grains could clearly be seen under a lens. Mention must also be -made of a circumstance which possibly might lead to the introduction -of foreign pollen. It occurs, for instance, in some rare cases that -certain parts of an otherwise quite normally developed flower wither, -resulting in a partial exposure of the fertilising organs. A defective -development of the keel has also been observed, owing to which the -stigma and anthers remained partially uncovered[30]. It also sometimes -happens that the pollen does not reach full perfection. In this event -there occurs a gradual lengthening of the pistil during the blooming -period, until the stigmatic tip protrudes at the point of the keel. -This remarkable appearance has also been observed in hybrids of -_Phaseolus_ and _Lathyrus_. - - [29] [It is somewhat surprising that no mention is made of Thrips, - which swarm in Pea flowers. I had come to the conclusion that this is - a real source of error and I see Laxton held the same opinion.] - - [30] [This also happens in Sweet Peas.] - -The risk of false impregnation by foreign pollen is, however, a very -slight one with _Pisum_, and is quite incapable of disturbing the -general result. Among more than 10,000 plants which were carefully -examined there were only a very few cases where an indubitable false -impregnation had occurred. Since in the greenhouse such a case was -never remarked, it may well be supposed that _Bruchus pisi_, and -possibly also the described abnormalities in the floral structure, were -to blame. - - -THE FORMS OF THE HYBRIDS.[31] - - [31] [Mendel throughout speaks of his cross-bred Peas as “hybrids,” a - term which many restrict to the offspring of two distinct _species_. - He, as he explains, held this to be only a question of degree.] - -Experiments which in previous years were made with ornamental plants -have already afforded evidence that the hybrids, as a rule, are not -exactly intermediate between the parental species. With some of the -more striking characters, those, for instance, which relate to the form -and size of the leaves, the pubescence of the several parts, &c., the -intermediate, indeed, was nearly always to be seen; in other cases, -however, one of the two parental characters was so preponderant that it -was difficult, or quite impossible, to detect the other in the hybrid. - -This is precisely the case with the Pea hybrids. In the case of each -of the seven crosses the hybrid-character resembles[32] that of one of -the parental forms so closely that the other either escapes observation -completely or cannot be detected with certainty. This circumstance is -of great importance in the determination and classification of the -forms under which the offspring of the hybrids appear. Henceforth in -this paper those characters which are transmitted entire, or almost -unchanged in the hybridisation, and therefore in themselves constitute -the characters of the hybrid, are termed the _dominant_, and those -which become latent in the process _recessive_. The expression -“recessive” has been chosen because the characters thereby designated -withdraw or entirely disappear in the hybrids, but nevertheless -reappear unchanged in their progeny, as will be demonstrated later on. - - [32] [Note that Mendel, with true penetration, avoids speaking of the - hybrid-character as “transmitted” by either parent, thus escaping the - error pervading modern views of heredity.] - -It was furthermore shown by the whole of the experiments that it is -perfectly immaterial whether the dominant character belong to the -seed-bearer or to the pollen parent; the form of the hybrid remains -identical in both cases. This interesting fact was also emphasised by -Gärtner, with the remark that even the most practised expert is not in -a position to determine in a hybrid which of the two parental species -was the seed or the pollen plant[33]. - - [33] [Gärtner, p. 223.] - -Of the differentiating characters which were used in the experiments -the following are dominant: - -1. The round or roundish form of the seed with or without shallow -depressions. - -2. The yellow colouring of the seed albumen [cotyledons]. - -3. The grey, grey-brown, or leather-brown colour of the seed-coat, in -connection with violet-red blossoms and reddish spots in the leaf axils. - -4. The simply inflated form of the pod. - -5. The green colouring of the unripe pod in connection with the same -colour in the stems, the leaf-veins and the calyx. - -6. The distribution of the flowers along the stem. - -7. The greater length of stem. - -With regard to this last character it must be stated that the longer -of the two parental stems is usually exceeded by the hybrid, which is -possibly only attributable to the greater luxuriance which appears in -all parts of plants when stems of very different length are crossed. -Thus, for instance, in repeated experiments, stems of 1 ft. and 6 ft. -in length yielded without exception hybrids which varied in length -between 6 ft. and 7-1/2 ft. - -The hybrid seeds in the experiments with seed-coat are often more -spotted, and the spots sometimes coalesce into small bluish-violet -patches. The spotting also frequently appears even when it is absent as -a parental character. - -The hybrid forms of the seed-shape and of the albumen are developed -immediately after the artificial fertilisation by the mere influence of -the foreign pollen. They can, therefore, be observed even in the first -year of experiment, whilst all the other characters naturally only -appear in the following year in such plants as have been raised from -the crossed seed. - - -THE FIRST GENERATION [BRED] FROM THE HYBRIDS. - -In this generation there reappear, together with the dominant -characters, also the recessive ones with their full peculiarities, -and this occurs in the definitely expressed average proportion of -three to one, so that among each four plants of this generation three -display the dominant character and one the recessive. This relates -without exception to all the characters which were embraced in the -experiments. The angular wrinkled form of the seed, the green colour of -the albumen, the white colour of the seed-coats and the flowers, the -constrictions of the pods, the yellow colour of the unripe pod, of the -stalk of the calyx, and of the leaf venation, the umbel-like form of -the inflorescence, and the dwarfed stem, all reappear in the numerical -proportion given without any essential alteration. _Transitional forms -were not observed in any experiment._ - -Once the hybrids resulting from reciprocal crosses are fully -formed, they present no appreciable difference in their subsequent -development, and consequently the results [of the reciprocal crosses] -can be reckoned together in each experiment. The relative numbers -which were obtained for each pair of differentiating characters are as -follows: - - Expt. 1. Form of seed.--From 253 hybrids 7,324 seeds were obtained in - the second trial year. Among them were 5,474 round or roundish ones - and 1,850 angular wrinkled ones. Therefrom the ratio 2·96 to 1 is - deduced. - - Expt. 2. Colour of albumen.--258 plants yielded 8,023 seeds, 6,022 - yellow, and 2,001 green; their ratio, therefore, is as 3·01 to 1. - -In these two experiments each pod yielded usually both kinds of seed. -In well-developed pods which contained on the average six to nine -seeds, it often occurred that all the seeds were round (Expt. 1) or -all yellow (Expt. 2); on the other hand there were never observed more -than five angular or five green ones in one pod. It appears to make no -difference whether the pods are developed early or later in the hybrid -or whether they spring from the main axis or from a lateral one. In -some few plants only a few seeds developed in the first formed pods, -and these possessed exclusively one of the two characters, but in the -subsequently developed pods the normal proportions were maintained -nevertheless. - -As in separate pods, so did the distribution of the characters vary in -separate plants. By way of illustration the first ten individuals from -both series of experiments may serve[34]. - - [34] [It is much to be regretted that Mendel does not give the - complete series individually. No one who repeats such experiments - should fail to record the _individual_ numbers, which on seriation - are sure to be full of interest.] - - Experiment 1. Experiment 2. - Form of Seed. Colour of Albumen. -Plants. Round. Angular. Yellow. Green. - - 1 45 12 25 11 - 2 27 8 32 7 - 3 24 7 14 5 - 4 19 10 70 27 - 5 32 11 24 13 - 6 26 6 20 6 - 7 88 24 32 13 - 8 22 10 44 9 - 9 28 6 50 14 - 10 25 7 44 18 - -As extremes in the distribution of the two seed characters in one -plant, there were observed in Expt. 1 an instance of 43 round and only -2 angular, and another of 14 round and 15 angular seeds. In Expt. 2 -there was a case of 32 yellow and only 1 green seed, but also one of 20 -yellow and 19 green. - -These two experiments are important for the determination of the -average ratios, because with a smaller number of experimental plants -they show that very considerable fluctuations may occur. In counting -the seeds, also, especially in Expt. 2, some care is requisite, since -in some of the seeds of many plants the green colour of the albumen is -less developed, and at first may be easily overlooked. The cause of the -partial disappearance of the green colouring has no connection with the -hybrid-character of the plants, as it likewise occurs in the parental -variety. This peculiarity is also confined to the individual and is -not inherited by the offspring. In luxuriant plants this appearance -was frequently noted. Seeds which are damaged by insects during their -development often vary in colour and form, but, with a little practice -in sorting, errors are easily avoided. It is almost superfluous -to mention that the pods must remain on the plants until they are -thoroughly ripened and have become dried, since it is only then that -the shape and colour of the seed are fully developed. - - Expt. 3. Colour of the seed-coats.--Among 929 plants 705 bore - violet-red flowers and grey-brown seed-coats; 224 had white flowers - and white seed-coats, giving the proportion 3·15 to 1. - - Expt. 4. Form of pods.--Of 1,181 plants 882 had them simply inflated, - and in 299 they were constricted. Resulting ratio, 2·95 to 1. - - Expt. 5. Colour of the unripe pods.--The number of trial plants was - 580, of which 428 had green pods and 152 yellow ones. Consequently - these stand in the ratio 2·82 to 1. - - Expt. 6. Position of flowers.--Among 858 cases 651 blossoms were - axial and 207 terminal. Ratio, 3·14 to 1. - - Expt. 7. Length of stem.--Out of 1,064 plants, in 787 cases the - stem was long, and in 277 short. Hence a mutual ratio of 2·84 to - 1. In this experiment the dwarfed plants were carefully lifted and - transferred to a special bed. This precaution was necessary, as - otherwise they would have perished through being overgrown by their - tall relatives. Even in their quite young state they can be easily - picked out by their compact growth and thick dark-green foliage. - -If now the results of the whole of the experiments be brought together, -there is found, as between the number of forms with the dominant and -recessive characters, an average ratio of 2·98 to 1, or 3 to 1. - -The dominant character can have here a _double signification_--viz. -that of a parental-character, or a hybrid-character[35]. In which -of the two significations it appears in each separate case can only -be determined by the following generation. As a parental character -it must pass over unchanged to the whole of the offspring; as a -hybrid-character, on the other hand, it must observe the same behaviour -as in the first generation. - - [35] [This paragraph presents the view of the hybrid-character as - something incidental to the hybrid, and not “transmitted” to it--a - true and fundamental conception here expressed probably for the first - time.] - - -THE SECOND GENERATION [BRED] FROM THE HYBRIDS. - -Those forms which in the first generation maintain the recessive -character do not further vary in the second generation as regards this -character; they remain constant in their offspring. - -It is otherwise with those which possess the dominant character in -the first generation [bred from the hybrids]. Of these _two_-thirds -yield offspring which display the dominant and recessive characters -in the proportion of 3 to 1, and thereby show exactly the same ratio -as the hybrid forms, while only _one_-third remains with the dominant -character constant. - -The separate experiments yielded the following results:-- - - Expt. 1.--Among 565 plants which were raised from round seeds of - the first generation, 193 yielded round seeds only, and remained - therefore constant in this character; 372, however, gave both round - and angular seeds, in the proportion of 3 to 1. The number of the - hybrids, therefore, as compared with the constants is 1·93 to 1. - - Expt. 2.--Of 519 plants which were raised from seeds whose albumen - was of yellow colour in the first generation, 166 yielded exclusively - yellow, while 353 yielded yellow and green seeds in the proportion - of 3 to 1. There resulted, therefore, a division into hybrid and - constant forms in the proportion of 2·13 to 1. - - For each separate trial in the following experiments 100 plants - were selected which displayed the dominant character in the first - generation, and in order to ascertain the significance of this, ten - seeds of each were cultivated. - - Expt. 3.--The offspring of 36 plants yielded exclusively grey-brown - seed-coats, while of the offspring of 64 plants some had grey-brown - and some had white. - - Expt. 4.--The offspring of 29 plants had only simply inflated pods; - of the offspring of 71, on the other hand, some had inflated and some - constricted. - - Expt. 5.--The offspring of 40 plants had only green pods; of the - offspring of 60 plants some had green, some yellow ones. - - Expt. 6.--The offspring of 33 plants had only axial flowers; of the - offspring of 67, on the other hand, some had axial and some terminal - flowers. - - Expt. 7.--The offspring of 28 plants inherited the long axis, and - those of 72 plants some the long and some the short axis. - -In each of these experiments a certain number of the plants came -constant with the dominant character. For the determination of the -proportion in which the separation of the forms with the constantly -persistent character results, the two first experiments are of especial -importance, since in these a larger number of plants can be compared. -The ratios 1·93 to 1 and 2·13 to 1 gave together almost exactly the -average ratio of 2 to 1. The sixth experiment has a quite concordant -result; in the others the ratio varies more or less, as was only to be -expected in view of the smaller number of 100 trial plants. Experiment -5, which shows the greatest departure, was repeated, and then in lieu -of the ratio of 60 and 40 that of 65 and 35 resulted. _The average -ratio of 2 to 1 appears, therefore, as fixed with certainty._ It is -therefore demonstrated that, of those forms which possess the dominant -character in the first generation, in two-thirds the hybrid character -is embodied, while one-third remains constant with the dominant -character. - -The ratio of 3 to 1, in accordance with which the distribution of the -dominant and recessive characters results in the first generation, -resolves itself therefore in all experiments into the ratio of 2 : -1 : 1 if the dominant character be differentiated according to its -significance as a hybrid character or a parental one. Since the members -of the first generation spring directly from the seed of the hybrids, -_it is now clear that the hybrids form seeds having one or other of the -two differentiating characters, and of these one-half develop again the -hybrid form, while the other half yield plants which remain constant -and receive the dominant or recessive characters [respectively] in -equal numbers_. - - -THE SUBSEQUENT GENERATIONS [BRED] FROM THE HYBRIDS. - -The proportions in which the descendants of the hybrids develop and -split up in the first and second generations presumably hold good for -all subsequent progeny. Experiments 1 and 2 have already been carried -through six generations, 3 and 7 through five, and 4, 5, and 6 through -four, these experiments being continued from the third generation with -a small number of plants, and no departure from the rule has been -perceptible. The offspring of the hybrids separated in each generation -in the ratio of 2 : 1 : 1 into hybrids and constant forms. - -If _A_ be taken as denoting one of the two constant characters, for -instance the dominant, _a_, the recessive, and _Aa_ the hybrid form in -which both are conjoined, the expression - - _A_ + 2_Aa_ + _a_ - -shows the terms in the series for the progeny of the hybrids of two -differentiating characters. - -The observation made by Gärtner, Kölreuter, and others, that hybrids -are inclined to revert to the parental forms, is also confirmed by the -experiments described. It is seen that the number of the hybrids which -arise from one fertilisation, as compared with the number of forms -which become constant, and their progeny from generation to generation, -is continually diminishing, but that nevertheless they could not -entirely disappear. If an average equality of fertility in all plants -in all generations be assumed, and if, furthermore, each hybrid forms -seed of which one-half yields hybrids again, while the other half is -constant to both characters in equal proportions, the ratio of numbers -for the offspring in each generation is seen by the following summary, -in which _A_ and _a_ denote again the two parental characters, and _Aa_ -the hybrid forms. For brevity’s sake it may be assumed that each plant -in each generation furnishes only 4 seeds. - - Ratios. -Generation _A_ _Aa_ _a_ _A_ :_Aa_ : _a_ - - 1 1 2 1 1 : 2 : 1 - 2 6 4 6 3 : 2 : 3 - 3 28 8 28 7 : 2 : 7 - 4 120 16 120 15 : 2 : 15 - 5 496 32 496 31 : 2 : 31 - _n_ 2^{_n_}-1 : 2 : 2^{_n_}-1 - -In the tenth generation, for instance, 2^{_n_}-1 = 1023. There result, -therefore, in each 2,048 plants which arise in this generation 1,023 -with the constant dominant character, 1,023 with the recessive -character, and only two hybrids. - - -THE OFFSPRING OF HYBRIDS IN WHICH SEVERAL DIFFERENTIATING CHARACTERS -ARE ASSOCIATED. - -In the experiments above described plants were used which differed only -in one essential character[36]. The next task consisted in ascertaining -whether the law of development discovered in these applied to each -pair of differentiating characters when several diverse characters are -united in the hybrid by crossing. As regards the form of the hybrids -in these cases, the experiments showed throughout that this invariably -more nearly approaches to that one of the two parental plants which -possesses the greater number of dominant characters. If, for instance, -the seed plant has a short stem, terminal white flowers, and simply -inflated pods; the pollen plant, on the other hand, a long stem, -violet-red flowers distributed along the stem, and constricted pods; -the hybrid resembles the seed parent only in the form of the pod; in -the other characters it agrees with the pollen parent. Should one of -the two parental types possess only dominant characters, then the -hybrid is scarcely or not at all distinguishable from it. - - [36] [This statement of Mendel’s in the light of present knowledge - is open to some misconception. Though his work makes it evident that - such varieties may exist, it is very unlikely that Mendel could - have had seven pairs of varieties such that the members of each - pair differed from each other in _only_ one considerable character - (_wesentliches Merkmal_). The point is probably of little theoretical - or practical consequence, but a rather heavy stress is thrown on - “_wesentlich_.”] - -Two experiments were made with a larger number of plants. In the first -experiment the parental plants differed in the form of the seed and -in the colour of the albumen; in the second in the form of the seed, -in the colour of the albumen, and in the colour of the seed-coats. -Experiments with seed characters give the result in the simplest and -most certain way. - -In order to facilitate study of the data in these experiments, the -different characters of the seed plant will be indicated by _A_, _B_, -_C_, those of the pollen plant by _a_, _b_, _c_, and the hybrid forms -of the characters by _Aa_, _Bb_, and _Cc_. - -Expt. 1.--_AB_, seed parents; _ab_, pollen parents; - _A_, form round; _a_, form angular; - _B_, albumen yellow. _b_, albumen green. - -The fertilised seeds appeared round and yellow like those of the seed -parents. The plants raised therefrom yielded seeds of four sorts, which -frequently presented themselves in one pod. In all 556 seeds were -yielded by 15 plants, and of these there were:-- - - 315 round and yellow, - 101 angular and yellow, - 108 round and green, - 32 angular and green. - -All were sown the following year. Eleven of the round yellow seeds did -not yield plants, and three plants did not form seeds. Among the rest: - -38 had round yellow seeds _AB_ -65 round yellow and green seeds _ABb_ -60 round yellow and angular yellow seeds _AaB_ -138 round yellow and green, angular yellow - and green seeds _AaBb_. - -From the angular yellow seeds 96 resulting plants bore seed, of which: - -28 had only angular yellow seeds _aB_ -68 angular yellow and green seeds _aBb_. - -From 108 round green seeds 102 resulting plants fruited, of which: - -35 had only round green seeds _Ab_ -67 round and angular green seeds _Aab_. - -The angular green seeds yielded 30 plants which bore seeds all of like -character; they remained constant _ab_. - -The offspring of the hybrids appeared therefore under nine different -forms, some of them in very unequal numbers. When these are collected -and co-ordinated we find: - - 38 plants with the sign _AB_ - 35 " " " _Ab_ - 28 " " " _aB_ - 30 " " " _ab_ - 65 " " " _ABb_ - 68 " " " _aBb_ - 60 " " " _AaB_ - 67 " " " _Aab_ -138 " " " _AaBb_. - -The whole of the forms may be classed into three essentially different -groups. The first embraces those with the signs _AB_, _Ab_, _aB_, and -_ab_ : they possess only constant characters and do not vary again -in the next generation. Each of these forms is represented on the -average thirty-three times. The second group embraces the signs _ABb_, -_aBb_, _AaB_, _Aab_ : these are constant in one character and hybrid -in another, and vary in the next generation only as regards the hybrid -character. Each of these appears on an average sixty-five times. The -form _AaBb_ occurs 138 times: it is hybrid in both characters, and -behaves exactly as do the hybrids from which it is derived. - -If the numbers in which the forms belonging to these classes appear be -compared, the ratios of 1, 2, 4 are unmistakably evident. The numbers -32, 65, 138 present very fair approximations to the ratio numbers of -33, 66, 132. - -The developmental series consists, therefore, of nine classes, of which -four appear therein always once and are constant in both characters; -the forms _AB_, _ab_, resemble the parental forms, the two others -present combinations between the conjoined characters _A_, _a_, _B_, -_b_, which combinations are likewise possibly constant. Four classes -appear always twice, and are constant in one character and hybrid -in the other. One class appears four times, and is hybrid in both -characters. Consequently the offspring of the hybrids, if two kinds of -differentiating characters are combined therein, are represented by the -expression - - _AB_ + _Ab_ + _aB_ + _ab_ + 2_ABb_ + 2_aBb_ + 2_AaB_ + 2_Aab_ + 4_AaBb_. - -This expression is indisputably a combination series in which the two -expressions for the characters _A_ and _a_, _B_ and _b_, are combined. -We arrive at the full number of the classes of the series by the -combination of the expressions: - - _A_ + 2_Aa_ + _a_ - _B_ + 2_Bb_ + _b_. - -Second Expt. - -_ABC_, seed parents; _abc_, pollen parents; - _A_, form round; _a_, form angular; - _B_, albumen yellow; _b_, albumen green; - _C_, seed-coat grey-brown. _c_, seed-coat white. - -This experiment was made in precisely the same way as the previous -one. Among all the experiments it demanded the most time and trouble. -From 24 hybrids 687 seeds were obtained in all: these were all either -spotted, grey-brown or grey-green, round or angular[37]. From these in -the following year 639 plants fruited, and, as further investigation -showed, there were among them: - - 8 plants _ABC_. 22 plants _ABCc_. 45 plants _ABbCc_. -14 " _ABc_. 17 " _AbCc_. 36 " _aBbCc_. - 9 " _AbC_. 25 " _aBCc_. 38 " _AaBCc_. -11 " _Abc_. 20 " _abCc_. 40 " _AabCc_. - 8 " _aBC_. 15 " _ABbC_. 49 " _AabbC_. -10 " _aBc_. 18 " _ABbc_. 48 " _AaBbc_. -10 " _abC_. 19 " _aBbC_. - 7 " _abc_. 24 " _aBbc_. - 14 " _AaBC_. 78 " _AaBbCc_. - 18 " _AaBc_. - 20 " _AabC_. - 16 " _Aabc_. - - [37] [Note that Mendel does not state the cotyledon-colour of the - first crosses in this case; for as the coats were thick, it could not - have been seen without opening or peeling the seeds.] - -The whole expression contains 27 terms. Of these 8 are constant in all -characters, and each appears on the average 10 times; 12 are constant -in two characters, and hybrid in the third; each appears on the average -19 times; 6 are constant in one character and hybrid in the other two; -each appears on the average 43 times. One form appears 78 times and is -hybrid in all of the characters. The ratios 10, 19, 43, 78 agree so -closely with the ratios 10, 20, 40, 80, or 1, 2, 4, 8, that this last -undoubtedly represents the true value. - -The development of the hybrids when the original parents differ -in three characters results therefore according to the following -expression: - - _ABC_ + _ABc_ + _AbC_ + _Abc_ + _aBC_ + _aBc_ + _abC_ + _abc_ + - 2 _ABCc_ + 2 _AbCc_ + 2 _aBCc_ + 2 _abCc_ + 2 _ABbC_ + 2 _ABbc_ + - 2 _aBbC_ + 2 _aBbc_ + 2 _AaBC_ + 2 _AaBc_ + 2 _AabC_ + 2 _Aabc_ + - 4 _ABbCc_ + 4 _aBbCc_ + 4 _AaBCc_ + 4 _AabCc_ + 4 _AaBbC_ + - 4 _AaBbc_ + 8 _AaBbCc_. - -Here also is involved a combination series in which the expressions for -the characters _A_ and _a_, _B_ and _b_, _C_ and _c_, are united. The -expressions - - _A_ + 2 _Aa_ + _a_ - _B_ + 2 _Bb_ + _b_ - _C_ + 2 _Cc_ + _c_ - -give all the classes of the series. The constant combinations which -occur therein agree with all combinations which are possible between -the characters _A_, _B_, _C_, _a_, _b_, _c_; two thereof, _ABC_ and -_abc_, resemble the two original parental stocks. - -In addition, further experiments were made with a smaller number -of experimental plants in which the remaining characters by twos -and threes were united as hybrids: all yielded approximately the -same results. There is therefore no doubt that for the whole of -the characters involved in the experiments the principle applies -that _the offspring of the hybrids in which several essentially -different characters are combined represent the terms of a series -of combinations, in which the developmental series for each pair of -differentiating characters are associated_. It is demonstrated at the -same time that _the relation of each pair of different characters -in hybrid union is independent of the other differences in the two -original parental stocks_. - -If _n_ represent the number of the differentiating characters in -the two original stocks, 3^{_n_} gives the number of terms of the -combination series, 4^{_n_} the number of individuals which belong to -the series, and 2^{_n_} the number of unions which remain constant. -The series therefore embraces, if the original stocks differ in four -characters, 3^4 = 81 of classes, 4^4 = 256 individuals, and 2^4 = 16 -constant forms; or, which is the same, among each 256 offspring of the -hybrids there are 81 different combinations, 16 of which are constant. - -All constant combinations which in Peas are possible by the combination -of the said seven differentiating characters were actually obtained -by repeated crossing. Their number is given by 2^7 = 128. Thereby is -simultaneously given the practical proof _that the constant characters -which appear in the several varieties of a group of plants may be -obtained in all the associations which are possible according to the -[mathematical] laws of combination, by means of repeated artificial -fertilisation_. - -As regards the flowering time of the hybrids, the experiments are -not yet concluded. It can, however, already be stated that the -period stands almost exactly between those of the seed and pollen -parents, and that the constitution of the hybrids with respect to -this character probably happens in the same way as in the case of the -other characters. The forms which are selected for experiments of this -class must have a difference of at least twenty days from the middle -flowering period of one to that of the other; furthermore, the seeds -when sown must all be placed at the same depth in the earth, so that -they may germinate simultaneously. Also, during the whole flowering -period, the more important variations in temperature must be taken into -account, and the partial hastening or delaying of the flowering which -may result therefrom. It is clear that this experiment presents many -difficulties to be overcome and necessitates great attention. - -If we endeavour to collate in a brief form the results arrived at, we -find that those differentiating characters which admit of easy and -certain recognition in the experimental plants, all behave exactly -alike in their hybrid associations. The offspring of the hybrids of -each pair of differentiating characters are, one-half, hybrid again, -while the other half are constant in equal proportions having the -characters of the seed and pollen parents respectively. If several -differentiating characters are combined by cross-fertilisation in a -hybrid, the resulting offspring form the terms of a combination series -in which the permutation series for each pair of differentiating -characters are united. - -The uniformity of behaviour shown by the whole of the characters -submitted to experiment permits, and fully justifies, the acceptance of -the principle that a similar relation exists in the other characters -which appear less sharply defined in plants, and therefore could not -be included in the separate experiments. An experiment with peduncles -of different lengths gave on the whole a fairly satisfactory result, -although the differentiation and serial arrangement of the forms could -not be effected with that certainty which is indispensable for correct -experiment. - - -THE REPRODUCTIVE CELLS OF HYBRIDS. - -The results of the previously described experiments induced further -experiments, the results of which appear fitted to afford some -conclusions as regards the composition of the egg and pollen cells of -hybrids. An important matter for consideration is afforded in _Pisum_ -by the circumstance that among the progeny of the hybrids constant -forms appear, and that this occurs, too, in all combinations of the -associated characters. So far as experience goes, we find it in every -case confirmed that constant progeny can only be formed when the egg -cells and the fertilising pollen are of like character, so that both -are provided with the material for creating quite similar individuals, -as is the case with the normal fertilisation of pure species[38]. We -must therefore regard it as essential that exactly similar factors are -at work also in the production of the constant forms in the hybrid -plants. Since the various constant forms are produced in _one_ plant, -or even in _one_ flower of a plant, the conclusion appears logical -that in the ovaries of the hybrids there are formed as many sorts of -egg cells, and in the anthers as many sorts of pollen cells, as there -are possible constant combination forms, and that these egg and pollen -cells agree in their internal composition with those of the separate -forms. - - [38] [“False hybridism” was of course unknown to Mendel.] - -In point of fact it is possible to demonstrate theoretically that -this hypothesis would fully suffice to account for the development of -the hybrids in the separate generations, if we might at the same time -assume that the various kinds of egg and pollen cells were formed in -the hybrids on the average in equal numbers[39]. - - [39] [This and the preceding paragraph contain the essence of the - Mendelian principles of heredity.] - -In order to bring these assumptions to an experimental proof, the -following experiments were designed. Two forms which were constantly -different in the form of the seed and the colour of the albumen were -united by fertilisation. - -If the differentiating characters are again indicated as _A_, _B_, _a_, -_b_, we have: - -_AB_, seed parent; _ab_, pollen parent; - _A_, form round; _a_, form angular; - _B_, albumen yellow. _b_, albumen green. - -The artificially fertilised seeds were sown together with several seeds -of both original stocks, and the most vigorous examples were chosen for -the reciprocal crossing. There were fertilised: - - 1. The hybrids with the pollen of _AB_. - 2. The hybrids " " _ab_. - 3. _AB_ " " the hybrids. - 4. _ab_ " " the hybrids. - -For each of these four experiments the whole of the flowers on three -plants were fertilised. If the above theory be correct, there must be -developed on the hybrids egg and pollen cells of the forms _AB_, _Ab_, -_aB_, _ab_, and there would be combined:-- - -1. The egg cells _AB_, _Ab_, _aB_, _ab_ with the pollen cells _AB_. - -2. The egg cells _AB_, _Ab_, _aB_, _ab_ with the pollen cells _ab_. - -3. The egg cells _AB_ with the pollen cells _AB_, _Ab_, _aB_, _ab_. - -4. The egg cells _ab_ with the pollen cells _AB_, _Ab_, _aB_, _ab_. - -From each of these experiments there could then result only the -following forms:-- - - 1. _AB_, _ABb_, _AaB_, _AaBb_. - 2. _AaBb_, _Aab_, _aBb_, _ab_. - 3. _AB_, _ABb_, _AaB_, _AaBb_. - 4. _AaBb_, _Aab_, _aBb_, _ab_. - -If, furthermore, the several forms of the egg and pollen cells of the -hybrids were produced on an average in equal numbers, then in each -experiment the said four combinations should stand in the same ratio -to each other. A perfect agreement in the numerical relations was, -however, not to be expected, since in each fertilisation, even in -normal cases, some egg cells remain undeveloped or subsequently die, -and many even of the well-formed seeds fail to germinate when sown. The -above assumption is also limited in so far that, while it demands the -formation of an equal number of the various sorts of egg and pollen -cells, it does not require that this should apply to each separate -hybrid with mathematical exactness. - -The first and second experiments had primarily the object of proving -the composition of the hybrid egg cells, while the third and fourth -experiments were to decide that of the pollen cells[40]. As is shown by -the above demonstration the first and second experiments and the third -and fourth experiments should produce precisely the same combinations, -and even in the second year the result should be partially visible in -the form and colour of the artificially fertilised seed. In the first -and third experiments the dominant characters of form and colour, _A_ -and _B_, appear in each union, and are also partly constant and partly -in hybrid union with the recessive characters _a_ and _b_, for which -reason they must impress their peculiarity upon the whole of the seeds. -All seeds should therefore appear round and yellow, if the theory be -justified. In the second and fourth experiments, on the other hand, -one union is hybrid in form and in colour, and consequently the seeds -are round and yellow; another is hybrid in form, but constant in the -recessive character of colour, whence the seeds are round and green; -the third is constant in the recessive character of form but hybrid in -colour, consequently the seeds are angular and yellow; the fourth is -constant in both recessive characters, so that the seeds are angular -and green. In both these experiments there were consequently four sorts -of seed to be expected--viz. round and yellow, round and green, angular -and yellow, angular and green. - - [40] [To prove, namely, that both were similarly differentiated, and - not one or other only.] - -The crop fulfilled these expectations perfectly. There were obtained in -the - - 1st Experiment, 98 exclusively round yellow seeds; - 3rd " 94 " " " " - -In the 2nd Experiment, 31 round and yellow, 26 round and green, 27 -angular and yellow, 26 angular and green seeds. - -In the 4th Experiment, 24 round and yellow, 25 round and green, 22 -angular and yellow, 27 angular and green seeds. - -A favourable result could now scarcely be doubted; the next generation -must afford the final proof. From the seed sown there resulted for the -first experiment 90 plants, and for the third 87 plants which fruited: -these yielded for the-- - -1st Exp. 3rd Exp. - 20 25 round yellow seeds _AB_ - 23 19 round yellow and green seeds _ABb_ - 25 22 round and angular yellow seeds _AaB_ - 22 21 round and angular green and yellow seeds _AaBb_ - -In the second and fourth experiments the round and yellow seeds yielded -plants with round and angular yellow and green seeds, _AaBb_. - -From the round green seeds plants resulted with round and angular green -seeds, _Aab_. - -The angular yellow seeds gave plants with angular yellow and green -seeds, _aBb_. - -From the angular green seeds plants were raised which yielded again -only angular and green seeds, _ab_. - -Although in these two experiments likewise some seeds did not -germinate, the figures arrived at already in the previous year were not -affected thereby, since each kind of seed gave plants which, as regards -their seed, were like each other and different from the others. There -resulted therefore from the - -2nd Exp. 4th Exp. - 31 24 plants of the form _AaBb_ - 26 25 " " _Aab_ - 27 22 " " _aBb_ - 26 27 " " _ab_ - -In all the experiments, therefore, there appeared all the forms which -the proposed theory demands, and also in nearly equal numbers. - -In a further experiment the characters of floral colour and length of -stem were experimented upon, and selection so made that in the third -year of the experiment each character ought to appear in half of all -the plants if the above theory were correct. _A_, _B_, _a_, _b_ serve -again as indicating the various characters. - -_A_, violet-red flowers. _a_, white flowers. -_B_, axis long. _b_, axis short. - -The form _Ab_ was fertilised with _ab_, which produced the hybrid -_Aab_. Furthermore, _aB_ was also fertilised with _ab_, whence the -hybrid _aBb_. In the second year, for further fertilisation, the hybrid -_Aab_ was used as seed parent, and hybrid _aBb_ as pollen parent. - -Seed parent, _Aab_. Pollen parent, _aBb_. -Possible egg cells, _Abab_. Pollen cells, _aBab_. - -From the fertilisation between the possible egg and pollen cells four -combinations should result, viz.:-- - - _AaBb_ + _aBb_ + _Aab_ + _ab_. - -From this it is perceived that, according to the above theory, in the -third year of the experiment out of all the plants - - Half should have violet-red flowers (_Aa_), Classes 1, 3 - " " " white flowers (_a_) " 2, 4 - " " " a long axis (_Bb_) " 1, 2 - " " " a short axis (_b_) " 3, 4 - -From 45 fertilisations of the second year 187 seeds resulted, of which -only 166 reached the flowering stage in the third year. Among these the -separate classes appeared in the numbers following:-- - - Class. Colour of flower. Stem. - 1 violet-red long 47 times - 2 white long 40 " - 3 violet-red short 38 " - 4 white short 41 " - -There consequently appeared-- - - The violet-red flower colour (_Aa_) in 85 plants. - " white " " (_a_) in 81 " - " long stem (_Bb_) in 87 " - " short " (_b_) in 79 " - -The theory adduced is therefore satisfactorily confirmed in this -experiment also. - -For the characters of form of pod, colour of pod, and position of -flowers experiments were also made on a small scale, and results -obtained in perfect agreement. All combinations which were possible -through the union of the differentiating characters duly appeared, and -in nearly equal numbers. - -Experimentally, therefore, the theory is justified _that the pea -hybrids form egg and pollen cells which, in their constitution, -represent in equal numbers all constant forms which result from the -combination of the characters when united in fertilisation_. - -The difference of the forms among the progeny of the hybrids, as well -as the respective ratios of the numbers in which they are observed, -find a sufficient explanation in the principle above deduced. The -simplest case is afforded by the developmental series of each pair -of differentiating characters. This series is represented by the -expression _A_ + 2_Aa_ + _a_, in which _A_ and _a_ signify the forms -with constant differentiating characters, and _Aa_ the hybrid form -of both. It includes in three different classes four individuals. In -the formation of these, pollen and egg cells of the form _A_ and _a_ -take part on the average equally in the fertilisation; hence each form -[occurs] twice, since four individuals are formed. There participate -consequently in the fertilisation-- - - The pollen cells _A_ + _A_ + _a_ + _a_ - The egg cells _A_ + _A_ + _a_ + _a_. - -It remains, therefore, purely a matter of chance which of the two sorts -of pollen will become united with each separate egg cell. According, -however, to the law of probability, it will always happen, on the -average of many cases, that each pollen form _A_ and _a_ will unite -equally often with each egg cell form _A_ and _a_, consequently one of -the two pollen cells _A_ in the fertilisation will meet with the egg -cell _A_ and the other with an egg cell _a_, and so likewise one pollen -cell _a_ will unite with an egg cell _A_, and the other with egg cell -_a_. - -Pollen cells _A_ _A_ _a_ _a_ - | \ / | - | \ / | - | x | - | / \ | - | / \ | - \|/ \/ \/ \|/ -Egg cells _A_ _A_ _a_ _a_ - -The result of the fertilisation may be made clear by putting the signs -for the conjoined egg and pollen cells in the form of fractions, those -for the pollen cells above and those for the egg cells below the line. -We then have - - _A_/_A_ + _A_/_a_ + _a_/_A_ + _a_/_a_. - -In the first and fourth term the egg and pollen cells are of like kind, -consequently the product of their union must be constant, viz. _A_ and -_a_; in the second and third, on the other hand, there again results a -union of the two differentiating characters of the stocks, consequently -the forms resulting from these fertilisations are identical with -those of the hybrid from which they sprang. _There occurs accordingly -a repeated hybridisation._ This explains the striking fact that the -hybrids are able to produce, besides the two parental forms, offspring -which are like themselves; _A_/_a_ and _a_/_A_ both give the same union -_Aa_, since, as already remarked above, it makes no difference in the -result of fertilisation to which of the two characters the pollen or -egg cells belong. We may write then-- - - _A_/_A_ + _A_/_a_ + _a_/_A_ + _a_/_a_ = _A_ + 2_Aa_ + _a_. - -This represents the average result of the self-fertilisation of the -hybrids when two differentiating characters are united in them. In -solitary flowers and in solitary plants, however, the ratios in which -the forms of the series are produced may suffer not inconsiderable -fluctuations[41]. Apart from the fact that the numbers in which both -sorts of egg cells occur in the seed vessels can only be regarded as -equal on the average, it remains purely a matter of chance which of -the two sorts of pollen may fertilise each separate egg cell. For this -reason the separate values must necessarily be subject to fluctuations, -and there are even extreme cases possible, as were described earlier -in connection with the experiments on the form of the seed and the -colour of the albumen. The true ratios of the numbers can only be -ascertained by an average deduced from the sum of as many single values -as possible; the greater the number the more are merely chance elements -eliminated. - - [41] [Whether segregation by such units is more than purely - fortuitous could probably be determined by seriation.] - -The developmental series for hybrids in which two kinds of -differentiating characters are united contains among sixteen -individuals nine different forms, viz., _AB_ + _Ab_ + _aB_ + -_ab_ + 2_ABb_ + 2_aBb_ + 2_AaB_ + 2_Aab_ + 4_AaBb_. Between the -differentiating characters of the original stocks _Aa_ and _Bb_ four -constant combinations are possible, and consequently the hybrids -produce the corresponding four forms of egg and pollen cells _AB_, -_Ab_, _aB_, _ab_, and each of these will on the average figure four -times in the fertilisation, since sixteen individuals are included in -the series. Therefore the participators in the fertilisation are-- - -Pollen cells _AB_ + _AB_ + _AB_ + _AB_ + _Ab_ + _Ab_ + _Ab_ + _Ab_ + - _aB_ + _aB_ + _aB_ + _aB_ + _ab_ + _ab_ + _ab_ + _ab_. - -Egg cells _AB_ + _AB_ + _AB_ + _AB_ + _Ab_ + _Ab_ + _Ab_ + _Ab_ + - _aB_ + _aB_ + _aB_ + _aB_ + _ab_ + _ab_ + _ab_ + _ab_. - -In the process of fertilisation each pollen form unites on an average -equally often with each egg cell form, so that each of the four pollen -cells _AB_ unites once with one of the forms of egg cell _AB_, _Ab_, -_aB_, _ab_. In precisely the same way the rest of the pollen cells -of the forms _Ab_, _aB_, _ab_ unite with all the other egg cells. We -obtain therefore-- - -_AB_/_AB_ + _AB_/_Ab_ + _AB_/_aB_ + _AB_/_ab_ + _Ab_/_AB_ + _Ab_/_Ab_ + -_Ab_/_aB_ + _Ab_/_ab_ + _aB_/_AB_ + _aB_/_Ab_ + _aB_/_aB_ + _aB_/_ab_ + -_ab_/_AB_ + _ab_/_Ab_ + _ab_/_aB_ + _ab_/_ab_, - -or - -_AB_ + _ABb_ + _AaB_ + _AaBb_ + _ABb_ + _Ab_ + _AaBb_ + _Aab_ + _AaB_ + -_AaBb_ + _aB_ + _aBb_ + _AaBb_ + _Aab_ + _aBb_ + _ab_ = _AB_ + _Ab_ + -_aB_ + _ab_ + 2_ABb_ + 2_aBb_ + 2_AaB_ + 2_Aab_ + 4_AaBb_[42]. - - [42] [In the original the sign of equality (=) is here represented by - +, evidently a misprint.] - -In precisely similar fashion is the developmental series of hybrids -exhibited when three kinds of differentiating characters are conjoined -in them. The hybrids form eight various kinds of egg and pollen -cells--_ABC_, _ABc_, _AbC_, _Abc_, _aBC_, _aBc_, _abC_, _abc_--and each -pollen form unites itself again on the average once with each form of -egg cell. - -The law of combination of different characters which governs the -development of the hybrids finds therefore its foundation and -explanation in the principle enunciated, that the hybrids produce egg -cells and pollen cells which in equal numbers represent all constant -forms which result from the combinations of the characters brought -together in fertilisation. - - -EXPERIMENTS WITH HYBRIDS OF OTHER SPECIES OF PLANTS. - -It must be the object of further experiments to ascertain whether -the law of development discovered for _Pisum_ applies also to the -hybrids of other plants. To this end several experiments were recently -commenced. Two minor experiments with species of _Phaseolus_ have been -completed, and may be here mentioned. - -An experiment with _Phaseolus vulgaris_ and _Phaseolus nanus_ gave -results in perfect agreement. _Ph. nanus_ had together with the dwarf -axis simply inflated green pods. _Ph. vulgaris_ had, on the other hand, -an axis 10 feet to 12 feet high, and yellow coloured pods, constricted -when ripe. The ratios of the numbers in which the different forms -appeared in the separate generations were the same as with _Pisum_. -Also the development of the constant combinations resulted according to -the law of simple combination of characters, exactly as in the case of -_Pisum_. There were obtained-- - - Constant Axis Colour of Form of - combinations the unripe pods. the ripe pods. - - 1 long green inflated - 2 " " constricted - 3 " yellow inflated - 4 " " constricted - 5 short green inflated - 6 " " constricted - 7 " yellow inflated - 8 " " constricted - -The green colour of the pod, the inflated forms, and the long axis -were, as in _Pisum_, dominant characters. - -Another experiment with two very different species of _Phaseolus_ had -only a partial result. _Phaseolus nanus_, L., served as seed parent, -a perfectly constant species, with white flowers in short racemes and -small white seeds in straight, inflated, smooth pods; as pollen parent -was used _Ph. multiflorus_, W., with tall winding stem, purple-red -flowers in very long racemes, rough, sickle-shaped crooked pods, and -large seeds which bore black flecks and splashes on a peach-blood-red -ground. - -The hybrids had the greatest similarity to the pollen parent, but the -flowers appeared less intensely coloured. Their fertility was very -limited; from seventeen plants, which together developed many hundreds -of flowers, only forty-nine seeds in all were obtained. These were of -medium size, and were flecked and splashed similarly to those of _Ph. -multiflorus_, while the ground colour was not materially different. The -next year forty-four plants were raised from these seeds, of which only -thirty-one reached the flowering stage. The characters of _Ph. nanus_, -which had been altogether latent in the hybrids, reappeared in various -combinations; their ratio, however, with relation to the dominant -characters was necessarily very fluctuating owing to the small number -of trial plants. With certain characters, as in those of the axis and -the form of pod, it was, however, as in the case of _Pisum_, almost -exactly 1 : 3. - -Insignificant as the results of this experiment may be as regards -the determination of the relative numbers in which the various -forms appeared, it presents, on the other hand, the phenomenon of a -remarkable change of colour in the flowers and seed of the hybrids. In -_Pisum_ it is known that the characters of the flower- and seed-colour -present themselves unchanged in the first and further generations, and -that the offspring of the hybrids display exclusively the one or the -other of the characters of the original stocks[43]. It is otherwise -in the experiment we are considering. The white flowers and the -seed-colour of _Ph. nanus_ appeared, it is true, at once in the first -generation [_from_ the hybrids] in one fairly fertile example, but the -remaining thirty plants developed flower colours which were of various -grades of purple-red to pale violet. The colouring of the seed-coat was -no less varied than that of the flowers. No plant could rank as fully -fertile; many produced no fruit at all; others only yielded fruits from -the flowers last produced, which did not ripen. From fifteen plants -only were well-developed seeds obtained. The greatest disposition to -infertility was seen in the forms with preponderantly red flowers, -since out of sixteen of these only four yielded ripe seed. Three of -these had a similar seed pattern to _Ph. multiflorus_, but with a more -or less pale ground colour; the fourth plant yielded only one seed of -plain brown tint. The forms with preponderantly violet coloured flowers -had dark brown, black-brown, and quite black seeds. - - [43] [This is the only passage where Mendel can be construed as - asserting universal dominance for _Pisum_; and even here, having - regard to the rest of the paper, it is clearly unfair to represent - him as predicating more than he had seen in his own experiments. - Moreover in flower and seed-coat colour (which is here meant), using - his characters dominance must be almost universal, if not quite.] - -The experiment was continued through two more generations under -similar unfavourable circumstances, since even among the offspring of -fairly fertile plants there were still some which were less fertile -or even quite sterile. Other flower- and seed-colours than those -cited did not subsequently present themselves. The forms which in the -first generation [bred from the hybrids] contained one or more of the -recessive characters remained, as regards these, constant without -exception. Also of those plants which possessed violet flowers and -brown or black seed, some did not vary again in these respects in -the next generation; the majority, however, yielded, together with -offspring exactly like themselves, some which displayed white flowers -and white seed-coats. The red flowering plants remained so slightly -fertile that nothing can be said with certainty as regards their -further development. - -Despite the many disturbing factors with which the observations had -to contend, it is nevertheless seen by this experiment that the -development of the hybrids, with regard to those characters which -concern the form of the plants, follows the same laws as does _Pisum_. -With regard to the colour characters, it certainly appears difficult -to perceive a substantial agreement. Apart from the fact that from the -union of a white and a purple-red colouring a whole series of colours -results, from purple to pale violet and white, the circumstance is a -striking one that among thirty-one flowering plants only one received -the recessive character of the white colour, while in _Pisum_ this -occurs on the average in every fourth plant. - -Even these enigmatical results, however, might probably be explained -by the law governing _Pisum_ if we might assume that the colour of -the flowers and seeds of _Ph. multiflorus_ is a combination of two -or more entirely independent colours, which individually act like -any other constant character in the plant. If the flower colour A -were a combination of the individual characters _A_{1} + _A_{2} + -... which produce the total impression of a purple colouration, then -by fertilisation with the differentiating character, white colour, -_a_, there would be produced the hybrid unions _A_{1}_a_ + _A_{2}_a_ -+ ... and so would it be with the corresponding colouring of the -seed-coats[44]. According to the above assumption, each of these hybrid -colour unions would be independent, and would consequently develop -quite independently from the others. It is then easily seen that -from the combination of the separate developmental series a perfect -colour-series must result. If, for instance, _A_ = _A_{1} + _A_{2}, -then the hybrids _A_{1}_a_ and _A_{2}_a_ form the developmental series-- - - _A_{1} + 2_A_{1}_a_ + _a_ - _A_{2} + 2_A_{2}_a_ + _a_. - - [44] [It appears to me clear that this expression is incorrectly - given, and the argument regarding compound characters is consequently - not legitimately developed. The original compound character should - be represented as _A_{1}_A_{2}_A_{3} ... which when fertilised by - _a_{1} gives _A_{1}_A_{2}_A_{3} ... a as the hybrid of the first - generation. Mendel practically tells us these were all alike, - and there is nothing to suggest that they were diverse. When on - self-fertilisation, they break up, they will produce the gametes he - specifies; but they may also produce _A_{1}_A_{1} and _A_{2}_A_{2}, - _A_{1}_A_{2}_a_, &c., thereby introducing terms of a nature different - from any indicated by him. That this point is one of the highest - significance, both practical and theoretical, is evident at once.] - -The members of this series can enter into nine different combinations, -and each of these denotes another colour[45]-- - - 1 _A_{1}A_{2}_ 2 _A_{1}aA_{2}_ 1 _A_{2}a_ - 2 _A_{1}A_{2}a_ 4 _A_{1}aA_{2}a_ 2 _A_{2}aa_ - 1 _A_{1}a_ 2 _A_{1}aa_ 1 _aa_. - - [45] [It seems very doubtful if the zygotes are correctly represented - by the terms _A_{1}aA_{2}a_, _A_{2}aa_, _A_{1}aa_; for in the hybrids - _A_{1}a_, &c. the allelomorphs _A_{1}_ and _a_, &c. should by - hypothesis be separated in the gametes.] - -The figures prescribed for the separate combinations also indicate how -many plants with the corresponding colouring belong to the series. -Since the total is sixteen, the whole of the colours are on the average -distributed over each sixteen plants, but, as the series itself -indicates, in unequal proportions. - -Should the colour development really happen in this way, we could offer -an explanation of the case above described, viz. that the white flowers -and seed-coat colour only appeared once among thirty-one plants of the -first generation. This colouring appears only once in the series, and -could therefore also only be developed once in the average in each -sixteen, and with three colour characters only once even in sixty-four -plants. - -It must, however, not be forgotten that the explanation here attempted -is based on a mere hypothesis, only supported by the very imperfect -result of the experiment just described. It would, however, be well -worth while to follow up the development of colour in hybrids by -similar experiments, since it is probable that in this way we might -learn the significance of the extraordinary variety in the colouring of -our ornamental flowers. - -So far, little at present is known with certainty beyond the fact that -the colour of the flowers in most ornamental plants is an extremely -variable character. The opinion has often been expressed that the -stability of the species is greatly disturbed or entirely upset by -cultivation, and consequently there is an inclination to regard the -development of cultivated forms as a matter of chance devoid of rules; -the colouring of ornamental plants is indeed usually cited as an -example of great instability. It is, however, not clear why the simple -transference into garden soil should result in such a thorough and -persistent revolution in the plant organism. No one will seriously -maintain that in the open country the development of plants is ruled -by other laws than in the garden bed. Here, as there, changes of type -must take place if the conditions of life be altered, and the species -possesses the capacity of fitting itself to its new environment. It is -willingly granted that by cultivation the origination of new varieties -is favoured, and that by man’s labour many varieties are acquired -which, under natural conditions, would be lost; but nothing justifies -the assumption that the tendency to the formation of varieties is so -extraordinarily increased that the species speedily lose all stability, -and their offspring diverge into an endless series of extremely -variable forms. Were the change in the conditions of vegetation the -sole cause of variability we might expect that those cultivated plants -which are grown for centuries under almost identical conditions would -again attain constancy. That, as is well known, is not the case, -since it is precisely under such circumstances that not only the -most varied but also the most variable forms are found. It is only -the _Leguminosæ_, like _Pisum_, _Phaseolus_, _Lens_, whose organs of -fertilisation are protected by the keel, which constitute a noteworthy -exception. Even here there have arisen numerous varieties during a -cultural period of more than 1000 years; these maintain, however, under -unchanging environments a stability as great as that of species growing -wild. - -It is more than probable that as regards the variability of cultivated -plants there exists a factor which so far has received little -attention. Various experiments force us to the conclusion that our -cultivated plants, with few exceptions, are _members of various hybrid -series_, whose further development in conformity with law is changed -and hindered by frequent crossings _inter se_. The circumstance must -not be overlooked that cultivated plants are mostly grown in great -numbers and close together, affording the most favourable conditions -for reciprocal fertilisation between the varieties present and the -species itself. The probability of this is supported by the fact -that among the great array of variable forms solitary examples are -always found, which in one character or another remain constant, if -only foreign influence be carefully excluded. These forms develop -precisely as do those which are known to be members of the compound -hybrid series. Also with the most susceptible of all characters, that -of colour, it cannot escape the careful observer that in the separate -forms the inclination to vary is displayed in very different degrees. -Among plants which arise from _one_ spontaneous fertilisation there -are often some whose offspring vary widely in the constitution and -arrangement of the colours, while others furnish forms of little -deviation, and among a greater number solitary examples occur which -transmit the colour of the flowers unchanged to their offspring. The -cultivated species of _Dianthus_ afford an instructive example of -this. A white-flowered example of _Dianthus caryophyllus_, which itself -was derived from a white-flowered variety, was shut up during its -blooming period in a greenhouse; the numerous seeds obtained therefrom -yielded plants entirely white-flowered like itself. A similar result -was obtained from a subspecies, with red flowers somewhat flushed with -violet, and one with flowers white, striped with red. Many others, on -the other hand, which were similarly protected, yielded progeny which -were more or less variously coloured and marked. - -Whoever studies the colouration which results in ornamental plants -from similar fertilisation can hardly escape the conviction that here -also the development follows a definite law which possibly finds -its expression _in the combination of several independent colour -characters_. - - -CONCLUDING REMARKS. - -It can hardly fail to be of interest to compare the observations made -regarding _Pisum_ with the results arrived at by the two authorities -in this branch of knowledge, Kölreuter and Gärtner, in their -investigations. According to the opinion of both, the hybrids in outer -appearance present either a form intermediate between the original -species, or they closely resemble either the one or the other type, and -sometimes can hardly be discriminated from it. From their seeds usually -arise, if the fertilisation was effected by their own pollen, various -forms which differ from the normal type. As a rule, the majority of -individuals obtained by one fertilisation maintain the hybrid form, -while some few others come more like the seed parent, and one or other -individual approaches the pollen parent. This, however, is not the case -with all hybrids without exception. With some the offspring have more -nearly approached, some the one and some the other, original stock, -or they all incline more to one or the other side; while with others -_they remain perfectly like the hybrid_ and continue constant in their -offspring. The hybrids of varieties behave like hybrids of species, but -they possess greater variability of form and a more pronounced tendency -to revert to the original type. - -With regard to the form of the hybrids and their development, as a rule -an agreement with the observations made in _Pisum_ is unmistakable. It -is otherwise with the exceptional cases cited. Gärtner confesses even -that the exact determination whether a form bears a greater resemblance -to one or to the other of the two original species often involved -great difficulty, so much depending upon the subjective point of view -of the observer. Another circumstance could, however, contribute to -render the results fluctuating and uncertain, despite the most careful -observation and differentiation; for the experiments plants were mostly -used which rank as good species and are differentiated by a large -number of characters. In addition to the sharply defined characters, -where it is a question of greater or less similarity, those characters -must also be taken into account which are often difficult to define -in words, but yet suffice, as every plant specialist knows, to give -the forms a strange appearance. If it be accepted that the development -of hybrids follows the law which is valid for _Pisum_, the series -in each separate experiment must embrace very many forms, since the -number of the components, as is known, increases with the number of -the differentiating characters in _cubic ratio_. With a relatively -small number of experimental-plants the result therefore could only be -approximately right, and in single cases might fluctuate considerably. -If, for instance, the two original stocks differ in seven characters, -and 100 and 200 plants were raised from the seeds of their hybrids to -determine the grade of relationship of the offspring, we can easily see -how uncertain the decision must become, since for seven differentiating -characters the combination series contains 16,384 individuals under -2187 various forms; now one and then another relationship could assert -its predominance, just according as chance presented this or that form -to the observer in a majority of cases. - -If, furthermore, there appear among the differentiating characters at -the same time dominant characters, which are transferred entire or -nearly unchanged to the hybrids, then in the terms of the developmental -series that one of the two original stocks which possesses the -majority of dominant characters must always be predominant. In the -experiment described relative to _Pisum_, in which three kinds of -differentiating characters were concerned, all the dominant characters -belonged to the seed parent. Although the terms of the series in their -internal composition approach both original stock plants equally, -in this experiment the type of the seed parent obtained so great -a preponderance that out of each sixty-four plants of the first -generation fifty-four exactly resembled it, or only differed in one -character. It is seen how rash it may be under such circumstances to -draw from the external resemblances of hybrids conclusions as to their -internal nature. - -Gärtner mentions that in those cases where the development was regular -among the offspring of the hybrids the two original species were not -reproduced, but only a few closely approximating individuals. With -very extended developmental series it could not in fact be otherwise. -For seven differentiating characters, for instance, among more than -16,000 individuals--offspring of the hybrids--each of the two original -species would occur only once. It is therefore hardly possible that -these should appear at all among a small number of experimental plants; -with some probability, however, we might reckon upon the appearance in -the series of a few forms which approach them. - -We meet with an _essential difference_ in those hybrids which remain -constant in their progeny and propagate themselves as truly as the pure -species. According to Gärtner, to this class belong the _remarkably -fertile hybrids_ _Aquilegia atropurpurea canadensis_, _Lavatera -pseudolbia thuringiaca_, _Geum urbano-rivale_, and some _Dianthus_ -hybrids; and, according to Wichura, the hybrids of the Willow species. -For the history of the evolution of plants this circumstance is of -special importance, since constant hybrids acquire the status of -new species. The correctness of this is evidenced by most excellent -observers, and cannot be doubted. Gärtner had opportunity to follow -up _Dianthus Armeria deltoides_ to the tenth generation, since it -regularly propagated itself in the garden. - -With _Pisum_ it was shown by experiment that the hybrids form egg and -pollen cells of _different_ kinds, and that herein lies the reason -of the variability of their offspring. In other hybrids, likewise, -whose offspring behave similarly we may assume a like cause; for -those, on the other hand, which remain constant the assumption appears -justifiable that their fertilising cells are all alike and agree with -the foundation-cell [fertilised ovum] of the hybrid. In the opinion of -renowned physiologists, for the purpose of propagation one pollen cell -and one egg cell unite in Phanerogams[46] into a single cell, which -is capable by assimilation and formation of new cells to become an -independent organism. This development follows a constant law, which -is founded on the material composition and arrangement of the elements -which meet in the cell in a vivifying union. If the reproductive cells -be of the same kind and agree with the foundation cell [fertilised -ovum] of the mother plant, then the development of the new individual -will follow the same law which rules the mother plant. If it chance -that an egg cell unites with a _dissimilar_ pollen cell, we must then -assume that between those elements of both cells, which determine -the mutual differences, some sort of compromise is effected. The -resulting compound cell becomes the foundation of the hybrid organism, -the development of which necessarily follows a different scheme from -that obtaining in each of the two original species. If the compromise -be taken to be a complete one, in the sense, namely, that the hybrid -embryo is formed from cells of like kind, in which the differences are -_entirely and permanently accommodated_ together, the further result -follows that the hybrids, like any other stable plant species, remain -true to themselves in their offspring. The reproductive cells which are -formed in their seed vessels and anthers are of one kind, and agree -with the fundamental compound cell [fertilised ovum]. - - [46] In _Pisum_ it is placed beyond doubt that for the formation of - the new embryo a perfect union of the elements of both fertilising - cells must take place. How could we otherwise explain that among - the offspring of the hybrids both original types reappear in equal - numbers and with all their peculiarities? If the influence of the - egg cell upon the pollen cell were only external, if it fulfilled - the _rôle_ of a nurse only, then the result of each artificial - fertilisation could be no other than that the developed hybrid - should exactly resemble the pollen parent, or at any rate do so very - closely. This the experiments so far have in no wise confirmed. An - evident proof of the complete union of the contents of both cells is - afforded by the experience gained on all sides that it is immaterial, - as regards the form of the hybrid, which of the original species is - the seed parent or which the pollen parent. - -With regard to those hybrids whose progeny is _variable_ we may perhaps -assume that between the differentiating elements of the egg and pollen -cells there also occurs a compromise, in so far that the formation of a -cell as foundation of the hybrid becomes possible; but, nevertheless, -the arrangement between the conflicting elements is only temporary and -does not endure throughout the life of the hybrid plant. Since in the -habit of the plant no changes are perceptible during the whole period -of vegetation, we must further assume that it is only possible for -the differentiating elements to liberate themselves from the enforced -union when the fertilising cells are developed. In the formation of -these cells all existing elements participate in an entirely free and -equal arrangement, in which it is only the differentiating ones which -mutually separate themselves. In this way the production would be -rendered possible of as many sorts of egg and pollen cells as there are -combinations possible of the formative elements. - -The attribution attempted here of the essential difference in the -development of hybrids to _a permanent or temporary union_ of the -differing cell elements can, of course, only claim the value of an -hypothesis for which the lack of definite data offers a wide field. -Some justification of the opinion expressed lies in the evidence -afforded by _Pisum_ that the behaviour of each pair of differentiating -characters in hybrid union is independent of the other differences -between the two original plants, and, further, that the hybrid -produces just so many kinds of egg and pollen cells as there are -possible constant combination forms. The differentiating characters -of two plants can finally, however, only depend upon differences in -the composition and grouping of the elements which exist in the -foundation-cells [fertilised ova] of the same in vital interaction[47]. - - [47] “_Welche in den Grundzellen derselben in lebendiger - Wechselwirkung stehen._” - -Even the validity of the law formulated for _Pisum_ requires still to -be confirmed, and a repetition of the more important experiments is -consequently much to be desired, that, for instance, relating to the -composition of the hybrid fertilising cells. A differential [element] -may easily escape the single observer[48], which although at the outset -may appear to be unimportant, may yet accumulate to such an extent -that it must not be ignored in the total result. Whether the variable -hybrids of other plant species observe an entire agreement must also -be first decided experimentally. In the meantime we may assume that in -material points a difference in principle can scarcely occur, since the -unity in the developmental plan of organic life is beyond question. - - [48] “_Dem einzelnen Beobachter kann leicht ein Differenziale - entgehen._” - -In conclusion, the experiments carried out by Kölreuter, Gärtner, -and others with respect to _the transformation of one species into -another by artificial fertilisation_ merit special mention. A special -importance has been attached to these experiments, and Gärtner reckons -them among “the most difficult of all in hybridisation.” - -If a species _A_ is to be transformed into a species _B_, both must be -united by fertilisation and the resulting hybrids then be fertilised -with the pollen of _B_; then, out of the various offspring resulting, -that form would be selected which stood in nearest relation to _B_ and -once more be fertilised with _B_ pollen, and so continuously until -finally a form is arrived at which is like _B_ and constant in its -progeny. By this process the species _A_ would change into the species -_B_. Gärtner alone has effected thirty such experiments with plants of -genera _Aquilegia_, _Dianthus_, _Geum_, _Lavatera_, _Lychnis_, _Malva_, -_Nicotiana_, and _Œnothera_. The period of transformation was not alike -for all species. While with some a triple fertilisation sufficed, -with others this had to be repeated five or six times, and even in -the same species fluctuations were observed in various experiments. -Gärtner ascribes this difference to the circumstance that “the specific -[_typische_] force by which a species, during reproduction, effects -the change and transformation of the maternal type varies considerably -in different plants, and that, consequently, the periods within which -the one species is changed into the other must also vary, as also the -number of generations, so that the transformation in some species is -perfected in more, and in others in fewer generations.” Further, the -same observer remarks “that in these transformation experiments a good -deal depends upon which type and which individual be chosen for further -transformation.” - -If it may be assumed that in these experiments the constitution of -the forms resulted in a similar way to that of _Pisum_, the entire -process of transformation would find a fairly simple explanation. -The hybrid forms as many kinds of egg cells as there are constant -combinations possible of the characters conjoined therein, and one -of these is always of the same kind as the fertilising pollen cells. -Consequently there always exists the possibility with all such -experiments that even from the second fertilisation there may result a -constant form identical with that of the pollen parent. Whether this -really be obtained depends in each separate case upon the number of -the experimental plants, as well as upon the number of differentiating -characters which are united by the fertilisation. Let us, for -instance, assume that the plants selected for experiment differed in -three characters, and the species _ABC_ is to be transformed into the -other species _abc_ by repeated fertilisation with the pollen of the -latter; the hybrids resulting from the first cross form eight different -kinds of egg cells, viz.: - -_ABC_, _ABc_, _AbC_, _aBC_, _Abc_, _aBc_, _abC_, _abc_. - -These in the second year of experiment are united again with the pollen -cells _abc_, and we obtain the series - -_AaBbCc_ + _AaBbc_ + _AabCc_ + _aBbCc_ + _Aabc_ + _aBbc_ + _abCc_ + -_abc_. - -Since the form _abc_ occurs once in the series of eight components, -it is consequently little likely that it would be missing among the -experimental plants, even were these raised in a smaller number, -and the transformation would be perfected already by a second -fertilisation. If by chance it did not appear, then the fertilisation -must be repeated with one of those forms nearest akin, _Aabc_, _aBbc_, -_abCc_. It is perceived that such an experiment must extend the farther -_the smaller the number of experimental plants and the larger the -number of differentiating characters_ in the two original species; -and that, furthermore, in the same species there can easily occur a -delay of one or even of two generations such as Gärtner observed. -The transformation of widely divergent species could generally only -be completed in five or six years of experiment, since the number of -different egg cells which are formed in the hybrid increases in square -ratio with the number of differentiating characters. - -Gärtner found by repeated experiments that the respective period of -transformation varies in many species, so that frequently a species -_A_ can be transformed into a species _B_ a generation sooner -than can species _B_ into species _A_. He deduces therefrom that -Kölreuter’s opinion can hardly be maintained that “the two natures -in hybrids are perfectly in equilibrium.” It appears, however, that -Kölreuter does not merit this criticism, but that Gärtner rather has -overlooked a material point, to which he himself elsewhere draws -attention, viz. that “it depends which individual is chosen for further -transformation.” Experiments which in this connection were carried out -with two species of _Pisum_ demonstrated that as regards the choice of -the fittest individuals for the purpose of further fertilisation it -may make a great difference which of two species is transformed into -the other. The two experimental plants differed in five characters, -while at the same time those of species _A_ were all dominant and -those of species _B_ all recessive. For mutual transformation _A_ -was fertilised with pollen of _B_, and _B_ with pollen of _A_, and -this was repeated with both hybrids the following year. With the -first experiment _B_/_A_ there were eighty-seven plants available in -the third year of experiment for the selections of individuals for -further crossing, and these were of the possible thirty-two forms; -with the second experiment _A_/_B_ seventy-three plants resulted, -which _agreed throughout perfectly in habit with the pollen parent_; -in their internal composition, however, they must have been just as -varied as the forms of the other experiment. A definite selection was -consequently only possible with the first experiment; with the second -some plants selected at random had to be excluded. Of the latter only -a portion of the flowers were crossed with the _A_ pollen, the others -were left to fertilise themselves. Among each five plants which were -selected in both experiments for fertilisation there agreed, as the -following year’s culture showed, with the pollen parent:-- - - 1st Experiment. 2nd Experiment. - 2 plants -- in all characters - 3 " -- " 4 " - -- 2 plants " 3 " - -- 2 " " 2 " - -- 1 plant " 1 character - -In the first experiment, therefore, the transformation was completed; -in the second, which was not continued further, two more fertilisations -would probably have been required. - -Although the case may not frequently occur that the dominant characters -belong exclusively to one or the other of the original parent plants, -it will always make a difference which of the two possesses the -majority. If the pollen parent shows the majority, then the selection -of forms for further crossing will afford a less degree of security -than in the reverse case, which must imply a delay in the period of -transformation, provided that the experiment is only considered as -completed when a form is arrived at which not only exactly resembles -the pollen plant in form, but also remains as constant in its progeny. - -Gärtner, by the results of these transformation experiments, was led to -oppose the opinion of those naturalists who dispute the stability of -plant species and believe in a continuous evolution of vegetation. He -perceives in the complete transformation of one species into another -an indubitable proof that species are fixed within limits beyond which -they cannot change. Although this opinion cannot be unconditionally -accepted we find on the other hand in Gärtner’s experiments a -noteworthy confirmation of that supposition regarding variability of -cultivated plants which has already been expressed. - -Among the experimental species there were cultivated plants, such as -_Aquilegia atropurpurea_ and _canadensis_, _Dianthus caryophyllus_, -_chinensis_, and _japonicus_, _Nicotiana rustica_ and _paniculata_, and -hybrids between these species lost none of their stability after four -or five generations[49]. - - [49] [The argument of these two last paragraphs appears to be that - though the general mutability of natural species might be doubtful, - yet among cultivated plants the transference of characters may be - accomplished, and may occur by integral steps until one species is - definitely “transformed” into the other.] - - - - -ON HIERACIUM-HYBRIDS OBTAINED BY ARTIFICIAL FERTILISATION - -By G. Mendel. - -(_Communicated to the Meeting 9 June, 1869[50]._) - -[50] [Published in _Verh. naturf. Ver. Brünn, Abhandlungen_, VIII. -1869, p. 26, which appeared in 1870.] - - -Although I have already undertaken many experiments in fertilisation -between species of _Hieracium_, I have only succeeded in obtaining the -following 6 hybrids, and only from one to three specimens of them. - - _H. Auricula_ ♀ × _H. aurantiacum_ ♂ - _H. Auricula_ ♀ × _H. Pilosella_ ♂ - _H. Auricula_ ♀ × _H. pratense_ ♂ - _H. echioides_[51] ♀ × _H. aurantiacum_ ♂ - _H. præaltum_ ♀ × _H. flagellare_ Rchb. ♂ - _H. præaltum_ ♀ × _H. aurantiacum_ ♂ - - [51] The plant used in this experiment is not exactly the typical _H. - echioides_. It appears to belong to the series transitional to _H. - præaltum_, but approaches more nearly to _H. echioides_ and for this - reason was reckoned as belonging to the latter. - -The difficulty of obtaining a larger number of hybrids is due to the -minuteness of the flowers and their peculiar structure. On account of -this circumstance it was seldom possible to remove the anthers from -the flowers chosen for fertilisation without either letting pollen -get on to the stigma or injuring the pistil so that it withered away. -As is well known, the anthers are united to form a tube, which closely -embraces the pistil. As soon as the flower opens, the stigma, already -covered with pollen, protrudes. In order to prevent self-fertilisation -the anther-tube must be taken out before the flower opens, and for this -purpose the bud must be slit up with a fine needle. If this operation -is attempted at a time when the pollen is mature, which is the case two -or three days before the flower opens, it is seldom possible to prevent -self-fertilisation; for with every care it is not easily possible to -prevent a few pollen grains getting scattered and communicated to -the stigma. No better result has been obtained hitherto by removing -the anthers at an earlier stage of development. Before the approach -of maturity the tender pistil and stigma are exceedingly sensitive -to injury, and even if they are not actually injured, they generally -wither and dry up after a little time if deprived of their protecting -investments. I hope to obviate this last misfortune by placing the -plants after the operation for two or three days in the damp atmosphere -of a greenhouse. An experiment lately made with _H. Auricula_ treated -in this way gave a good result. - -To indicate the object with which these fertilisation experiments were -undertaken, I venture to make some preliminary remarks respecting the -genus _Hieracium_. This genus possesses such an extraordinary profusion -of distinct forms that no other genus of plants can compare with it. -Some of these forms are distinguished by special peculiarities and -may be taken as type-forms of species, while all the rest represent -intermediate and transitional forms by which the type-forms are -connected together. The difficulty in the separation and delimitation -of these forms has demanded the close attention of the experts. -Regarding no other genus has so much been written or have so many and -such fierce controversies arisen, without as yet coming to a definite -conclusion. It is obvious that no general understanding can be arrived -at, so long as the value and significance of the intermediate and -transitional forms is unknown. - -Regarding the question whether and to what extent hybridisation plays -a part in the production of this wealth of forms, we find very various -and conflicting views held by leading botanists. While some of them -maintain that this phenomenon has a far-reaching influence, others, for -example, Fries, will have nothing to do with hybrids in _Hieracia_. -Others take up an intermediate position; and while granting that -hybrids are not rarely formed between the species in a wild state, -still maintain that no great importance is to be attached to the fact, -on the ground that they are only of short duration. The [suggested] -causes of this are partly their restricted fertility or complete -sterility; partly also the knowledge, obtained by experiment, that in -hybrids self-fertilisation is always prevented if pollen of one of -the parent-forms reaches the stigma. On these grounds it is regarded -as inconceivable that _Hieracium_ hybrids can constitute and maintain -themselves as fully fertile and constant forms when growing near their -progenitors. - -The question of the origin of the numerous and constant intermediate -forms has recently acquired no small interest since a famous -_Hieracium_ specialist has, in the spirit of the Darwinian teaching, -defended the view that these forms are to be regarded as [arising] from -the transmutation of lost or still existing species. - -From the nature of the subject it is clear that without an exact -knowledge of the structure and fertility of the hybrids and the -condition of their offspring through several generations no one -can undertake to determine the possible influence exercised by -hybridisation over the multiplicity of intermediate forms in -_Hieracium_. The condition of the _Hieracium_ hybrids in the range -we are concerned with must necessarily be determined by experiments; -for we do not possess a complete theory of hybridisation, and we -may be led into erroneous conclusions if we take rules deduced from -observation of certain other hybrids to be Laws of hybridisation, and -try to apply them to _Hieracium_ without further consideration. If by -the experimental method we can obtain a sufficient insight into the -phenomenon of hybridisation in _Hieracium_, then by the help of the -experience which has been collected respecting the structural relations -of the wild forms, a satisfactory judgment in regard to this question -may become possible. - -Thus we may express the object which was sought after in these -experiments. I venture now to relate the very slight results which I -have as yet obtained with reference to this object. - - -1. Respecting the structure of the hybrids, we have to record the -striking phenomenon that the forms hitherto obtained by similar -fertilisation are not identical. The hybrids _H. præaltum_ ♀ x _H. -aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ are each -represented by two, and _H. Auricula_ ♀ x _H. pratense_ ♂ by three -individuals, while as to the remainder only one of each has been -obtained. - -If we compare the individual characters of the hybrids with the -corresponding characters of the two parent types, we find that they -sometimes present intermediate structures, but are sometimes so near -to one of the parent characters that the [corresponding] character -of the other has receded considerably or almost evades observation. -So, for instance, we see in one of the two forms of _H. Auricula_ ♀ x -_H. aurantiacum_ ♂ pure yellow disc-florets; only the petals of the -marginal florets are on the outside tinged with red to a scarcely -noticeable degree: in the other on the contrary the colour of these -florets comes very near to _H. aurantiacum_, only in the centre of the -disc the orange red passes into a deep golden-yellow. This difference -is noteworthy, for the flower-colour in _Hieracium_ has the value of a -constant character. Other similar cases are to be found in the leaves, -the peduncles, &c. - -If the hybrids are compared with the parent types as regards the sum -total of their characters, then the two forms of _H. præaltum_ ♀ x _H. -aurantiacum_ ♂ constitute approximately intermediate forms which do -not agree in certain characters. On the contrary in _H. Auricula_ ♀ -x _H. aurantiacum_ ♂ and in _H. Auricula_ ♀ x _H. pratense_ ♂ we see -the forms widely divergent, so that one of them is nearer to the one -and the other to the other parental type, while in the case of the -last-named hybrid there is still a third which is almost precisely -intermediate between them. - -The conviction is then forced on us that we have here only single terms -in an unknown series which may be formed by the direct action of the -pollen of one species on the egg-cells of another. - - -2. With a single exception the hybrids in question form seeds capable -of germination. _H. echioides_ ♀ x _H. aurantiacum_ ♂ may be described -as fully fertile; _H. præaltum_ ♀ x _H. flagellare_ ♂ as fertile; _H. -præaltum_ ♀ x _H. aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. pratense_ ♂ -as partially fertile; _H. Auricula_ ♀ x _H. Pilosella_ ♂ as slightly -fertile, and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ as unfertile. Of the -two forms of the last named hybrid, the red-flowered one was completely -sterile, but from the yellow-flowered one a single well-formed seed -was obtained. Moreover it must not pass unmentioned that among the -seedlings of the partially fertile hybrid _H. præaltum_ ♀ x _H. -aurantiacum_ ♂ there was one plant which possessed full fertility. - - -[3.] As yet the offspring produced by self-fertilisation of the hybrids -have not varied, but agree in their characters both with each other and -with the hybrid plant from which they were derived. - -From _H. præaltum_ ♀ x _H. flagellare_ ♂ two generations have flowered; -from _H. echioides_ ♀ x _H. aurantiacum_ ♂, _H. præaltum_ ♀ x _H. -aurantiacum_ ♂, _H. Auricula_ ♀ x _H. Pilosella_ ♂ one generation in -each case has flowered. - - -4. The fact must be declared that in the case of the fully fertile -hybrid _H. echioides_ ♀ x _H. aurantiacum_ ♂ the pollen of the parent -types was not able to prevent self-fertilisation, though it was applied -in great quantity to the stigmas protruding through the anther-tubes -when the flowers opened. - -From two flower-heads treated in this way seedlings were produced -resembling this hybrid plant. A very similar experiment, carried -out this summer with the partially fertile _H. præaltum_ ♀ x _H. -aurantiacum_ ♂ led to the conclusion that those flower-heads in which -pollen of the parent type or of some other species had been applied -to the stigmas, developed a notably larger number of seeds than those -which had been left to self-fertilisation alone. The explanation of -this result must only be sought in the circumstance that as a large -part of the pollen-grains of the hybrid, examined microscopically, -show a defective structure, a number of egg-cells capable of -fertilisation do not become fertilised by their own pollen in the -ordinary course of self-fertilisation. - -It not rarely happens that in fully fertile species in the wild state -the formation of the pollen fails, and in many anthers not a single -good grain is developed. If in these cases seeds are nevertheless -formed, such fertilisation must have been effected by foreign pollen. -In this way hybrids may easily arise by reason of the fact that many -forms of insects, notably the industrial Hymenoptera, visit the flowers -of _Hieracia_ with great zeal and are responsible for the pollen -which easily sticks to their hairy bodies reaching the stigmas of -neighbouring plants. - -From the few facts that I am able to contribute it will be evident -the work scarcely extends beyond its first inception. I must express -some scruple in describing in this place an account of experiments -just begun. But the conviction that the prosecution of the proposed -experiments will demand a whole series of years, and the uncertainty -whether it will be granted to me to bring the same to a conclusion have -determined me to make the present communication. By the kindness of Dr -Nägeli, the Munich Director, who was good enough to send me species -which were wanting, especially from the Alps, I am in a position to -include a larger number of forms in my experiments. I venture to hope -even next year to be able to contribute something more by way of -extension and confirmation of the present account. - -If finally we compare the described result, still very uncertain, -with those obtained by crosses made between forms of _Pisum_, which -I had the honour of communicating in the year 1865, we find a very -real distinction. In _Pisum_ the hybrids, obtained from the immediate -crossing of two forms, have in all cases the same type, but their -posterity, on the contrary, are variable and follow a definite law in -their variations. In _Hieracium_ according to the present experiments -the exactly opposite phenomenon seems to be exhibited. Already in -describing the _Pisum_ experiments it was remarked that there are also -hybrids whose posterity do not vary, and that, for example, according -to Wichura the hybrids of _Salix_ reproduce themselves like pure -species. In _Hieracium_ we may take it we have a similar case. Whether -from this circumstance we may venture to draw the conclusion that the -polymorphism of the genera _Salix_ and _Hieracium_ is connected with -the special condition of their hybrids is still an open question, which -may well be raised but not as yet answered. - - - - -A DEFENCE OF MENDEL’S PRINCIPLES OF HEREDITY. - - “_The most fertile men of science have made blunders, and their - consciousness of such slips has been retribution enough; it is only - their more sterile critics who delight to dwell too often and too - long on such mistakes._” BIOMETRIKA, 1901. - - -INTRODUCTORY. - -On the rediscovery and confirmation of Mendel’s Law by de Vries, -Correns, and Tschermak two years ago, it became clear to many -naturalists, as it certainly is to me, that we had found a principle -which is destined to play a part in the Study of Evolution comparable -only with the achievement of Darwin--that after the weary halt of forty -years we have at last begun to march. - -If we look back on the post-Darwinian period we recognize one notable -effort to advance. This effort--fruitful as it proved, memorable as it -must ever be--was that made by Galton when he enuntiated his Law of -Ancestral Heredity, subsequently modified and restated by Karl Pearson. -Formulated after long and laborious inquiry, this principle beyond -question gives us an expression including and denoting many phenomena -in which previously no regularity had been detected. But to practical -naturalists it was evident from the first that there are great groups -of facts which could not on any interpretation be brought within the -scope of Galton’s Law, and that by no emendation could that Law be -extended to reach them. The existence of these phenomena pointed to a -different physiological conception of heredity. Now it is precisely -this conception that Mendel’s Law enables us to form. Whether the -Mendelian principle can be extended so as to include some apparently -Galtonian cases is another question, respecting which we have as yet no -facts to guide us, but we have certainly no warrant for declaring such -an extension to be impossible. - -Whatever answer the future may give to that question, it is clear from -this moment that every case which obeys the Mendelian principle is -removed finally and irretrievably from the operations of the Law of -Ancestral Heredity. - -At this juncture Professor Weldon intervenes as a professed exponent -of Mendel’s work. It is not perhaps to a devoted partisan of the Law -of Ancestral Heredity that we should look for the most appreciative -exposition of Mendel, but some bare measure of care and accuracy in -representation is demanded no less in justice to fine work, than by the -gravity of the issue. - -Professor Weldon’s article appears in the current number of -_Biometrika_, Vol. I. Pt. II. which reached me on Saturday, Feb. 8. -The paper opens with what purports to be a restatement of Mendel’s -experiments and results. In this “restatement” a large part of Mendel’s -experiments--perhaps the most significant--are not referred to at -all. The perfect simplicity and precision of Mendel’s own account are -destroyed; with the result that the reader of Professor Weldon’s paper, -unfamiliar with Mendel’s own memoir, can scarcely be blamed if he fail -to learn the essence of the discovery. Of Mendel’s conception of the -hybrid as a distinct entity with characters proper to itself, apart -from inheritance--the most novel thing in the whole paper--Professor -Weldon gives no word. Upon this is poured an undigested mass of -miscellaneous “facts” and statements from which the reader is asked -to conclude, first, that a proposition attributed to Mendel regarding -dominance of one character is not of “general”[52] application, and -finally that “all work based on Mendel’s method” is “vitiated” by a -“fundamental mistake,” namely “the neglect of ancestry[53].” - - [52] The words “general” and “universal” appear to be used by - Professor Weldon as interchangeable. Cp. Weldon, p. 235 and - elsewhere, with Abstract given below. - - [53] These words occur p. 252: “The fundamental mistake which - vitiates all work based upon Mendel’s method is the neglect of - ancestry, and the attempt to regard the whole effect upon offspring - produced by a particular parent, as due to the existence in the - parent of particular structural characters, &c.” As a matter of fact - the view indicated in these last words is especially repugnant to the - Mendelian principle, as will be seen. - -To find a parallel for such treatment of a great theme in biology we -must go back to those writings of the orthodox which followed the -appearance of the “Origin of Species.” - -On 17th December 1900 I delivered a Report to the Evolution Committee -of the Royal Society on the experiments in Heredity undertaken by Miss -E. R. Saunders and myself. This report has been offered to the Society -for publication and will I understand shortly appear. In it we have -attempted to show the extraordinary significance of Mendel’s principle, -to point out what in his results is essential and what subordinate, the -ways in which the principle can be extended to apply to a diversity -of more complex phenomena--of which some are incautiously cited by -Professor Weldon as conflicting facts--and lastly to suggest a few -simple terms without which (or some equivalents) the discussion of such -phenomena is difficult. Though it is impossible here to give an outline -of facts and reasoning there set out at length, I feel that his article -needs an immediate reply. Professor Weldon is credited with exceptional -familiarity with these topics, and his paper is likely to be accepted -as a sufficient statement of the case. Its value will only be known to -those who have either worked in these fields themselves or have been at -the trouble of thoughtfully studying the original materials. - -The nature of Professor Weldon’s article may be most readily indicated -if I quote the summary of it issued in a paper of abstracts sent out -with Review copies of the Part. This paper was most courteously sent to -me by an editor of _Biometrika_ in order to call my attention to the -article on Mendel, a subject in which he knew me to be interested. The -abstract is as follows. - - “Few subjects have excited so much interest in the last year or two - as the laws of inheritance in hybrids. Professor W. F. R. Weldon - describes the results obtained by Mendel by crossing races of Peas - which differed in one or more of seven characters. From a study of - the work of other observers, and from examination of the ‘Telephone’ - group of hybrids, the conclusion is drawn that Mendel’s results - do not justify any general statement concerning inheritance in - cross-bred Peas. A few striking cases of other cross-bred plants and - animals are quoted to show that the results of crossing cannot, as - Mendel and his followers suggest, be predicted from a knowledge of - the characters of the two parents crossed without knowledge of the - more remote ancestry.” - -Such is the judgment a fellow-student passes on this mind - - “_Voyaging through strange seas of thought alone._” - -The only conclusion which most readers could draw from this abstract -and indeed from the article it epitomizes, is that Mendel’s discovery -so far from being of paramount importance, rests on a basis which -Professor Weldon has shown to be insecure, and that an error has come -in through disregard of the law of Ancestral Heredity. On examining the -paper it is perfectly true that Professor Weldon is careful nowhere -directly to question Mendel’s facts or his interpretation of them, -for which indeed in some places he even expresses a mild enthusiasm, -but there is no mistaking the general purpose of the paper. It must -inevitably produce the impression that the importance of the work -has been greatly exaggerated and that supporters of current views on -Ancestry may reassure themselves. That this is Professor Weldon’s own -conclusion in the matter is obvious. After close study of his article -it is evident to me that Professor Weldon’s criticism is baseless and -for the most part irrelevant, and I am strong in the conviction that -the cause which will sustain damage from this debate is not that of -Mendel. - - -I. THE MENDELIAN PRINCIPLE OF PURITY OF GERM-CELLS AND THE LAWS OF -HEREDITY BASED ON ANCESTRY. - -Professor Weldon’s article is entitled “Mendel’s Laws of Alternative -Inheritance in Peas.” This title expresses the scope of Mendel’s work -and discovery none too precisely and even exposes him to distinct -misconception. - -To begin with, it says both too little and too much. Mendel did -certainly determine Laws of Inheritance in peas--not precisely the -laws Professor Weldon has been at the pains of drafting, but of that -anon. Having done so, he knew what his discovery was worth. He saw, -and rightly, that he had found a principle which _must_ govern a wide -area of phenomena. He entitles his paper therefore “_Versuche über -Pflanzen-Hybriden_,” or, Experiments in Plant-Hybridisation. - -Nor did Mendel start at first with any particular intention respecting -Peas. He tells us himself that he wanted to find the laws of -inheritance in _hybrids_, which he suspected were definite, and that -after casting about for a suitable subject, he found one in peas, for -the reasons he sets out. - -In another respect the question of title is much more important. By -the introduction of the word “Alternative” the suggestion is made that -the Mendelian principle applies peculiarly to cases of “alternative” -inheritance. Mendel himself makes no such limitation in his earlier -paper, though perhaps by rather remote implication in the second, to -which the reader should have been referred. On the contrary, he wisely -abstains from prejudicial consideration of unexplored phenomena. - - * * * * * - -To understand the significance of the word “alternative” as introduced -by Professor Weldon we must go back a little in the history of these -studies. In the year 1897 Galton formally announced the Law of -Ancestral Heredity referred to in the _Introduction_, having previously -“stated it briefly and with hesitation” in _Natural Inheritance_, -p. 134. In 1898 Professor Pearson published his modification and -generalisation of Galton’s Law, introducing a correction of admitted -theoretical importance, though it is not in question that the principle -thus restated is fundamentally not very different from Galton’s[54]. -_It is an essential part of the Galton-Pearson Law of Ancestral -Heredity that in calculating the probable structure of each descendant -the structure of each several ancestor must be brought to account._ - - [54] I greatly regret that I have not a precise understanding of the - basis of the modification proposed by Pearson. His treatment is in - algebraical form and beyond me. Nevertheless I have every confidence - that the arguments are good and the conclusion sound. I trust it may - not be impossible for him to provide the non-mathematical reader with - a paraphrase of his memoir. The arithmetical differences between the - original and the modified law are of course clear. - -Professor Weldon now tells us that these two papers of Galton and of -Professor Pearson have “given us an expression for the effects of -_blended_ inheritance which seems likely to prove generally applicable, -though the constants of the equations which express the relation -between divergence from the mean in one generation, and that in -another, may require modification in special cases. Our knowledge of -_particulate_ or mosaic inheritance, and of _alternative_ inheritance, -is however still rudimentary, and there is so much contradiction -between the results obtained by different observers, that the evidence -available is difficult to appreciate.” - -But Galton stated (p. 401) in 1897 that his statistical law of heredity -“appears to be universally applicable to bi-sexual descent.” Pearson -in re-formulating the principle in 1898 made no reservation in regard -to “alternative” inheritance. On the contrary he writes (p. 393) that -“if Mr Galton’s law can be firmly established, _it is a complete -solution, at any rate to a first approximation, of the whole problem -of heredity_,” and again (p. 412) that “it is highly probable that -it [this law] is the simple descriptive statement which brings into -a single focus all the complex lines of hereditary influence. If -Darwinian evolution be natural selection combined with _heredity_, -then the single statement which embraces the whole field of heredity -must prove almost as epoch-making as the law of gravitation to the -astronomer[55].” - - [55] I have searched Professor Pearson’s paper in vain for any - considerable reservation regarding or modification of this general - statement. Professor Pearson enuntiates the law as “only correct - on certain limiting hypotheses,” but he declares that of these the - most important is “the absence of reproductive selection, i.e. the - negligible correlation of fertility with the inherited character, and - the absence of sexual selection.” The case of in-and-in breeding is - also reserved. - -As I read there comes into my mind that other fine passage where -Professor Pearson warns us - - “There is an insatiable desire in the human breast to resume in some - short formula, some brief statement, the facts of human experience. - It leads the savage to ‘account’ for all natural phenomena by - deifying the wind and the stream and the tree. It leads civilized - man, on the other hand, to express his emotional experience in works - of art, and his physical and mental experience in the formulae or - so-called laws of science[56].” - - [56] K. Pearson, _Grammar of Science_, 2nd ed. 1900, p. 36. - -No naturalist who had read Galton’s paper and had tried to apply it to -the facts he knew could fail to see that here was a definite advance. -We could all perceive phenomena that were in accord with it and there -was no reasonable doubt that closer study would prove that accord to -be close. It was indeed an occasion for enthusiasm, though no one -acquainted with the facts of experimental breeding could consider the -suggestion of universal application for an instant. - -But two years have gone by, and in 1900 Pearson writes[57] that the -values obtained from the Law of Ancestral Heredity - - [57] _Grammar of Science_, 2nd ed. 1900, p. 480. - - “seem to fit the observed facts fairly well in the case of _blended_ - inheritance. In other words we have a certain amount of evidence in - favour of the conclusion: _That whenever the sexes are equipotent, - blend their characters and mate pangamously, all characters will be - inherited at the same rate_,” - -or, again in other words, that the Law of Ancestral Heredity after -the glorious launch in 1898 has been home for a complete refit. The -top-hamper is cut down and the vessel altogether more manageable; -indeed she looks trimmed for most weathers. Each of the qualifications -now introduced wards off whole classes of dangers. Later on (pp. 487–8) -Pearson recites a further list of cases regarded as exceptional. “All -characters will be inherited at the same rate” might indeed almost be -taken to cover the results in Mendelian cases, though the mode by which -those results are arrived at is of course wholly different. - -Clearly we cannot speak of the Law of Gravitation now. Our Tycho Brahe -and our Kepler, with the yet more distant Newton, are appropriately -named as yet to come[58]. - - [58] _Phil. Trans._ 1900, vol. 195, A, p. 121. - -But the truth is that even in 1898 such a comparison was scarcely -happy. Not to mention moderns, these high hopes had been finally -disposed of by the work of the experimental breeders such as Kölreuter, -Knight, Herbert, Gärtner, Wichura, Godron, Naudin, and many more. To -have treated as non-existent the work of this group of naturalists, -who alone have attempted to solve the problems of heredity and -species--Evolution, as we should now say--by the only sound -method--_experimental breeding_--to leave out of consideration almost -the whole block of evidence collected in _Animals and Plants_--Darwin’s -finest legacy as I venture to declare--was unfortunate on the part of -any exponent of Heredity, and in the writings of a professed naturalist -would have been unpardonable. But even as modified in 1900 the Law -of Ancestral Heredity is heavily over-sparred, and any experimental -breeder could have increased Pearson’s list of unconformable cases by -as many again. - -But to return to Professor Weldon. He now repeats that the Law of -Ancestral Heredity seems likely to prove generally applicable to -_blended_ inheritance, but that the case of _alternative_ inheritance -is for the present reserved. We should feel more confidence in -Professor Weldon’s exposition if he had here reminded us that the -special case which fitted Galton’s Law so well that it emboldened -him to announce that principle as apparently “universally applicable -to bi-sexual descent” was one of _alternative_ inheritance--namely -the coat-colour of Basset-hounds. Such a fact is, to say the least, -ominous. Pearson, in speaking (1900) of this famous case of Galton’s, -says that these phenomena of alternative inheritance must be treated -separately (from those of blended inheritance)[59], and for them he -deduces a proposed “_law of reversion_,” based of course on ancestry. -He writes, “In both cases we may speak of a law of ancestral heredity, -but the first predicts the probable character of the individual -produced by a given ancestry, while the second tells us the -percentages of the total offspring which on the average revert to each -ancestral type[60].” - - [59] “If this be done, we shall, I venture to think, keep not only - our minds, but our points for observation, clearer; and further, the - failure of Mr Galton’s statement in the one case will not in the - least affect its validity in the other.” Pearson (32), p. 143. - - [60] _Grammar of Science_, 1900, p. 494. See also Pearson, _Proc. - Roy. Soc._ 1900, LXVI. pp. 142–3. - -With the distinctions between the original Law of Ancestral Heredity, -the modified form of the same law, and the Law of Reversion, important -as all these considerations are, we are not at present concerned. - -For the Mendelian principle of heredity asserts a proposition -absolutely at variance with all the laws of ancestral heredity, however -formulated. In those cases to which it applies strictly, this principle -declares that the cross-breeding of parents _need_ not diminish -the purity of their germ-cells or consequently the purity of their -offspring. When in such cases individuals bearing opposite characters, -_A_ and _B_, are crossed, the germ-cells of the resulting cross-bred, -_AB_, are each to be bearers either of character A or of character _B_, -not both. - -Consequently when the cross-breds breed either together or with the -pure forms, individuals will result of the forms _AA_, _AB_, _BA_, -_BB_[61]. Of these the forms _AA_ and _BB_, formed by the union of -similar germs, are stated to be as pure as if they had had no cross in -their pedigree, and henceforth their offspring will be no more likely -to depart from the _A_ type or the _B_ type respectively, than those of -any other originally pure specimens of these types. - - [61] On an average of cases, in equal numbers, as Mendel found. - -Consequently in such examples it is _not_ the fact that each ancestor -must be brought to account as the Galton-Pearson Law asserts, and we -are clearly dealing with a physiological phenomenon not contemplated by -that Law at all. - -Every case therefore which obeys the Mendelian principle is in direct -contradiction to the proposition to which Professor Weldon’s school is -committed, and it is natural that he should be disposed to consider -the Mendelian principle as applying especially to “alternative” -inheritance, while the law of Galton and Pearson is to include the -phenomenon of blended inheritance. The latter, he tells us, is “the -most usual case,” a view which, if supported by evidence, might not be -without value. - -It is difficult to blame those who on first acquaintance concluded -Mendel’s principle can have no strict application save to alternative -inheritance. Whatever blame there is in this I share with Professor -Weldon and those whom he follows. Mendel’s own cases were almost all -alternative; also the fact of dominance is very dazzling at first. But -that was two years ago, and when one begins to see clearly again, it -does not look so certain that the real essence of Mendel’s discovery, -the purity of germ-cells in respect of certain characters, may not -apply also to some phenomena of blended inheritance. The analysis of -this possibility would take us to too great length, but I commend to -those who are more familiar with statistical method, the consideration -of this question: whether dominance being absent, indefinite, or -suppressed, the phenomena of heritages completely blended in the -zygote, may not be produced by gametes presenting Mendelian purity of -characters. A brief discussion of this possibility is given in the -Introduction, p. 31. - -Very careful inquiry would be needed before such a possibility could -be negatived. For example, we know that the Laws based on Ancestry -can apply to _alternative_ inheritance; witness the case of the -Basset-hounds. Here there is no simple Mendelian dominance; but are we -sure there is no purity of germ-cells? The new conception goes a long -way and it may well reach to such facts as these. - -But for the present we will assume that Mendel’s principle applies only -to _certain phenomena of alternative inheritance_, which is as far as -our warrant yet runs. - -No close student of the recent history of evolutionary thought needs -to be told what the attitude of Professor Weldon and his followers -has been towards these same disquieting and unwelcome phenomena of -alternative inheritance and discontinuity in variation. Holding at -first each such fact for suspect, then treating them as rare and -negligible occurrences, he and his followers have of late come slowly -to accede to the facts of discontinuity a bare and grudging recognition -in their scheme of evolution[62]. - - [62] Read in this connexion Pearson, K., _Grammar of Science_, 2nd - ed. 1900, pp. 390–2. - - Professor Weldon even now opens his essay with the statement--or - perhaps reminiscence--that “it is perfectly possible and indeed - probable that the difference between these forms of inheritance - [blended, mosaic, and alternative] is only one of degree.” This may - be true; but reasoning favourable to this proposition could equally - be used to prove the difference between mechanical mixture and - chemical combination to be a difference of degree. - -Therefore on the announcement of that discovery which once and for all -ratifies and consolidates the conception of discontinuous variation, -and goes far to define that of alternative inheritance, giving a finite -body to what before was vague and tentative, it is small wonder if -Professor Weldon is disposed to criticism rather than to cordiality. - - * * * * * - -We have now seen what is the essence of Mendel’s discovery based on a -series of experiments of unequalled simplicity which Professor Weldon -does not venture to dispute. - - -II. MENDEL AND THE CRITIC’S VERSION OF HIM. - -_The “Law of Dominance.”_ - -I proceed to the question of dominance which Professor Weldon treats as -a prime issue, almost to the virtual concealment of the great fact of -gametic purity. - -Cross-breds in general, _AB_ and _BA_, named above, may present many -appearances. They may all be indistinguishable from _A_, or from _B_; -some may appear _A_’s and some _B_’s; they may be patchworks of both; -they may be blends presenting one or many grades between the two; and -lastly they _may have an appearance special to themselves_ (_being in -the latter case, as it often happens, “reversionary”_), a possibility -which Professor Weldon does not stop to consider, though it is the clue -that may unravel many of the facts which mystify him now. - -Mendel’s discovery became possible because he worked with regular -cases of the first category, in which he was able to recognize that -_one_ of each of the pairs of characters he studied _did_ thus prevail -and _was_ “dominant” in the cross-bred to the exclusion of the other -character. This fact, which is still an accident of particular cases, -Professor Weldon, following some of Mendel’s interpreters, dignifies by -the name of the “Law of Dominance,” though he omits to warn his reader -that Mendel states no “Law of Dominance” whatever. The whole question -whether one or other character of the antagonistic pair is dominant -though of great importance is logically a subordinate one. It depends -on the specific nature of the varieties and individuals used, sometimes -probably on the influence of external conditions and on other factors -we cannot now discuss. There is as yet no universal law here perceived -or declared. - -Professor Weldon passes over the proof of the purity of the germ-cells -lightly enough, but this proposition of dominance, suspecting its -weakness, he puts prominently forward. Briefest equipment will -suffice. Facing, as he supposes, some new pretender--some local -Theudas--offering the last crazy prophecy,--any argument will do -for such an one. An eager gathering in an unfamiliar literature, a -scrutiny of samples, and he will prove to us with small difficulty that -dominance of yellow over green, and round over wrinkled, is irregular -even in peas after all; that in the sharpness of the discontinuity -exhibited by the variations of peas there are many grades; that many -of these grades co-exist in the same variety; that some varieties may -perhaps be normally intermediate. All these propositions are supported -by the production of a collection of evidence, the quality of which we -shall hereafter consider. “Enough has been said,” he writes (p. 240), -“to show the grave discrepancy between the evidence afforded by -Mendel’s own experiments and that obtained by other observers, equally -competent and trustworthy.” - -We are asked to believe that Professor Weldon has thus discovered “a -fundamental mistake” vitiating all that work, the importance of which, -he elsewhere tells us, he has “no wish to belittle.” - - -III. THE FACTS IN REGARD TO DOMINANCE OF CHARACTERS IN PEAS. - -Professor Weldon refers to no experiments of his own and presumably -has made none. Had he done so he would have learnt many things about -dominance in peas, whether of the yellow cotyledon-colour or of the -round form, that might have pointed him to caution. - -In the year 1900 Messrs Vilmorin-Andrieux & Co. were kind enough to -send to the Cambridge Botanic Garden on my behalf a set of samples -of the varieties of _Pisum_ and _Phaseolus_, an exhibit of which -had greatly interested me at the Paris Exhibition of that year. In -the past summer I grew a number of these and made some preliminary -cross-fertilizations among them (about 80 being available for these -deductions) with a view to a future study of certain problems, -Mendelian and others. In this work I had the benefit of the assistance -of Miss Killby of Newnham College. Her cultivations and crosses were -made independently of my own, but our results are almost identical. The -experience showed me, what a naturalist would expect and practical men -know already, that _a great deal turns on the variety used_; that some -varieties are very sensitive to conditions while others maintain their -type sturdily; that in using certain varieties Mendel’s experience -as to dominance is regularly fulfilled, while in the case of other -varieties irregularities and even some contradictions occur. That the -dominance of yellow cotyledon-colour over green, and the dominance of -the smooth form over the wrinkled, is a _general_ truth for _Pisum -sativum_ appears at once; that it is a universal truth I cannot believe -any competent naturalist would imagine, still less assert. Mendel -certainly never did. When he speaks of the “law” or “laws” that he -has established for _Pisum_ he is referring to his own discovery of -the purity of the germ-cells, that of the statistical distribution of -characters among them, and the statistical grouping of the different -germ-cells in fertilization, and not to the “Law of Dominance” which he -never drafted and does not propound. - -The issue will be clearer if I here state briefly what, as far -as my experience goes, are the facts in regard to the characters -_cotyledon-colour_ and _seed-shapes_ in peas. I have not opportunity -for more than a passing consideration of the _seed-coats_ of pure -forms[63]; that is a maternal character, a fact I am not sure Professor -Weldon fully appreciates. Though that may be incredible, it is evident -from many passages that he has not, in quoting authorities, considered -the consequences of this circumstance. - - [63] The whole question as to seed-coat colour is most complex. - Conditions of growth and ripening have a great effect on it. Mr - Arthur Sutton has shown me samples of _Ne Plus Ultra_ grown in - England and abroad. This pea has yellow cotyledons with seed-coats - either yellow or “blue.” The foreign sample contained a much greater - proportion of the former. He told me that generally speaking this is - the case with samples ripened in a hot, dry climate. - - Unquestionable Xenia appears occasionally, and will be spoken of - later. Moreover to experiment with such a _plant_-character an extra - generation has to be sown and cultivated. Consequently the evidence - is meagre. - - -_The normal characters: colour of cotyledons and seed-coats._ - -Culinary peas (_P. sativum_, omitting purple sorts) can primarily be -classified on colour into two groups, yellow and green. In the green -certain pigmentary matters persist in the ripe seed which disappear -or are decomposed in the yellow as the seed ripens. But it may be -observed that the “green” class itself is treated as of two divisions, -_green_ and _blue_. In the seedsmen’s lists the classification is made -on the _external appearance_ of the seed, without regard to whether -the colour is due to the seed-coat, the cotyledons, or both. As a rule -perhaps yellow coats contain yellow cotyledons, and green coats green -cotyledons, though yellow cotyledons in green coats are common, e.g. -_Gradus_, of which the cotyledons are yellow while the seed-coats -are about as often green as yellow (or “white,” as it is called -technically). Those called “blue” consist mostly of seeds which have -green cotyledons seen through transparent skins, or yellow cotyledons -combined with green skins. The skins may be roughly classified into -thin and transparent, or thick and generally at some stage pigmented. -In numerous varieties the colour of the cotyledon is wholly yellow, -or wholly green. Next there are many varieties which are constant in -habit and other properties but have seeds belonging to these two colour -categories in various proportions. How far these proportions are known -to be constant I cannot ascertain. - -Of such varieties showing mixture of _cotyledon_-colours nearly all can -be described as dimorphic in colour. For example in Sutton’s _Nonpareil -Marrowfat_ the cotyledons are almost always _either_ yellow _or_ green, -with some piebalds, and the colours of the seed-coats are scarcely -less distinctly dimorphic. In some varieties which exist in both -colours intermediates are so common that one cannot assert any regular -dimorphism[64]. - - [64] Knowing my interest in this subject Professor Weldon was so good - as to forward to me a series of his peas arranged to form a scale of - colours and shapes, as represented in his Plate I. I have no doubt - that the use of such colour-scales will much facilitate future study - of these problems. - -There are some varieties which have cotyledons green and intermediate -shading to greenish yellow, like _Stratagem_ quoted by Professor -Weldon. Others have yellow and intermediate shading to yellowish green, -such as McLean’s _Best of all_[65]. I am quite disposed to think -there may be truly monomorphic varieties with cotyledons permanently -of intermediate colour only, but so far I have not seen one[66]. The -variety with greatest _irregularity_ (apart from regular dimorphism) -in cotyledon-colour I have seen is a sample of “_mange-tout à rames, à -grain vert_,” but it was a good deal injured by weevils (_Bruchus_), -which always cause irregularity or change of colour. - - [65] I notice that Vilmorin in the well-known _Plantes Potagères_, - 1883, classifies the intermediate-coloured peas with the _green_. - - [66] Similarly though _tall_ and _dwarf_ are Mendelian characters, - peas occur of all heights and are usually classified as tall, - half-dwarfs, and dwarfs. - -Lastly in some varieties there are many piebalds or mosaics. - -From what has been said it will be evident that the description of a -pea in an old book as having been green, blue, white, and so forth, -unless the cotyledon-colour is distinguished from seed-coat colour, -needs careful consideration before inferences are drawn from it. - - -_Shape._ - -In regard to shape, if we keep to ordinary shelling peas, the facts are -somewhat similar, but as shape is probably more sensitive to conditions -than cotyledon-colour (not than _seed-coat_ colour) there are -irregularities to be perhaps ascribed to this cause. Broadly, however, -there are two main divisions, round and wrinkled. It is unquestioned -that between these two types every intermediate occurs. Here again -a vast number of varieties can be at once classified into round and -wrinkled (the classification commonly used), others are intermediate -normally. Here also I suspect some fairly clear sub-divisions might be -made in the wrinkled group and in the round group too, but I would not -assert this as a fact. - -I cannot ascertain from botanists what is the nature of the difference -between round and wrinkled peas, though no doubt it will be easily -discovered. In maize the round seeds contain much unconverted starch, -while in the wrinkled or sugar-maize this seems to be converted in -great measure as the seed ripens; with the result that, on drying, the -walls collapse. In such seeds we may perhaps suppose that the process -of conversion, which in round seeds takes place on germination, is -begun earlier, and perhaps the variation essentially consists in the -premature appearance of the converting ferment. It would be most rash -to suggest that such a process may be operating in the pea, for the -phenomenon may have many causes; but however that may be, there is -evidently a difference of such a nature that when the water dries out -of the seed on ripening, its walls collapse[67]; and this collapse may -occur in varying degrees. - - [67] Wrinkling must of course be distinguished further from the - squaring due to the peas pressing against each other in the pod. - - In connexion with these considerations I may mention that Vilmorin - makes the interesting statement that most peas retain their vitality - three years, dying as a rule rapidly after that time is passed, - though occasionally seeds seven or eight years old are alive; but - that _wrinkled_ peas germinate as a rule less well than round, and do - not retain their vitality so long as the round. Vilmorin-Andrieux, - _Plantes Potagères_, 1883, p. 423. Similar statements regarding the - behaviour of wrinkled peas in India are made by Firminger, _Gardening - for India_, 3rd ed. 1874, p. 146. - -In respect of _shape_ the seeds of a variety otherwise stable are -as a rule fairly uniform, the co-existence of both shapes and of -intermediates between them in the same variety is not infrequent. As -Professor Weldon has said, _Telephone_ is a good example of an extreme -case of mixture of both colours and shapes. _William I._ is another. -It may be mentioned that regular dimorphism in respect of shape is -not so common as dimorphism in respect of colour. Of great numbers of -varieties seen at Messrs Suttons’ I saw none so distinctly dimorphic in -shape as _William I._ which nevertheless contains all grades commonly. - -So far I have spoken of the shapes of ordinary English culinary peas. -But if we extend our observations to the shapes of _large-seeded_ peas, -which occur for the most part among the sugar-peas (_mange-touts_), -of the “grey” peas with coloured flowers, etc., there are fresh -complications to be considered. - -Professor Weldon does not wholly avoid these (as Mendel did in regard -to shape) and we will follow him through his difficulties hereafter. -For the present let me say that the classes _round_ and _wrinkled_ are -not readily applicable to those other varieties and are not so applied -either by Mendel or other practical writers on these subjects. To use -the terms indicated in the Introduction, _seed-shape_ depends on more -than one pair of allelomorphs--possibly on several. - - -_Stability and Variability._ - -Generally speaking peas which when seen in bulk are monomorphic in -colour and shape, will give fairly true and uniform offspring (but -such strict monomorphism is rather exceptional). Instances to the -contrary occur, and in my own brief experience I have seen some. -In a row of _Fill-basket_ grown from selected seed there were two -plants of different habit, seed-shape, etc. Each bore pods with seeds -few though large and round. Again _Blue Peter_ (blue and round) and -_Laxton’s Alpha_ (blue and wrinkled), grown in my garden and left to -nature uncovered, have each given a considerable proportion of seeds -with _yellow_ cotyledons, about 20% in the case of _Laxton’s Alpha_. -The distribution of these on the plants I cannot state. The plants -bearing them in each case sprang from green-cotyledoned seeds taken -from samples containing presumably unselected green seeds only. A part -of this exceptional result may be due to crossing, but heterogeneity of -conditions[68] especially in or after ripening is a more likely cause, -hypotheses I hope to investigate next season. Hitherto I had supposed -the crossing, if any, to be done by _Bruchus_ or Thrips, but Tschermak -also suspects _Megachile_, the leaf-cutter bee, which abounds in my -garden. - - [68] Cotyledon-colour is not nearly so sensitive to ordinary changes - in conditions as coat-colour, provided the coat be uninjured. But - even in monomorphic _green_ varieties, a seed which for any cause has - burst on ripening, has the exposed parts of its cotyledons _yellow_. - The same may be the case in seeds of green varieties injured by - _Bruchus_ or birds. These facts make one hesitate before denying - the effects of conditions on the cotyledon-colour even of uninjured - seeds, and the variation described above may have been simply - weathering. The seeds were gathered very late and many were burst in - _Laxton’s Alpha_. I do not yet know they are alive. - -Whatever the cause, these irregularities may undoubtedly occur; and if -they be proved to be largely independent of crossing and conditions, -this will in nowise vitiate the truth of the Mendelian principle. -For in that case it may simply be variability. Such true variation, -or sporting, in the pea is referred to by many observers. Upon this -subject I have received most valuable facts from Mr Arthur Sutton, -who has very kindly interested himself in these inquiries. He tells -me that several highly bred varieties, selected with every possible -care, commonly throw a small but constant proportion of poor and almost -vetch-like plants, with short pods and small round seeds, which are -hoed out by experienced men each year before ripening. Other high-class -varieties always, wherever grown, and when far from other sorts, -produce a small percentage of some one or more definite “sports.” Of -these peculiar sports he has sent me a collection of twelve, taken from -as many standard varieties, each “sport” being represented by eight -seeds, which though quite distinct from the type agree with each other -in almost all cases. - -In two cases, he tells me, these seed-sports sown separately have -been found to give plants identical with the standard type and must -therefore be regarded as sports in _seed characters_ only; in other -cases change of plant-type is associated with the change of seed-type. - -In most standard varieties these definite sports are not very common, -but in a few they are common enough to require continual removal by -selection[69]. - - [69] It is interesting to see that in at least one case the same--or - practically the same--variety has been independently produced by - different raisers, as we now perceive, by the fortuitous combination - of similar allelomorphs. _Sutton’s Ringleader_ and _Carter’s First - Crop_ (and two others) are cases in point, and it is peculiarly - instructive to see that in the discussion of these varieties when - they were new, one of the points indicating their identity was taken - to be the fact that they produced _the same “rogues.”_ See _Gard. - Chron._ 1865, pp. 482 and 603; 1866, p. 221; 1867, pp. 546 and 712. - - Rimpau quotes Blomeyer (_Kultur der Landw. Nutzpflanzen_, Leipzig, - 1889, pp. 357 and 380) to the effect that _purple_-flowered plants - with _wrinkled_ seeds may spring as direct sports from peas with - _white_ flowers and _round_ seeds. I have not seen a copy of - Blomeyer’s work. Probably this “wrinkling” was “indentation.” - -I hope before long to be able to give statistical details and -experiments relating to this extraordinarily interesting subject. As -de Vries writes in his fine work _Die Mutationstheorie_ (I. p. 580), -“a study of the seed-differences of inconstant, or as they are -called, ‘still’ unfixed varieties, is a perfect treasure-house of new -discoveries.” - -Let us consider briefly the possible significance of these facts in the -light of Mendelian teaching. First, then, it is clear that as regards -most of such cases the hypothesis is not excluded that these recurring -sports may be due to the fortuitous concurrence of certain scarcer -hypallelomorphs, which may either have been free in the original parent -varieties from which the modern standard forms were raised, or may have -been freed in the crossing to which the latter owe their origin (see -p. 28). This possibility raises the question whether, if we could make -“_pure_ cultures” of the gametes, any variations of this nature would -ever occur. This may be regarded as an unwarrantable speculation, but -it is not wholly unamenable to the test of experiments. - -But variability, in the sense of division of gonads into heterogeneous -gametes, may surely be due to causes other than crossing. This we -cannot doubt. Cross-fertilization of the zygote producing those gametes -is _one_ of the causes of such heterogeneity among them. We cannot -suppose it to be the sole cause of this phenomenon. - -When Mendel asserts the purity of the germ-cells of cross-breds he -cannot be understood to mean that they are _more pure_ than those of -the original parental races. These must have varied in the past. The -wrinkled seed arose from the round, the green from the yellow (or _vice -versâ_, if preferred), and probably numerous intermediate forms from -both. - -The variations, or as I provisionally conceive it, that differentiant -division among the gametes of which variation (neglecting environment) -is the visible expression, has arisen and can arise at one or more -points of time, and we have no difficulty in believing it to occur now. -In many cases we have clear evidence that it does. Crossing,--dare -we call it asymmetrical fertilization?--is _one_ of the causes of -the production of heterogeneous gametes--the result of divisions -qualitatively differentiant and perhaps asymmetrical[70]. - - [70] The asymmetries here conceived may of course be combined in - an inclusive symmetry. Till the differentiation can be optically - recognized in the gametes we shall probably get no further with this - part of the problem. - -There are other causes and we have to find them. Some years ago I -wrote that consideration of the causes of variation was in my judgment -premature[71]. Now that through Mendel’s work we are clearing our -minds as to the fundamental nature of “gametic” variation, the time is -approaching when an investigation of such causes may be not unfruitful. - - [71] _Materials for the Study of Variation_, 1894, p. 78. - -Of _variation_ as distinct from _transmission_ why does Professor -Weldon take no heed? He writes (p. 244): - - “If Mendel’s statements were universally valid, even among Peas, the - characters of the seeds in the numerous hybrid races now existing - should fall into one or other of a few definite categories, which - should not be connected by intermediate forms.” - -Now, as I have already pointed out, Mendel made no pretence of -universal statement: but had he done so, the conclusion, which -Professor Weldon here suggests should follow from such a universal -statement, is incorrectly drawn. Mendel is concerned with the laws of -_transmission of existing characters_, not with _variation_, which he -does not discuss. - -Nevertheless Professor Weldon has some acquaintance with the general -fact of variability in certain peas, which he mentions (p. 236), but -the bearing of this fact on the difficulty he enuntiates escapes him. - - -_Results of crossing in regard to seed characters: normal and -exceptional._ - -The conditions being the same, the question of the characters of the -cross-bred zygotes which we will call _AB_’s depends primarily on the -specific nature of the varieties which are crossed to produce them. It -is unnecessary to point out that if all _AB_’s are to look alike, both -the varieties _A_ and _B_ must be _pure_--not in the common sense of -descended, as far as can be traced, through individuals identical with -themselves, but pure in the Mendelian sense, that is to say that each -must be at that moment producing only homogeneous gametes bearing the -same characters _A_ and _B_ respectively. Purity of pedigree in the -breeder’s sense is a distinct matter altogether. The length of time--or -if preferred--the number of generations through which a character of a -variety has remained pure, alters the probability of its _dominance_, -i.e. its appearance when a gamete bearing it meets another bearing an -antagonistic character, no more, so far as we are yet aware, than the -length of time a stable element has been isolated alters the properties -of the chemical compound which may be prepared from it. - -Now when individuals (bearing contrary characters), pure in the sense -indicated, are crossed together, the question arises, What will be the -appearance of the first cross individuals? Here again, _generally -speaking_, when thoroughly green cotyledons are crossed with thoroughly -yellow cotyledons, the first-cross seeds will have yellow cotyledons; -when fully round peas are crossed with fully wrinkled the first result -will _generally speaking_ be _round_, often with slight pitting as -Mendel has stated. This has been the usual experience of Correns, -Tschermak, Mendel, and myself[72] and, as we shall see, the amount of -clear and substantial evidence to the contrary is still exceedingly -small. But as any experienced naturalist would venture to predict, -there is no _universal_ rule in the matter. As Professor Weldon himself -declares, had there been such a universal rule it would surely have -been notorious. He might further have reflected that in Mendel’s day, -when hybridisation was not the _terra incognita_ it has since become, -the assertion of such universal propositions would have been peculiarly -foolish. Mendel does not make it; but Professor Weldon perceiving the -inherent improbability of the assertion conceives at once that Mendel -_must_ have made it, and if Mendel doesn’t say so in words then he must -have implied it. As a matter of fact Mendel never treats dominance as -more than an incident in his results, merely using it as a means to an -end, and I see no reason to suppose he troubled to consider to what -extent the phenomenon is or is not universal--a matter with which he -had no concern. - - [72] The varieties used were _Express_, _Laxton’s Alpha_, - _Fillbasket_, _McLean’s Blue Peter_, _Serpette nain blanc_, _British - Queen_, _très nain de Bretagne_, Sabre, _mange-tout_ Debarbieux, and - a large “grey” sugar-pea, _pois sans parchemin géant à très large - cosse_. Not counting the last two, five are round and three are - wrinkled. As to cotyledons, six have yellow and four have green. In - about 80 crosses I saw no exception to dominance of yellow; but one - apparently clear case of dominance of wrinkled and some doubtful ones. - -Of course there may be exceptions. As yet we cannot detect the causes -which control them, though injury, impurity, accidental crossing, -mistakes of various kinds, account for many. Mendel himself says, for -instance, that unhealthy or badly grown plants give uncertain results. -Nevertheless there seems to be a true residuum of exceptions not to be -explained away. I will recite some that I have seen. In my own crosses -I have seen green × green give yellow four times. This I incline -to attribute to conditions or other disturbance, for the natural -pods of these plants gave several yellows. At Messrs Suttons’ I saw -second-generation seeds got by allowing a cross of _Sutton’s Centenary_ -(gr. wr.) × _Eclipse_ (gr. rd.) to go to seed; the resulting seeds were -both green and _yellow_, wrinkled and round. But in looking at a sample -of _Eclipse_ I found a few _yellow_ seeds, say two per cent., which may -perhaps be the explanation. Green wrinkled × green round _may_ give all -wrinkled, and again wrinkled × wrinkled may give _round_[73]. Of this -I saw a clear case--supposing no mistake to have occurred--at Messrs -Suttons’. Lastly we have the fact that in exceptional cases crossing -two forms--apparently pure in the strict sense--may give a mixture in -the _first_ generation. There are doubtless examples also of unlikeness -between reciprocals, and of this too I have seen one putative case[74]. - - [73] Professor Weldon may take this as a famous blow for Mendel, till - he realizes what is meant by Mendel’s “Hybrid-character.” - - [74] In addition to those spoken of later, where the great difference - between reciprocals is due to the _maternal_ characters of the seeds. - -Such facts thus set out for the first cross-bred generation may without -doubt be predicated for subsequent generations. - -What then is the significance of the facts? - - -_Analysis of exceptions._ - -Assuming that all these “contradictory” phenomena happened truly as -alleged, and were not pathological or due to error--an explanation -which seems quite inadequate--there are at least four possible accounts -of such diverse results--each valid, without any appeal to ancestry. - - 1. That dominance may exceptionally fail--or in other words - be created on the side which is elsewhere recessive. For this - exceptional failure we have to seek exceptional causes. The - artificial _creation_ of dominance (in a character usually recessive) - has not yet to my knowledge been demonstrated experimentally, but - experiments are begun by which such evidence may conceivably be - obtained. - - 2. There may be what is known to practical students of evolution as - the _false hybridism of Millardet_, or in other words, fertilisation - with--from unknown causes--transmission of none or of only some - of the characters of one pure parent. The applicability of this - hypothesis to the colours and shapes of peas is perhaps remote, - but we may notice that it is one possible account of those rare - cases where two pure forms give a _mixed_ result in the first - generation, even assuming the gametes of each pure parent to be truly - monomorphic as regards the character they bear. The applicability of - this suggestion can of course be tested by study of the subsequent - generations, self-fertilised or fertilised by similar forms produced - in the same way. In the case of a _genuine_ false-hybrid the lost - characters will not reappear in the posterity. - - 3. The result may not be a case of transmission at all as it is at - present conceived, but of the creation on crossing of something - _new_. Our _AB_’s may have one or more characters _peculiar - to themselves_. We may in fact have made a distinct “mule” or - heterozygote form. Where this is the case, there are several - subordinate possibilities we need not at present pursue. - - 4. There may be definite _variation_ (distinct from that proper to - the “mule”) consequent on causes we cannot yet surmise (see pp. 125 - and 128). - -The above possibilities are I believe at the present time the only -ones that need to be considered in connexion with these exceptional -cases[75]. They are all of them capable of experimental test and in -certain instances we are beginning to expect the conclusion. - - [75] I have not here considered the case in which male and female - elements of a pure variety are not homologous and the variety is a - _permanent_ monomorphic “mule.” Such a phenomenon, when present, will - prove itself in reciprocal crossing. I know no such case in peas for - certain. - - -_The “mule” or heterozygote._ - -There can be little doubt that in many cases it is to the third -category that the phenomena belong. An indication of the applicability -of this reasoning will generally be found in the fact that in such -“mule” forms the colour or the shape of the seeds will be recognizably -peculiar and proper to the specimens themselves, as distinct from their -parents, and we may safely anticipate that when those seeds are grown -the plants will show some character which is recognizable as novel. The -_proof_ that the reasoning may apply can as yet only be got by finding -that the forms in question cannot breed true even after successive -selections, but constantly break up into the same series of forms[76]. - - [76] It will be understood that a “mule” form is quite distinct from - what is generally described as a “blend.” One certain criterion of - the “mule” form is the fact that it cannot be fixed, see p. 25. There - is little doubt that Laxton had such a “mule” form when he speaks - of “the remarkably fine but unfixable pea, Evolution.” _J. R. Hort. - Soc._ XII. 1890, p. 37 (_v. infra_). - -This conception of the “mule” form, or “hybrid-character” as Mendel -called it, though undeveloped, is perfectly clear in his work. He -says that the dominant character may have two significations, it may -be either a parental character or a hybrid-character, and it must be -differentiated according as it appears in the one capacity or the -other. He does not regard the character displayed by the hybrid, -whether dominant or other, _as a thing inherited from or transmitted by -the pure parent at all, but as the peculiar function or property of the -hybrid_. When this conception has been fully understood and appreciated -in all its bearings it will be found to be hardly less fruitful than -that of the purity of the germ-cells. - -The two parents are two--let us say--substances[77] represented -by corresponding gametes. These gametes unite to form a new -“substance”--the cross-bred zygote. This has its own properties and -structure, just as a chemical compound has, and the properties of this -new “substance” are _not more strictly_ traceable to, or “inherited” -from, those of the two parents than are those of a new chemical -compound “inherited” from those of the component elements. If the -case be one in which the gametes are pure, the new “substance” is not -represented by them, but the compound is again dissociated into its -components, each of which is separately represented by gametes. - - [77] Using the word metaphorically. - -The character of the cross-bred zygote may be anything. It may be -something we have seen before in one or other of the parents, it may -be intermediate between the two, or it may be something new. All -these possibilities were known to Mendel and he is perfectly aware -that his principle is equally applicable to all. The first case is -his “dominance.” That he is ready for the second is sufficiently -shown by his brief reference to time of flowering considered as a -character (p. 65). The hybrids, he says, flower at a time _almost -exactly intermediate_ between the flowering times of the parents, -and he remarks that the development of the hybrids in this case -probably happens in the same way as it does in the case of the other -characters[78]. - - [78] “_Ueber die Blüthezeit der Hybriden sind die Versuche noch - nicht abgeschlossen. So viel kann indessen schon angegeben werden, - dass dieselbe fast genau in der Mitte zwischen jener der Samen- und - Pollenpflanze steht, und die Entwicklung der Hybriden bezüglich - dieses Merkmales wahrscheinlich in der nämlichen Weise erfolgt, wie - es für die übrigen Merkmale der Fall ist._” Mendel, p. 23. - -That he was thoroughly prepared for the third possibility appears -constantly through the paper, notably in the argument based on the -_Phaseolus_ hybrids, and in the statement that the hybrid between talls -and dwarfs is generally taller than the tall parent, having increased -height as its “hybrid-character.” - -All this Professor Weldon has missed. In place of it he offers us the -_sententia_ that no one can expect to understand these phenomena if he -neglect ancestry. This is the idle gloss of the scribe, which, if we -erase it not thoroughly, may pass into the text. - -Enough has been said to show how greatly Mendel’s conception of -heredity was in advance of those which pass current at the present -day; I have here attempted the barest outline of the nature of the -“hybrid-character,” and I have not sought to indicate the conclusions -that we reach when the reasoning so clear in the case of the hybrid is -applied to the pure forms and their own characters. - -In these considerations we reach the very base on which all conceptions -of heredity and variation must henceforth rest, and that it is now -possible for us to attempt any such analysis is one of the most -far-reaching consequences of Mendel’s principle. Till two years ago no -one had made more than random soundings of this abyss. - -I have briefly discussed these possibilities to assist the reader in -getting an insight into Mendel’s conceptions. But in dealing with -Professor Weldon we need not make this excursion; for his objection -arising from the absence of uniform regularity in dominance is not in -point. - -The soundness of Mendel’s work and conclusions would be just as -complete if dominance be found to fail often instead of rarely. For -it is perfectly certain that varieties _can_ be chosen in such a way -that the dominance of one character over its antagonist is so regular a -phenomenon that it _can_ be used in the way Mendel indicates. He chose -varieties, in fact, in which a known character _was_ regularly dominant -and it is because he did so that he made his discovery[79]. When -Professor Weldon speaks of the existence of fluctuation and diversity -in regard to dominance as proof of a “grave discrepancy” between -Mendel’s facts and those of other observers[80], he merely indicates -the point at which his own misconceptions began. - - [79] As has been already shown the discovery could have been made - equally well and possibly with greater rapidity in a case in which - the hybrid had a character distinct from either parent. The cases - that would _not_ have given a clear result are those where there is - irregular dominance of one or other parent. - - [80] Weldon, p. 240. - -From Mendel’s style it may be inferred that if he had meant to state -universal dominance in peas he would have done so in unequivocal -language. Let me point out further that of the 34 varieties he -collected for study, he discarded 12 as not amenable to his -purposes[81]. He tells us he would have nothing to do with characters -which were not sharp, but of a “more or less” description. As the -34 varieties are said to have all come true from seed, we may -fairly suppose that the reason he discarded twelve was that they -were unsuitable for his calculations, having either ill-defined and -intermediate characters, or possibly defective and irregular dominance. - - [81] See p. 43. - - -IV. PROFESSOR WELDON’S COLLECTION OF “OTHER EVIDENCE CONCERNING -DOMINANCE IN PEAS.” - -_A. In regard to cotyledon colour: Preliminary._ - -I have been at some pains to show how the contradictory results, no -doubt sometimes occurring, on which Professor Weldon lays such stress, -may be comprehended without any injury to Mendel’s main conclusions. -This excursion was made to save trouble with future discoverers of -exceptions, though the existence of such facts need scarcely disturb -many minds. As regards the dominance of yellow cotyledon-colour over -green the whole number of genuine unconformable cases is likely to -prove very small indeed, though in regard to the dominance of round -shape over wrinkled we may be prepared for more discrepancies. Indeed -my own crosses alone are sufficient to show that in using some -varieties irregularities are to be expected. Considering also that -the shapes of peas depend unquestionably on more than one pair of -allelomorphs I fully expect regular blending in some cases. - -As however it may be more satisfactory to the reader and to Professor -Weldon if I follow him through his “contradictory” evidence I will -endeavour to do so. Those who have even a slight practical acquaintance -with the phenomena of heredity will sympathize with me in the -difficulty I feel in treating this section of his arguments with that -gravity he conceives the occasion to demand. - -In following the path of the critic it will be necessary for me to -trouble the reader with a number of details of a humble order, but the -journey will not prove devoid of entertainment. - -Now exceptions are always interesting and suggestive things, and -sometimes hold a key to great mysteries. Still when a few exceptions -are found disobeying rules elsewhere conformed to by large classes of -phenomena it is not an unsafe course to consider, with such care as -the case permits, whether the exceptions may not be due to exceptional -causes, or failing such causes whether there may be any possibility of -error. But to Professor Weldon, an exception is an exception--and as -such may prove a very serviceable missile; so he gathers them as they -were “smooth stones from the brook.” - -Before examining the quality of this rather miscellaneous ammunition I -would wish to draw the non-botanical reader’s attention to one or two -facts of a general nature. - -For our present purpose the seed of a pea may be considered as -consisting of two parts, the _embryo with its cotyledons_, enclosed in -a _seed-coat_. It has been known for about a century that this coat or -skin is a _maternal_ structure, being part of the mother plant just -as much as the pods are, and consequently not belonging to the next -generation at all. If then any changes take place in it consequent -on fertilisation, they are to be regarded not as in any sense a -transmission of character by heredity, but rather as of the nature -of an “infection.” If on the other hand it is desired to study the -influence of hereditary transmission on seed-coat characters, then the -crossed seeds must be sown and the seed-coats of their seeds studied. -Such infective changes in maternal tissues have been known from early -times, a notable collection of them having been made especially by -Darwin; and for these cases Focke suggested the convenient word -_Xenia_. With this familiar fact I would not for a moment suppose -Professor Weldon unacquainted, though it was with some surprise that I -found in his paper no reference to the phenomenon. - -For as it happens, xenia is not at all a rare occurrence with _certain -varieties_ of peas; though in them, as I believe is generally the case -with this phenomenon, it is highly irregular in its manifestations, -being doubtless dependent on slight differences of conditions during -ripening. - -The coats of peas differ greatly in different varieties, being -sometimes thick and white or yellow, sometimes thick and highly -pigmented with green or other colours, in both of which cases it -may be impossible to judge the cotyledon-colour without peeling -off the opaque coat; or the coats may be very thin, colourless and -transparent, so that the cotyledon-colour is seen at once. It was -such a transparent form that Mendel says he used for his experiments -with cotyledon-colour. In order to see xenia a pea with a _pigmented_ -seed-coat should be taken as seed-parent, and crossed with a variety -having a different cotyledon-colour. There is then a fair chance of -seeing this phenomenon, but much still depends on the variety. For -example, _Fillbasket_ has green cotyledons and seed-coat green except -near the hilar surface. Crossed with _Serpette nain blanc_ (yellow -cotyledons and yellow coat) this variety gave three pods with 17 seeds -in which the seed-coats were almost full yellow (xenia). Three other -pods (25 seeds), similarly produced, showed slight xenia, and one pod -with eight seeds showed little or none. - -On the other hand _Fillbasket_ fertilised with _nain de Bretagne_ -(yellow cotyledons, seed-coats yellow to yellowish green) gave six pods -with 39 seeds showing slight xenia, distinct in a few seeds but absent -in most. - -Examples of xenia produced by the contrary proceeding, namely -fertilising a yellow pea with a green, may indubitably occur and I -have seen doubtful cases; but as by the nature of the case these are -_negative_ phenomena, i.e. the seed-coat remaining greenish and _not_ -going through its normal maturation changes, they must always be -equivocal, and would require special confirmation before other causes -were excluded. - -Lastly, the special change (xenia) Mendel saw in “grey” peas, -appearance or increase of purple pigment in the thick coats, following -crossing, is common but also irregular. - -If a _transparent_ coated form be taken as seed-parent there is no -appreciable xenia, so far as I know, and such a phenomenon would -certainly be paradoxical[82]. - - [82] In some transparent coats there is pigment, but so little as a - rule that xenia would be scarcely noticeable. - -In this connection it is interesting to observe that Giltay, whom -Professor Weldon quotes as having obtained purely Mendelian results, -got no xenia though searching for it. If the reader goes carefully -through Giltay’s numerous cases, he will find, _almost_ without doubt, -that none of them were such as produce it. _Reading Giant_, as Giltay -states, has a _transparent_ skin, and the only xenia likely to occur -in the other cases would be of the peculiar and uncertain kind seen in -using “grey” peas. Professor Weldon notes that Giltay, who evidently -worked with extreme care, _peeled_ his seeds before describing them, -a course which Professor Weldon, not recognizing the distinction -between the varieties with opaque and transparent coats, himself wisely -recommends. The coincidence of the peeled seeds giving simple Mendelian -results is one which might have alarmed a critic less intrepid than -Professor Weldon. - -Bearing in mind, then, that the coats of peas may be transparent or -opaque; and in the latter case may be variously pigmented, green, grey, -reddish, purplish, etc.; that in any of the latter cases there may or -may not be xenia; the reader will perceive that to use the statements -of an author, whether scientific or lay, to the effect that on crossing -varieties he obtained peas of such and such colours _without specifying -at all whether the coats were transparent or whether the colours he saw -were coat- or cotyledon-colours_ is a proceeding fraught with peculiar -and special risks. - -(1) _Gärtner’s cases._ Professor Weldon gives, as exceptions, a series -of Gärtner’s observations. Using several varieties, amongst them -_Pisum sativum macrospermum_, a “grey” pea, with coloured flowers and -seed-coats[83], he obtained results partly Mendelian and partly, as now -alleged, contradictory. The latter consist of seeds “dirty yellow” and -“yellowish green,” whereas it is suggested they should have been simply -yellow. - - [83] Usually correlated characters, as Mendel knew. - -Now students of this department of natural history will know that -these same observations of Gärtner’s, whether rightly or wrongly, have -been doing duty for more than half a century as stock illustrations -of xenia. In this capacity they have served two generations of -naturalists. The ground nowadays may be unfamiliar, but others have -travelled it before and recorded their impressions. Darwin, for -example, has the following passage[84]: - - [84] _Animals and Plants_, 2nd ed. 1885, p. 428. - - “These statements led Gärtner, who was highly sceptical on the - subject, carefully to try a long series of experiments; he selected - the most constant varieties, and the results conclusively showed - _that the colour of the skin of the pea_ is modified when pollen of a - differently coloured variety is used.” (The italics are mine.) - -In the true spirit of inquiry Professor Weldon doubtless reflected, - - “’Tis not _Antiquity_ nor _Author_, - That makes _Truth Truth_, altho’ _Time’s Daughter_”; - -but perhaps a word of caution to the reader that another interpretation -exists would have been in place. It cannot be without amazement -therefore that we find him appropriating these examples as referring to -cotyledon-colour, with never a hint that the point is doubtful. - -Giltay, without going into details, points out the ambiguity[85]. As -Professor Weldon refers to the writings both of Darwin and Giltay, -it is still more remarkable that he should regard the phenomenon as -clearly one of cotyledon-colour and not coat-colour as Darwin and many -other writers have supposed. - - [85] “_Eine andere Frage ist jedoch, ob der Einfluss des Pollens auf - den Keim schon äusserlich an diesen letzteren sichtbar sein kann. - Darwin führt mehrere hierher gehörige Fälle an, und wahrscheinlich - sind auch die Resultate der von Gärtner über diesen Gegenstand - ausgeführten Experimente hier zu erwähnen, wenn es auch nicht ganz - deutlich ist, ob der von Gärtner erwähnte directe Einfluss des - Pollens sich nur innerhalb der Grenzen des Keimes merklich macht oder - nicht._” p. 490. - -Without going further it would be highly improbable that Gärtner -is speaking solely or even chiefly of the cotyledons, from the -circumstance that these observations are given as evidence of “_the -influence of foreign pollen on the female organs_”; and that Gärtner -was perfectly aware of the fact that the coat of the seed was a -maternal structure is evident from his statement to that effect on -p. 80. - -To go into the whole question in detail would require considerable -space; but indeed it is unnecessary to labour the point. The reader who -examines Gärtner’s account with care, especially the peculiar phenomena -obtained in the case of the “grey” pea (_macrospermum_), with specimens -before him, will have no difficulty in recognizing that Gärtner is -simply describing the seeds _as they looked in their coats_, and is not -attempting to distinguish cotyledon-characters and coat-characters. -If he had peeled them, which in the case of “grey” peas would be -_absolutely necessary_ to see cotyledon-colour, he must surely have -said so. - -Had he done so, he would have found the cotyledons full yellow in every -ripe seed; for I venture to assert that anyone who tries, as we have, -crosses between a yellow-cotyledoned “grey” pea, such as Gärtner’s was, -with any pure green variety will see that there is no question whatever -as to absolute dominance of the yellow cotyledon-character here, more -striking than in any other case. If exceptions are to be looked for, -they will not be found _there_; and, except in so far as they show -simple dominance of yellow, Gärtner’s observations cannot be cited in -this connection at all. - - -(2) _Seton’s case._ Another exception given by Professor Weldon is much -more interesting and instructive. It is the curious case of Seton[86]. -Told in the words of the critic it is as follows:-- - - “Mr Alexander Seton crossed the flowers of _Dwarf Imperial_, ‘a - well-known green variety of the Pea,’ with the pollen of ‘a white - free-growing variety.’ Four hybrid seeds were obtained, ‘which did - not differ in appearance from the others of the female parent.’ - These seeds therefore did _not_ obey the law of dominance, or if the - statement be preferred, greenness became dominant in this case. The - seeds were sown, and produced plants bearing ‘green’ and ‘white’ - seeds side by side in the same pod. An excellent coloured figure of - one of these pods is given (_loc. cit._ Plate 9, Fig. 1), and is the - only figure I have found which illustrates segregation of colours in - hybrid Peas of the second generation.” - - [86] Appendix to paper of Goss, _Trans. Hort. Soc._ v. 1822, pub. - 1824 (_not_ 1848, as given by Professor Weldon), p. 236. - -Now if Professor Weldon had applied to this case the same independence -of judgment he evinced in dismissing Darwin’s interpretation of -Gärtner’s observations, he might have reached a valuable result. -Knowing how difficult it is to give all the points in a brief citation, -I turned up the original passage, where I find it stated that the mixed -seeds of the second generation “were all completely either of one -colour or the other, none of them having an intermediate tint, as Mr -Seton had expected.” The utility of this observation of the absence of -intermediates, is that it goes some way to dispose of the suggestion of -xenia as a cause contributing to the result. - -Moreover, feeling perfectly clear, from the fact of the absence of -intermediates, that the case must be one of simple dominance in -spite of first appearances, I suggest the following account with -every confidence that it is the true one. There have been several -“_Imperials_,” though _Dwarf Imperial_, in a form which I can feel -sure is Seton’s form, I have not succeeded in seeing; but from -Vilmorin’s description that the peas when ripe are “_franchement -verts_” I feel no doubt it was a green pea _with a green skin_. If it -had had a transparent skin this description would be inapplicable. -Having then a green skin, which may be assumed with every probability -of truth, the seeds, even though the cotyledons were yellow, might, -especially if examined fresh, be indistinguishable from those of -the maternal type. Next from the fact of the mixture in the second -generation we learn that the _semi-transparent seed-coat of the -paternal form was dominant_ as a plant-character, and indeed the -coloured plate makes this fairly evident. It will be understood that -this explanation is as yet suggestive, but from the facts of the -second generation, any supposition that there was real irregularity in -dominance in this case is out of the question[87]. - - [87] Since the above passage was written I find the “_Imperials_” - described in “Report of Chiswick Trials,” _Proc. R. Hort. Soc._ 1860, - I. p. 340, as “skin thick”; and on p. 360 “skin thick, blue”; which - finally disposes of this “exception.” - - -(3) _Tschermak’s exceptions._ These are a much more acceptable lot -than those we have been considering. Tschermak was thoroughly alive -to the seed-coat question and consequently any exception stated as an -unqualified fact on his authority must be accepted. The nature of these -cases we shall see. Among the many varieties he used, some being _not_ -monomorphic, it would have been surprising if he had not found true -irregularities in dominance. - - -(3 _a_) _Buchsbaum case._ This variety, growing in the open, gave -once a pod in which _every seed but one was green_. In stating this -case Professor Weldon refers to _Buchsbaum_ as “a yellow-seeded -variety.” Tschermak[88], however, describes it as having “_gelbes, -öfters gelblich-grünes Speichergewebe_” (cotyledons); and again -says the cotyledon-colour is “_allerdings gerade bei Buchsbaum zur -Spontanvariation nach gelb-grün neigend!_” The (!) is Tschermak’s. -Therefore Professor Weldon can hardly claim _Buchsbaum_ as -“yellow-seeded” without qualification. - - [88] (36), p. 502 and (37), p. 663. - -_Buchsbaum_ in fact is in all probability a blend-form and certainly -not a true, stable yellow. One of the green seeds mentioned above grew -and gave 15 _yellows_ and three _greens_, and the result showed pretty -clearly, as Tschermak says, that there had been an accidental cross -with a tall green. - -On another occasion _Telephone_ ♀ (another impure green) × _Buchsbaum_ -gave four _yellow smooth and_ two _green wrinkled_, but one [? both: -the grammar is obscure] of the greens did not germinate[89]. - - [89] Professor Weldon should have alluded to this. _Dead_ seeds have - no bearing on these questions, seeing that their characters may be - pathological. The same seeds are later described as “_wie Telephone - selbst_,” so, apart from the possibility of death, they may also have - been self-fertilised. - - -(3 _b_) _Telephone cases._ _Telephone_, crossed with at least one -yellow variety (_Auvergne_) gave all or some green or greenish. These -I have no doubt are good cases of “defective dominance” of yellow. -But it must be noted that _Telephone is an impure green_. Nominally a -green, it is as Professor Weldon has satisfied himself, very irregular -in colour, having many intermediates shading to pure yellow and many -piebalds. It is the variety from which alone Professor Weldon made -his colour-scale. _I desire therefore to call special attention to -the fact that Telephone, though not a pure green, Tschermak’s sample -being as he says “gelblichweiss grün,” a yellowish-white-green in -cotyledon-colour, is the variety which has so far contributed the -clearest evidence of the green colour dominating in its crosses with a -yellow_; and that _Buchsbaum_ is probably a similar case. To this point -we shall return. It may not be superfluous to mention also that one -cross between _Fillbasket_ (a thorough _green_) and _Telephone_ gave -three _yellowish_ green seeds (Tschermak, (36), p. 501). - - -(3 _c_) _Couturier cases._ This fully yellow variety in crosses with -two fully green sorts gave seeds either yellow or greenish yellow. In -one case _Fillbasket_ ♀ fertilised by _Couturier_ gave mixed seeds, -green and yellow. For any evidence to the contrary, the green in this -case may have been self-fertilised. Nevertheless, taking the evidence -together, I think it is most likely that _Couturier_ is a genuine -case of imperfect dominance of yellow. If so, it is the only true -“exception” in crosses between stable forms. - - * * * * * - -We have now narrowed down Professor Weldon’s exceptions to dominance of -cotyledon-colour to two varieties, one yellow (_Couturier_), and one -yellow “tending to green” (_Buchsbaum_), which show imperfect dominance -of yellow; and one variety, _Telephone_, an impure and irregular green, -which shows occasional but uncertain dominance of _green_. - -What may be the meaning of the phenomenon shown by the unstable or -mosaic varieties we cannot tell; but I venture to suggest that when we -more fully appreciate the nature and genesis of the gametes, it will be -found that the peculiarities of heredity seen in these cases have more -in common with those of “false hybridism” (see p. 34) than with any -true failure of dominance. - -Before, however, feeling quite satisfied in regard even to this -residuum of exceptions, one would wish to learn the subsequent fate -of these aberrant seeds and how their offspring differed from that of -their sisters. One only of them can I yet trace, viz. the green seed -from _Telephone_ ♀ × _Buchsbaum_ ♂, which proved a veritable “green -dominant.” As for the remainder, Tschermak promises in his first -paper to watch them. But in his second paper the only passage I can -find relating to them declares that perhaps some of the questionable -cases he mentioned in his first paper “_are attributable to similar -isolated anomalies in dominance; some proved themselves by subsequent -cultivation to be cases of accidental self-fertilisation; others failed -to germinate_[90].” I may warn those interested in these questions, -that in estimating changes due to ripening, _dead_ seeds are not -available. - - [90] “_Vielleicht sind einige der l.c. 507 bis 508 erwähnten - fraglichen Fälle auf ähnliche vereinzelte Anomalien der - Merkmalswerthigkeit zu beziehen; einige erwiesen sich allerdings beim - Anbau als Producte ungewollter Selbstbefruchtung, andere keimten - nicht._” - - -_B. Seed-coats and shapes._ - -1. _Seed-coats._ Professor Weldon lays some stress on the results -obtained by Correns[91] in crossing a pea having green cotyledons and a -thin almost colourless coat (_grüne späte Erfurter Folger-erbse_) with -two purple-flowered varieties. The latter are what are known in England -as “grey” peas, though the term grey is not generally appropriate. - - [91] Regarding this case I have to thank Professor Correns for a - good deal of information which he kindly sent me in response to my - inquiry. I am thus able to supplement the published account in some - particulars. - -In these varieties the cotyledon-colour is yellow and the coats are -usually highly coloured or orange-brown. In reciprocal crosses Correns -found no change from the maternal seed-coat-colour or seed-shape. -On sowing these peas he obtained plants bearing peas which, using -the terminology of Mendel and others, he speaks of as the “first -generation.” - -These peas varied in the colour of their seed-coats from an almost -colourless form slightly tinged with green like the one parent to the -orange-brown of the other parent. The seeds varied in this respect not -only from plant to plant, but from pod to pod, and from seed to seed, -as Professor Correns has informed me. - -The peas with more highly-coloured coats were sown and gave rise to -plants with seeds showing the whole range of seed-coat-colours again. - -Professor Weldon states that in this case neither the law of -dominance nor the law of segregation was observed; and the same is -the opinion of Correns, who, as I understand, inclines to regard the -colour-distribution as indicating a “mosaic” formation. This is perhaps -conceivable; and in that case the statement that there was no dominance -would be true, and it would also be true that the unit of segregation, -if any, was smaller than the individual plant and may in fact be the -individual seed. - -A final decision of this question is as yet impossible. Nevertheless -from Professor Correns I have learnt one point of importance, namely, -that the coats of all these seeds were _thick_, like that of the -coloured and as usual dominant form. There is no “mosaic” of coats -like one parent and coats like the other, though there may be a mosaic -of colours. In regard to the distribution of _colour_ however the -possibility does not seem to me excluded that we are here dealing -with changes influenced by conditions. I have grown a “grey” pea and -noticed that the seed-coats ripened in my garden differ considerably -and not quite uniformly from those received from and probably -ripened in France, mine being mostly pale and greyish, instead of -reddish-brown. We have elsewhere seen (p. 120) that pigments of the -seed-coat-colour may be very sensitive to conditions, and slight -differences of moisture, for example, may in some measure account -for the differences in colour. Among my crosses I have a pod of such -“grey” peas fertilised by _Laxton’s Alpha_ (green cotyledons, coat -transparent). It contained five seeds, of which four were _red-brown -on one side_ and grey with purple specks on the other. The fifth was -of the grey colour on both sides. I regard this difference not as -indicating segregation of character but merely as comparable with the -difference between the two sides of a ripe apple, and I have little -doubt that Correns’ case may be of the same nature[92]. Phenomena -somewhat similar to these will be met with in Laxton’s case of the -“maple” seeded peas (see p. 161). - - [92] Mr Hurst, of Burbage, tells me that in varieties having coats - green or white, e.g. _American Wonder_, the white coats are mostly - from early, the green from later pods, the tints depending on - conditions and exposure. - - -2. _Seed-shapes._ Here Professor Weldon has three sets of alleged -exceptions to the rule of dominance of round shape over wrinkled. The -first are Rimpau’s cases, the second are Tschermak’s cases, the third -group are cases of “grey” peas, which we will treat in a separate -section (see pp. 153 and 158). - -(_a_) _Rimpau’s cases._ Professor Weldon quotes Rimpau as having -crossed wrinkled and round peas[93] and found the second hybrid -generation dimorphic as usual. The wrinkled peas were selected and -sown and gave wrinkled peas _and round_ peas, becoming “true” to the -wrinkled character in one case only in the fifth year, while in the -second case--that of a _Telephone_ cross--there was a mixture of round -and wrinkled similarly resulting from _wrinkled_ seed for two years, -but the experiment was not continued. - - [93] In the first case _Knight’s Marrow_ with _Victoria_, both ways; - in the second _Victoria_ with _Telephone_, both ways. - -These at first sight look like genuine exceptions. In reality, however, -they are capable of a simple explanation. It must be remembered that -Rimpau was working in ignorance of Mendel’s results, was not testing -any rule, and was not on the look out for irregularities. Now all who -have crossed wrinkled and round peas on even a moderate scale will have -met with the fact that there is frequently _some_ wrinkling in the -cross-bred seeds. Though round when compared with the true wrinkled, -these are often somewhat more wrinkled than the round type, and in -irregular degrees. For my own part I fully anticipate that we may find -rare cases of complete blending in this respect though I do not as yet -know one. - -Rimpau gives a photograph of eight peas (Fig. 146) which he says -represent the wrinkled form derived from this cross. It is evident -that these are not from _one pod_ but a miscellaneous selection. On -close inspection it will be seen that while the remainder are shown -with their _cotyledon_-surfaces upwards, the two peas at the lower -end of the row are represented with their _hilar_-surfaces upwards. -Remembering this it will be recognized that these two lower peas are in -fact _not_ fully wrinkled peas but almost certainly _round_ “hybrids,” -and the depression is merely that which is often seen in round peas -(such as _Fillbasket_), squared by mutual pressure. Such peas, when -sown, might of course give some round. - -As Tschermak writes ((37), p. 658), experience has shown him that -cross-bred seeds with character transitional between “round” and -“wrinkled” behave as hybrids, and have both wrinkled and round -offspring, and he now reckons them accordingly with the round dominants. - -Note further the fact that Rimpau found the wrinkled form came true -in the _fifth_ year, while the round gave at first more, later fewer, -wrinkleds, not coming true till the _ninth_ year. This makes it quite -clear that there _was_ dominance of the round form, but that the -heterozygotes were not so sharply distinguishable from the two pure -forms as to be separated at once by a person not on the look-out for -the distinctions. Nevertheless there _was_ sufficient difference to -lead to a practical distinction of the cross-breds both from the pure -dominants and from the pure recessives. - -The _Telephone_ case may have been of the same nature; though, as we -have seen above, this pea is peculiar in its colour-heredity and may -quite well have followed a different rule in shape also. As stated -before, the wrinkled offspring were not cultivated after the third -year, but the _round_ seeds are said to have still given some wrinkleds -in the eighth year after the cross, as would be expected in a simple -Mendelian case. - -(_b_) _Tschermak’s cases._ The cases Professor Weldon quotes from -Tschermak all relate to crosses with _Telephone_ again, and this fact -taken with the certainty that the colour-heredity of _Telephone_ is -abnormal makes it fairly clear that there is here something of a really -exceptional character. What the real nature of the exception is, and -how far it is to be taken as contradicting the “law of dominance,” is -quite another matter. - - -3. _Other phenomena, especially regarding seed-shapes, in the case of -“grey” peas. Modern evidence._ Professor Weldon quotes from Tschermak -the interesting facts about the “grey” pea, _Graue Riesen_, but does -not attempt to elucidate them. He is not on very safe ground in -adducing these phenomena as conflicting with the “law of dominance.” -Let us see whither we are led if we consider these cases. On p. 124 -I mentioned that the classes round and wrinkled do not properly hold -if we try to extend them to large-seeded sorts, and that these cases -require separate consideration. In many of such peas, which usually -belong either to the classes of sugar-peas (_mange-touts_) or “grey” -peas (with coloured flowers), the seeds would be rather described as -irregularly indented, lumpy or stony[94], than by any use of the terms -round or wrinkled. One sugar-pea (_Debarbieux_) which I have used -has large flattish, smooth, yellow seeds with white skins, and this -also in its crossings follows the rules about to be described for the -large-seeded “grey” peas. - - [94] Gärtner’s _macrospermum_ was evidently one of these, though - from the further account (p. 498) it was probably more wrinkled. - There are of course _mange-touts_ which have perfectly round seeds. - Mendel himself showed that the _mange-tout_ character, the soft - constricted pod, was transferable. There are also _mange-touts_ - with fully wrinkled seeds and “grey” peas with small seeds (see - Vilmorin-Andrieux, _Plantes Potagères_, 1883). - -In the large “grey” peas the most conspicuous feature is the seed-coat, -which is grey, brownish, or of a bright reddish colour. Such seed-coats -are often speckled with purple, and on boiling these seed-coats turn -dark brown. They are in fact the very peas used by Mendel in making up -his third pair of characters. Regarding them Professor Weldon, stating -they may be considered separately, writes as follows:-- - - “Tschermak has crossed _Graue Riesen_ with five races of _P. - sativum_, and he finds that the form of the first hybrid seeds - _follows the female parent_, so that if races of _P. sativum_ - with round smooth seeds be crossed with _Graue Riesen_ (which has - flattened, feebly wrinkled seeds) the hybrids will be round and - smooth or flattened and wrinkled, as the _P. sativum_ or the _Graue - Riesen_ is used as female parent[95]. There is here a more complex - phenomenon than at first sight appears; because if the flowers of the - first hybrid generation are self-fertilised, the resulting seeds of - the second generation invariably resemble those of the _Graue Riesen_ - in shape, although in colour they follow Mendel’s law of segregation!” - - [95] Correns found a similar result. - -From this account who would not infer that we have here some mystery -which does not accord with the Mendelian principles? As a matter of -fact the case is dominance in a perfectly obvious if distinct form. - -_Graue Riesen_, a large grey sugar-pea, the _pois sans parchemin -géant_ of the French seedsmen, has full-yellow cotyledons and a highly -coloured seed-coat of varying tints. In shape the seed is somewhat -flattened with irregular slight indentations, lightly wrinkled if -the term be preferred. Tschermak speaks of it in his first paper as -“_Same flach, zusammengedrückt_”--a flat, compressed seed; in his -second paper as “_flache, oft schwach gerunzelte Cotyledonen-form_,” -or cotyledon-shape, flat, often feebly wrinkled, as Professor Weldon -translates. - -First-crosses made from this variety, each with a different form -of _P. sativum_, are stated on the authority of Tschermak’s five -cases, to follow exclusively the maternal seed-shape. From “_schwach -gerunzelte_,” “feebly wrinkled,” Professor Weldon easily passes to -“wrinkled,” and tells us that according as a round _sativum_ or the -_Graue Riesen_ is used as mother, the first-cross seeds “will be round -and smooth or flattened and wrinkled.” - -As a matter of fact, however, the seeds of _Graue Riesen_ though -_slightly_ wrinkled do not belong to the “wrinkled” class; but if the -classification “wrinkled” and “round” is to be extended to such peas at -all, they belong to the _round_. Mendel is careful to state that his -_round_ class are “either spherical or roundish, the depressions on the -surface, when there are any, always slight”; while the “wrinkled” class -are “irregularly angular, deeply wrinkled[96].” - - [96] “_Entweder kugelrund oder rundlich, die Einsenkungen, wenn - welche an der Oberfläche vorkommen, immer nur seicht, oder sie sind - unregelmässig kantig, tief runzlig_ (_P. quadratum_).” - -On this description alone it would be very likely that _Graue Riesen_ -should fall into the _round_ class, and as such it behaves in its -crosses, _being dominant over wrinkled_ (see Nos. 3 and 6, below). I -can see that in this case Professor Weldon has been partly misled by -expressions of Tschermak’s, but the facts of the second generation -should have aroused suspicion. Neither author notices that as all -five varieties crossed by Tschermak with _Graue Riesen_ were _round_, -the possibilities are not exhausted. Had Tschermak tried a really -wrinkled _sativum_ with _Graue Riesen_ he would have seen this obvious -explanation. - -As some of my own few observations of first-crosses bear on this point -I may quote them, imperfect though they are. - -I grew the purple-flowered sugar-pea “_Pois sans parchemin géant à très -large cosse_,” a soft-podded “_mange-tout_” pea, flowers and seed-coats -coloured, from Vilmorin’s, probably identical with _Graue Riesen_. - - 1. One flower of this variety fertilised with _Pois très nain de - Bretagne_ (very small seed; yellow cotyledons; very round) gave - seven seeds indistinguishable (in their coats) from those of the - mother, save for a doubtful increase in purple pigmentation of coats. - - 2. Fertilised by _Laxton’s Alpha_ (green; wrinkled; coats - transparent), two flowers gave 11 seeds exactly as above, the purple - being in this case clearly increased. - - In the following the purple sugar-pea was _father_. - - 3. _Laxton’s Alpha_ (green; wrinkled; coats transparent) fertilised - by the purple sugar-pea gave one pod of four seeds with yellow - cotyledons and _round_ form. - - 4. _Fillbasket_ (green; smooth but squared; coats green) fertilised - by the _purple_ sugar-pea gave one pod with six seeds, yellow - cotyledons[97]; _Fillbasket_ size and shape; but the normally green - coat yellowed near _the hilum_ by xenia. - - [97] The colour is the peculiarly deep yellow of the “grey” - _mange-tout_. - -5. _Express_ (“blue”-green cotyledons and transparent skins; round) -fertilised with _purple sugar-pea_ gave one pod with four seeds, yellow -cotyledons, shape round, much as in _Fillbasket_. - -6. _British Queen_ (yellow cotyledons, wrinkled, white coats) ♀ × -purple sugar-pea gave two pods with seven seeds, cotyledons yellow, -coats _tinged greenish_ (xenia?), all _round_. - -So much for the “_Purple_” sugar-pea. - -I got similar results with _Mange-tout Debarbieux_. This is a -soft-podded _Mange-tout_ or sugar-pea, with white flowers, large, -flattish, smooth seeds, scarcely dimpled; yellow cotyledons. - -7. _Debarbieux_ fertilised by _Serpette nain blanc_ (yellow cotyledons; -wrinkled; white skin; dwarf) gave one pod with six seeds, size and -shape of _Debarbieux_, with slight dimpling. - -8. _Debarbieux_ by _nain de Bretagne_ (very small; yellow cotyledons; -very round) gave three pods, 12 seeds, all yellow cotyledons, of which -two pods had eight seeds identical in shape with _Debarbieux_, while -the third had four seeds like _Debarbieux_ but more dimpled. The -reciprocal cross gave two seeds exactly like _nain de Bretagne_. - -But it may be objected that the shape of this large grey pea is very -peculiar[98]; and that it maintains its type remarkably when fertilised -by many distinct varieties though its pollen effects little or no -change in them; for, so long as round varieties of _sativum_ are -used as mothers, this is true as we have seen. But when once it is -understood that in _Graue Riesen_ there is no question of wrinkling, -seeing that the variety behaves as a _round_ variety, the shape and -especially the size of the seed must be treated as a maternal property. - - [98] It is certainly subject to considerable changes according - to conditions. Those ripened in my garden are without exception - much larger and flatter than Vilmorin’s seeds (now two years old) - from which they grew. The colour of the coats is also much duller. - These changes are just what is to be expected from the English - climate--taken with the fact that my sample of this variety was late - sown. - -_Why_ the distinction between the shape of _Graue Riesen_ and that of -ordinary round peas should be a matter of maternal physiology we do -not know. The question is one for the botanical chemist. But there -is evidently very considerable regularity, the seeds borne by the -_cross-breds_ exhibiting the form of the “grey” pea, which is then -a dominant character as much as the seed-coat characters are. And -that is what Tschermak’s _Graue Riesen_ crosses actually did, thereby -exhibiting dominance in a very clear form. To interject these cases as -a mystery without pointing out how easily they can be reconciled with -the “law of dominance” may throw an unskilled reader into gratuitous -doubt. - -Finally, since _the wrinkled peas_, _Laxton’s Alpha_ and _British -Queen_, _pollinated by a large flat mange-tout, witness Nos. 3 and 6 -above_, became round in both cases where this experiment was made, we -here merely see the usual dominance of the non-wrinkled character; -though of course if a _round_-seeded mother be used there can be no -departure from the maternal shape, as far as roundness is concerned. - -Correns’ observations on the shapes of a “grey” pea crossed with a -round shelling pea, also quoted by Professor Weldon as showing no -dominance of roundness, are of course of the same nature as those just -discussed. - - -_C. Evidence of Knight and Laxton._ - -In the last two sections we have seen that in using peas of the “grey” -class, i.e. with brown, red, or purplish coats, special phenomena are -to be looked for, and also that in the case of large “indented” peas, -the phenomena of size and shape may show some divergence from that -simple form of the phenomenon of dominance seen when ordinary round and -wrinkled are crossed. Here the fuller discussion of these phenomena -must have been left to await further experiment, were it not that we -have other evidence bearing on the same questions. - -The first is that of Knight’s well-known experiments, long familiar but -until now hopelessly mysterious. I have not space to quote the various -interpretations which Knight and others have put upon them, but as the -Mendelian principle at once gives a complete account of the whole, -this is scarcely necessary, though the matter is full of historical -interest. - -Crossing a white pea with a very large grey purple-flowered form -Knight (21) found that the peas so produced “were not in any sensible -degree different from those afforded by other plants of the same -[white] variety; owing, I imagine, to the external covering of the -seed (as I have found in other plants) being furnished entirely by -the female[99].” All grew very tall[100], and had colours of male -parent[101]. The seeds they produced were dark grey[102]. - - [99] Thus avoiding the error of Seton, see p. 144. There is no xenia - perhaps because the seed-coat of mother was a transparent coat. - - [100] As heterozygotes often do. - - [101] Dominance of the purple form. - - [102] Dominance of the grey coat as a maternal character. - -“I had frequent occasion to observe, in this plant [the hybrid], a -stronger tendency to produce purple blossoms, and coloured seeds, -than white ones; for when I introduced the farina of a purple blossom -into a white one, the whole of the seeds in the succeeding year -became coloured [viz. _DR_ × _D_ giving _DD_ and _DR_]; but, when I -endeavoured to discharge this colour, by reversing the process, a part -only of them afforded plants with white blossoms; this part sometimes -occupying one end of the pod, and being at times irregularly intermixed -with those which, when sown, retained their colour” [viz. _DR_ × _R_ -giving _DR_ and _RR_] (draws conclusions, now obviously erroneous[103]). - - [103] Sherwood’s view (_J. R. Hort. Soc._ XXII. p. 252) that this was - the origin of the “Wrinkled” pea, seems very dubious. - -In this account we have nothing not readily intelligible in the light -of Mendel’s hypothesis. - -The next evidence is supplied by an exceptionally complete record of -a most valuable experiment made by Laxton[104]. The whole story is -replete with interest, and as it not only carries us on somewhat beyond -the point reached by Mendel, but furnishes an excellent illustration of -how his principles may be applied, I give the whole account in Laxton’s -words, only altering the paragraphing for clearness, and adding a -commentary. The paper appears in _Jour. Hort. Soc._ N.S. III. 1872, -p. 10, and very slightly abbreviated in _Jour. of Hort._ XVIII. 1870, -p. 86. Some points in the same article do not specially relate to this -section, but for simplicity I treat the whole together. - - [104] It will be well known to all practical horticulturalists that Laxton, -originally of Stamford, made and brought out a large number of the best -known modern peas. The firm is now in Bedford. - -It is not too much to say that two years ago the whole of this story -would have been a maze of bewildering confusion. There are still some -points in it that we cannot fully comprehend, for the case is one of -far more than ordinary complexity, but the general outlines are now -clear. In attempting to elucidate the phenomena it will be remembered -that there are no statistics (those given being inapplicable), and the -several offspring are only imperfectly referred to the several classes -of seeds. This being so, our rationale cannot hope to be complete. -Laxton states that as the seeds of peas are liable to change colour -with keeping, for this and other reasons he sent to the Society a part -of the seeds resulting from his experiment before it was brought to a -conclusion. - - “The seeds exhibited were derived from a single experiment. Amongst - these seeds will be observed some of several remarkable colours, - including black, violet, purple-streaked and spotted, maple, grey, - greenish, white, and almost every intermediate tint, the varied - colours being apparently produced on the outer coat or envelope of - the cotyledons only. - - The peas were selected for their colours, &c., from the third year’s - sowing in 1869 of the produce of a cross in 1866 of the early round - white-seeded and white-flowered garden variety “Ringleader,” which - is about 2-1/2 ft. in height, fertilised by the pollen of the common - purple-flowered “maple” pea, which is taller than “Ringleader,” and - has slightly indented seeds. I effected impregnation by removing - the anthers of the seed-bearer, and applying the pollen at an early - stage. This cross produced a pod containing five round white peas, - exactly like the ordinary “Ringleader” seeds[105]. - - [105] A round white ♀ × grey ♂ giving the usual result, round, - “white” (yellow) seeds. - -In 1867 I sowed these seeds, and all five produced tall purple-flowered -purplish-stemmed plants[106], and the seeds, with few exceptions, -had all maple or brownish-streaked envelopes of various shades; the -remainder had entirely violet or deep purple-coloured envelopes[107]: -in shape the peas were partly indented; but a few were round[108]. -Some of the plants ripened off earlier than the “maple,” which, in -comparison with “Ringleader,” is a late variety; and although the -pods were in many instances partially abortive, the produce was very -large[109]. - - [106] Tall heterozygotes, with normal dominance of purple flowers. - - [107] Here we see dominance of the _pigmented_ seed-coat as a - maternal character over _white_ seed-coat. The colours of the - seed-coats are described as essentially two: maple or brown-streaked, - and violet, the latter being a small minority. As the sequel shows, - the latter are heterozygotes, not breeding true. Now Mendel found, - and the fact has been confirmed both by Correns and myself, that - crossing a grey pea which is capable of producing purple leads to - such production as a form of xenia. - - We have here therefore in the purple seeds the union of dissimilar - gametes, with production of xenia. But as the brown-streaked - seeds are also in part heterozygous, the splitting of a compound - allelomorph has probably taken place, though without precise - statistics and allotment of offspring among the several seeds the - point is uncertain. The colour of seed-coats in “grey” peas and - probably “maples” also is, as was stated on p. 150, sensitive to - conditions, but the whole difference between “maples” and purple is - too much to attribute safely to such irregularity. “Maple” is the - word used to describe certain seed-coats which are pigmented with - intricate brown mottlings on a paler buff ground. In French they are - _perdrix_. - - [108] This is not, as it stands, explicable. It seems from this point - and also from what follows that if the account is truly given, some - of the plants may have been mosaic with segregation of characters in - particular flowers; but see subsequent note. - - [109] As, commonly, in heterozygotes when fertile. - -In 1868 I sowed the peas of the preceding year’s growth, and selected -various plants for earliness, productiveness, &c. Some of the plants -had light-coloured stems and leaves; these all showed white flowers, -and produced round white seeds[110]. Others had purple flowers, showed -the purple on the stems and at the axils of the stipules, and produced -seeds with maple, grey, purple-streaked, or mottled, and a few only, -again, with violet-coloured envelopes. Some of the seeds were round, -some partially indented[111]. The pods on each plant, in the majority -of instances, contained peas of like characters; but in a few cases -the peas in the same pod varied slightly, and in some instances a -pod or two on the same plant contained seeds all distinct from the -remainder[112]. The white-flowered plants were generally dwarfish, -of about the height of “Ringleader”; but the coloured-flowered sorts -varied altogether as to height, period of ripening, and colour and -shape of seed[113]. Those seeds with violet-coloured envelopes produced -nearly all maple- or parti-coloured seeds, and only here and there one -with a violet-coloured envelope; that colour, again, appeared only -incidentally, and in a like degree in the produce of the maple-coloured -seeds[114]. - - [110] Recessive in flower-colour, seed-coat colour, and in seed-shape - as a maternal character: pure recessives as the sequel proved. - - [111] These are then a mixture of pure dominants and cross-bred - dominants, and are now inextricably confused. This time the round - seeds may have been all on particular plants--showing recessive - seed-shape as a maternal character. It seems just possible that - this fact suggested the idea of “round” seeds on the _coloured_ - plants in the last generation. Till that result is confirmed it - should be regarded as very doubtful on the evidence. But we cannot - at the present time be sure how much difference there was between - these round seeds and the _normal_ maples in point of shape; and - on the whole it seems most probable that the roundness was a mere - fluctuation, such as commonly occurs among the peas with large - indented seeds. - - [112] Is this really evidence of segregation of characters, the - flower being the unit? In any case the possibility makes the - experiment well worth repeating, especially as Correns has seen a - phenomenon conceivably similar. - - [113] Being a mixture of heterozygotes (probably involving several - pairs of allelomorphs) and homozygotes. - - [114] This looks as if the violet colour was merely due to - irregularity of xenia. - -In 1869 the seeds of various selections of the previous year were again -sown separately; and the white-seeded peas again produced only plants -with white flowers and round white seeds[115]. Some of the coloured -seeds, which I had expected would produce purple-flowered plants, -produced plants with white flowers and round white seeds only[116]; the -majority, however, brought plants with purple flowers and with seeds -principally marked with purple or grey, the maple- or brown-streaked -being in the minority[117]. On some of the purple-flowered plants -were again a few pods with peas differing entirely from the remainder -on the same plant. In some pods the seeds were all white, in others -all black, and in a few, again, all violet[118]; but those plants -which bore maple-coloured seeds seemed the most constant and fixed -in character of the purple-flowered seedlings[119], and the purplish -and grey peas, being of intermediate characters, appeared to vary -most[120]. The violet-coloured seeds again produced almost invariably -purplish, grey, or maple peas, the clear violet colour only now and -then appearing, either wholly in one pod or on a single pea or two in -a pod. All the seeds of the purple-flowered plants were again either -round or only partially indented; and the plants varied as to height -and earliness. In no case, however, does there seem to have been an -intermediate-coloured flower; for although in some flowers I thought -I found the purple of a lighter shade, I believe this was owing to -light, temperature, or other circumstances, and applied equally to the -parent maple. I have never noticed a single tinted white flower nor an -indented white seed in either of the three years’ produce. The whole -produce of the third sowing consisted of seeds of the colours and in -the approximate quantities in order as follows,--viz.: 1st, white, -about half; 2nd, purplish, grey, and violet (intermediate colours), -about three-eighths; and, 3rd, maple, about one-eighth. - - [115] Pure recessives. - - [116] Pure recessives in coats showing maternal dominant character. - - [117] Now recognized as pure homozygotes. - - [118] This seems almost certainly segregation by flower-units, and is - as yet inexplicable on any other hypothesis. Especially paradoxical - is the presence of “white” seeds on these plants. The impression is - scarcely resistible that some remarkable phenomenon of segregation - was really seen here. - - [119] Being now homozygotes. - - [120] Being heterozygotes exclusively. - -From the above I gather that the white-flowered white-seeded pea is -(if I may use the term) an original variety well fixed and distinct -entirely from the maple, that the two do not thoroughly intermingle -(for whenever the white flower crops out, the plant and its parts all -appear to follow exactly the characters of the white pea), and that the -maple is a cross-bred variety which has become somewhat permanent and -would seem to include amongst its ancestors one or more bearing seeds -either altogether or partly violet- or purple-coloured; for although -this colour does not appear on the seed of the “maple,” it is very -potent in the variety, and appears in many parts of the plant and its -offspring from cross-fertilised flowers, sometimes on the external -surface or at the sutures of the pods of the latter, at others on -the seeds and stems, and very frequently on the seeds; and whenever -it shows itself on any part of the plant, the flowers are invariably -purple. My deductions have been confirmed by intercrosses effected -between the various white-, blue-, some singularly bright green-seeded -peas which I have selected, and the maple- and purple-podded and the -purple-flowered sugar peas, and by reversing those crosses. - -I have also deduced from my experiments, in accordance with the -conclusions of the late Mr Knight and others, that the colours of the -envelopes of the seeds of peas immediately resulting from a cross are -never changed[121]. I find, however, that the colour and probably the -substance of the cotyledons are sometimes, but not always, changed -by the cross fertilisation of two different varieties; and I do not -agree with Mr Knight that the form and size of the seeds produced are -unaltered[122]; for I have on more than one occasion observed that -the cotyledons in the seeds directly resulting from a cross of a blue -wrinkled pea fertilised by the pollen of a white round variety have -been of a greenish-white colour[123], and the seeds nearly round[124] -and larger or smaller according as there may have been a difference in -the size of the seeds of the two varieties[125]. - - [121] The nature of this mistake is now clear; for as stated above - xenia is only likely to occur when the maternal seed-coat is - pigmented. The violet coats in this experiment are themselves cases - of xenia. - - [122] Knight, it was seen, crossed round ♀ × indented ♂ and - consequently got no change of form. - - [123] Cotyledons seen through coat. - - [124] Ordinary dominance of round. - - [125] This is an extraordinary statement to be given as a general - truth. There are sometimes indications of this kind, but certainly - the facts are not usually as here stated. - -I have also noticed that a cross between a round white and a blue -wrinkled pea will in the third and fourth generations (second and third -years’ produce) at times bring forth blue round, blue wrinkled, white -round and white wrinkled peas in the same pods, that the white round -seeds, when again sown, will produce only white round seeds, that -the white wrinkled seeds will, up to the fourth or fifth generation, -produce both blue and white wrinkled and round peas, that the blue -round peas will produce blue wrinkled and round peas, but that the -blue wrinkled peas will bear only blue wrinkled seeds[126]. This -would seem to indicate that the white round and the blue wrinkled peas -are distinct varieties derived from ancestors respectively possessing -one only of those marked qualities; and, in my opinion, the white -round peas trace their origin to a dwarfish pea having white flowers -and round white seeds, and the blue wrinkled varieties to a tall -variety, having also white flowers but blue wrinkled seeds. It is -also noticeable, that from a single cross between two different peas -many hundreds of varieties, not only like one or both parents and -intermediate, but apparently differing from either, may be produced -in the course of three or four years (the shortest time which I have -ascertained it takes to attain the climax of variation in the produce -of cross-fertilised peas, and until which time it would seem useless -to expect a fixed seedling variety to be produced[127]), although a -reversion to the characters of either parent, or of any one of the -ancestors, may take place at an earlier period. - - [126] If we were obliged to suppose that this is a matured conclusion - based on detailed observation it would of course constitute the most - serious “exception” yet recorded. But it is clear that the five - statements are not mutually consistent. We have dominance of round - white in first cross. - - In the second generation blue wrinkled give only blue wrinkled, and - blue round give blue wrinkled and round, in accordance with general - experience. But we are told that white round give _only_ white round. - This would be true of some white rounds, but not, according to - general experience, of all. Lastly we are told _white wrinkled give - all four classes_. If we had not been just told by Laxton that the - first cross showed dominance of white round, and that blue wrinkled - and blue round give the Mendelian result, I should hesitate in face - of this positive statement, but as it is inconsistent with the rest - of the story I think it is unquestionably an error of statement. The - context, and the argument based on the maple crosses show clearly - also what was in Laxton’s mind. He plainly expected the characters - of the original pure varieties to separate out according to their - original combinations, and this expectation confused his memory and - general impressions. This, at least, until any such result is got - by a fresh observer, using strict methods, is the only acceptable - account. - - Of the same nature is the statement given by the late Mr Masters - to Darwin (_Animals and Plants_, I. p. 318) that blue round, white - round, blue wrinkled, and white wrinkled, all reproduced all four - sorts during successive years. Seeing that one sort would give - all four, and two would give two kinds, without special counting - such an impression might easily be produced. There are the further - difficulties due to seed-coat colour, and the fact that the - distinction between round and wrinkled may need some discrimination. - The sorts are not named, and the case cannot be further tested. - - [127] See later. - -These circumstances do not appear to have been known to Mr Knight, as -he seems to have carried on his experiments by continuing to cross -his seedlings in the year succeeding their production from a cross -and treating the results as reliable; whereas it is probable that the -results might have been materially affected by the disturbing causes -then in existence arising from the previous cross fertilisation, and -which, I consider, would, in all cases where either parent has not -become fixed or permanent, lead to results positively perplexing -and uncertain, and to variations almost innumerable. I have again -selected, and intend to sow, watch, and report; but as the usual -climax of variation is nearly reached in the recorded experiment, I -do not anticipate much further deviation, except in height and period -of ripening--characters which are always very unstable in the pea. -There are also important botanical and other variations and changes -occurring in cross-fertilised peas to which it is not my province -here to allude; but in conclusion I may, perhaps, in furtherance of -the objects of this paper, be permitted to inquire whether any light -can, from these observations or other means, be thrown upon the origin -of the cultivated kinds of peas, especially the “maple” variety, and -also as to the source whence the violet and other colours which appear -at intervals on the seeds and in the offspring of cross-fertilised -purple-flowered peas are derived.” - -The reader who has closely followed the preceding passage will begin -to appreciate the way in which the new principles help us to interpret -these hitherto paradoxical phenomena. Even in this case, imperfectly -recorded as it is, we can form a fairly clear idea of what was taking -place. If the “round” seeds really occurred as a distinct class, on -the heterozygotes as described, it is just possible that the fact may -be of great use hereafter. - -We are still far from understanding maternal seed-form--and perhaps -size--as a dominant character. So far, as Miss Saunders has pointed out -to me, it appears to be correlated with a thick and coloured seed-coat. - - * * * * * - -We have now seen the nature of Professor Weldon’s collection of -contradictory evidence concerning dominance in peas. He tells us: -“Enough has been said to show the grave discrepancy between the -evidence afforded by Mendel’s experiments and that obtained by -observers equally trustworthy.” - -He proceeds to a discussion of the _Telephone_ and _Telegraph_ group -and recites facts, which I do not doubt for a moment, showing that -in this group of peas--which have unquestionably been more or less -“blend” or “mosaic” forms from their beginning--the “laws of dominance -and segregation” do not hold. Professor Weldon’s collection of the -facts relating to _Telephone_, &c. has distinct value, and it is the -chief addition he makes to our knowledge of these phenomena. The merit -however of this addition is diminished by the erroneous conclusion -drawn from it, as will be shown hereafter. Meanwhile the reader who -has studied what has been written above on the general questions of -stability, “purity,” and “universal” dominance, will easily be able to -estimate the significance of these phenomena and their applicability to -Mendel’s hypotheses. - - -_D. Miscellaneous cases in other plants and animals_. - -Professor Weldon proceeds: - - “In order to emphasize the need that the ancestry of the parents, - used in crossing, should be considered in discussing the results of a - cross, it may be well to give one or two more examples of fundamental - inconsistency between different competent observers.” - -The “one or two” run to three, viz. Stocks (hoariness and colour); -_Datura_ (character of fruits and colour of flowers); and lastly -colours of Rats and Mice. Each of these subjects, as it happens, -has been referred to in the forthcoming paper by Miss Saunders and -myself. _Datura_ and _Matthiola_ have been subjected to several years’ -experiment and I venture to refer the reader who desires to see whether -the facts are or are not in accord with Mendel’s expectation and how -far there is “fundamental inconsistency” amongst them to a perusal of -our work. - -But as Professor Weldon refers to some points that have not been -explicitly dealt with there, it will be safer to make each clear as we -proceed. - - -1. _Stocks_ (_Matthiola_). Professor Weldon quotes Correns’ observation -that glabrous Stocks crossed with hoary gave offspring all hoary, while -Trevor Clarke thus obtained some hoary and some glabrous. As there are -some twenty different sorts of Stocks[128] it is not surprising that -different observers should have chanced on different materials and -obtained different results. Miss Saunders has investigated laws of -heredity in Stocks on a large scale and an account of her results is -included in our forthcoming Report. Here it must suffice to say that -the cross hoary ♀ × glabrous ♂ always gave offspring all hoary except -once: that the cross glabrous ♀ × hoary ♂ of several types gave all -hoary; _but_ the same cross using other hoary types did frequently -give a mixture, some of the offspring being hoary, others glabrous. -Professor Weldon might immediately decide that here was the hoped for -phenomenon of “reversed” dominance, due to ancestry, but here again -that hypothesis is excluded. For the glabrous (recessive) cross-breds -were _pure_, and produced on self-fertilisation glabrous plants only, -being in fact, almost beyond question, “false hybrids” (see p. 34), -a specific phenomenon which has nothing to do with the question of -dominance. - - [128] The number in Haage and Schmidt’s list exceeds 200, counting - colour-varieties. - -Professor Weldon next suggests that there is discrepancy between the -observations as to flower-colour. He tells us that Correns found -_violet_ Stocks crossed with “_yellowish white_” gave violet or shades -of violet flaked together. According to Professor Weldon - - “On the other hand Nobbe crossed a number of varieties of _M. annua_ - in which the flowers were white, violet, carmine-coloured, crimson - or dark blue. These were crossed in various ways, and before a cross - was made the colour of each parent was matched by a mixture of dry - powdered colours which was preserved. In every case the hybrid flower - was of an intermediate colour, which could be matched by mixing the - powders which recorded the parental colours. The proportions in which - the powders were mixed are not given in each [any] case, but it is - clear that the colours blended[129].” - - [129] The original passage is in _Landwirths. Versuchstationen_, - 1888, XXXV. [_not_ XXXIV.], p. 151. - -On comparing Professor Weldon’s version with the originals we find -the missing explanations. Having served some apprenticeship to the -breeding of Stocks, we, here, are perhaps in a better position to take -the points, but it is to me perfectly inexplicable how in such a simple -matter as this he can have gone wrong. - -Note then - -(1) That Nobbe does _not_ specify _which_ colours he crossed together, -beyond the fact that _white_ was crossed with each fertile form. -The _crimson_ form (_Karmoisinfarbe_), being double to the point of -sterility, was not used. There remain then, white, carmine, and two -purples (violet, “dark blue”). When _white_ was crossed with either of -these, Nobbe says the colour becomes _paler_, whichever sort gave the -pollen. Nobbe does not state that he crossed _carmine_ with the purples. - -(2) Professor Weldon gives no qualification in his version. Nobbe -however states that he found it very difficult to distinguish the -result of crossing _carmine with white_ from that obtained by crossing -_dark blue or violet with white_[130], thereby nullifying Professor -Weldon’s statement that in every case the cross was a simple mixture -of the parental colours--a proposition sufficiently disproved by Miss -Saunders’ elaborate experiments. - - [130] “_Es ist sogar sehr schwierig, einen Unterschied in der Farbe - der Kreuzungsprodukte von Karmin und Weiss gegenüber Dunkelblau oder - Violett und Weiss zu erkennen._” - -(3) Lately the champion of the “importance of small variations,” -Professor Weldon now prefers to treat the distinctions between -established varieties as negligible fluctuations instead of specific -phenomena[131]. Therefore when Correns using “_yellowish white_” -obtained one result and Nobbe using “_white_” obtained another, -Professor Weldon hurries to the conclusion that the results are -comparable and therefore contradictory. Correns however though calling -his flowers _gelblich-weiss_ is careful to state that they are -described by Haage and Schmidt (the seed-men) as “_schwefel-gelb_” or -sulphur-yellow. The topics Professor Weldon treats are so numerous that -we cannot fairly expect him to be personally acquainted with all; still -had he _looked_ at Stocks before writing, or even at the literature -relating to them, he would have easily seen that these yellow Stocks -are a thoroughly distinct form[132]; and in accordance with this fact -it would be surprising if they had not a distinctive behaviour in their -crosses. To use our own terminology their colour character depends -almost certainly on a _compound_ allelomorph. Consequently there is no -evidence of contradiction in the results, and appeal to ancestry is as -unnecessary as futile. - - [131] See also the case of _Buchsbaum_, p. 146, which received - similar treatment. - - [132] One of the peculiarities of most _double_ “sulphur” races is - that the singles they throw are _white_. See Vilmorin, _Fleurs de - pleine Terre_, 1866, p. 354, _note_. In _Wien. Ill. Gartenztg._ 1891, - p. 74, mention is made of a new race with singles also “sulphur,” - cp. _Gartenztg._ 1884, p. 46. Messrs Haage and Schmidt have kindly - written to me that this new race has the alleged property, but that - six other yellow races (two distinct colours) throw their singles - white. - - -2. _Datura._ As for the evidence on _Datura_, I must refer the reader -again to the experiments set forth in our Report. - -The phenomena obey the ordinary Mendelian rules with accuracy. There -are (as almost always where discontinuous variation is concerned) -occasional cases of “mosaics,” a phenomenon which has nothing to do -with “ancestry.” - - -3. _Colours of Rats and Mice._ Professor Weldon reserves his -collection of evidence on this subject for the last. In it we reach an -indisputable contribution to the discussion--a reference to Crampe’s -papers, which together constitute without doubt the best evidence yet -published, respecting colour-heredity in an animal. So far as I have -discovered, the only previous reference to these memoirs is that of -Ritzema Bos[133], who alludes to them in a consideration of the alleged -deterioration due to in-breeding. - - [133] _Biol. Cblt._ XIV. 1894, p. 79. - -Now Crampe through a long period of years made an exhaustive study of -the peculiarities of the colour-forms of Rats, white, black, grey and -their piebalds, as exhibited in Heredity. - -Till the appearance of Professor Weldon’s article Crampe’s work was -unknown to me, and all students of Heredity owe him a debt for putting -it into general circulation. My attention had however been called -by Dr Correns to the interesting results obtained by von Guaita, -experimenting with crosses originally made between albino _mice_ and -piebald Japanese waltzing mice. This paper also gives full details of -an elaborate investigation admirably carried out and recorded. - -In the light of modern knowledge both these two researches furnish -material of the most convincing character demonstrating the Mendelian -principles. It would be a useful task to go over the evidence they -contain and rearrange it in illustration of the laws now perceived. To -do this here is manifestly impossible, and it must suffice to point -out that the albino is a simple recessive in both cases (the waltzing -character in mice being also a recessive), and that the “wild grey” -form is one of the commonest heterozygotes--there appearing, like -the yellow cotyledon-colour of peas, _in either of two capacities_, -i.e. as a pure form, or as the heterozygote form of one or more -combinations[134]. - - [134] The various “contradictions” which Professor Weldon suggests - exist between Crampe, von Guaita and Colladon can almost certainly be - explained by this circumstance. For Professor Weldon “wild-coloured” - mice, however produced, are “wild-coloured” mice and no more (see - Introduction). - -Professor Weldon refers to both Crampe and von Guaita, whose results -show an essential harmony in the fact that both found _albino_ an -obvious recessive, pure almost without exception, while the coloured -forms show various phenomena of dominance. Both found heterozygous -colour-types. He then searches for something that looks like a -contradiction. Of this there is no lack in the works of Johann von -Fischer (11)--an authority of a very different character--whom he -quotes in the following few words: - - “In both rats and mice von Fischer says that piebald rats crossed - with albino varieties of their species, give piebald young if the - father only is piebald, white young if the mother only is piebald.” - -But this is doing small justice to the completeness of Johann von -Fischer’s statement, which is indeed a proposition of much more amazing -import. - -That investigator in fact began by a study of the cross between the -albino Ferret and the Polecat, as a means of testing whether they were -two species or merely varieties. The cross, he found, was in colour and -form a blend of the parental types. Therefore, he declares, the Ferret -and the Polecat are two distinct species, because, “as everybody ought -to know,” - - “_The result of a cross between albino and normal [of one species] is - always a constant one, namely an offspring like the father at least - in colour_[135],” - - [135] “Das Resultat einer Kreuzung zwischen Albino- und Normal-form - ist stets, also, constant, ein dem Vater mindestens in der Färbung - gleiches Junge.” This law is predicated for the case in which both - parents belong to the same species. - -whereas in _crosses_ (between species) this is _not_ the case. - -And again, after reciting that the Ferret-Polecat crosses gave -intermediates, he states: - - “But all this is _not_ the case in crosses between albinos and normal - animals within the species, in which always and without any exception - the young resemble the father in colour[136].” - - [136] “Dieses Alles ist aber _nie_ der Fall bei Kreuzungen unter - Leucismen und normalen Thieren innerhalb der Species, bei denen - _stets und ohne jede Ausnahme die Jungen in Färbung dem Vater - gleichen_.” - -These are admirable illustrations of what is meant by a “_universal_” -proposition. But von Fischer doesn’t stop here. He proceeds to -give a collection of evidence in proof of this truth which he says -“ought to be known to everyone.” He has observed the fact in regard -to albino mole, albino shrew (_Sorex araneus_), melanic squirrel -(_Sciurus vulgaris_), albino ground-squirrel (_Hypudaeus terrestris_), -albino hamster, albino rats, albino mice, piebald (grey-and-white or -black-and-white) mice and rats, partially albino sparrow, and we are -even presented with two cases in Man. No single exception was known to -von Fischer[137]. - - [137] He even withdraws two cases of his own previously published, - in which grey and albino mice were alleged to have given mixtures, - saying that this result must have been due to the broods having been - accidentally mixed by the servants in his absence. - -In his subsequent paper von Fischer declares that from matings of rats -in which the mothers were grey and the fathers albino he bred 2017 pure -albinos; and from albino mothers and grey fathers 3830 normal greys. -“Not a single individual varied in any respect, or was in any way -intermediate.” - -With piebalds the same result is asserted, save that certain melanic -forms appeared. Finally von Fischer repeats his laws already reached, -giving them now in this form: _that if the offspring of a cross show -only the colour of the father, then the parents are varieties of -one species; but if the colour of the offspring be intermediate or -different from that of the father, then the parents belong to distinct -species_. - -The reader may have already gathered that we have here that bane of -the advocate--the witness who proves too much. But why does Professor -Weldon confine von Fischer to the few modest words recited above? That -author has--so far as colour is concerned--a complete law of heredity -supported by copious “observations.” Why go further? - -Professor Weldon “brings forth these strong reasons” of the rats and -mice with the introductory sentence: - - “Examples might easily be multiplied, but as before, I have chosen - rather to cite a few cases which rest on excellent authority, than to - quote examples which may be doubted. I would only add one case among - animals, in which the evidence concerning the inheritance of colour - is affected by the ancestry of the varieties used.” - -So once again Professor Weldon suggests that his laws of ancestry will -explain even the discrepancies between von Fischer on the one hand and -Crampe and von Guaita on the other but he does not tell us how he -proposes to apply them. - -In the cross between the albino and the grey von Fischer tells us that -both colours appear in the offspring, but always, without exception or -variation, that of the father only, in 5847 individuals. - -Surely, the law of ancestry, if he had a moment’s confidence in it, -might rather have warned Professor Weldon that von Fischer’s results -were wrong somewhere, of which there cannot be any serious doubt. -The precise source of error is not easy to specify, but probably -carelessness and strong preconception of the expected result were -largely responsible, though von Fischer says he did all the recording -most carefully himself. - -Such then is the evidence resting “on excellent authority”: may we some -day be privileged to see the “examples which may be doubted”? - -The case of mice, invoked by Professor Weldon, has also been referred -to in our Report. Its extraordinary value as illustrating Mendel’s -principles and the beautiful way in which that case may lead on to -extensions of those principles are also there set forth (see the -present Introduction, p. 25). Most if not all of such “conflicting” -evidence can be reconciled by the steady application of the -Mendelian principle that the progeny will be constant when--and only -when[138]--_similar_ gametes meet in fertilisation, apart from any -question of the characters of the parent which produces those gametes. - - [138] Excluding “false hybridisations.” - - -V. PROFESSOR WELDON’S QUOTATIONS FROM LAXTON. - -In support of his conclusions Professor Weldon adduces two passages -from Laxton, some of whose testimony we have just considered. This -further evidence of Laxton is so important that I reproduce it in full. -The first passage, published in 1866, is as follows:-- - - “The results of experiments in crossing the Pea tend to show that - the colour of the immediate offspring or second generation sometimes - follows that of the female parent, is sometimes intermediate between - that and the male parent, and is sometimes distinct from both; and - although at times it partakes of the colour of the male, it has - not been ascertained by the experimenter ever to follow the exact - colour of the male parent[139]. In shape, the seed frequently has an - intermediate character, but as often follows that of either parent. - In the second generation, in a single pod, the result of a cross - of Peas different in shape and colour, the seeds are sometimes all - intermediate, sometimes represent either or both parents in shape - or colour, and sometimes both colours and characters, with their - intermediates, appear. The results also seem to show that the third - generation or the immediate offspring of a cross, frequently varies - from its parents in a limited manner--usually in one direction - only, but that the fourth generation produces numerous and wider - variations[140]; the seed often reverting partly to the colour and - character of its ancestors of the first generation, partly partaking - of the various intermediate colours and characters, and partly - sporting quite away from any of its ancestry.” - - [139] This is of course on account of the maternal seed characters. - Unless the coat-characters are treated separately from the - cotyledon-characters Laxton’s description is very accurate. Both - this and the statements respecting the “shape” of the seeds, a term - which as used by Laxton means much more than merely “wrinkled” and - “smooth,” are recognizably true as general statements. - - [140] Separation of hypallelomorphs. - -Here Professor Weldon’s quotation ceases. It is unfortunate he did -not read on into the very next sentence with which the paragraph -concludes:-- - - “These sports appear to become fixed and permanent in the next - and succeeding generations; and the tendency to revert and sport - thenceforth seems to become checked if not absolutely stopped[141].” - - [141] The combinations being exhausted. Perhaps Professor Weldon - thought his authority was here lapsing into palpable nonsense! - -Now if Professor Weldon instead of leaving off on the word “ancestry” -had noticed this passage, I think his article would never have been -written. - -Laxton proceeds:-- - - “The experiments also tend to show that the height of the plant - is singularly influenced by crossing; a cross between two dwarf - peas, commonly producing some dwarf and some tall [? in the second - generation]; but on the other hand, a cross between two tall peas - does not exhibit a tendency to diminution in height. - - “No perceptible difference appears to result from reversing the - parents; the influence of the pollen of each parent at the climax or - fourth generation producing similar results[142].” - - [142] Laxton constantly refers to this conception of the “climax” - of--as we now perceive--analytical variation and recombination. Many - citations could be given respecting his views on this “climax” (cp. - p. 167). - -The significance of this latter testimony I will presently discuss. - -Professor Weldon next appeals to a later paper of Laxton’s published in -1890. From it he quotes this passage: - - “By means, however, of cross-fertilisation alone, and unless it be - followed by careful and continuous selection, the labours of the - cross-breeder, instead of benefiting the gardener, may lead to utter - confusion,” - -Here again the reader would have gained had Professor Weldon, instead -of leaving off at the comma, gone on to the end of the paragraph, which -proceeds thus:-- - - “because, as I have previously stated, the Pea under ordinary - conditions is much given to sporting and reversion, for when two - dissimilar old or fixed varieties have been cross-fertilised, - three or four generations at least must, under the most favourable - circumstances, elapse before the progeny will become fixed or - settled; and from one such cross I have no doubt that, by sowing - every individual Pea produced during the three or four generations, - hundreds of different varieties may be obtained; but as might be - expected, I have found that where the two varieties desired to be - intercrossed are unfixed, confusion will become confounded[143], - and the variations continue through many generations, the number at - length being utterly incalculable.” - - [143] Further subdivision and recombination of hypallelomorphs. - -Professor Weldon declares that Laxton’s “experience was altogether -different from that of Mendel.” The reader will bear in mind that when -Laxton speaks of fixing a variety he is not thinking particularly of -seed-characters, but of all the complex characters, fertility, size, -flavour, season of maturity, hardiness, etc., which go to make a -serviceable pea. Considered carefully, Laxton’s testimony is so closely -in accord with Mendelian expectation that I can imagine no chance -description in non-Mendelian language more accurately stating the -phenomena. - -Here we are told in unmistakable terms the breaking up of the original -combination of characters on crossing, their re-arrangement, that -at the fourth or fifth generation the possibilities of sporting -[sub-division of compound allelomorphs and re-combinations of them?] -are exhausted, that there are then definite forms which if selected -are thenceforth fixed [produced by union of similar gametes?] -that it takes longer to select some forms [dominants?] than others -[recessives?], that there may be “mule” forms[144] or forms which -cannot be fixed at all[145] [produced by union of dissimilar gametes?]. - - [144] For instance the _talls_ produced by crossing _dwarfs_ are such - “mules.” Tschermak found in certain cases distinct increase in height - in such a case, though not always (p. 531). - - [145] “The remarkably fine but unfixable pea _Evolution_.” Laxton, p. - 37. - -But Laxton tells us more than this. He shows us that numbers of -varieties may be obtained--hundreds--“incalculable numbers.” Here -too if Professor Weldon had followed Mendel with even moderate care -he would have found the secret. For in dealing with the crosses of -_Phaseolus_ Mendel clearly forecasts the conception of _compound -characters themselves again consisting of definite units_, all of which -may be separated and re-combined in the possible combinations, laying -for us the foundation of the new science of Analytical Biology. - -How did Professor Weldon, after reading Mendel, fail to perceive these -principles permeating Laxton’s facts? Laxton must have seen the very -things that Mendel saw, and had he with his other gifts combined that -penetration which detects a great principle hidden in the thin mist of -“exceptions,” we should have been able to claim for him that honour -which must ever be Mendel’s in the history of discovery. - -When Laxton speaks of selection and the need for it, he means, what -the raiser of new varieties almost always means, the selection of -_definite_ forms, not impalpable fluctuations. When he says that -without selection there will be utter confusion, he means--to use -Mendelian terms--that the plant which shows the desired combination of -characters must be chosen and bred from, and that if this be not done -the grower will have endless combinations mixed together in his stock. -If however such a selection be made in the fourth or fifth generation -the breeder may very possibly have got a fixed form--namely, one that -will breed true[146]. On the other hand he may light on one that does -not breed true, and in the latter case it may be that the particular -type he has chosen is not represented in the gametes and will _never_ -breed true, though selected to the end of time. Of all this Mendel has -given us the simple and final account. - - [146] Apart from fresh original variations, and perhaps in some cases - imperfect homozygosis of some hypallelomorphs. - -At Messrs Sutton and Sons, to whom I am most grateful for unlimited -opportunities of study, I have seen exactly such a case as this. For -many years Messrs Sutton have been engaged in developing new strains -of the Chinese Primrose (_Primula sinensis_, hort.). Some thirty -thoroughly distinct and striking varieties (not counting the _Stellata_ -or “Star” section) have already been produced which breed true or -very nearly so. In 1899 Messrs Sutton called my attention to a strain -known as “Giant Lavender,” a particularly fine form with pale magenta -or lavender flowers, telling me that it had never become fixed. On -examination it appeared that self-fertilised seed saved from this -variety gave some magenta-reds, some lavenders, and some which are -white on opening but tinge with very faint pink as the flower matures. - -On counting these three forms in two successive years the following -figures appeared. Two separately bred batches raised from “Giant -Lavender” were counted in each year. - - Magenta Lavender White - red faintly tinged - - 1901 1st batch 19 27 14 - " 2nd " 9 20 9 - 1902 1st " 12 23 11 - " 2nd " 14 26 11 - -- -- -- - 54 96 45 - -The numbers 54 : 96 : 45 approach the ratio 1 : 2 : 1 so nearly that -there can be no doubt we have here a simple case of Mendelian laws, -operating without definite dominance, but rather with blending. - -When Laxton speaks of the “remarkably fine but unfixable pea -_Evolution_” we now know for the first time exactly what the phenomenon -meant. It, like the “Giant Lavender,” was a “mule” form, not -represented by germ-cells, and in each year arose by “self-crossing.” - -This is only one case among many similar ones seen in the Chinese -Primrose. In others there is no doubt that more complex factors are at -work, the subdivision of compound characters, and so on. The history -of the “Giant Lavender” goes back many years and is not known with -sufficient precision for our purposes, but like all these forms it -originated from crossings among the old simple colour varieties of -_sinensis_. - - -VI. THE ARGUMENT BUILT ON EXCEPTIONS. - -So much for the enormous advance that the Mendelian principles already -permit us to make. But what does Professor Weldon offer to substitute -for all this? Nothing. - -Professor Weldon suggests that a study of ancestry will help us. Having -recited Tschermak’s exceptions and the great irregularities seen in -the _Telephone_ group, he writes: - - “Taking these results together with Laxton’s statements, and with the - evidence afforded by the _Telephone_ group of hybrids, I think we can - only conclude that segregation of seed-characters is not of universal - occurrence among cross-bred peas, and that when it does occur, it may - or may not follow Mendel’s law.” - -Premising that when pure types are used the exceptions form but a small -part of the whole, and that any supposed absence of “segregation” may -have been _variation_, this statement is perfectly sound. He proceeds:-- - - “The law of segregation, like the law of dominance, appears therefore - to hold only for races of _particular ancestry_ [my italics]. In - special cases, other formulae expressing segregation have been - offered, especially by De Vries and by Tschermak for other plants, - but these seem as little likely to prove generally valid as Mendel’s - formula itself. - - “The fundamental mistake which vitiates all work based upon Mendel’s - method is the neglect of ancestry, and the attempt to regard the - whole effect upon offspring, produced by a particular parent, as due - to the existence in the parent of particular structural characters; - while the contradictory results obtained by those who have observed - the offspring of parents identical in certain characters show clearly - enough that not only the parents themselves, but their race, that - is their ancestry, must be taken into account before the result of - pairing them can be predicted.” - -In this passage the Mendelian view is none too precisely represented. -I should rather have said that it was from Mendel, first of all men, -that we have learnt _not_ to regard the effects produced on offspring -“as due to the existence in the parent of particular structural -characters.” We have come rather to disregard the particular structure -of the parent except in so far as it may give us a guide as to the -nature of its gametes. - -This indication, if taken in the positive sense--as was sufficiently -shown in considering the significance of the “mule” form or -“hybrid-character”--we now know may be absolutely worthless, and in any -unfamiliar case is very likely to be so. Mendel has proved that the -inheritance from individuals of _identical ancestry_ may be entirely -different: that from identical ancestry, without new variation, may -be produced three kinds of individuals (in respect of each pair of -characters), namely, individuals capable of transmitting one type, or -another type, or both: moreover that the statistical relations of these -three classes of individuals to each other will in a great number of -cases be a definite one: and of all this he shows a complete account. - -Professor Weldon cannot deal with any part of this phenomenon. He does -little more than allude to it in passing and point out exceptional -cases. These he suggests a study of ancestry will explain. - -As a matter of fact a study of ancestry will give little guide--perhaps -none--even as to the probability of the phenomenon of dominance of a -character, none as to the probability of normal “purity” of germ-cells. -Still less will it help to account for fluctuations in dominance, or -irregularities in “purity.” - - -_Ancestry and Dominance._ - -In a series of astonishing paragraphs (pp. 241–2) Professor Weldon -rises by gradual steps, from the exceptional facts regarding occasional -dominance of green colour in _Telephone_ to suggest that the _whole -phenomenon of dominance may be attributable to ancestry_, and that -in fact one character has no natural dominance over another, apart -from what has been created by selection of ancestry. This piece of -reasoning, one of the most remarkable examples of special pleading -to be met with in scientific literature, must be read as a whole. -I reproduce it entire, that the reader may appreciate this curious -effort. The remarks between round parenthetical marks are Professor -Weldon’s, those between crotchets are mine. - - “Mendel treats such characters as yellowness of cotyledons and the - like as if the condition of the character in two given parents - determined its condition in all their subsequent offspring[147]. Now - it is well known to breeders, and is clearly shown in a number of - cases by Galton and Pearson, that the condition of an animal does - not as a rule depend upon the condition of any one pair of ancestors - alone, but in varying degrees upon the condition of all its ancestors - in every past generation, the condition in each of the half-dozen - nearest generations having a quite sensible effect. Mendel does - not take the effect of differences of ancestry into account, but - considers that any yellow-seeded pea, crossed with any green-seeded - pea, will behave in a certain definite way, whatever the ancestry - of the green and yellow peas may have been. (He does not say this - in words, but his attempt to treat his results as generally true of - the characters observed is unintelligible unless this hypothesis be - assumed.) The experiments afford no evidence which can be held to - justify this hypothesis. His observations on cotyledon colour, for - example, are based upon 58 cross-fertilised flowers, all of which - were borne upon ten plants; and we are not even told whether these - ten plants included individuals from more than two races. - - [147] Mendel, on the contrary, disregards the “condition of the - character” in the parent altogether; but is solely concerned with the - nature of the characters of the _gametes_. - -“The many thousands of individuals raised from these ten plants -afford an admirable illustration of the effect produced by crossing -a few pairs of plants of known ancestry; but while they show this -perhaps better than any similar experiment, they do not afford the data -necessary for a statement as to the behaviour of yellow-seeded peas in -general, whatever their ancestry, when crossed with green-seeded peas -of any ancestry. [Mendel of course makes no such statement.] - -“When this is remembered, the importance of the exceptions to dominance -of yellow cotyledon-colour, or of smooth and rounded shape of seeds, -observed by Tschermak, is much increased; because although they form a -small percentage of his whole result, they form a very large percentage -of the results obtained with peas of certain races. [Certainly.] -The fact that _Telephone_ behaved in crossing on the whole like a -green-seeded race of exceptional dominance shows that something other -than the mere character of the parental generation operated in this -case. Thus in eight out of 27 seeds from the yellow _Pois d’Auvergne_ ♀ -× _Telephone_ ♂ the cotyledons were yellow with green patches; the -reciprocal cross gave two green and one yellow-and-green seed out of -the whole ten obtained; and the cross _Telephone_ ♀ × (yellow-seeded) -_Buchsbaum_[148] ♂ gave on one occasion two green and four yellow seeds. - - [148] Regarding this “exception” see p. 146. - -“So the cross _Couturier_ (orange-yellow) ♀ × the green-seeded -_Express_ ♂ gave a number of seeds intermediate in colour. (It is not -clear from Tschermak’s paper whether _all_ the seeds were of this -colour, but certainly some of them were.) The green _Plein le Panier_ -[_Fillbasket_] ♀ × _Couturier_ ♂ in three crosses always gave either -seeds of colour intermediate between green and yellow, or some yellow -and some green seeds in the same pod. The cross reciprocal to this was -not made; but _Express_ ♀ × _Couturier_ ♂ gave 22 seeds of which four -were yellowish green[149]. - - [149] See p. 148. - -“These facts show _first_ that Mendel’s law of dominance conspicuously -fails for crosses between certain races, while it appears to hold -for others; and _secondly_ that the intensity of a character in one -generation of a race is no trustworthy measure of its dominance in -hybrids. The obvious suggestion is that the behaviour of an individual -when crossed depends largely upon the characters of its ancestors[150]. -When it is remembered that peas are normally self-fertilised, and that -more than one named variety may be selected out of the seeds of a -single hybrid pod, it is seen to be probable that Mendel worked with a -very definite combination of ancestral characters, and had no proper -basis for generalisation about yellow and green peas of any ancestry” -[which he never made]. - - [150] Where was that “logician,” the “consulting-partner,” when this - piece of reasoning passed the firm? - -Let us pause a moment before proceeding to the climax. Let the reader -note we have been told of _two_ groups of cases in which dominance of -yellow failed or was irregular. (Why are not Gärtner’s and Seton’s -“exceptions” referred to here?) In one of these groups _Couturier_ -was always one parent, either father or mother, and were it not for -Tschermak’s own obvious hesitation in regard to his own exceptions -(see p. 148), I would gladly believe that _Couturier_--a form I do not -know--may be an exceptional variety. _How_ Professor Weldon proposes -to explain its peculiarities by reference to ancestry he omits to tell -us. The _Buchsbaum_ case is already disposed of, for on Tschermak’s -showing, it is an unstable form. - -Happily, thanks to Professor Weldon, we know rather more of the third -case, that of _Telephone_, which, whether as father or mother, was -frequently found by Tschermak to give either green, greenish, or -patchwork-seeds when crossed with yellow varieties. It behaves, in -short, “like a green-seeded pea of exceptional dominance,” as we are -now told. For this dominant quality of _Telephone’s_ greenness we are -asked to account _by appeal to its ancestry_. May we not expect, -then, this _Telephone_ to be--if not a pure-bred green pea from time -immemorial--at least as pure-bred as other green peas which do _not_ -exhibit dominance of green at all? Now, what is _Telephone_? Do not let -us ask too much. Ancestry takes a lot of proving. We would not reject -him “_parce qu’il n’avait que soixante & onze quartiers, & que le reste -de son arbre généalogique avait été perdu par l’injure du tems_.” - -But with stupefaction we learn from Professor Weldon himself that -_Telephone_ is the very variety which he takes _as his type of a -permanent and incorrigible mongrel_, a character it thoroughly deserves. - -From _Telephone_ he made his colour scale! Tschermak declares the -cotyledons to be “yellowish or whitish green, often entirely bright -yellow[151].” So little is it a thorough-bred green pea, that it cannot -always keep its own self-fertilised offspring green. Not only is this -pea a parti-coloured mongrel, but Professor Weldon himself quotes -Culverwell that as late as 1882 both _Telegraph_ and _Telephone_ “will -always come from one sort, more especially from the green variety”; and -again regarding a supposed good sample of _Telegraph_ that “Strange to -say, although the peas were taken from one lot, those sown in January -produced a great proportion of the light variety known as _Telephone_. -These were of every shade of light green up to white, and could have -been shown for either variety,” _Gard. Chron._ 1882 (2), p. 150. This -is the variety whose green, it is suggested, partially “dominates” -over the yellow of _Pois d’Auvergne_, a yellow variety which has a -clear lineage of about a century, and probably more. If, therefore, -the facts regarding _Telephone_ have any bearing on the significance -of ancestry, they point the opposite way from that in which Professor -Weldon desires to proceed. - - [151] “_Speichergewebe gelblich--oder weisslich--grün, manchmal auch - vollständig hellgelb._” Tschermak (36), p. 480. - -In view of the evidence, the conclusion is forced upon me that the -suggestion that “ancestry” may explain the facts regarding _Telephone_ -has no meaning behind it, but is merely a verbal obstacle. Two words -more on _Telephone_. On p. 147 I ventured to hint that if we try to -understand the nature of the appearance of green in the offspring -of _Telephone_ bred with yellow varieties, we are more likely to do -so by comparing the facts with those of false hybridisation than -with fluctuations in dominance. In this connection I would call the -reader’s attention to a point Professor Weldon misses, that Tschermak -_also got yellowish-green seeds from Fillbasket (green) crossed with -Telephone_. I suggest therefore that _Telephone’s_ allelomorphs may be -in part transmitted to its offspring in a state which needs no union -with any corresponding allelomorph of the other gamete, just as may -the allelomorphs of “false hybrids.” It would be quite out of place -here to pursue this reasoning, but the reader acquainted with special -phenomena of heredity will probably be able fruitfully to extend it. -It will be remembered that we have already seen the further fact that -the behaviour of _Telephone_ in respect to seed-shape was also peculiar -(see p. 152). - -Whatever the future may decide on this interesting question it is -evident that with _Telephone_ (and possibly _Buchsbaum_) we are -encountering a _specific_ phenomenon, which calls for specific -elucidation and not a case simply comparable with or contradicting the -evidence of dominance in general. - -In this excursion we have seen something more of the “exceptions.” -Many have fallen, but some still stand, though even as to part of -the remainder Tschermak entertains some doubts, and, it will be -remembered, cautions his reader that of his exceptions some may be -self-fertilisations, and some did not germinate[152]. Truly a slender -basis to carry the coming structure! - - [152] In his latest publication on this subject, the notes to the - edition of Mendel in Ostwald’s _Klassiker_ (pp. 60–61), Tschermak, - who has seen more true exceptions than any other observer, thus - refers to them. As to dominance:--“_Immerhin kommen vereinzelt - auch zweifellose Fälle von Merkmalmischung, d. h. Uebergangsformen - zwischen gelber und grüner Farbe, runder und runzeliger Form vor, - die sich in weiteren Generationen wie dominantmerkmalige Mischlinge - verhalten._” As to purity of the extracted recessives:--_Ganz - vereinzelt scheinen Ausnahmsfälle vorzukommen._" - - Küster (22) also in a recent note on Mendelism points out, with - reason, that the number of “exceptions” to dominance that we shall - find, depends simply on the stringency with which the supposed “law” - is drawn. The same writer remarks further that Mendel makes no such - rigid definition of dominance as his followers have done. - -But Professor Weldon cannot be warned. He told us the “law of dominance -conspicuously fails for crosses between certain races.” Thence the -start. I venture to give the steps in this impetuous argument. There -are exceptions[153]--a fair number if we count the bad ones--there -may be more--must be more--_are_ more--no doubt many more: so to the -brink. Then the bold leap: may there not be as many cases one way as -the other? We have not tried half the sorts of Peas yet. There is still -hope. True we know dominance of many characters in some hundreds of -crosses, using some twenty varieties--not to speak of other plants and -animals--but we _do_ know some exceptions, of which a few are still -good. So dominance may yet be all a myth, built up out of the petty -facts those purblind experimenters chanced to gather. Let us take wider -views. Let us look at fields more propitious--more what we would have -them be! Let us turn to eye-colour: at least there is no dominance in -that. Thus Professor Weldon, telling us that Mendel “had no proper -basis for generalisation about yellow and green peas of any ancestry,” -proceeds to this lamentable passage:-- - - [153] If the “logician-consulting-partner” will successfully apply - this _Fallacia acervalis_, the “method of the vanishing heap,” to - dominant peas, he will need considerable leisure. - - “Now in such a case of alternative inheritance as that of human - eye-colour, it has been shown that a number of pairs of parents, one - of whom has dark and the other blue eyes, will produce offspring of - which nearly one half are dark-eyed, nearly one half are blue-eyed, - a small but sensible percentage being children with mosaic eyes, - the iris being a patch-work of lighter and darker portions. But the - dark-eyed and light-eyed children are not equally distributed among - all families; and it would almost certainly be possible, by selecting - cases of marriage between men and women of appropriate ancestry, - to demonstrate for their families a law of dominance of dark over - light eye-colour, or of light over dark. Such a law might be as - valid for the families of selected ancestry as Mendel’s laws are for - his peas and for other peas of probably similar ancestral history, - but it would fail when applied to dark and light-eyed parents in - general,--that is, to parents of any ancestry who happen to possess - eyes of given colour.” - -The suggestion amounts to this: that because there are exceptions -to dominance in peas; and because by some stupendous coincidence, -or still more amazing incompetence, a bungler might have thought he -found dominance of one eye-colour whereas really there was none[154]; -therefore Professor Weldon is at liberty to suggest there is a fair -chance that Mendel and all who have followed him have either been -the victims of this preposterous coincidence not once, but again and -again; or else persisted in the same egregious and perfectly gratuitous -blunder. Professor Weldon is skilled in the Calculus of Chance: will he -compute the probabilities in favour of his hypothesis? - - [154] I have no doubt there is no universal dominance in eye-colour. - Is it _quite_ certain there is no dominance at all? I have searched - the works of Galton and Pearson relating to this subject without - finding a clear proof. If there is in them material for this decision - may perhaps be pardoned for failing to discover it, since the - tabulations are not prepared with this point in view. Reference to - the original records would soon clear up the point. - - -_Ancestry and purity of germ-cells._ - -To what extent ancestry is likely to elucidate dominance we have now -seen. We will briefly consider how laws derived from ancestry stand in -regard to segregation of characters among the gametes. - -For Professor Weldon suggests that his view of ancestry will explain -the facts not only in regard to dominance and its fluctuations but -in regard to the purity of the germ-cells. He does not apply this -suggestion in detail, for its error would be immediately exposed. In -every strictly Mendelian case the _ancestry_ of the pure extracted -recessives or dominants, arising from the breeding of first crosses, is -identical with that of the impure dominants [or impure recessives in -cases where they exist]. Yet the posterity of each is wholly different. -The pure extracted forms, in these simplest cases, are no more likely -to produce the form with which they have been crossed than was their -pure grandparent; while the impure forms break up again into both -grand-parental forms. - -Ancestry does not touch these facts in the least. They and others -like them have been a stumbling-block to all naturalists. Of such -paradoxical phenomena Mendel now gives us the complete and final -account. Will Professor Weldon indicate how he proposes to regard them? - - * * * * * - -Let me here call the reader’s particular attention to that section -of Mendel’s experiments to which Professor Weldon does not so much -as allude. Not only did Mendel study the results of allowing his -cross-breds (_DR_’s) to fertilise themselves, giving the memorable ratio - - 1 _DD_ : 2 _DR_ : 1 _RR_, - -but he fertilised those cross-breds (_DR_’s) both with the pure -dominant (_D_) and with the pure recessive (_R_) varieties -reciprocally, obtaining in the former case the ratio - - 1 _DD_ : 1 _DR_ - -and in the latter the ratio - - 1 _DR_ : 1 _RR_. - -The _DD_ group and the _RR_ group thus produced giving on -self-fertilisation pure _D_ offspring and pure _R_ offspring -respectively, while the _DR_ groups gave again - - 1 _DD_ : 2 _DR_ : 1 _RR_. - -How does Professor Weldon propose to deal with these results, and by -what reasoning can he suggest that considerations of ancestry are to be -applied to them? If I may venture to suggest what was in Mendel’s mind -when he applied this further test to his principles it was perhaps some -such considerations as the following. Knowing that the cross-breds on -self-fertilisation give - - 1 _DD_ : 2 _DR_ : 1 _RR_ - -three explanations are possible: - - (_a_) These cross-breds may produce pure _D_ germs of both sexes and - pure _R_ germs of both sexes on an average in equal numbers. - - (_b_) _Either_ the female, _or_ the male, gametes may be _alone_ - differentiated according to the allelomorphs, into pure _D_’s, pure - _R_’s, and crosses _DR_ or _RD_, the gametes of the other sex being - homogeneous and neutral in regard to those allelomorphs. - - (_c_) There may be some neutralisation or cancelling between - characters in _fertilisation_ occurring in such a way that the - well-known ratios resulted. The absence of and inability to transmit - the _D_ character in the _RR_’s, for instance, might have been due - not to the original purity of the germs constituting them, but to - some condition incidental to or connected with fertilisation. - -It is clear that Mendel realized (_b_) as a possibility, for he says -_DR_ was fertilised with the pure forms to test the composition of its -egg-cells, but the reciprocal crosses were made to test the composition -of the pollen of the hybrids. Readers familiar with the literature -will know that both Gärtner and Wichura had in many instances shown -that the offspring of crosses in the form (_a_ × _b_) ♀ × _c_ ♂ were -less variable than those of crosses in the form _a_ ♀ × (_b_ × _c_) -♂, &c. This important fact in many cases is observed, and points to -differentiation of characters occurring frequently among the male -gametes when it does not occur or is much less marked among the -maternal gametes. Mendel of course knew this, and proceeded to test for -such a possibility, finding by the result that differentiation was the -same in the gametes of both sexes[155]. - - [155] See Wichura (46), pp. 55–6. - -Of hypotheses (_b_) and (_c_) the results of recrossing with the two -pure forms dispose; and we can suggest no hypothesis but (_a_) which -gives an acceptable account of the facts. - -It is the purity of the “extracted” recessives and the “extracted” -dominants--primarily the former, as being easier to recognize--that -constitutes the real proof of the validity of Mendel’s principle. - -Using this principle we reach immediately results of the most -far-reaching character. These theoretical deductions cannot be further -treated here--but of the practical use of the principle a word may be -said. Where-ever there is marked dominance of one character the breeder -can at once get an indication of the amount of trouble he will have in -getting his cross-bred true to either dominant or recessive character. -He can only thus forecast the future of the race in regard to each such -pair of characters taken severally, but this is an immeasurable advance -on anything we knew before. More than this, it is certain that in some -cases he will be able to detect the “mule” or heterozygous forms by -the statistical frequency of their occurrence or by their structure, -especially when dominance is absent, and sometimes even in cases -where there is distinct dominance. With peas, the practical seedsman -cares, as it happens, little or nothing for those simple characters -of seed-structure, &c. that Mendel dealt with. He is concerned with -size, fertility, flavour, and numerous similar characters. It is to -these that Laxton (invoked by Professor Weldon) primarily refers, when -he speaks of the elaborate selections which are needed to fix his -novelties. - -We may now point tentatively to the way in which some even of these -complex cases may be elucidated by an extension of Mendel’s principle, -though we cannot forget that there are other undetected factors at work. - - -_The value of the appeal to Ancestry._ - -But it may be said that Professor Weldon’s appeal to ancestry calls -for more specific treatment. When he suggests ancestry as “one great -reason” for the different properties displayed by different races or -individuals, and as providing an account of other special phenomena of -heredity, he is perhaps not to be taken to mean any definite ancestry, -known or hypothetical. He may, in fact, be using the term “ancestry” -merely as a brief equivalent signifying the previous history of the -race or individual in question. But if such a plea be put forward, the -real utility and value of the appeal to ancestry is even less evident -than before. - -Ancestry, as used in the method of Galton and Pearson, means a -definite thing. The whole merit of that method lies in the fact that -by it a definite accord could be proved to exist between the observed -characters and behaviour of specified descendants and the ascertained -composition of their pedigree. Professor Weldon in now attributing -the observed peculiarities of _Telephone_ &c. to conjectural -peculiarities of pedigree--if this be his meaning--renounces all -that had positive value in the reference to ancestry. His is simply -an appeal to ignorance. The introduction of the word “ancestry” -in this sense contributes nothing. The suggestion that ancestry -might explain peculiarities means no more than “we do not know how -peculiarities are to be explained.” So Professor Weldon’s phrase “peas -of probably similar ancestral history[156]” means “peas probably -similar”; when he speaks of Mendel having obtained his results with -“a few pairs of plants of known ancestry[157],” he means “a few -pairs of known plants” and no more; when he writes that “the law of -segregation, like the law of dominance appears to hold only for races -of particular ancestry[158],” the statement loses nothing if we write -simply “for particular races.” We all know--the Mendelian, best of -all--that particular races and particular individuals may, even though -indistinguishable by any other test, exhibit peculiarities in heredity. - - [156] See above, p. 192. - - [157] See above, p. 187. - - [158] See above, p. 184. - -But though on analysis those introductions of the word “ancestry” -are found to add nothing, yet we can feel that as used by Professor -Weldon they are intended to mean a great deal. Though the appeal may -be confessedly to ignorance, the suggestion is implied that if we did -know the pedigrees of these various forms we should then have some -real light on their present structure or their present behaviour in -breeding. Unfortunately there is not the smallest ground for even this -hope. - -As Professor Weldon himself tells us[159], conclusions from pedigree -must be based on the conditions of the several ancestors; and even more -categorically (p. 244), “_The degree to which a parental character -affects offspring depends not only upon its development in the -individual parent, but on its degree of development in the ancestors -of that parent._” [My italics.] Having rehearsed this profession of -an older faith Professor Weldon proceeds to stultify it in his very -next paragraph. For there he once again reminds us that _Telephone_, -the mongrel pea of recent origin, which does not breed true to -seed characters, has yet manifested the peculiar power of stamping -the recessive characters on its cross-bred offspring, though pure -and stable varieties that have exhibited the same characters in a -high degree for generations have _not_ that power. As we now know, -the presence or absence of a character in a progenitor _may_ be no -indication whatever as to the probable presence of the character in the -offspring; for the characters of the latter depend on gametic and not -on zygotic differentiation. - - [159] See above, p. 186. - -The problem is of a different order of complexity from that which -Professor Weldon suggests, and facts like these justify the affirmation -that if we could at this moment bring together the whole series of -individuals forming the pedigree of _Telephone_, or of any other plant -or animal known to be aberrant as regards heredity, we should have no -more knowledge of the nature of these aberrations; no more prescience -of the moment at which they would begin, or of their probable modes -of manifestation; no more criterion in fact as to the behaviour such -an individual would exhibit in crossing[160], or solid ground from -which to forecast its posterity, than we have already. We should learn -then--what we know already--that at some particular point of time its -peculiar constitution was created, and that its peculiar properties -then manifested themselves: how or why this came about, we should no -more comprehend with the full ancestral series before us, than we -can in ignorance of the ancestry. Some cross-breds follow Mendelian -segregation; others do not. In some, palpable dominance appears; in -others it is absent. - - [160] Beyond an indication as to the homogeneity or “purity” of its - gametes at a given time. - -If there were no ancestry, there would be no posterity. But to answer -the question _why_ certain of the posterity depart from the rule which -others follow, we must know, not the ancestry, but how it came about -_either_ that at a certain moment a certain gamete divided from its -fellows in a special and unwonted fashion; _or_, though the words -are in part tautological, the reason why the union of two particular -gametes in fertilisation took place in such a way that gametes having -new specific properties resulted[161]. No one yet knows how to use the -facts of ancestry for the elucidation of these questions, or how to get -from them a truth more precise than that contained in the statement -that a diversity of specific consequences (in heredity) may follow an -apparently single specific disturbance. Rarely even can we see so much. -The appeal to ancestry, as introduced by Professor Weldon, masks the -difficulty he dare not face. - - [161] May there be a connection between the extraordinary fertility - and success of the _Telephone_ group of peas, and the peculiar - frequency of a blended or mosaic condition of their allelomorphs? - The conjecture may be wild, but it is not impossible that the two - phenomena may be interdependent. - -In other words, it is the _cause of variation_ we are here seeking. -To attack that problem no one has yet shown the way. Knowledge of a -different order is wanted for that task; and a compilation of ancestry, -valuable as the exercise may be, does not provide that particular kind -of knowledge. - -Of course when once we have discovered by experiment that--say, -_Telephone_--manifests a peculiar behaviour in heredity, we can perhaps -make certain forecasts regarding it with fair correctness; but that -any given race or individual will behave in such a way, is a fact not -deducible from its ancestry, for the simple reason that organisms -of identical ancestry may behave in wholly distinct, though often -definite, ways. - -It is from this hitherto hopeless paradox that Mendel has begun at last -to deliver us. The appeal to ancestry is a substitution of darkness for -light. - - -VII. THE QUESTION OF ABSOLUTE PURITY OF GERM-CELLS. - -But let us go back to the cases of defective “purity” and consider how -the laws of ancestry stand in regard to them. It appears from the facts -almost certain that purity may sometimes be wanting in a character -which elsewhere usually manifests it. - -Here we approach a question of greater theoretical consequence to the -right apprehension of the part borne by Mendelian principles in the -physiology of heredity. We have to consider the question whether the -purity of the gametes in respect of one or other antagonistic character -is or is likely to be in case of _any_ given character a _universal_ -truth? The answer is unquestionably--No--but for reasons in which -“ancestry” plays no part[162]. - - [162] This discussion leaves “false hybridism” for separate - consideration. - -Hoping to interest English men of science in the Mendelian discoveries -I offered in November 1900 a paper on this subject to “Nature.” The -article was of some length and exceeded the space that the Editor could -grant without delay. I did not see my way to reduce it without injury -to clearness, and consequently it was returned to me. At the time our -own experiments were not ready for publication and it seemed that all I -had to say would probably be common knowledge in the next few weeks, so -no further attempt at publication was made. - -In that article I discussed this particular question of the absolute -purity of the germ-cells, showing how, on the analogy of other -bud-variations, it is almost certain that the germ-cells, even in -respect to characters normally Mendelian, may on occasion present the -same mixture of characters, whether apparently blended or mosaic, -which we know so well elsewhere. Such a fact would in nowise -diminish the importance of Mendel’s discovery. The fact that mosaic -peach-nectarines occur is no refutation of the fact that the _total_ -variation is common. Just as there may be trees with several such -mosaic fruits, so there may be units, whether varieties, individual -plants, flowers or gonads, or other structural units, bearing mosaic -egg-cells or pollen grains. Nothing is more likely or more in -accordance with analogy than that by selecting an individual producing -germs of blended or mosaic character, a race could be established -continuing to produce such germs. Persistence of such blends or mosaics -in _asexual_ reproduction is well-known to horticulturists; for example -“bizarre” carnations, oranges streaked with “blood”-orange character, -and many more. In the famous paper of Naudin, who came nearer to the -discovery of the Mendelian principle than any other observer, a paper -quoted by Professor Weldon, other examples are given. These forms, once -obtained, can be multiplied _by division_; and there is no reason why -a zygote formed by the union of mosaic or blended germs, once arisen, -should not in the cell-divisions by which its gametes are formed, -continue to divide in a similar manner and produce germs like those -which united to form that zygote. The irregularity, once begun, may -continue for an indefinite number of divisions. - -I am quite willing to suppose, with Professor Weldon (p. 248), that the -pea _Stratagem_ may, as he suggests, be such a case. I am even willing -to accept provisionally as probable that when two gametes, themselves -of mosaic or blended character, meet together in fertilisation, they -are more likely to produce gametes of mosaic or blended character than -of simply discontinuous character. Among Messrs Sutton’s Primulas -there are at least two striking cases of “flaked” or “bizarre” unions -of bright colours and white which reproduce themselves by seed with -fair constancy, though Mendelian purity in respect of these colours -is elsewhere common in the varieties (I suspect mosaics of “false -hybridism” among allelomorphs in some of these cases). Similarly Galton -has shown that though children having one light-eyed and one dark-eyed -parent generally have eyes either light or dark, the comparatively rare -medium eye-coloured persons when they mate together frequently produce -children with medium eye-colour. - -In this connection it may be worth while to allude to a point of some -practical consequence. We know that when pure dominant--say yellow--is -crossed with pure recessive--say green--the dominance of yellow is -seen; and we have every reason to believe this rule generally (_not_ -universally) true for pure varieties of peas. But we notice that in -the case of a form like the pea, depending on human selection for its -existence, it might be possible in a few years for the races with pure -seed characters to be practically supplanted by the “mosaicized” races -like the _Telephone_ group, if the market found in these latter some -specially serviceable quality. In the maincrop peas I suspect this very -process is taking place[163]. After such a revolution it might be -possible for a future experimenter to conclude that _Pisum sativum_ was -by nature a “mosaicized” species in these respects, though the mosaic -character may have arisen once in a seed or two as an exceptional -phenomenon. When the same reasoning is extended to wild forms depending -on other agencies for selection, some interesting conclusions may be -reached. - - [163] Another practical point of the same nature arises from the - great variability which these peas manifest in plant- as well as - seed-characters. Mr Hurst of Burbage tells me that in _e.g._ _William - the First_, a pea very variable in seed-characters also, tall plants - may be so common that they have to be rogued out even when the - variety is grown for the vegetable market, and that the same is true - of several such varieties. It seems by no means improbable that it is - by such roguing that the unstable mosaic or blend-form is preserved. - In a thoroughly stable variety such as _Ne Plus Ultra_ roguing is - hardly necessary even for the seed-market. - - Mr N. N. Sherwood in his useful account of the origin and races - of peas (_Jour. R. Hort. Soc._ XXII. 1899, p. 254) alludes to the - great instability of this class of pea. To Laxton, he says, “we are - indebted for a peculiar type of Pea, a round seed with a very slight - indent, the first of this class sent out being _William the First_, - the object being to get a very early blue-seeded indented Pea of the - same earliness as the Sangster type with a blue seed, or in other - words with a Wrinkled Pea flavour. This type of Pea is most difficult - to keep true on account of the slight taint of the Wrinkled Pea in - the breed, which causes it to run back to the Round variety.” - -But in Mendelian cases we are concerned primarily not with the product -of gametes of blended character, but with the consequences of the -union of gametes already discontinuously dissimilar. The existence of -pure Mendelian gametes for given characters is perfectly compatible -with the existence of blended or mosaic gametes for similar characters -elsewhere, but this principle enables us to form a comprehensive and -fruitful conception of the relation of the two phenomena to each other. -As I also pointed out, through the imperfection of our method which -does not yet permit us to _see_ the differentiation among the gametes -though we know it exists, we cannot yet as a rule obtain certain proof -of the impurity of the gametes (except perhaps in the case of mosaics) -as distinct from evidence of imperfect dominance. If however the case -be one of a “mule” form, distinct from either parent, and not merely -of dominance, there is no _a priori_ reason why even this may not be -possible; for we should be able to distinguish the results of breeding -first crosses together into _four_ classes: two pure forms, one or -more blend or mosaic forms, and “mule” forms. Such a study could as -yet only be attempted in simplest cases: for where we are concerned -with a compound allelomorph capable of resolution, the combinations -of the integral components become so numerous as to make this finer -classification practically inapplicable. - -But in many cases--perhaps a majority--though by Mendel’s statistical -method we can perceive the fluctuations in the numbers of the several -products of fertilisation, we shall not know whether abnormalities in -the distribution of those products are due to a decline in dominance, -or to actual impurity of the gametes. We shall have further to -consider, as affecting the arithmetical results, the possibility of -departure from the rule that each kind of gamete is produced in equal -numbers[164]; also that there may be the familiar difficulties in -regard to possible selection and assortative matings among the gametes. - - [164] In dealing with cases of decomposition or resolution of - compound characters this consideration is of highest importance. - -I have now shown how the mosaic and blend-forms are to be regarded in -the light of the Mendelian principle. What has Professor Weldon to say -in reference to them? His suggestion is definite enough--that a study -of ancestry will explain the facts: _how_, we are not told. - -In speaking of the need of study of the characters of the _race_ he -is much nearer the mark, but when he adds “that is their ancestry,” -he goes wide again. When _Telephone_ does not truly divide the -antagonistic characters among its germ-cells this fact is in nowise -simply traceable to its having originated in a cross--a history it -shares with almost all the peas in the market--but to its own peculiar -nature. In such a case imperfect dominance need not surprise us. - -What we need in all these phenomena is a knowledge of the properties -of each race, or variety, as we call it in peas. We must, as I have -often pleaded, study the properties of each form no otherwise than the -chemist does the properties of his substances, and thus only can we -hope to work our way through these phenomena. _Ancestry_ holds no key -to these facts; for the same ancestry is common to own brothers and -sisters endowed with dissimilar properties and producing dissimilar -posterity. To the knowledge of the properties of each form and the laws -which it obeys there are no short cuts. We have no periodic law to -guide us. Each case must as yet be separately worked out. - -We can scarcely avoid mention of a further category of phenomena -that are certain to be adduced in opposition to the general truth -of the purity of the extracted forms. It is a fact well known to -breeders that a highly-bred stock may, unless selections be continued, -“degenerate.” This has often been insisted on in regard to peas. I -have been told of specific cases by Messrs Sutton and Sons, instances -which could be multiplied. Surely, will reply the supporters of the -theory of Ancestry, this is simply impurity in the extracted stocks -manifesting itself at last. Such a conclusion by no means follows, and -the proof that it is inapplicable is obtained from the fact that the -“degeneration,” or variation as we should rather call it, need not -lead to the production of any proximate ancestor of the selected stock -at all, but immediately to a new form, or to one much more remote--in -the case of some high class peas, _e.g._, to the form which Mr Sutton -describes as “vetch-like,” with short pods, and a very few small round -seeds, two or three in a pod. Such plants are recognized by their -appearance and are rigorously hoed out every year before seeding. - -To appreciate the meaning of these facts we must go back to what was -said above on the nature of compound characters. We can perceive that, -as Mendel showed, the integral characters of the varieties can be -dissociated and re-combined in any combination. More than that; certain -integral characters can be resolved into further integral components, -by _analytical_ variations. What is taking place in this process of -resolution we cannot surmise, but we may liken the consequences of -that process to various phenomena of analysis seen elsewhere. To -continue the metaphor we may speak of return to the vetch-like type as -a _synthetical_ variation: well remembering that we know nothing of -any _substance_ being subtracted in the former case or added in the -latter, and that the phenomenon is more likely to be primarily one of -alteration in arrangement than in substance. - -A final proof that nothing is to be looked for from an appeal to -ancestry is provided by the fact--of which the literature of variation -contains numerous illustrations--that such newly synthesised forms, -instead of themselves producing a large proportion of the high class -variety which may have been their ancestor for a hundred generations, -may produce almost nothing but individuals like themselves. A subject -fraught with extraordinary interest will be the determination whether -by crossing these newly synthesised forms with their parent, or -another pure form, we may not succeed in reproducing a great part -of the known series of components afresh. The pure parental form, -produced, or extracted, by “analytical” breeding, would not in ordinary -circumstances be capable of producing the other components from which -it has been separated; but by crossing it with the “synthesised” -variety it is not impossible that these components would again -reappear. If this can be shown to be possible we shall have entirely -new light on the nature of variation and stability. - - -CONCLUSION. - -I trust what I have written has convinced the reader that we are, -as was said in opening, at last beginning to move. Professor Weldon -declares he has “no wish to belittle the importance of Mendel’s -achievement”; he desires “simply to call attention to a series of -facts which seem to him to suggest fruitful lines of inquiry.” In -this purpose I venture to assist him, for I am disposed to think that -unaided he is--to borrow Horace Walpole’s phrase--about as likely to -light a fire with a wet dish-clout as to kindle interest in Mendel’s -discoveries by his tempered appreciation. If I have helped a little in -this cause my time has not been wasted. - -In these pages I have only touched the edge of that new country which -is stretching out before us, whence in ten years’ time we shall -look back on the present days of our captivity. Soon every science -that deals with animals and plants will be teeming with discovery, -made possible by Mendel’s work. The breeder, whether of plants or -of animals, no longer trudging in the old paths of tradition, will -be second only to the chemist in resource and in foresight. Each -conception of life in which heredity bears a part--and which of them is -exempt?--must change before the coming rush of facts. - - - - -BIBLIOGRAPHY. - - - 1. CORRENS, C. G. Mendel’s Regel über das Verhalten der - Nachkommenschaft der Rassenbastarde, _Ber. deut. bot. Ges._, XVIII., - 1900, p. 158. - - 2. ---- Gregor Mendel’s “Versuche über Pflanzen-Hybriden” und die - Bestätigung ihrer Ergebnisse durch die neuesten Untersuchungen, _Bot. - Ztg._, 1900, p. 229. - - 3. ---- Ueber Levkoyenbastarde zur Kenntniss der Grenzen der - Mendel’schen Regeln, _Bot. Cblt._, 1900, Vol. LXXXIV., p. 97. - - 4. ---- Bastarde zwischen Maisrassen, mit besonderer Berücksichtigung - der Xenien, _Bibliotheca Botanica_, Hft. 53, 1901. - - 5. CRAMPE. Kreuzungen zwischen Wanderratten verschiedener Farbe, - _Landwirths. Jahrb._, VI., 1877, p. 384. - - 6. ---- Zucht-Versuche mit zahmen Wanderratten. 1. Resultate der - Zucht in Verwandtschaft, _ibid._, XII., 1883, p. 389. 2. Resultate - der Kreuzung der zahmen Ratten mit wilden, _ibid._, XIII., 1884, - p. 699. - - 7. ---- Die Gesetze der Vererbung der Farbe, _ibid._, XIV., p. 539. - - 8. DARWIN, C. _Variation of Animals and Plants under Domestication_, - ed. 2, I., pp. 348 and 428. - - 9. FISCHER, JOHANN VON. Die Säugethiere des S^t Petersburger - Gouvernements, _Zool. Garten_, X., 1869, p. 336. - - 10. ---- Iltis (_Mustela putorius_) und Frett (_Mustela furo_), - _ibid._, XIV., 1873, p. 108. - - 11. FISCHER, JOHANN VON. Beobachtungen über Kreuzungen verschiedener - Farbenspielarten innerhalb einer Species, _ibid._, XV., 1874, p. 361. - - 12. FOCKE, W. O. _Die Pflanzen-Mischlinge_, Bornträger, Berlin, 1881. - - 13. ---- Ueber dichotype Gewächse. _Oesterr. bot. Ztschr._, XVIII., - 1868, p. 139. - - 14. GALTON, F. _Natural Inheritance_, Macmillan and Co., London, 1889. - - 15. ---- The Average Contribution of each several Ancestor to the - total Heritage of the Offspring, _Proc. Roy. Soc._, LXI., 1897, - p. 401. - - 16. GÄRTNER, C. F. VON. _Versuche und Beobachtungen über die - Bastarderzeugung im Pflanzenreich_, Stuttgart, 1849. - - 17. GITAY, E. Ueber den directen Einfluss des Pollens auf Frucht- und - Samenbildung, _Pringsheim’s JB. d. wiss. Bot._, XXV., 1893, p. 489. - - 18. GODRON, D. A. Des Hybrides Végétaux, etc. _Ann. Sci. Nat. Bot._, - Ser. 4, XIX., 1863, p. 135, and a series of papers in _Mém. Acad. - Stanislas_, Nancy, 1864, 1865, and especially 1872. - - 19. GUAITA, G. VON. Versuche mit Kreuzungen von verschiedenen Rassen - der Hausmaus, _Ber. d. naturf. Ges. Freiburg_, X., 1898, p. 317. - - 20. ---- Zweite Mittheilung, etc., _ibid._, XI., 1900, p. 131. - - 21. KNIGHT, T. A. An account of some experiments on the Fecundation - of Vegetables, _Phil. Trans._, 1799, Pt. II., p. 195. - - 122. KÜSTER, E. Die Mendel’schen Regeln, ihre ursprüngliche Fassung - und ihre moderne Ergänzungen, _Biol. Cblt._, XXII., 1902, p. 129. - - 23. LAXTON, T. Observations on the variations effected by crossing in - the colour and character of the seed of Peas, _Internat. Hort. Exhib. - and Bot. Congr._, Report, 1866, p. 156. - - 24. ---- Notes on some Changes and Variations in the Offspring of - Cross-fertilized Peas, _Jour. Hort. Soc._, N.S. III., 1872, p. 10. - - 25. LAXTON, T. Improvement amongst Peas, _Jour. Hort. Soc._, 1890, - XII., 1, p. 29. - - 26. MENDEL, GREGOR JOHANN. Versuche über Pflanzen-Hybriden, _Verh. - naturf. Ver. in Brünn_, Band IV., 1865, _Abhandlungen_, p. 1; - reprinted in _Flora_, 1901, and in Ostwald’s _Klassiker d. exakten - Wiss._ English translation in _Jour. R. Hort. Soc._, 1901, XXVI. - - 27. ---- Ueber einige aus künstlicher Befruchtung gewonnenen - Hieracium-Bastarde, _ibid._, VIII., 1869, _Abhandlungen_, p. 26. - - 28. MILLARDET. Note sur l’hybridation sans croisement, ou fausse - hybridation, _Mém. Soc. Sci. Bordeaux_, Ser. 4, IV., 1894, p. 347. - - 29. C. NAUDIN. Nouvelles recherches sur l’Hybridité dans les - Végétaux, _Nouv. Arch. Mus._, I., 1865, p. 25. - - 30. ---- _Ann. sci. nat., Bot._, Ser. 4, XIX., p. 180. - - 31. PEARSON, KARL. On the Law of Ancestral Heredity, _Proc. Roy. - Soc._, LXII., 1898, p. 386. - - 32. ---- On the Law of Reversion, _ibid._, LXVI., 1900, p. 140. - - 33. ---- _The Grammar of Science_, second edition, London, A. and - Charles Black, 1900. - - 34. ---- Mathematical Contributions to the Theory of Evolution. VIII. - On the Inheritance of Characters not capable of exact Measurement, - _Phil. Trans. Roy. Soc._, 1900, Vol. 195, p. 79. - - 35. RIMPAU. Kreuzungsprodukte landw. Kulturpflanzen, _Landw. Jahrb._, - XX., 1891. - - 36. TSCHERMAK, E. Ueber künstliche Kreuzung bei _Pisum sativum_, - _Ztschrft. f. d. landwirths. Versuchswesen in Oesterr._, 1900, III., - p. 465. - - 37. ---- Weitere Beiträge über Verschiedenwerthigkeit der Merkmale - bei Kreuzung von Erbsen and Bohnen, _ibid._, 1901, IV., 641; - _abstract in Ber. deut. bot. Ges._, 1901, XIX., p. 35. - - 38. TSCHERMAK, E. Ueber Züchtung neuer Getreiderassen mittelst - künstlicher Kreuzung, _ibid._, 1901, IV., p. 1029. - - 39. VILMORIN-ANDRIEUX AND CO. _Les Plantes Potagères_, 1st ed. 1883; - 2nd ed. 1891. - - 40. VRIES, H. DE. Sur la loi de disjonction des hybrides, _Comptes - Rendus_, 26 March, 1900. - - 41. ---- Das Spaltungsgesetz der Bastarde, _Ber. deut. bot. Ges._, - 1900, XVIII., p. 83. - - 42. ---- Ueber erbungleiche Kreuzungen, _ibid._, p. 435. - - 43. ---- Sur les unités des caractères spécifiques et leur - application à l’étude des hybrides, _Rev. Gén. de Bot._, 1900, XII., - p. 257. See also by the same author, _Intracellulare Pangenesis_, - Jena, 1889, in which the conception of unit-characters is clearly set - forth. - - 44. ---- _Die Mutationstheorie_, Vol. I., Leipzig, 1901. - - 45. WELDON, W. F. R. Mendel’s Laws of Alternative Inheritance in - Peas, _Biometrika_, I., Pt. ii., 1902, p. 228. - - 46. WICHURA, MAX. Die Bastardbefruchtung im Pflanzenreich, erläutert - an den Bastarden der Weiden, Breslau, 1865. - - -_Received as this sheet goes to press:--_ - - CORRENS, C. Die Ergebnisse der neuesten Bastardforschungen - für die Vererbungslehre, _Ber. deut. bot. Ges._, XIX., - Generalversammlungs-Heft 1. - - ---- Ueber den Modus und den Zeitpunkt der Spaltung der Anlagen bei - den Bastarden vom Erbsen-Typus, _Bot. 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font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of Mendel's principles of heredity, by William Bateson</p> -<div style='display:block; margin:1em 0'> -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online -at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you -are not located in the United States, you will have to check the laws of the -country where you are located before using this eBook. -</div> - -<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Title: Mendel's principles of heredity</p> -<p style='display:block; margin-left:2em; text-indent:0; margin-top:0; margin-bottom:1em;'>A defence</p> -<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Author: William Bateson</p> -<p style='display:block; text-indent:0; margin:1em 0'>Release Date: November 15, 2022 [eBook #69362]</p> -<p style='display:block; text-indent:0; margin:1em 0'>Language: English</p> - <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em; text-align:left'>Produced by: Thiers Halliwell, ellinora, Bryan Ness and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/American Libraries.)</p> -<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK MENDEL'S PRINCIPLES OF HEREDITY ***</div> - -<div class="transnote"> -<p><b><a id="Transcribers_notes"></a>Transcriber’s notes</b>:</p> - -<p>The text of this e-book has mostly been preserved in its original -form. One spelling error was corrected (considertion → consideration) -and a few missing full stops inserted, but inconsistent hyphenation -was left unchanged. To help readers navigate the book more easily, -hyperlinks have been added to footnotes, the table of contents, and -internal cross-references. Footnotes have been numbered and moved to -the end of the book.</p> - -<p class="epubonly">The text contains tables and symbols that might not -display faithfully on small reading devices.</p> - -<p class="epubonly">The cover image of the book was created by the -transcriber and is placed in the public domain.</p> -</div> - -<p class="tac fs160 ls01em">MENDEL’S<br /> -PRINCIPLES OF HEREDITY</p> - - -<div class="publisher"> -<div><b>London</b>: <span class="fs110 ls01em ws05em lh17em">C. J. CLAY <span class="lowercase smcap">AND</span> SONS,</span></div> -<div class="ws03em lh15em">CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,<br /> -AVE MARIA LANE,</div> -<div class="fs60 mtb1em">AND</div> -<div class="ws03em lh15em">H. K. LEWIS, 136, GOWER STREET, W.C.</div> - -<div class="figcenter illowp70" id="colophon" style="max-width: 6.25em;"> - <img class="w100" src="images/colophon.jpg" alt="" /> -</div> - -<div class="fs80 lh15em ws03em mt1em"> -<b>Glasgow</b>: 50, WELLINGTON STREET.<br /> -<b>Leipzig</b>: F. A. BROCKHAUS.<br /> -<b>New York</b>: THE MACMILLAN COMPANY.<br /> -<b>Bombay and Calcutta</b>: MACMILLAN AND CO., <span class="smcap">Ltd.</span> -</div> - -<div class="fs80 mtb6em"> -[<i>All Rights reserved.</i>] -</div> - -</div> - - - -<div class="figcenter illowp52" id="frontispiece" style="max-width: 28.125em;"> - <img class="w100" src="images/frontispiece.jpg" alt="" /> - <div class="center fs95 lh17em ws03em mt15em"> -GREGOR MENDEL<br /> -Abbot of Brünn<br /> -Born 1822. Died 1884. -</div> - -<p class="tac fs70 ws03em"><i>From a photograph kindly supplied by the Very Rev. Dr Janeischek, -the present Abbot.</i></p> -</div> - - -<div class="titlepage"> -<h1> -<span class="t1">MENDEL’S<br /> - -PRINCIPLES OF HEREDITY</span><br /> - -<span class="t2">A DEFENCE</span></h1> - -<div class="tp1">BY</div> - -<div class="tp2">W. BATESON, M.A., F.R.S.</div> - -<div class="tp3"><i>WITH A TRANSLATION OF MENDEL’S ORIGINAL<br /> -PAPERS ON HYBRIDISATION.</i></div> - -<div class="tp4">CAMBRIDGE:<br /> -AT THE UNIVERSITY PRESS.<br /> -1902</div> -</div> - - - -<div class="tac lh13em ws03em mtb6em"> -<span class="fs90"><b>Cambridge</b>:</span><br /> -<span class="fs70">PRINTED BY J. AND C. F. CLAY,<br /> -AT THE UNIVERSITY PRESS.</span> -</div> - -<p><span class="pagenum" id="Page_v">v</span></p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="PREFACE">PREFACE.</h2> -</div> - - -<p>In the Study of Evolution progress had well-nigh -stopped. The more vigorous, perhaps also -the more prudent, had left this field of science -to labour in others where the harvest is less precarious -or the yield more immediate. Of those who -remained some still struggled to push towards truth -through the jungle of phenomena: most were content -supinely to rest on the great clearing Darwin made -long since.</p> - -<p>Such was our state when two years ago it was -suddenly discovered that an unknown man, Gregor -Johann Mendel, had, alone, and unheeded, broken off -from the rest—in the moment that Darwin was at -work—and cut a way through.</p> - -<p>This is no mere metaphor, it is simple fact. Each -of us who now looks at his own patch of work sees -Mendel’s clue running through it: whither that clue -will lead, we dare not yet surmise.</p> - -<p>It was a moment of rejoicing, and they who had -heard the news hastened to spread them and take the<span class="pagenum" id="Page_vi">vi</span> -instant way. In this work I am proud to have borne -my little part.</p> - -<p>But every gospel must be preached to all alike. -It will be heard by the Scribes, by the Pharisees, by -Demetrius the Silversmith, and the rest. Not lightly -do men let their occupation go; small, then, would -be our wonder, did we find the established prophet -unconvinced. Yet, is it from misgiving that Mendel -had the truth, or merely from indifference, that no -naturalist of repute, save Professor Weldon, has risen -against him?</p> - -<p>In the world of knowledge we are accustomed to -look for some strenuous effort to understand a new -truth even in those who are indisposed to believe. -It was therefore with a regret approaching to indignation -that I read Professor Weldon’s <span class="nowrap">criticism<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">1</a></span>. -Were such a piece from the hand of a junior it -might safely be neglected; but coming from Professor -Weldon there was the danger—almost the certainty—that -the small band of younger men who are thinking -of research in this field would take it they had learnt -the gist of Mendel, would imagine his teaching exposed -by Professor Weldon, and look elsewhere for -lines of work.</p> - -<p>In evolutionary studies we have no Areopagus. -With us it is not—as happily it is with Chemistry,<span class="pagenum" id="Page_vii">vii</span> -Physics, Physiology, Pathology, and other well-followed -sciences—that an open court is always -sitting, composed of men themselves workers, keenly -interested in every new thing, skilled and well versed -in the facts. Where this is the case, doctrine is soon -tried and the false trodden down. But in our sparse -and apathetic community error mostly grows unheeded, -choking truth. That fate must not befall -Mendel now.</p> - -<p>It seemed imperative that Mendel’s own work -should be immediately put into the hands of all who -will read it, and I therefore sought and obtained the -kind permission of the Royal Horticultural Society to -reprint and modify the translation they had already -caused to be made and published in their Journal. -To this I add a translation of Mendel’s minor paper -of later date. As introduction to the subject, the -same Society has authorized me to reprint with -alterations a lecture on heredity delivered before -them in 1900. For these privileges my warm thanks -are due. The introduction thus supplied, composed -originally for an audience not strictly scientific, is far -too slight for the present purpose. A few pages are -added, but I have no time to make it what it should -be, and I must wait for another chance of treating -the whole subject on a more extended scale. It will -perhaps serve to give the beginner the slight<span class="pagenum" id="Page_viii">viii</span> -assistance which will prepare him to get the most -from Mendel’s own memoir.</p> - - -<p class="mt15em">The next step was at once to defend Mendel from -Professor Weldon. That could only be done by -following this critic from statement to statement in -detail, pointing out exactly where he has gone wrong, -what he has misunderstood, what omitted, what introduced -in error. With such matters it is easy to -deal, and they would be as nothing could we find in his -treatment some word of allusion to the future; some -hint to the ignorant that this is a very big thing; -some suggestion of what it all <i>may</i> mean if it <i>be</i> -true.</p> - -<p>Both to expose each error and to supply effectively -what is wanting, within the limits of a brief article, -written with the running pen, is difficult. For simplicity -I have kept almost clear of reference to facts -not directly connected with the text, and have foregone -recital of the now long list of cases, both of plants -and animals, where the Mendelian principles have -already been perceived. These subjects are dealt -with in a joint Report to the Evolution Committee of -the Royal Society, made by Miss E. R. Saunders and -myself, now in the Press. To Miss Saunders who -has been associated with me in this work for several -years I wish to express my great indebtedness. Much<span class="pagenum" id="Page_ix">ix</span> -of the present article has indeed been written in -consultation with her. The reader who seeks fuller -statement of facts and conceptions is referred to the -writings of other naturalists who have studied the -phenomena at first hand (of which a bibliography is -appended) and to our own Report.</p> - -<p>I take this opportunity of acknowledging the -unique facilities generously granted me, as representative -of the Evolution Committee, by Messrs -Sutton and Sons of Reading, to watch some of the -many experiments they have in progress, to inspect -their admirable records, and to utilise these facts for -the advancement of the science of heredity. My -studies at Reading have been for the most part -confined to plants other than those immediately the -subject of this discussion, but some time ago I availed -myself of a kind permission to examine their stock of -peas, thus obtaining information which, with other -facts since supplied, has greatly assisted me in treating -this subject.</p> - -<p class="mt15em">I venture to express the conviction, that if the -facts now before us are carefully studied, it will become -evident that the experimental study of heredity, -pursued on the lines Mendel has made possible, is -second to no branch of science in the certainty and -magnitude of the results it offers. This study has<span class="pagenum" id="Page_x">x</span> -one advantage which no other line of scientific inquiry -possesses, in that the special training necessary for -such work is easily learnt in the practice of it, and -can be learnt in no other way. All that is needed is -the faithful resolve to scamp nothing.</p> - -<p>If a tenth part of the labour and cost now devoted -by leisured persons, in this country alone, to the -collection and maintenance of species of animals and -plants which have been collected a hundred times -before, were applied to statistical experiments in -heredity, the result in a few years would make a -revolution not only in the industrial art of the breeder -but in our views of heredity, species and variation. -We have at last a brilliant method, and a solid basis -from which to attack these problems, offering an -opportunity to the pioneer such as occurs but seldom -even in the history of modern science.</p> - -<p>We have been told of late, more than once, that -Biology must become an <i>exact</i> science. The same is -my own fervent hope. But exactness is not always -attainable by numerical precision: there have been -students of Nature, untrained in statistical nicety, -whose instinct for truth yet saved them from perverse -inference, from slovenly argument, and from misuse -of authorities, reiterated and grotesque.</p> - -<p>The study of variation and heredity, in our ignorance -of the causation of those phenomena, <i>must</i> be<span class="pagenum" id="Page_xi">xi</span> -built of statistical data, as Mendel knew long ago; -but, as he also perceived, the ground must be prepared -by specific experiment. The phenomena of -heredity and variation are specific, and give loose and -deceptive answers to any but specific questions. That -is where our <i>exact</i> science will begin. Otherwise we -may one day see those huge foundations of “biometry” -in ruins.</p> - -<p>But Professor Weldon, by coincidence a vehement -preacher of precision, in his haste to annul this first -positive achievement of the precise method, dispenses -for the moment even with those unpretending forms -of precision which conventional naturalists have usefully -practised. His essay is a strange symptom of -our present state. The facts of variation and heredity -are known to so few that anything passes for evidence; -and if only a statement, or especially a conclusion, be -negative, neither surprise nor suspicion are aroused. -An author dealing in this fashion with subjects commonly -studied, of which the literature is familiar and -frequently verified, would meet with scant respect. -The reader who has the patience to examine Professor -Weldon’s array of objections will find that almost all -are dispelled by no more elaborate process than a -reference to the original records.</p> - -<p>With sorrow I find such an article sent out to -the world by a Journal bearing, in any association,<span class="pagenum" id="Page_xii">xii</span> -the revered name of Francis Galton, or under the -high sponsorship of Karl Pearson. I yield to no one -in admiration of the genius of these men. Never -can we sufficiently regret that those great intellects -were not trained in the profession of the naturalist.</p> - -<p>Mr Galton suggested that the new scientific firm -should have a mathematician and a biologist as -partners, and—soundest advice—a logician retained -as <span class="nowrap">consultant<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">2</a></span>. Biologist surely must one partner be, -but it will never do to have him sleeping. In many -well-regulated occupations there are persons known -as “knockers-up,” whose thankless task it is to rouse -others from their slumber, and tell them work-time is -come round again. That part I am venturing to play -this morning, and if I have knocked a trifle loud, it is -because there is need.</p> - -<p class="fs95 ml2em"><i>March, 1902.</i></p> - -<hr class="chap x-ebookmaker-drop" /> - -<div> -<div class="chapter"> -<p><span class="pagenum" id="Page_xiii">xiii</span></p> -<h2 class="nobreak" id="CONTENTS">CONTENTS.</h2> -</div> - -<div class="center"> -<table id="toc"> -<tr><td class="tac ptb06" colspan="2">INTRODUCTION.</td></tr> - -<tr><td class="tal pb03" colspan="2"><span class="smcap">The Problems of Heredity and their Solution</span>, pp. <a href="#Page_1">1</a>–39.</td></tr> - -<tr><td class="taj pl2i35" colspan="2">Preliminary statement of Mendel’s principles, <a href="#Page_8">8</a>. Relation -of Mendel’s discovery to the law of Ancestral -Heredity, <a href="#Page_19">19</a>. <i>Heterozygote</i> and <i>Homozygote</i>, <a href="#Page_23">23</a>. New -conceptions necessitated by Mendel’s discovery, <a href="#Page_26">26</a>. Simple -alternative characters, or <i>allelomorphs</i>, <a href="#Page_27">27</a>. <i>Compound -allelomorphs</i> and their components, <a href="#Page_29">29</a>. Analytical Variations, <a href="#Page_29">29</a>. -Relation of Mendel’s principle to continuous -variation, <a href="#Page_32">32</a>. Dominance, <a href="#Page_32">32</a>. Non-Mendelian phenomena, <a href="#Page_33">33</a>. -False hybrids of Millardet, <a href="#Page_34">34</a>. Brief historical -notice, <a href="#Page_36">36</a>.</td></tr> - -<tr><td class="tal ptb06" colspan="2">MENDEL’S EXPERIMENTS IN PLANT HYBRIDISATION, pp. <a href="#Page_40">40</a>–95.</td></tr> - -<tr><td class="taj pl2i35" colspan="2">Introductory Remarks, <a href="#Page_40">40</a>. Selection of Experimental -Plants, <a href="#Page_42">42</a>. Division and Arrangement of Experiments, <a href="#Page_44">44</a>. -Characters selected, <a href="#Page_45">45</a>. Number of first crosses, <a href="#Page_47">47</a>. -Possible sources of error, <a href="#Page_47">47</a>. Forms of the Hybrids, <a href="#Page_49">49</a>. -Dominant and recessive, <a href="#Page_49">49</a>.</td></tr> - -<tr><td class="taj pl2i35" colspan="2">First generation bred from the Hybrids, <a href="#Page_51">51</a>. Numbers -of each form in offspring, <a href="#Page_52">52</a>. Second generation bred from -the Hybrids, <a href="#Page_55">55</a>. Subsequent generations bred from the -Hybrids, <a href="#Page_57">57</a>.</td></tr> - -<tr><td class="taj pl2i35" colspan="2">Offspring of Hybrids in which several differentiating -characters are associated, <a href="#Page_59">59</a>. The reproductive cells of -the Hybrids, <a href="#Page_66">66</a>. Statement of Mendel’s essential deductions, <a href="#Page_67">67</a>. -Experiments to determine constitution of germ-cells, <a href="#Page_68">68</a>. -Statement of purity of germ-cells, <a href="#Page_72">72</a>.</td></tr> - -<tr><td class="taj pl2i35" colspan="2">Experiments with <i>Phaseolus</i>, <a href="#Page_76">76</a>. Compound characters, <a href="#Page_80">80</a>. -Concluding Remarks, <a href="#Page_84">84</a>.</td></tr> - -<tr><td class="tal ptb06" colspan="2">MENDEL’S EXPERIMENTS WITH HIERACIUM, <a href="#Page_96">96</a>–103.</td></tr> - -<tr><td class="tac pt15" colspan="2">A DEFENCE OF MENDEL’S PRINCIPLES OF HEREDITY, <a href="#Page_104">104</a>–208.<span class="pagenum" id="Page_xiv">xiv</span></td></tr> - -<tr><td class="tal pb03" colspan="2"><i>Introductory</i>, <a href="#Page_104">104</a>.</td></tr> - -<tr><td class="tal vat pl05hi">I.</td><td class="tal pl05hi"><span class="smcap">The Mendelian Principle of Purity of Germ-cells -and the Laws of Heredity based on Ancestry</span>, <a href="#Page_108">108</a>.</td></tr> - -<tr><td class="tal vat">II.</td><td class="tal pl05hi"><span class="smcap">Mendel and the critic’s version of him.</span></td></tr> - -<tr><td></td><td class="tal ti15">The Law of Dominance, <a href="#Page_117">117</a>.</td></tr> - -<tr><td class="tal vat">III.</td><td class="tal pl05hi"><span class="smcap">The facts in regard to Dominance of Characters in -Peas</span>, <a href="#Page_119">119</a>.</td></tr> - -<tr><td></td><td class="taj ti15">The normal characters: colours of cotyledons and seed-coats, <a href="#Page_120">120</a>. -Shape, <a href="#Page_122">122</a>. Stability and variability, <a href="#Page_124">124</a>. -Results of crossing in regard to seed-characters: normal and -exceptional, <a href="#Page_129">129</a>. Analysis of exceptions, <a href="#Page_132">132</a>. The “mule” -or heterozygote, <a href="#Page_133">133</a>.</td></tr> - -<tr><td class="tal vat">IV.</td><td class="tal pl05hi"><span class="smcap">Professor Weldon’s collection of “Other evidence -concerning Dominance in Peas.”</span></td></tr> - -<tr><td></td><td class="taj ti15">A. In regard to cotyledon colour: Preliminary, <a href="#Page_137">137</a>. -Xenia, <a href="#Page_139">139</a>. (1) Gärtner’s cases, <a href="#Page_141">141</a>. (2) Seton’s case, <a href="#Page_143">143</a>. -(3) Tschermak’s exceptions, <a href="#Page_145">145</a>. (3<i>a</i>) <i>Buchsbaum</i> case, <a href="#Page_145">145</a>. -(3<i>b</i>) <i>Telephone</i> cases, <a href="#Page_146">146</a>. (3<i>c</i>) <i>Couturier</i> cases, <a href="#Page_147">147</a>.</td></tr> - -<tr><td></td><td class="taj ti15">B. Seed-coats and Shapes. 1. Seed-coats, <a href="#Page_148">148</a>. 2. Seed-shapes: -(<i>a</i>) Rimpau’s cases, <a href="#Page_150">150</a>. (<i>b</i>) Tschermak’s cases, <a href="#Page_152">152</a>. -3. Other phenomena, especially regarding seed-shapes, in -the case of “grey” peas. Modern evidence, <a href="#Page_153">153</a>.</td></tr> - -<tr><td></td><td class="tal ti15">C. Evidence of Knight and Laxton, <a href="#Page_158">158</a>.</td></tr> - -<tr><td></td><td class="tal ti15">D. Miscellaneous cases in other plants and animals:</td></tr> - -<tr><td></td><td class="tal pl3">1. Stocks (<i>Matthiola</i>). Hoariness, <a href="#Page_169">169</a>. Flower-colour, <a href="#Page_170">170</a>.</td></tr> - -<tr><td></td><td class="tal pl3">2. <i>Datura</i>, <a href="#Page_172">172</a>.</td></tr> - -<tr><td></td><td class="tal pl3">3. Colours of Rats and Mice, <a href="#Page_173">173</a>.</td></tr> - -<tr><td class="tal vat">V.</td><td class="tal pl05hi"><span class="smcap">Professor Weldon’s quotations from Laxton</span>, <a href="#Page_178">178</a>.</td></tr> - -<tr><td></td><td class="tal ti15">Illustration from <i>Primula sinensis</i>, <a href="#Page_182">182</a>.</td></tr> - -<tr><td class="tal vat">VI.</td><td class="tal pl05hi"><span class="smcap">The Argument built on exceptions</span>, <a href="#Page_183">183</a>.</td></tr> - -<tr><td></td><td class="tal ti15">Ancestry and Dominance, <a href="#Page_185">185</a>.</td></tr> - -<tr><td></td><td class="tal ti15">Ancestry and purity of germ-cells, <a href="#Page_193">193</a>.</td></tr> - -<tr><td></td><td class="tal ti15">The value of the appeal to Ancestry, <a href="#Page_197">197</a>.</td></tr> - -<tr><td class="tal vat">VII. </td><td class="tal pl05hi"><span class="smcap">The question of absolute purity of germ-cells</span>, <a href="#Page_201">201</a>.</td></tr> - -<tr><td></td><td class="tal"><span class="smcap">Conclusion</span>, <a href="#Page_208">208</a>.</td></tr> -</table> -</div> -</div> - -<p><span class="pagenum hide" id="Page_xv">xv</span></p> - - -<hr class="tb x-ebookmaker-drop" /> - - -<p class="tac fs110">ERRATA.</p> - - -<div class="ml2em"> -<p class="fs95"> -p. <a href="#Page_22">22</a>, par. 3, line 2, for “falls” read “fall.”<br /> -p. <a href="#Page_63">63</a>, line 12, for “<i>AabbC</i>” read “<i>AaBbc</i>.”<br /> -p. <a href="#Page_66">66</a>, in heading, for “<span class="lowercase smcap">OF HYBRIDS</span>” read “<span class="lowercase smcap">OF THE HYBRIDS</span>.”<br /> -</p> -</div> - -<p class="fs95 mt2em"><i>Note to</i> p. <a href="#Page_125">125</a>. None of the yellow seeds produced by <i>Laxton’s -Alpha</i> germinated, though almost all the green seeds sown gave -healthy plants. The same was found in the case of <i>Express</i>, another -variety which bore some yellow seeds. In the case of <i>Blue Peter</i>, on -the contrary, the yellow seeds have grown as well as the green ones. -Few however were <i>wholly</i> yellow. Of nine yellow seeds produced by -crossing green varieties together (p. <a href="#Page_131">131</a>), six did not germinate, -and three which did gave weak and very backward plants. Taken -together, this evidence makes it scarcely doubtful that the yellow colour -in these cases was pathological, and almost certainly due to exposure -after ripening.</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_1">1</span></p> - -<h2 class="nobreak" id="THE_PROBLEMS_OF_HEREDITY_AND">THE PROBLEMS OF HEREDITY AND -THEIR SOLUTION<span class="nowrap"><a id="FNanchor_3" href="#Footnote_3" class="fnanchor"><span class="fs70">3</span></a></span>.</h2> -</div> - - -<p>An exact determination of the laws of heredity will -probably work more change in man’s outlook on the -world, and in his power over nature, than any other -advance in natural knowledge that can be clearly foreseen.</p> - -<p>There is no doubt whatever that these laws can be -determined. In comparison with the labour that has been -needed for other great discoveries we may even expect -that the necessary effort will be small. It is rather -remarkable that while in other branches of physiology -such great progress has of late been made, our knowledge -of the phenomena of heredity has increased but little; -though that these phenomena constitute the basis of -all evolutionary science and the very central problem -of natural history is admitted by all. Nor is this due -to the special difficulty of such inquiries so much as to -general neglect of the subject.</p> - -<p><span class="pagenum" id="Page_2">2</span></p> - -<p>It is in the hope of inducing others to follow these -lines of investigation that I take the problems of heredity -as the subject of this lecture to the Royal Horticultural -Society.</p> - -<p>No one has better opportunities of pursuing such -work than horticulturists and stock breeders. They are -daily witnesses of the phenomena of heredity. Their -success also depends largely on a knowledge of its laws, -and obviously every increase in that knowledge is of -direct and special importance to them.</p> - -<p>The want of systematic study of heredity is due -chiefly to misapprehension. It is supposed that such -work requires a lifetime. But though for adequate study -of the complex phenomena of inheritance long periods -of time must be necessary, yet in our present state of -deep ignorance almost of the outline of the facts, observations -carefully planned and faithfully carried out for -even a few years may produce results of great value. In -fact, by far the most appreciable and definite additions -to our knowledge of these matters have been thus -obtained.</p> - -<p>There is besides some misapprehension as to the -kind of knowledge which is especially wanted at this -time, and as to the modes by which we may expect to -obtain it. The present paper is written in the hope that -it may in some degree help to clear the ground of these -difficulties by a preliminary consideration of the question, -How far have we got towards an exact knowledge of -heredity, and how can we get further?</p> - -<p>Now this is pre-eminently a subject in which we -must distinguish what we <i>can</i> do from what we want -to do. We <i>want</i> to know the whole truth of the matter; -we want to know the physical basis, the inward and<span class="pagenum" id="Page_3">3</span> -essential nature, “the causes,” as they are sometimes -called, of heredity: but we want also to know the laws -which the outward and visible phenomena obey.</p> - -<p>Let us recognise from the outset that as to the essential -nature of these phenomena we still know absolutely -nothing. We have no glimmering of an idea as to what -constitutes the essential process by which the likeness -of the parent is transmitted to the offspring. We can -study the processes of fertilisation and development in -the finest detail which the microscope manifests to us, -and we may fairly say that we have now a considerable -grasp of the visible phenomena; but of the nature of -the physical basis of heredity we have no conception -at all. No one has yet any suggestion, working hypothesis, -or mental picture that has thus far helped in -the slightest degree to penetrate beyond what we see. -The process is as utterly mysterious to us as a flash of -lightning is to a savage. We do not know what is the -essential agent in the transmission of parental characters, -not even whether it is a material agent or not. Not only -is our ignorance complete, but no one has the remotest -idea how to set to work on that part of the problem. -We are in the state in which the students of physical -science were, in the period when it was open to anyone -to believe that heat was a material substance or not, as -he chose.</p> - -<p>But apart from any conception of the essential modes -of transmission of characters, we <i>can</i> study the outward -facts of the transmission. Here, if our knowledge is -still very vague, we are at least beginning to see how -we ought to go to work. Formerly naturalists were -content with the collection of numbers of isolated instances -of transmission—more especially, striking and peculiar<span class="pagenum" id="Page_4">4</span> -cases—the sudden appearance of highly prepotent forms, -and the like. We are now passing out of that stage. -It is not that the interest of particular cases has in -any way diminished—for such records will always have -their value—but it has become likely that general expressions -will be found capable of sufficiently wide application -to be justly called “laws” of heredity. That this -is so was till recently due almost entirely to the work of -Mr F. Galton, to whom we are indebted for the first -systematic attempt to enuntiate such a law.</p> - -<p>All laws of heredity so far propounded are of a -statistical character and have been obtained by statistical -methods. If we consider for a moment what is actually -meant by a “law of heredity” we shall see at once why -these investigations must follow statistical methods. For -a “law” of heredity is simply an attempt to declare -the course of heredity under given conditions. But if -we attempt to predicate the course of heredity we have -to deal with conditions and groups of causes wholly -unknown to us, whose presence we cannot recognize, -and whose magnitude we cannot estimate in any particular -case. The course of heredity in particular cases -therefore cannot be foreseen.</p> - -<p>Of the many factors which determine the degree -to which a given character shall be present in a given -individual only one is usually known to us, namely, -the degree to which that character is present in the -parents. It is common knowledge that there is not that -close correspondence between parent and offspring which -would result were this factor the only one operating; -but that, on the contrary, the resemblance between the -two is only an uncertain one.</p> - -<p>In dealing with phenomena of this class the study<span class="pagenum" id="Page_5">5</span> -of single instances reveals no regularity. It is only by -collection of facts in great numbers, and by statistical -treatment of the mass, that any order or law can be -perceived. In the case of a chemical reaction, for instance, -by suitable means the conditions can be accurately reproduced, -so that in every individual case we can predict -with certainty that the same result will occur. But with -heredity it is somewhat as it is in the case of the rainfall. -No one can say how much rain will fall to-morrow in -a given place, but we can predict with moderate accuracy -how much will fall next year, and for a period of years -a prediction can be made which accords very closely with -the truth.</p> - -<p>Similar predictions can from statistical data be made as -to the duration of life and a great variety of events, the -conditioning causes of which are very imperfectly understood. -It is predictions of this kind that the study of -heredity is beginning to make possible, and in that sense -laws of heredity can be perceived.</p> - -<p>We are as far as ever from knowing <i>why</i> some characters -are transmitted, while others are not; nor can anyone yet -foretell which individual parent will transmit characters to -the offspring, and which will not; nevertheless the progress -made is distinct.</p> - -<p>As yet investigations of this kind have been made in -only a few instances, the most notable being those of -Galton on human stature, and on the transmission of -colours in Basset hounds. In each of these cases he has -shown that the expectation of inheritance is such that a -simple arithmetical rule is approximately followed. The -rule thus arrived at is that of the whole heritage of the -offspring the two parents together on an average contribute -one half, the four grandparents one-quarter, the eight<span class="pagenum" id="Page_6">6</span> -great-grandparents one-eighth, and so on, the remainder -being contributed by the remoter ancestors.</p> - -<p>Such a law is obviously of practical importance. In -any case to which it applies we ought thus to be able to -predict the degree with which the purity of a strain may -be increased by selection in each successive generation.</p> - -<p>To take a perhaps impossibly crude example, if a -seedling show any particular character which it is desired -to fix, on the assumption that successive self-fertilisations -are possible, according to Galton’s law the expectation of -purity should be in the first generation of self-fertilisation -1 in 2, in the second generation 3 in 4, in the third 7 in 8, -and so <span class="nowrap">on<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">4</a></span>.</p> - -<p>But already many cases are known to which the rule in -any simple form will not apply. Galton points out that -it takes no account of individual prepotencies. There are, -besides, numerous cases in which on crossing two varieties -the character of one variety almost always appears in each -member of the first cross-bred generation. Examples of -these will be familiar to those who have experience in such -matters. The offspring of the Polled Angus cow and the -Shorthorn bull is almost invariably polled or with very -small loose “scurs.” Seedlings raised by crossing <i>Atropa -belladonna</i> with the yellow-fruited variety have without -exception the blackish-purple fruits of the type. In several -hairy species when a cross with a glabrous variety is made, -the first cross-bred generation is altogether <span class="nowrap">hairy<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">5</a></span>.</p> - -<p>Still more numerous are examples in which the characters -of one variety very largely, though not exclusively, predominate -in the offspring.</p> - -<p><span class="pagenum" id="Page_7">7</span></p> - -<p>These large classes of exceptions—to go no further—indicate -that, as we might in any case expect, the principle -is not of universal application, and will need various -modifications if it is to be extended to more complex cases -of inheritance of varietal characters. No more useful work -can be imagined than a systematic determination of the -precise “law of heredity” in numbers of particular cases.</p> - -<p>Until lately the work which Galton accomplished stood -almost alone in this field, but quite recently remarkable -additions to our knowledge of these questions have been -made. In the year 1900 Professor de Vries published -a brief <span class="nowrap">account<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">6</a></span> of experiments which he has for several -years been carrying on, giving results of the highest value.</p> - -<p>The description is very short, and there are several -points as to which more precise information is necessary -both as to details of procedure and as to statement of -results. Nevertheless it is impossible to doubt that the -work as a whole constitutes a marked step forward, and -the full publication which is promised will be awaited with -great interest.</p> - -<p>The work relates to the course of heredity in cases -where definite varieties differing from each other in some -<i>one</i> definite character are crossed together. The cases are -all examples of discontinuous variation: that is to say, -cases in which actual intermediates between the parent -forms are not usually produced on <span class="nowrap">crossing<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">7</a></span>. It is shown -that the subsequent posterity obtained by self-fertilising -these cross-breds or hybrids, or by breeding them with each -other, break up into the original parent forms according to -fixed numerical rule.</p> -<p><span class="pagenum" id="Page_8">8</span></p> -<p>Professor de Vries begins by reference to a remarkable -memoir by Gregor <span class="nowrap">Mendel<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">8</a></span>, giving the results of his -experiments in crossing varieties of <i>Pisum sativum</i>. These -experiments of Mendel’s were carried out on a large scale, -his account of them is excellent and complete, and the -principles which he was able to deduce from them will -certainly play a conspicuous part in all future discussions -of evolutionary problems. It is not a little remarkable -that Mendel’s work should have escaped notice, and been -so long forgotten.</p> - -<p>For the purposes of his experiments Mendel selected -seven pairs of characters as follows:—</p> - -<p>1. Shape of ripe seed, whether round; or angular and -wrinkled.</p> - -<p>2. Colour of “endosperm” (cotyledons), whether some -shade of yellow; or a more or less intense green.</p> - -<p>3. Colour of the seed-skin, whether various shades of -grey and grey-brown; or white.</p> - -<p>4. Shape of seed-pod, whether simply inflated; or -deeply constricted between the seeds.</p> - -<p>5. Colour of unripe pod, whether a shade of green; or -bright yellow.</p> - -<p>6. Nature of inflorescence, whether the flowers are -arranged along the axis of the plant; or are terminal and -form a kind of umbel.</p> - -<p>7. Length of stem, whether about 6 or 7 ft. long, or -about <span class="nowrap"> <span class="fraction"><span class="fnum">3</span><span class="bar">/</span><span class="fden">4</span></span></span> to <span class="nowrap">1 <span class="fraction"><span class="fnum">1</span><span class="bar">/</span><span class="fden">2</span></span></span> ft.</p> - -<p>Large numbers of crosses were made between Peas differing -in respect of <i>one</i> of each of these pairs of characters.<span class="pagenum" id="Page_9">9</span> -It was found that in each case the offspring of the cross -exhibited the character of one of the parents in almost -undiminished intensity, and intermediates which could not -be at once referred to one or other of the parental forms -were not found.</p> - -<p>In the case of each pair of characters there is thus -one which in the first cross prevails to the exclusion of the -other. This prevailing character Mendel calls the <i>dominant</i> -character, the other being the <i>recessive</i> <span class="nowrap">character<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">9</a></span>.</p> - -<p>That the existence of such “dominant” and “recessive” -characters is a frequent phenomenon in cross-breeding, is -well known to all who have attended to these subjects.</p> - -<p>By letting the cross-breds fertilise themselves Mendel -next raised another generation. In this generation were -individuals which showed the dominant character, but also -individuals which presented the recessive character. Such -a fact also was known in a good many instances. But -Mendel discovered that in this generation the numerical -proportion of dominants to recessives is on an average of -cases approximately constant, being in fact <i>as three to one</i>. -With very considerable regularity these numbers were -approached in the case of each of his pairs of characters.</p> - -<p>There are thus in the first generation raised from the -cross-breds 75 per cent. dominants and 25 per cent. -recessives.</p> - -<p>These plants were again self-fertilised, and the offspring -of each plant separately sown. It next appeared that the -offspring of the recessives <i>remained pure recessive</i>, and -in subsequent generations never produced the dominant -again.</p> - -<p>But when the seeds obtained by self-fertilising the<span class="pagenum" id="Page_10">10</span> -dominants were examined and sown it was found that -the dominants were not all alike, but consisted of two -classes, (1) those which gave rise to pure dominants, and -(2) others which gave a mixed offspring, composed partly -of recessives, partly of dominants. Here also it was found -that the average numerical proportions were constant, those -with pure dominant offspring being to those with mixed -offspring as one to two. Hence it is seen that the 75 per -cent. dominants are not really of similar constitution, but -consist of twenty-five which are pure dominants and fifty -which are really cross-breds, though, like the cross-breds -raised by crossing the two original varieties, they only -exhibit the dominant character.</p> - -<p>To resume, then, it was found that by self-fertilising -the original cross-breds the same proportion was always -approached, namely—</p> - -<p class="ml2em"> -25 dominants, 50 cross-breds, 25 recessives, or 1<i>D</i> : 2<i>DR</i> : 1<i>R</i>. -</p> - -<p>Like the pure recessives, the pure dominants are -thenceforth pure, and only give rise to dominants in all -succeeding generations studied.</p> - -<p>On the contrary the fifty cross-breds, as stated above, -have mixed offspring. But these offspring, again, in their -numerical proportions, follow the same law, namely, that -there are three dominants to one recessive. The recessives -are pure like those of the last generation, but the dominants -can, by further self-fertilisation, and examination or cultivation -of the seeds produced, be again shown to be made -up of pure dominants and cross-breds in the same proportion -of one dominant to two cross-breds.</p> - -<p>The process of breaking up into the parent forms is -thus continued in each successive generation, the same<span class="pagenum" id="Page_11">11</span> -numerical law being followed so far as has yet been -observed.</p> - -<p>Mendel made further experiments with <i>Pisum sativum</i>, -crossing pairs of varieties which differed from each other -in <i>two</i> characters, and the results, though necessarily much -more complex, showed that the law exhibited in the simpler -case of pairs differing in respect of one character operated -here also.</p> - -<p>In the case of the union of varieties <i>AB</i> and <i>ab</i> -differing in two distinct pairs of characters, <i>A</i> and <i>a</i>, -<i>B</i> and <i>b</i>, of which <i>A</i> and <i>B</i> are dominant, <i>a</i> and <i>b</i> -recessive, Mendel found that in the first cross-bred generation -there was only <i>one</i> class of offspring, really <i>AaBb</i>.</p> - -<p>But by reason of the dominance of one character of -each pair these first crosses were hardly if at all distinguishable -from <i>AB</i>.</p> - -<p>By letting these <i>AaBb</i>’s fertilise themselves, only <i>four</i> -classes of offspring seemed to be produced, namely,</p> - -<div class="table ml2em"> -<div class="row fs110"><div class="cell tal"><i>AB</i></div><div class="cell tac"><div> showing </div></div><div class="cell tal"> both dominant characters.</div></div> -<div class="row fs110"><div class="cell tal"><i>Ab</i></div><div class="cell tac"><div>"</div></div><div class="cell tal">dominant <i>A</i> and recessive <i>b</i>.</div></div> -<div class="row fs110"><div class="cell tal"><i>aB</i></div><div class="cell tac"><div>"</div></div><div class="cell tal">recessive <i>a</i> and dominant <i>B</i>.</div></div> -<div class="row fs110"><div class="cell tal"><i>ab</i></div><div class="cell tac"><div>"</div></div><div class="cell tal"><span class="ilb">both recessive characters <i>a</i> and <i>b</i>.</span></div></div> -</div> - -<p>The numerical ratio in which these classes appeared -were also regular and approached the ratio</p> - -<p class="ml2em"> -9<i>AB</i> : 3<i>Ab</i> : 3<i>aB</i> : 1<i>ab</i>. -</p> - -<p>But on cultivating these plants and allowing them to -fertilise themselves it was found that the members of the</p> - -<table class="fs100 ml2em"> -<tr> -<td class="tal" colspan="4"><span class="smcap">Ratios</span></td> -</tr> -<tr class="pb06"> -<td class="tal pb06">1</td> -<td class="tar"></td> -<td class="tal"></td> -<td class="tal pl1 pb06"><i>ab</i> class produce only <i>ab</i>’s.</td> -</tr> -<tr> -<td class="tal pb06" rowspan="2">3</td> -<td class="tar vab pl1 pb06" rowspan="2"><img src="images/31x6bl.png" width="6" height="31" alt="" /></td> -<td class="tal">1</td> -<td class="tal pl1"><i>aB</i> class may produce either all <i>aB</i>’s,</td> -</tr> -<tr> -<td class="tal pb06">2</td> -<td class="tal pl2 pb06"><i>or</i> both <i>aB</i>’s and <i>ab</i>’s.<span class="pagenum" id="Page_12">12</span></td> -</tr> -<tr> -<td class="tal pb06" rowspan="2">3</td> -<td class="tar vab pl1 pb06" rowspan="2"><img src="images/31x6bl.png" width="6" height="31" alt="" /></td> -<td class="tal">1</td> -<td class="tal pl1"><i>Ab</i> class may produce either all <i>Ab</i>’s,</td> -</tr> -<tr> -<td class="tal pb06">2</td> -<td class="tal pl2 pb06"><i>or</i> both <i>Ab</i>’s and <i>ab</i>’s.</td> -</tr> -<tr> -<td class="tal" rowspan="5">9</td> -<td class="tar vab pl1" rowspan="5"><img src="images/93x6bl.png" width="6" height="93" alt="" /></td> -<td class="tal">1</td> -<td class="tal pl1"><i>AB</i> class may produce either all <i>AB</i>’s,</td> -</tr> -<tr> -<td class="tal">2</td> -<td class="tal pl2"><i>or</i> both <i>AB</i>’s and <i>Ab</i>’s,</td> -</tr> -<tr> -<td class="tal">2</td> -<td class="tal pl2"><i>or</i> both <i>AB</i>’s and <i>aB</i>’s,</td> -</tr> -<tr> -<td class="tal">4</td> -<td class="tal pl2"><i>or</i> all four possible classes again, namely,</td> -</tr> -<tr> -<td class="tal"></td> -<td class="tal pl2"><i>AB</i>’s, <i>Ab</i>’s, <i>aB</i>’s, and <i>ab</i>’s,</td> -</tr> -</table> - -<p>and the average number of members of each class will -approach the ratio 1 : 3 : 3 : 9 as indicated above.</p> - -<p>The details of these experiments and of others like -them made with <i>three</i> pairs of differentiating characters are -all set out in Mendel’s memoir.</p> - -<p>Professor de Vries has worked at the same problem in -some dozen species belonging to several genera, using pairs -of varieties characterised by a great number of characters: -for instance, colour of flowers, stems, or fruits, hairiness, -length of style, and so forth. He states that in all these -cases Mendel’s principles are followed.</p> - -<p>The numbers with which Mendel worked, though large, -were not large enough to give really smooth <span class="nowrap">results<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">10</a></span>; but -with a few rather marked exceptions the observations are -remarkably consistent, and the approximation to the numbers -demanded by the law is greatest in those cases where -the largest numbers were used. When we consider, besides, -that Tschermak and Correns announce definite confirmation -in the case of <i>Pisum</i>, and de Vries adds the evidence of his -long series of observations on other species and orders, -there can be no doubt that Mendel’s law is a substantial<span class="pagenum" id="Page_13">13</span> -reality; though whether some of the cases that depart -most widely from it can be brought within the terms of -the same principle or not, can only be decided by further -experiments.</p> - -<p>One may naturally ask, How can these results be -brought into harmony with the facts of hybridisation -hitherto known; and, if all this is true, how is it that -others who have carefully studied the phenomena of hybridisation -have not long ago perceived this law? The -answer to this question is given by Mendel at some length, -and it is, I think, satisfactory. He admits from the first -that there are undoubtedly cases of hybrids and cross-breds -which maintain themselves pure and do not break up. -Such examples are plainly outside the scope of his law. -Next he points out, what to anyone who has rightly -comprehended the nature of discontinuity in variation is -well known, that the variations in <i>each</i> character must be -<i>separately</i> regarded. In most experiments in crossing, -forms are taken which differ from each other in a multitude -of characters—some continuous, others discontinuous, -some capable of blending with their contraries, while others -are not. The observer on attempting to perceive any -regularity is confused by the complications thus introduced. -Mendel’s law, as he fairly says, could only appear -in such cases by the use of overwhelming numbers, which -are beyond the possibilities of practical experiment. Lastly, -no previous observer had applied a strict statistical method.</p> - -<p>Both these answers should be acceptable to those who -have studied the facts of variation and have appreciated -the nature of Species in the light of those facts. That -different species should follow different laws, and that the -same law should not apply to all characters alike, is exactly -what we have every right to expect. It will also be<span class="pagenum" id="Page_14">14</span> -remembered that the principle is only explicitly declared -to apply to discontinuous <span class="nowrap">characters<a id="FNanchor_11" href="#Footnote_11" class="fnanchor">11</a></span>. As stated also -it can only be true where reciprocal crossings lead to the -same result. Moreover, it can only be tested when there -is no sensible diminution in fertility on crossing.</p> - -<p>Upon the appearance of de Vries’ paper announcing the -“rediscovery” and confirmation of Mendel’s law and its -extension to a great number of cases two other observers -came forward almost simultaneously and independently -described series of experiments fully confirming Mendel’s -work. Of these papers the first is that of Correns, who -repeated Mendel’s original experiment with Peas having -seeds of different colours. The second is a long and very -valuable memoir of Tschermak, which gives an account of -elaborate researches into the results of crossing a number -of varieties of <i>Pisum sativum</i>. These experiments were in -many cases carried out on a large scale, and prove the -main fact enuntiated by Mendel beyond any possibility of -contradiction. The more exhaustive of these researches -are those of Tschermak on Peas and Correns on several -varieties of Maize. Both these elaborate investigations -have abundantly proved the general applicability of Mendel’s -law to the character of the plants studied, though both -indicate some few exceptions. The details of de Vries’ -experiments are promised in the second volume of his most -valuable <i>Mutationstheorie</i>. Correns in regard to Maize -and Tschermak in the case of <i>P. sativum</i> have obtained -further proof that Mendel’s law holds as well in the case of -varieties differing from each other in <i>two</i> pairs of characters, -one of each pair being dominant, though of course a more -complicated expression is needed in such <span class="nowrap">cases<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">12</a></span>.</p> -<p><span class="pagenum" id="Page_15">15</span></p> -<p>That we are in the presence of a new principle of the -highest importance is manifest. To what further conclusions -it may lead us cannot yet be foretold. But both -Mendel and the authors who have followed him lay stress -on one conclusion, which will at once suggest itself to -anyone who reflects on the facts. For it will be seen that -the results are such as we might expect if it be imagined -that the cross-bred plant produced pollen grains and egg-cells, -each of which bears only <i>one</i> of the alternative varietal -characters and not both. If this were so, and if on an -average the same number of pollen grains and egg-cells -transmit each of the two characters, it is clear that on a -random assortment of pollen grains and egg-cells Mendel’s -law would be obeyed. For 25 per cent. of “dominant” -pollen grains would unite with 25 per cent. “dominant” -egg-cells; 25 per cent. “recessive” pollen grains would -similarly unite with 25 per cent. “recessive” egg-cells; -while the remaining 50 per cent. of each kind would unite -together. It is this consideration which leads both Mendel -and those who have followed him to assert that these facts -of crossing prove that each egg-cell and each pollen grain -is pure in respect of each character to which the law -applies. It is highly desirable that varieties differing in -the form of their pollen should be made the subject of -these experiments, for it is quite possible that in such a -case strong confirmation of this deduction might be obtained. -[Preliminary trials made with reference to this -point have so far given negative results. Remembering -that a pollen grain is not a germ-cell, but only a bearer of<span class="pagenum" id="Page_16">16</span> -a germ-cell, the hope of seeing pollen grains differentiated -according to the characters they bear is probably remote. -Better hopes may perhaps be entertained in regard to -spermatozoa, or possibly female cells.]</p> - -<p>As an objection to the deduction of purity of germ-cells, -however, it is to be noted that though true intermediates -did not generally occur, yet the intensity in which the -characters appeared did vary in degree, and it is not easy -to see how the hypothesis of <i>perfect</i> purity in the reproductive -cells can be supported in such cases. Be this, -however, as it may, there is no doubt we are beginning to -get new lights of a most valuable kind on the nature of -heredity and the laws which it obeys. It is to be hoped -that these indications will be at once followed up by -independent workers. Enough has been said to show how -necessary it is that the subjects of experiment should be -chosen in such a way as to bring the laws of heredity to a -real test. For this purpose the first essential is that the -differentiating characters should be few, and that all avoidable -complications should be got rid of. Each experiment -should be reduced to its simplest possible limits. The -results obtained by Galton, and also the new ones especially -described in this paper, have each been reached by restricting -the range of observation to one character or group of characters, -and it is certain that by similar treatment our -knowledge of heredity may be rapidly extended.</p> - -<hr class="tb" /> - -<p>To the above popular presentation of the essential facts, -made for an audience not strictly scientific, some addition, -however brief, is called for. First, in regard to the law of -Ancestry, spoken of on p. <a href="#Page_5">5</a>. Those who are acquainted with -Pearson’s <i>Grammar of Science</i>, 2nd ed. published early in<span class="pagenum" id="Page_17">17</span> -1900, the same author’s paper in <i>Proc. R. S.</i> vol. 66, 1900, -p. 140, or the extensive memoir (pubd. Oct. 1900), on the -inheritance of coat-colour in horses and eye-colour in man -(<i>Phil. Trans.</i> 195, <span class="lowercase smcap">A</span>, 1900, p. 79), will not need to be told -that the few words I have given above constitute a most -imperfect diagram of the operations of that law as now developed. -Until the appearance of these treatises it was, -I believe, generally considered that the law of Ancestral -Heredity was to be taken as applying to phenomena like -these (coat-colour, eye-colour, &c.) where the inheritance -is generally <i>alternative</i>, as well as to the phenomena -of <i>blended</i> inheritance.</p> - -<p>Pearson, in the writings referred to, besides withdrawing -other large categories of phenomena from the scope of its -operations, points out that the law of Ancestral Heredity -does not satisfactorily express the cases of alternative -inheritance. He urges, and with reason, that these classes -of phenomena should be separately dealt with.</p> - -<p class="mt15em">The whole issue as regards the various possibilities of -heredity now recognized will be made clearer by a very brief -exposition of the several conceptions involved.</p> - -<p>If an organism producing germ-cells of a given constitution, -uniform in respect of the characters they bear, breeds -with another <span class="nowrap">organism<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">13</a></span> bearing <i>precisely similar</i> germ-cells, -the offspring resulting will, if the conditions are -identical, be uniform.</p> - -<p>In practice such a phenomenon is seen in <i>pure</i>-breeding. -It is true that we know no case in nature where all the -germ-cells are thus identical, and where no variation takes -place beyond what we can attribute to conditions, but we<span class="pagenum" id="Page_18">18</span> -know many cases where such a result is approached, and -very many where all the essential features which we regard -as constituting the characters of the breed are reproduced -with approximate certainty in every member of the pure-bred -race, which thus closely approach to uniformity.</p> - -<p>But if two germ-cells of dissimilar constitution unite -in fertilisation, what offspring are we to <span class="nowrap">expect<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">14</a></span>? First -let us premise that the answer to this question is known -experimentally to differ for many organisms and for many -classes of characters, and may almost certainly be in part -determined by external circumstances. But omitting the -last qualification, certain principles are now clearly detected, -though what principle will apply in any given case can only -be determined by direct experiment made with that case.</p> - -<p>This is the phenomenon of <i>cross</i>-breeding. As generally -used, this term means the union of members of dissimilar -varieties, or species: though when dissimilar <span class="nowrap">gametes<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">15</a></span> produced -by two individuals of the same variety unite in -fertilisation, we have essentially <i>cross</i>-breeding in respect -of the character or characters in which those gametes differ. -We will suppose, as before, that these two gametes bearing -properties unlike in respect of a given character, are borne -by different individuals.</p> - -<p>In the simplest case, suppose a gamete from an individual -presenting any character in intensity <i>A</i> unite in -fertilisation with another from an individual presenting -the same character in intensity <i>a</i>. For brevity’s sake we<span class="pagenum" id="Page_19">19</span> -may call the parent individuals <i>A</i> and <i>a</i>, and the resulting -zygote <i>Aa</i>. What will the structure of <i>Aa</i> be in regard to -the character we are considering?</p> - -<p>Up to Mendel no one proposed to answer this question -in any other way than by reference to the intensity of the -character in the progenitors, and <i>primarily</i> in the parents, -<i>A</i> and <i>a</i>, in whose bodies the gametes had been developed. -It was well known that such a reference gave a very poor -indication of what <i>Aa</i> would be. Both <i>A</i> and <i>a</i> may come -from a population consisting of individuals manifesting the -same character in various intensities. In the pedigree of -either <i>A</i> or <i>a</i> these various intensities may have occurred -few or many times. Common experience leads us to expect -the probability in regard to <i>Aa</i> to be influenced by this -history. The next step is that which Galton took. He -extended the reference beyond the immediate parents of -<i>Aa</i>, to its grandparents, great-grandparents, and so on, and -in the cases he studied he found that from a knowledge of -the intensity in which the given character was manifested -in each progenitor, even for some few generations back, a -fairly accurate prediction could be made, not as to the -character of any individual <i>Aa</i>, but as to the average -character of <i>Aa</i>’s of similar parentage, in general.</p> - -<p>But suppose that instead of individuals presenting one -character in differing intensities, two individuals breed -together distinguished by characters which we know to be -mutually exclusive, such as <i>A</i> and <i>B</i>. Here again we may -speak of the individuals producing the gametes as <i>A</i> and -<i>B</i>, and the resulting zygote as <i>AB</i>. What will <i>AB</i> be -like? The population here again may consist of many like -<i>A</i> and like <i>B</i>. These two forms may have been breeding -together indiscriminately, and there may have been many -or few of either type in the pedigree of either <i>A</i> or <i>B</i>.</p> - -<p><span class="pagenum" id="Page_20">20</span></p> - -<p>Here again Galton applied his method with remarkable -success. Referring to the progenitors of <i>A</i> and <i>B</i>, determining -how many of each type there were in the direct -pedigree of <i>A</i> and of <i>B</i>, he arrived at the same formula as -before, with the simple difference that instead of expressing -the probable average intensity of one character in several -individuals, the prediction is given in terms of the probable -number of <i>A</i>’s and <i>B</i>’s that would result on an average -when particular <i>A</i>’s and <i>B</i>’s of known pedigree breed -together.</p> - -<p>The law as Galton gives it is as follows:—</p> - -<p>“It is that the two parents contribute between them -on the average one-half, or (0·5) of the total heritage of -the offspring; the four grandparents, one-quarter, or (0·5)<sup>2</sup>; -the eight great-grandparents, one-eighth, or (0·5)<sup>3</sup>, and so -on. Then the sum of the ancestral contributions is expressed -by the series</p> - -<p class="tac"> -{(0·5) + (0·5)<sup>2</sup> + (0·5)<sup>3</sup>, &c.}, -</p> - -<p>which, being equal to 1, accounts for the whole heritage.”</p> - -<p>In the former case where <i>A</i> and <i>a</i> are characters which -can be denoted by reference to a common scale, the law -assumes of course that the inheritance will be, to use -Galton’s term, <i>blended</i>, namely that the zygote resulting -from the union of <i>A</i> with <i>a</i> will on the average be more -like <i>a</i> than if <i>A</i> had been united with <i>A</i>; and conversely -that an <i>Aa</i> zygote will on the average <i>be more like A than -an aa zygote would be</i>.</p> - -<p>But in the case of <i>A</i>’s and <i>B</i>’s, which are assumed to -be mutually exclusive characters, we cannot speak of -blending, but rather, to use Galton’s term, of <i>alternative</i> -inheritance.</p> - -<p>Pearson, finding that the law whether formulated thus,<span class="pagenum" id="Page_21">21</span> -or in the modified form in which he restated <span class="nowrap">it<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">16</a></span>, did not -express the phenomena of alternative inheritance known -to him with sufficient accuracy to justify its strict application -to them, and also on general grounds, proposed that -the phenomena of blended and alternative inheritance -should be treated apart—a <span class="nowrap">suggestion<a id="FNanchor_17" href="#Footnote_17" class="fnanchor">17</a></span> the wisdom of -which can scarcely be questioned.</p> - -<p>Now the law thus imperfectly set forth and every -modification of it is incomplete in one respect. It deals -only with the characters of the resulting zygotes and -predicates nothing in regard to the gametes which go to -form them. A good prediction may be made as to any -given group of zygotes, but the various possible constitutions -of the gametes are not explicitly treated.</p> - -<p>Nevertheless a definite assumption is implicitly made -regarding the gametes. It is not in question that differences -between these gametes may occur in respect of the heritage -they bear; yet it is assumed that these differences will be -distributed among the gametes of any individual zygote in -such a way that each gamete remains capable, on fertilisation, -of transmitting <i>all</i> the characters (both of the parent-zygote -and of its progenitors) to the zygote which it then -contributes to form (and to the posterity of that zygote) in -the intensity indicated by the law. Hence the gametes of -any individual are taken as collectively a fair sample of all -the racial characters in their appropriate intensities, and this -theory demands that there shall have been no qualitative -redistribution of characters among the gametes of any -zygote in such a way that some gametes shall be finally -excluded from partaking of and transmitting any specific<span class="pagenum" id="Page_22">22</span> -part of the heritage. The theory further demands—and -by the analogy of what we know otherwise not only of -animals and plants, but of physical or chemical laws, -perhaps this is the most serious assumption of all—that -the structure of the gametes shall admit of their being -capable of transmitting any character in any intensity -varying from zero to totality with equal ease; and that -gametes of each intensity are all equally likely to occur, -given a pedigree of appropriate arithmetical composition.</p> - -<p>Such an assumption appears so improbable that even -in cases where the facts seem as yet to point to this -conclusion with exceptional clearness, as in the case of -human stature, I cannot but feel there is still room for -reserve of judgment.</p> - -<p>However this may be, the Law of Ancestral Heredity, -and all modifications of it yet proposed, falls short in the -respect specified above, that <i>it does not directly attempt -to give any account of the distribution of the heritage among -the gametes</i> of any one individual.</p> - -<p>Mendel’s conception differs fundamentally from that -involved in the Law of Ancestral Heredity. The relation -of his hypothesis to the foregoing may be most easily -shown if we consider it first in application to the phenomena -resulting from the cross-breeding of two pure -varieties.</p> - -<p>Let us again consider the case of two varieties each displaying -the same character, but in the respective intensities -<i>A</i> and <i>a</i>. Each gamete of the <i>A</i> variety bears <i>A</i>, and -each gamete of the <i>a</i> variety bears <i>a</i>. When they unite in -fertilisation they form the zygote <i>Aa</i>. What will be its -characters? The Mendelian teaching would reply that -this can only be known by direct experiment with the two -forms <i>A</i> and <i>a</i>, and that the characters <i>A</i> and <i>a</i> perceived<span class="pagenum" id="Page_23">23</span> -in those two forms or varieties need not give any indication -as to the character of the zygote <i>Aa</i>. It may display the -character <i>A</i>, or <i>a</i>, or a character half way between the two, -or a character beyond <i>A</i> or below <i>a</i>. The character of <i>Aa</i> -is not regarded as a <i>heritage</i> transmitted to it by <i>A</i> and by -<i>a</i>, but as a character special and peculiar to <i>Aa</i>, just as -NaCl is not a body half way between sodium and chlorine, -or such that its properties can be predicted from or easily -stated in terms of theirs.</p> - -<p>If a concrete case may help, a tall pea <i>A</i> crossed with -a dwarf <i>a</i> often produces, not a plant having the height of -either <i>A</i> or <i>a</i>, but something <i>taller</i> than the pure tall -variety <i>A</i>.</p> - -<p>But if the case obeys the Mendelian principles—as does -that here quoted—then it can be declared <i>first</i> that the -gametes of <i>Aa</i> will not be bearers of the character proper to -<i>Aa</i>; but, generally speaking, each gamete will either bear -the pure <i>A</i> character or the pure <i>a</i> character. There will -in fact be a redistribution of the characters brought in by -the gametes which united to form the zygote <i>Aa</i>, such that -each gamete of <i>Aa</i> is pure, as the parental gametes were. -<i>Secondly</i> this redistribution will occur in such a way that, -of the gametes produced by such <i>Aa</i>’s, on an average -there will be equal numbers of <i>A</i> gametes and of <i>a</i> -gametes.</p> - -<p>Consequently if <i>Aa</i>’s breed together, the new <i>A</i> gametes -may meet each other in fertilisation, forming a zygote <i>AA</i>, -namely, the pure <i>A</i> variety again; similarly two <i>a</i> gametes -may meet and form <i>aa</i>, or the pure <i>a</i> variety again. But if -an <i>A</i> gamete meets an <i>a</i> it will once more form <i>Aa</i>, with -its special character. This <i>Aa</i> is the hybrid, or “mule” -form, or as I have elsewhere called it, the <i>heterozygote</i>, as -distinguished from <i>AA</i> or <i>aa</i> the <i>homozygotes</i>.</p> - -<p><span class="pagenum" id="Page_24">24</span></p> - -<p>Similarly if the two gametes of two varieties distinguished -by characters, <i>A</i> and <i>B</i>, which cannot be described -in terms of any common scale (such as for example the -“rose” and “single” combs of fowls) unite in fertilisation, -again the character of the mule form cannot be predicted. -Before the experiment is made the “mule” may present <i>any</i> -form. Its character or properties can as yet be no more -predicted than could those of the compounds of unknown -elements before the discovery of the periodic law.</p> - -<p>But again—if the case be Mendelian—the gametes borne -by <i>AB</i> will be either <i>A</i>’s or <i>B</i>’<span class="nowrap">s<a id="FNanchor_18" href="#Footnote_18" class="fnanchor">18</a></span>, and the cross-bred -<i>AB</i>’s breeding together will form <i>AA</i>’s, <i>AB</i>’s and <i> BB</i>’s. -Moreover, if as in the normal Mendelian case, <i>AB</i>’s bear on -an average equal numbers of <i>A</i> gametes and <i>B</i> gametes, the -numerical ratio of these resulting zygotes to each other will be</p> - -<p class="tac"> -1 <i>AA</i> : 2 <i>AB</i> : 1 <i>BB</i>. -</p> - -<p>We have seen that Mendel makes no prediction as to -the outward and visible characters of <i>AB</i>, but only as -to the essential constitution and statistical condition of its -gametes in regard to the characters <i>A</i> and <i>B</i>. Nevertheless -in a large number of cases the character of <i>AB</i> is known -to fall into one of three categories (omitting mosaics).</p> - -<p class="ml2em">(1) The cross-bred may almost always resemble one -of its pure parents so closely as to be practically -indistinguishable from that pure form, as in the -case of the yellow cotyledon-colour of certain varieties -of peas when crossed with green-cotyledoned varieties; -in which case the parental character, yellow, thus<span class="pagenum" id="Page_25">25</span> -manifested by the cross-bred is called “dominant” -and the parental character, green, not manifested, is -called recessive.</p> - -<p class="ml2em">(2) The cross-bred may present some condition -intermediate between the two parental forms, in -which case we may still retain the term “blend” -as applied to the zygote.</p> - -<p class="ml2em">Such an “intermediate” may be the apparent mean -between the two parental forms or be nearer to one -or other in any degree. Such a case is that of a -cross between a rich crimson Magenta Chinese Primrose -and a clear White, giving a flower of a colour -appropriately described as a “washy” magenta.</p> - -<p class="ml2em">(3) The cross-bred may present some form quite -different from that of either pure parent. Though, -as has been stated, nothing can be predicted of an unknown -case, we already know a considerable number -of examples of this nature in which the mule-form -<i>approaches sometimes with great accuracy to that of -a putative ancestor, near or remote</i>. It is scarcely -possible to doubt that several—though perhaps not -all—of Darwin’s “reversions on crossing” were of -this nature.</p> - -<p class="ml2em">Such a case is that of the “wild grey mouse” produced -by the union of an albino tame mouse and a piebald -Japanese <span class="nowrap">mouse<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">19</a></span>. These “reversionary” mice bred -together produce the parental tame types, some other -types, and “reversionary” mice again.</p> - -<p>From what has been said it will now be clear that the -applicability of the Mendelian hypothesis has, intrinsically,<span class="pagenum" id="Page_26">26</span> -nothing whatever to do with the question of the inheritance -being <i>blended</i> or <i>alternative</i>. In fact, as soon as the relation -of zygote characters to gamete characters is appreciated, it is -difficult to see any reason for supposing that the manifestation -of characters seen in the zygotes should give any -indication as to their mode of allotment among the gametes.</p> - -<p>On a previous occasion I pointed out that the terms -“Heredity” and “Inheritance” are founded on a misapplication -of metaphor, and in the light of our present -knowledge it is becoming clearer that the ideas of “transmission” -of a character by parent to offspring, or of there -being any “contribution” made by an ancestor to its posterity, -must only be admitted under the strictest reserve, -and merely as descriptive terms.</p> - -<p class="mt15em">We are now presented with some entirely new conceptions:—</p> - -<p class="ml2em">(1) The purity of the gametes in regard to certain -characters.</p> - -<p class="ml2em">(2) The distinction of all zygotes according as they are or -are not formed by the union of like or unlike gametes. -In the former case, apart from Variation, they breed -true when mated with their like; in the latter case -their offspring, collectively, will be heterogeneous.</p> - -<p class="ml2em">(3) If the zygote be formed by the union of dissimilar -gametes, we may meet the phenomenon of (<i>a</i>) dominant -and recessive characters; (<i>b</i>) a blend form; -(<i>c</i>) a form distinct from either parent, often -<span class="nowrap">reversionary<a id="FNanchor_20" href="#Footnote_20" class="fnanchor">20</a></span>.</p> - -<p><span class="pagenum" id="Page_27">27</span></p> - -<p>But there are additional and even more significant deductions -from the facts. We have seen that the gametes are -differentiated in respect of pure characters. Of these pure -characters there may <i>conceivably</i> be any number associated -together in one organism. In the pea Mendel detected at -least seven—not all seen by him combined in the same -plant, but there is every likelihood that they are all capable -of being thus combined.</p> - -<p>Each such character, which is capable of being dissociated -or replaced by its contrary, must henceforth be conceived -of as a distinct <i>unit-character</i>; and as we know that the -several unit-characters are of such a nature that any one -of them is capable of independently displacing or being displaced -by one or more alternative characters taken singly, -we may recognize this fact by naming such unit-characters -<i>allelomorphs</i>. So far, we know very little of any allelomorphs -existing otherwise than as <i>pairs</i> of contraries, but this is -probably merely due to experimental limitations and the -rudimentary state of our knowledge.</p> - -<p>In one case (combs of fowls) we know three characters, -<i>pea</i> comb, <i>rose</i> comb and <i>single</i> comb; of which <i>pea</i> and -<i>single</i>, or <i>rose</i> and <i>single</i>, behave towards each other as a -pair of allelomorphs, but of the behaviour of <i>pea</i> and <i>rose</i> -towards each other we know as yet nothing.</p> - -<p>We have no reason as yet for affirming that any -phenomenon properly described as <i>displacement</i> of one -allelomorph by another occurs, though the metaphor may -be a useful one. In all cases where <i>dominance</i> has been -perceived, we can affirm that the members of the allelomorphic -pair stand to each other in a relation the nature -<span class="pagenum" id="Page_28">28</span>of which we are as yet wholly unable to apprehend or -illustrate.</p> - -<p>To the new conceptions already enumerated we may -therefore add</p> - -<p class="ml2em">(4) <i>Unit-characters</i> of which some, <i>when once arisen by -Variation</i>, are alternative to each other in the constitution -of the gametes, according to a definite system.</p> - -<p>From the relations subsisting between these characters, -it follows that as each zygotic union of allelomorphs is <i>resolved</i> -on the formation of the gametes, no zygote can give -rise to gametes collectively representing more than <i>two</i> characters -allelomorphic to each other, apart from new variation.</p> - -<p>From the fact of the existence of the interchangeable -characters we must, for purposes of treatment, and to complete -the possibilities, necessarily form the conception of an -<i>irresoluble base</i>, though whether such a conception has any -objective reality we have no means as yet of determining.</p> - -<p>We have now seen that when the varieties <i>A</i> and <i>B</i> -are crossed together, the heterozygote, <i>AB</i>, produces -gametes bearing the pure <i>A</i> character and the pure <i>B</i> -character. In such a case we speak of such characters as -<i>simple</i> allelomorphs. In many cases however a more -complex phenomenon happens. The character brought in -on fertilisation by one or other parent may be of such a -nature that when the zygote, <i>AB</i>, forms its gametes, these -are not individually bearers merely of <i>A</i> and <i>B</i>, <i>but of a -number of characters themselves again integral</i>, which in, -say <i>A</i>, behaved as one character so long as its gametes -united in fertilisation with others like themselves, but on -cross-fertilisation are resolved and redistributed among the -gametes produced by the cross-bred zygote.</p> - -<p>In such a case we call the character <i>A</i> a <i>compound</i><span class="pagenum" id="Page_29">29</span> -allelomorph, and we can speak of the integral characters -which constitute it as <i>hypallelomorphs</i>. We ought to write -the heterozygote (<i>A A′ A″</i> . . .) <i>B</i> and the gametes produced -by it may be of the form <i>A</i>, <i>A′</i>, <i>A″</i>, <i>A‴</i>, . . . <i>B</i>. Or the -resolution may be incomplete in various degrees, as we -already suspect from certain instances; in which case we -may have gametes <i>A</i>, <i>A′ A″</i>, <i>A‴ A″″</i>, <i>A′ A″ A<sup>v</sup></i>, . . . <i>B</i>, and -so on. Each of these may meet a similar or a dissimilar -gamete in fertilisation, forming either a homozygote, or a -heterozygote with its distinct properties.</p> - -<p>In the case of compound allelomorphs we know as yet -nothing of the statistical relations of the several gametes.</p> - -<p>Thus we have the conception</p> - -<p class="ml2em">(5) <i>of a Compound character</i>, borne by one gamete, -transmitted entire as a single character so long as -fertilisation only occurs between like gametes, or is, -in other words, “symmetrical,” but if fertilisation -take place with a dissimilar gamete (or possibly by -other causes), resolved into integral constituent characters, -each separately transmissible.</p> - -<p>Next, as, by the union of the gametes bearing the -various hypallelomorphs with other such gametes, or with -gametes bearing simple allelomorphs, in fertilisation, a -number of new zygotes will be formed, such as may not have -been seen before in the breed: these will inevitably be -spoken of as <i>varieties</i>; and it is difficult not to extend the -idea of variation to them. To distinguish these from other -variations—which there must surely be—we may call them</p> - -<p class="ml2em">(6) <i>Analytical</i> variations in contradistinction to</p> - -<p class="ml2em">(7) <i>Synthetical</i> variations, occurring not by the -separation of pre-existing constituent-characters but -by the addition of new characters.</p> - -<p><span class="pagenum" id="Page_30">30</span></p> - -<p>Lastly, it is impossible to be presented with the fact -that in Mendelian cases the cross-bred produces on an -average <i>equal</i> numbers of gametes of each kind, that is to -say, a symmetrical result, without suspecting that this fact -must correspond with some symmetrical figure of distribution -of those gametes in the cell-divisions by which they are -produced.</p> - -<p class="mt15em">At the present time these are the main conceptions—though -by no means all—arising directly from Mendel’s -work. The first six are all more or less clearly embodied -by him, though not in every case developed in accordance -with modern knowledge. The seventh is not a Mendelian -conception, but the facts before us justify its inclusion in -the above list though for the present it is little more than -a mere surmise.</p> - -<p class="mt15em">In Mendelian cases it will now be perceived that all -the zygotes composing the population consist of a limited -number of possible types, each of definite constitution, -bearing gametes also of a limited and definite number of -types, and definite constitution in respect of pre-existing -characters. It is now evident that in such cases each -several progenitor need not be brought to account in -reckoning the probable characters of each descendant; -for the gametes of cross-breds are differentiated at each -successive generation, some parental (Mendelian) characters -being left out in the composition of each gamete produced -by a zygote arising by the union of bearers of opposite -allelomorphs.</p> - -<p>When from these considerations we return to the -phenomena comprised in the Law of Ancestral Heredity, -what certainty have we that the same conceptions are not -applicable there also?</p> - -<p><span class="pagenum" id="Page_31">31</span></p> - -<p>It has now been shown that the question whether in the -cross-bred zygotes in general the characters blend or are -mutually exclusive is an entirely subordinate one, and -distinctions with regard to the essential nature of heredity -based on these circumstances become irrelevant.</p> - -<p>In the case of a population presenting continuous -variation in regard to say, stature, it is easy to see how -purity of the gametes in respect of any intensities of -that character might not in ordinary circumstances be -capable of detection. There are doubtless more than -two pure gametic forms of this character, but there may -quite conceivably be six or eight. When it is remembered -that each heterozygous combination of any two -may have its own appropriate stature, and that such a -character is distinctly dependent on external conditions, -the mere fact that the observed curves of stature give -“chance distributions” is not surprising and may still be -compatible with purity of gametes in respect of certain -pure types. In peas (<i>P. sativum</i>), for example, from -Mendel’s work we know that the tall forms and the extreme -dwarf forms exhibit gametic purity. I have seen -at Messrs Sutton’s strong evidence of the same nature -in the case of the tall Sweet Pea (<i>Lathyrus odoratus</i>) -and the dwarf or procumbent “Cupid” form.</p> - -<p>But in the case of the Sweet Pea we know at least one -pure form of definitely intermediate height, and in the -case of <i>P. sativum</i> there are many. When the <i>extreme</i> -types breed together it will be remembered the heterozygote -commonly exceeds the taller in height. In the next -generation, since there is, in the case of extremes, so much -margin between the types of the two pure forms, the return -of the offspring to the three forms of which two are homozygous -and one heterozygous is clearly perceptible.</p> - -<p><span class="pagenum" id="Page_32">32</span></p> - -<p>If however instead of pure extreme varieties we were to -take a pair of varieties differing normally by only a foot or -two, we might, owing to the masking effects of conditions, -&c., have great difficulty in distinguishing the three forms -in the second generation. There would besides be twice as -many heterozygous individuals as homozygous individuals -of each kind, giving a symmetrical distribution of heights, -and who might not—in pre-Mendelian days—have accepted -such evidence—made still less clear by influence of conditions—as -proof of Continuous Variation both of zygotes -and gametes?</p> - -<p>Suppose, then, that instead of two pure types, we had -six or eight breeding together, each pair forming their own -heterozygote, there would be a very remote chance of such -purity or fixity of type whether of gamete or zygote being -detected.</p> - -<p><i>Dominance</i>, as we have seen, is merely a phenomenon -incidental to specific cases, between which no other common -property has yet been perceived. In the phenomena of -<i>blended</i> inheritance we clearly have no dominance. In the -cases of <i>alternative</i> inheritance studied by Galton and -Pearson there is evidently no <i>universal</i> dominance. From -the tables of Basset hound pedigrees there is clearly no -definite dominance of either of the coat-colours. In the case -of eye-colour the published tables do not, so far as I have -discovered, furnish the material for a decision, though it is -scarcely possible the phenomenon, even if only occasional, -could have been overlooked. We must take it, then, there -is no sensible dominance in these cases; but whether there -is or is not sensible gametic purity is an altogether different -question, which, so far as I can judge, is as yet untouched. -It may perfectly well be that we shall be compelled to -recognize that in many cases there is no such purity, and<span class="pagenum" id="Page_33">33</span> -that the characters may be carried by the gametes in any -proportion from zero to totality, just as some substances -may be carried in a solution in any proportion from zero -to saturation without discontinuous change of properties. -That this will be found true in <i>some</i> cases is, on any -hypothesis, certain; but to prove the fact for any given -case will be an exceedingly difficult operation, and I scarcely -think it has been yet carried through in such a way as to -leave no room for doubt.</p> - -<p>Conversely, the <i>absolute</i> and <i>universal</i> purity of the -gametes has certainly not yet been determined for any -case; not even in those cases where it looks most likely -that such universal purity exists. Impairment of such -purity we may conceive either to occur in the form of -mosaic gametes, or of gametes with blended properties. -On analogy and from direct evidence we have every right -to believe that gametes of both these classes may occur in -rare and exceptional cases, of as yet unexplored <span class="nowrap">nature<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">21</a></span>, -but such a phenomenon will not diminish the significance -of observed purity.</p> - -<p class="mt15em">We have now seen the essential nature of the Mendelian -principles and are able to appreciate the exact relation in -which they stand to the group of cases included in the Law -of Ancestral Heredity. In seeking any general indication -as to the common properties of the phenomena which are -already known to obey Mendelian principles we can as yet -point to none, and whether some such common features -exist or not is unknown.</p> - -<p class="mt15em">There is however one group of cases, definite though -as yet not numerous, where we know that the Mendelian<span class="pagenum" id="Page_34">34</span> -principles do not apply. These are the phenomena upon -which Mendel touches in his brief paper on <i>Hieracium</i>. -As he there states, the hybrids, if they are fertile at all, -produce offspring like themselves, not like their parents. -In further illustration of this phenomenon he cites Wichura’s -<i>Salix</i> hybrids. Perhaps some dozen other such illustrations -could be given which rest on good evidence. To these -cases the Mendelian principle will in nowise apply, nor is it -easy to conceive any modification of the law of ancestral -heredity which can express them. There the matter at -present rests. Among these cases, however, we perceive -several more or less common features. They are often, -though not always, hybrids between forms differing in -many characters. The first cross frequently is not the -exact intermediate between the two parental types, but -may as in the few <i>Hieracium</i> cases be irregular in this -respect. There is often some degree of sterility. In the -absence of fuller and statistical knowledge of such cases -further discussion is impossible.</p> - -<p class="mt15em">Another class of cases, untouched by any hypothesis of -heredity yet propounded, is that of the false hybrids of -Millardet, where we have fertilisation without transmission -of one or several parental characters. In these not only -does the first cross show, in some respect, the character or -characters of <i>one parent only</i>, but in its posterity <i>no reappearance -of the lost character or characters is observed</i>. -The nature of such cases is still quite obscure, but we have -to suppose that the allelomorph of one gamete only developes -after fertilisation to the exclusion of the corresponding allelomorph -of the other gamete, much—if the crudity of the -comparison may be pardoned—as occurs on the female side -in parthenogenesis without fertilisation at all.</p> - -<p><span class="pagenum" id="Page_35">35</span></p> - -<p>To these as yet altogether unconformable cases we can -scarcely doubt that further experiment will add many more. -Indeed we already have tolerably clear evidence that many -phenomena of inheritance are of a much higher order of -complexity. When the paper on <i>Pisum</i> was written -Mendel apparently inclined to the view that with modifications -his law might be found to include all the phenomena -of hybridisation, but in the brief subsequent paper on -<i>Hieracium</i> he clearly recognized the existence of cases of -a different nature. Those who read that contribution will -be interested to see that he lays down a principle which -may be extended from hybridisation to heredity in general, -that the laws of each new case must be determined by -separate experiment.</p> - -<p class="mt15em">As regards the Mendelian principles, which it is the -chief aim of this introduction to present clearly before the -reader, a professed student of variation will easily be able -to fill in the outline now indicated, and to illustrate the -various conceptions from phenomena already familiar. To -do this is beyond the scope of this short sketch. But -enough perhaps has now been said to show that by the -application of those principles we are enabled to reach and -deal in a comprehensive manner with phenomena of a -fundamental nature, lying at the very root of all conceptions -not merely of the physiology of reproduction -and heredity, but even of the essential nature of living -organisms; and I think that I used no extravagant words -when, in introducing Mendel’s work to the notice of readers -of the Royal Horticultural Society’s Journal, I ventured to -declare that his experiments are worthy to rank with those -which laid the foundation of the Atomic laws of Chemistry.</p> - -<p><span class="pagenum" id="Page_36">36</span></p> - -<hr class="tb" /> - -<p>As some biographical particulars of this remarkable -investigator will be welcome, I give the following brief -notice, first published by Dr Correns on the authority -of Dr von Schanz: Gregor Johann Mendel was born on -July 22, 1822, at Heinzendorf bei Odrau, in Austrian -Silesia. He was the son of well-to-do peasants. In 1843 -he entered as a novice the “Königinkloster,” an Augustinian -foundation in Altbrünn. In 1847 he was ordained priest. -From 1851 to 1853 he studied physics and natural science -at Vienna. Thence he returned to his cloister and became -a teacher in the Realschule at Brünn. Subsequently he -was made Abbot, and died January 6, 1884. The experiments -described in his papers were carried out in the -garden of his Cloister. Besides the two papers on hybridisation, -dealing respectively with <i>Pisum</i> and <i>Hieracium</i>, -Mendel contributed two brief notes to the <i>Verh. Zool. bot. -Verein</i>, Wien, on <i>Scopolia margaritalis</i> (1853, <span class="lowercase smcap">III.</span>, p. 116) -and on <i>Bruchus pisi</i> (<i>ibid.</i> 1854, <span class="lowercase smcap">IV.</span>, p. 27). In these -papers he speaks of himself as a pupil of Kollar.</p> - -<p>Mendel published in the Brünn journal statistical -observations of a meteorological character, but, so far -as I am aware, no others relating to natural history. -Dr Correns tells me that in the latter part of his life -he engaged in the Ultramontane Controversy. He was -for a time President of the Brünn <span class="nowrap">Society<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">22</a></span>.</p> - -<p>For the photograph of Mendel which forms the frontispiece -to this work, I am indebted to the Very Rev. Dr -Janeischek, the present Abbot of Brünn, who most kindly -supplied it for this purpose.</p> - -<p>So far as I have discovered there was, up to 1900, only -one reference to Mendel’s observations in scientific literature, -namely that of Focke, <i>Pflanzenmischlinge</i>, 1881, p. 109,<span class="pagenum" id="Page_37">37</span> -where it is simply stated that Mendel’s numerous experiments -on <i>Pisum</i> gave results similar to those obtained -by Knight, but that he believed he had found constant -numerical ratios among the types produced by hybridisation. -In the same work a similar brief reference is made to the -paper on <i>Hieracium</i>.</p> - -<p>It may seem surprising that a work of such importance -should so long have failed to find recognition and to become -current in the world of science. It is true that the journal -in which it appeared is scarce, but this circumstance has -seldom long delayed general recognition. The cause is -unquestionably to be found in that neglect of the experimental -study of the problem of Species which supervened -on the general acceptance of the Darwinian doctrines. The -problem of Species, as Kölreuter, Gärtner, Naudin, Wichura, -and the other hybridists of the middle of the nineteenth -century conceived it, attracted thenceforth no workers. The -question, it was imagined, had been answered and the -debate ended. No one felt much interest in the matter. -A host of other lines of work were suddenly opened up, and -in 1865 the more original investigators naturally found -those new methods of research more attractive than the -tedious observations of the hybridisers, whose inquiries -were supposed, moreover, to have led to no definite result.</p> - -<p>Nevertheless the total neglect of such a discovery is -not easy to account for. Those who are acquainted with -the literature of this branch of inquiry will know that the -French Academy offered a prize in 1861 to be awarded in -1862 on the subject “<i>Étudier les Hybrides végétaux au -point de vue de leur fécondité et de la perpétuité de leurs -caractères</i>.” This subject was doubtless chosen with -reference to the experiments of Godron of Nancy and -Naudin, then of Paris. Both these naturalists competed,<span class="pagenum" id="Page_38">38</span> -and the accounts of the work of Godron on <i>Datura</i> and -of Naudin on a number of species were published in the -years 1864 and 1865 respectively. Both, especially the -latter, are works of high consequence in the history of the -science of heredity. In the latter paper Naudin clearly -enuntiated what we shall henceforth know as the Mendelian -conception of the dissociation of characters of cross-breds -in the formation of the germ-cells, though apparently he -never developed this conception.</p> - -<p>In the year 1864, George Bentham, then President of -the Linnean Society, took these treatises as the subject of -his address to the Anniversary meeting on the 24 May, -Naudin’s work being known to him from an abstract, the -full paper having not yet appeared. Referring to the -hypothesis of dissociation which he fully described, he said -that it appeared to be new and well supported, but required -much more confirmation before it could be held as proven. -(<i>J. Linn. Soc., Bot.</i>, <span class="lowercase smcap">VIII.</span>, <i>Proc.</i>, p. XIV.)</p> - -<p>In 1865, the year of Mendel’s communication to the -Brünn Society, appeared Wichura’s famous treatise on his -experiments with <i>Salix</i> to which Mendel refers. There are -passages in this memoir which come very near Mendel’s -principles, but it is evident from the plan of his experiments -that Mendel had conceived the whole of his ideas before -that date.</p> - -<p>In 1868 appeared the first edition of Darwin’s <i>Animals -and Plants</i>, marking the very zenith of these studies, and -thenceforth the decline in the experimental investigation -of Evolution and the problem of Species has been steady. -With the rediscovery and confirmation of Mendel’s work -by de Vries, Correns and Tschermak in 1900 a new era -begins.</p> - -<p>That Mendel’s work, appearing as it did, at a moment<span class="pagenum" id="Page_39">39</span> -when several naturalists of the first rank were still occupied -with these problems, should have passed wholly unnoticed, -will always remain inexplicable, the more so as the Brünn -Society exchanged its publications with most of the -Academies of Europe, including both the Royal and -Linnean Societies.</p> - -<p>Naudin’s views were well known to Darwin and are -discussed in <i>Animals and Plants</i> (ed. 1885, <span class="lowercase smcap">II.</span>, p. 23); but, -put forward as they were without full proof, they could not -command universal credence. Gärtner, too, had adopted -opposite views; and Wichura, working with cases of -another order, had proved the fact that some hybrids breed -true. Consequently it is not to be wondered at that -Darwin was sceptical. Moreover, the Mendelian idea of -the “hybrid-character,” or heterozygous form, was unknown -to him, a conception without which the hypothesis of dissociation -of characters is quite imperfect.</p> - -<p>Had Mendel’s work come into the hands of Darwin, it -is not too much to say that the history of the development -of evolutionary philosophy would have been very different -from that which we have witnessed.</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_40">40</span></p> - -<h2 class="nobreak" id="EXPERIMENTS_IN_PLANT-HYBRIDISATION23">EXPERIMENTS IN PLANT-<span class="nowrap">HYBRIDISATION<a id="FNanchor_23" href="#Footnote_23" class="fnanchor"><span class="fs70">23</span></a></span>.</h2> -</div> - -<p class="tac"><span class="smcap">By Gregor Mendel.</span></p> - -<p class="tac fs90">(<i>Read at the Meetings of the 8th February -and 8th March, 1865.</i>)</p> - - -<p class="tac"><span class="smcap">Introductory Remarks.</span></p> - -<p>Experience of artificial fertilisation, such as is effected -with ornamental plants in order to obtain new variations -in colour, has led to the experiments which will here be -discussed. The striking regularity with which the same -hybrid forms always reappeared whenever fertilisation took -place between the same species induced further experiments -to be undertaken, the object of which was to follow up the -developments of the hybrids in their progeny.</p> - -<p>To this object numerous careful observers, such as -Kölreuter, Gärtner, Herbert, Lecoq, Wichura and others, -have devoted a part of their lives with inexhaustible -perseverance. Gärtner especially, in his work “Die Bastarderzeugung -im Pflanzenreiche” (The Production of -Hybrids in the Vegetable Kingdom), has recorded very -valuable observations; and quite recently Wichura published -the results of some profound investigations into the hybrids<span class="pagenum" id="Page_41">41</span> -of the Willow. That, so far, no generally applicable law -governing the formation and development of hybrids has -been successfully formulated can hardly be wondered at by -anyone who is acquainted with the extent of the task, and -can appreciate the difficulties with which experiments of -this class have to contend. A final decision can only be -arrived at when we shall have before us the results of -detailed experiments made on plants belonging to the most -diverse orders.</p> - -<p>Those who survey the work done in this department -will arrive at the conviction that among all the numerous -experiments made, not one has been carried out to such an -extent and in such a way as to make it possible to determine -the number of different forms under which the offspring of -hybrids appear, or to arrange these forms with certainty -according to their separate generations, or to definitely -ascertain their statistical <span class="nowrap">relations<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">24</a></span>.</p> - -<p>It requires indeed some courage to undertake a labour -of such far-reaching extent; it appears, however, to be the -only right way by which we can finally reach the solution -of a question the importance of which cannot be over-estimated -in connection with the history of the evolution -of organic forms.</p> - -<p>The paper now presented records the results of such -a detailed experiment. This experiment was practically -confined to a small plant group, and is now, after eight -years’ pursuit, concluded in all essentials. Whether the -plan upon which the separate experiments were conducted -and carried out was the best suited to attain the desired -end is left to the friendly decision of the reader.</p> - -<p><span class="pagenum" id="Page_42">42</span></p> - - -<h3><span class="smcap">Selection of the Experimental Plants.</span></h3> - -<p>The value and utility of any experiment are determined -by the fitness of the material to the purpose for which it is -used, and thus in the case before us it cannot be immaterial -what plants are subjected to experiment and in what manner -such experiments are conducted.</p> - -<p>The selection of the plant group which shall serve for -experiments of this kind must be made with all possible -care if it be desired to avoid from the outset every risk of -questionable results.</p> - -<p>The experimental plants must necessarily—</p> - -<p>1. Possess constant differentiating characters.</p> - -<p>2. The hybrids of such plants must, during the -flowering period, be protected from the influence of all -foreign pollen, or be easily capable of such protection.</p> - -<p>The hybrids and their offspring should suffer no marked -disturbance in their fertility in the successive generations.</p> - -<p>Accidental impregnation by foreign pollen, if it occurred -during the experiments and were not recognized, -would lead to entirely erroneous conclusions. Reduced -fertility or entire sterility of certain forms, such as occurs in -the offspring of many hybrids, would render the experiments -very difficult or entirely frustrate them. In order to discover -the relations in which the hybrid forms stand towards -each other and also towards their progenitors it appears to -be necessary that all members of the series developed in -each successive generation should be, <i>without exception</i>, -subjected to observation.</p> - -<p>At the very outset special attention was devoted to the -<i>Leguminosæ</i> on account of their peculiar floral structure.<span class="pagenum" id="Page_43">43</span> -Experiments which were made with several members of this -family led to the result that the genus <i>Pisum</i> was found to -possess the necessary conditions.</p> - -<p>Some thoroughly distinct forms of this genus possess -characters which are constant, and easily and certainly -recognisable, and when their hybrids are mutually crossed -they yield perfectly fertile progeny. Furthermore, a disturbance -through foreign pollen cannot easily occur, since -the fertilising organs are closely packed inside the keel and -the anther bursts within the bud, so that the stigma -becomes covered with pollen even before the flower opens. -This circumstance is of especial importance. As additional -advantages worth mentioning, there may be cited the easy -culture of these plants in the open ground and in pots, and -also their relatively short period of growth. Artificial -fertilisation is certainly a somewhat elaborate process, but -nearly always succeeds. For this purpose the bud is -opened before it is perfectly developed, the keel is removed, -and each stamen carefully extracted by means of forceps, -after which the stigma can at once be dusted over with the -foreign pollen.</p> - -<p>In all, thirty-four more or less distinct varieties of Peas -were obtained from several seedsmen and subjected to a -two years’ trial. In the case of one variety there were -remarked, among a larger number of plants all alike, a few -forms which were markedly different. These, however, did -not vary in the following year, and agreed entirely with -another variety obtained from the same seedsmen; the -seeds were therefore doubtless merely accidentally mixed. -All the other varieties yielded perfectly constant and -similar offspring; at any rate, no essential difference was -observed during two trial years. For fertilisation twenty-two -of these were selected and cultivated during the whole<span class="pagenum" id="Page_44">44</span> -period of the experiments. They remained constant without -any exception.</p> - -<p>Their systematic classification is difficult and uncertain. -If we adopt the strictest definition of a species, according -to which only those individuals belong to a species which -under precisely the same circumstances display precisely -similar characters, no two of these varieties could be referred -to one species. According to the opinion of experts, -however, the majority belong to the species <i>Pisum sativum</i>; -while the rest are regarded and classed, some as sub-species -of <i>P. sativum</i>, and some as independent species, such as -<i>P. quadratum</i>, <i>P. saccharatum</i>, and <i>P. umbellatum</i>. The -positions, however, which may be assigned to them in a -classificatory system are quite immaterial for the purposes -of the experiments in question. It has so far been found -to be just as impossible to draw a sharp line between the -hybrids of species and varieties as between species and -varieties themselves.</p> - - -<h3><span class="smcap">Division and Arrangement of the Experiments.</span></h3> - -<p>If two plants which differ constantly in one or several -characters be crossed, numerous experiments have demonstrated -that the common characters are transmitted unchanged -to the hybrids and their progeny; but each pair of -differentiating characters, on the other hand, unite in the -hybrid to form a new character, which in the progeny of the -hybrid is usually variable. The object of the experiment -was to observe these variations in the case of each pair of -differentiating characters, and to deduce the law according -to which they appear in the successive generations. The -experiment resolves itself therefore into just as many<span class="pagenum" id="Page_45">45</span> -separate experiments as there are constantly differentiating -characters presented in the experimental plants.</p> - -<p>The various forms of Peas selected for crossing showed -differences in the length and colour of the stem; in the -size and form of the leaves; in the position, colour, and -size of the flowers; in the length of the flower stalk; in the -colour, form, and size of the pods; in the form and size of -the seeds; and in the colour of the seed-coats and the -albumen [cotyledons]. Some of the characters noted do -not permit of a sharp and certain separation, since the -difference is of a “more or less” nature, which is often -difficult to define. Such characters could not be utilised -for the separate experiments; these could only be confined -to characters which stand out clearly and definitely in the -plants. Lastly, the result must show whether they, in -their entirety, observe a regular behaviour in their hybrid -unions, and whether from these facts any conclusion can -be come to regarding those characters which possess a -subordinate significance in the type.</p> - -<p>The characters which were selected for experiment relate:</p> - -<p>1. To the <i>difference in the form of the ripe seeds</i>. These -are either round or roundish, the wrinkling, when such occurs -on the surface, being always only shallow; or they are -irregularly angular and deeply wrinkled (<i>P. quadratum</i>).</p> - -<p>2. To the <i>difference in the colour of the seed albumen</i> -(endosperm<span class="nowrap">)<a id="FNanchor_25" href="#Footnote_25" class="fnanchor">25</a></span>. The albumen of the ripe seeds is either -pale yellow, bright yellow and orange coloured, or it -possesses a more or less intense green tint. This difference -of colour is easily seen in the seeds as their coats are -transparent.</p> -<p><span class="pagenum" id="Page_46">46</span></p> -<p>3. To the <i>difference in the colour of the seed-coat</i>. -This is either white, with which character white flowers -are constantly correlated; or it is grey, grey-brown, leather-brown, -with or without violet spotting, in which case the -colour of the standards is violet, that of the wings purple, -and the stem in the axils of the leaves is of a reddish tint. -The grey seed-coats become dark brown in boiling water.</p> - -<p>4. To the <i>difference in the form of the ripe pods</i>. These -are either simply inflated, never contracted in places; or -they are deeply constricted between the seeds and more or -less wrinkled (<i>P. saccharatum</i>).</p> - -<p>5. To the <i>difference in the colour of the unripe pods</i>. -They are either light to dark green, or vividly yellow, in -which colouring the stalks, leaf-veins, and calyx <span class="nowrap">participate<a id="FNanchor_26" href="#Footnote_26" class="fnanchor">26</a></span>.</p> - -<p>6. To the <i>difference in the position of the flowers</i>. -They are either axial, that is, distributed along the main -stem; or they are terminal, that is, bunched at the top of -the stem and arranged almost in a false umbel; in this -case the upper part of the stem is more or less widened in -section (<i>P. umbellatum</i><span class="nowrap">)<a id="FNanchor_27" href="#Footnote_27" class="fnanchor">27</a></span>.</p> - -<p>7. To the <i>difference in the length of the stem</i>. The -length of the <span class="nowrap">stem<a id="FNanchor_28" href="#Footnote_28" class="fnanchor">28</a></span> is very various in some forms; it is,<span class="pagenum" id="Page_47">47</span> -however, a constant character for each, in so far that healthy -plants, grown in the same soil, are only subject to unimportant -variations in this character.</p> - -<p>In experiments with this character, in order to be able to -discriminate with certainty, the long axis of 6–7 ft. was -always crossed with the short one of <span class="nowrap"><span class="fraction"><span class="fnum">3</span><span class="bar">/</span><span class="fden">4</span></span></span> ft. to <span class="nowrap">1 <span class="fraction"><span class="fnum">1</span><span class="bar">/</span><span class="fden">2</span></span></span> ft.</p> - -<p>Each two of the differentiating characters enumerated -above were united by cross-fertilisation. There were made -for the</p> - -<div class="table ml2em"> -<div class="row fs100"><div class="cell">1st</div><div class="cell"> trial </div><div class="cell">60 </div><div class="cell">fertilisations</div><div class="cell"> on </div><div class="cell">15</div><div class="cell">plants.</div></div> -<div class="row fs100"><div class="cell">2nd</div><div class="cell tac"><div>"</div></div><div class="cell">58</div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell">10 </div><div class="cell tac"><div>"</div></div></div> -<div class="row fs100"><div class="cell">3rd</div><div class="cell tac"><div>"</div></div><div class="cell">35</div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell">10 </div><div class="cell tac"><div>"</div></div></div> -<div class="row fs100"><div class="cell">4th</div><div class="cell tac"><div>"</div></div><div class="cell">40</div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell">10 </div><div class="cell tac"><div>"</div></div></div> -<div class="row fs100"><div class="cell">5th</div><div class="cell tac"><div>"</div></div><div class="cell">23</div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell"> 5 </div><div class="cell tac"><div>"</div></div></div> -<div class="row fs100"><div class="cell">6th</div><div class="cell tac"><div>"</div></div><div class="cell">34</div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell">10 </div><div class="cell tac"><div>"</div></div></div> -<div class="row fs100"><div class="cell">7th</div><div class="cell tac"><div>"</div></div><div class="cell">37</div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell">10 </div><div class="cell tac"><div>"</div></div></div> -</div> - -<p>From a larger number of plants of the same variety only -the most vigorous were chosen for fertilisation. Weakly -plants always afford uncertain results, because even in the -first generation of hybrids, and still more so in the subsequent -ones, many of the offspring either entirely fail to -flower or only form a few and inferior seeds.</p> - -<p>Furthermore, in all the experiments reciprocal crossings -were effected in such a way that each of the two varieties -which in one set of fertilisations served as seed-bearers in -the other set were used as pollen plants.</p> - -<p>The plants were grown in garden beds, a few also -in pots, and were maintained in their naturally upright -position by means of sticks, branches of trees, and strings -stretched between. For each experiment a number of pot -plants were placed during the blooming period in a greenhouse, -to serve as control plants for the main experiment<span class="pagenum" id="Page_48">48</span> -in the open as regards possible disturbance by insects. -Among the <span class="nowrap">insects<a id="FNanchor_29" href="#Footnote_29" class="fnanchor">29</a></span> which visit Peas the beetle <i>Bruchus -pisi</i> might be detrimental to the experiments should it -appear in numbers. The female of this species is known -to lay the eggs in the flower, and in so doing opens the -keel; upon the tarsi of one specimen, which was caught in -a flower, some pollen grains could clearly be seen under a -lens. Mention must also be made of a circumstance which -possibly might lead to the introduction of foreign pollen. -It occurs, for instance, in some rare cases that certain parts -of an otherwise quite normally developed flower wither, -resulting in a partial exposure of the fertilising organs. A -defective development of the keel has also been observed, -owing to which the stigma and anthers remained partially -<span class="nowrap">uncovered<a id="FNanchor_30" href="#Footnote_30" class="fnanchor">30</a></span>. It also sometimes happens that the pollen -does not reach full perfection. In this event there occurs -a gradual lengthening of the pistil during the blooming -period, until the stigmatic tip protrudes at the point of the -keel. This remarkable appearance has also been observed -in hybrids of <i>Phaseolus</i> and <i>Lathyrus</i>.</p> - -<p>The risk of false impregnation by foreign pollen is, -however, a very slight one with <i>Pisum</i>, and is quite -incapable of disturbing the general result. Among more -than 10,000 plants which were carefully examined there -were only a very few cases where an indubitable false -impregnation had occurred. Since in the greenhouse such -a case was never remarked, it may well be supposed that -<i>Bruchus pisi</i>, and possibly also the described abnormalities -in the floral structure, were to blame.</p> - -<p><span class="pagenum" id="Page_49">49</span></p> - - -<h3><span class="smcap">The Forms of the Hybrids.</span><a id="FNanchor_31" href="#Footnote_31" class="fnanchor"><span class="fs80">31</span></a></h3> - -<p>Experiments which in previous years were made with -ornamental plants have already afforded evidence that the -hybrids, as a rule, are not exactly intermediate between -the parental species. With some of the more striking -characters, those, for instance, which relate to the form -and size of the leaves, the pubescence of the several parts, -&c., the intermediate, indeed, was nearly always to be -seen; in other cases, however, one of the two parental -characters was so preponderant that it was difficult, or -quite impossible, to detect the other in the hybrid.</p> - -<p>This is precisely the case with the Pea hybrids. In -the case of each of the seven crosses the hybrid-character -<span class="nowrap">resembles<a id="FNanchor_32" href="#Footnote_32" class="fnanchor">32</a></span> that of one of the parental forms so closely that -the other either escapes observation completely or cannot -be detected with certainty. This circumstance is of great -importance in the determination and classification of the -forms under which the offspring of the hybrids appear. -Henceforth in this paper those characters which are transmitted -entire, or almost unchanged in the hybridisation, -and therefore in themselves constitute the characters of -the hybrid, are termed the <i>dominant</i>, and those which -become latent in the process <i>recessive</i>. The expression -“recessive” has been chosen because the characters thereby -designated withdraw or entirely disappear in the hybrids,<span class="pagenum" id="Page_50">50</span> -but nevertheless reappear unchanged in their progeny, as -will be demonstrated later on.</p> - -<p>It was furthermore shown by the whole of the experiments -that it is perfectly immaterial whether the dominant -character belong to the seed-bearer or to the pollen parent; -the form of the hybrid remains identical in both cases. This -interesting fact was also emphasised by Gärtner, with the -remark that even the most practised expert is not in a -position to determine in a hybrid which of the two parental -species was the seed or the pollen <span class="nowrap">plant<a id="FNanchor_33" href="#Footnote_33" class="fnanchor">33</a></span>.</p> - -<p>Of the differentiating characters which were used in the -experiments the following are dominant:</p> - -<p>1. The round or roundish form of the seed with or -without shallow depressions.</p> - -<p>2. The yellow colouring of the seed albumen [cotyledons].</p> - -<p>3. The grey, grey-brown, or leather-brown colour of -the seed-coat, in connection with violet-red blossoms and -reddish spots in the leaf axils.</p> - -<p>4. The simply inflated form of the pod.</p> - -<p>5. The green colouring of the unripe pod in connection -with the same colour in the stems, the leaf-veins and the calyx.</p> - -<p>6. The distribution of the flowers along the stem.</p> - -<p>7. The greater length of stem.</p> - -<p>With regard to this last character it must be stated -that the longer of the two parental stems is usually exceeded -by the hybrid, which is possibly only attributable to the -greater luxuriance which appears in all parts of plants -when stems of very different length are crossed. Thus, for -instance, in repeated experiments, stems of 1 ft. and 6 ft. -in length yielded without exception hybrids which varied -in length between 6 ft. and <span class="nowrap">7 <span class="fraction"><span class="fnum">1</span><span class="bar">/</span><span class="fden">2</span></span></span> ft.</p> - -<p><span class="pagenum" id="Page_51">51</span></p> - -<p>The hybrid seeds in the experiments with seed-coat are -often more spotted, and the spots sometimes coalesce into -small bluish-violet patches. The spotting also frequently -appears even when it is absent as a parental character.</p> - -<p>The hybrid forms of the seed-shape and of the albumen -are developed immediately after the artificial fertilisation -by the mere influence of the foreign pollen. They can, -therefore, be observed even in the first year of experiment, -whilst all the other characters naturally only appear in the -following year in such plants as have been raised from the -crossed seed.</p> - - -<h3><span class="smcap">The First Generation [Bred] from the Hybrids.</span></h3> - -<p>In this generation there reappear, together with the -dominant characters, also the recessive ones with their full -peculiarities, and this occurs in the definitely expressed -average proportion of three to one, so that among each -four plants of this generation three display the dominant -character and one the recessive. This relates without -exception to all the characters which were embraced in -the experiments. The angular wrinkled form of the seed, -the green colour of the albumen, the white colour of the -seed-coats and the flowers, the constrictions of the pods, -the yellow colour of the unripe pod, of the stalk of the -calyx, and of the leaf venation, the umbel-like form of the -inflorescence, and the dwarfed stem, all reappear in the -numerical proportion given without any essential alteration. -<i>Transitional forms were not observed in any experiment.</i></p> - -<p>Once the hybrids resulting from reciprocal crosses are -fully formed, they present no appreciable difference in their<span class="pagenum" id="Page_52">52</span> -subsequent development, and consequently the results [of -the reciprocal crosses] can be reckoned together in each -experiment. The relative numbers which were obtained for -each pair of differentiating characters are as follows:</p> - -<p>Expt. 1. Form of seed.—From 253 hybrids 7,324 seeds -were obtained in the second trial year. Among them were -5,474 round or roundish ones and 1,850 angular wrinkled -ones. Therefrom the ratio 2·96 to 1 is deduced.</p> - -<p>Expt. 2. Colour of albumen.—258 plants yielded 8,023 -seeds, 6,022 yellow, and 2,001 green; their ratio, therefore, -is as 3·01 to 1.</p> - -<p>In these two experiments each pod yielded usually both -kinds of seed. In well-developed pods which contained on -the average six to nine seeds, it often occurred that all the -seeds were round (Expt. 1) or all yellow (Expt. 2); on the -other hand there were never observed more than five angular -or five green ones in one pod. It appears to make no -difference whether the pods are developed early or later in -the hybrid or whether they spring from the main axis or -from a lateral one. In some few plants only a few seeds -developed in the first formed pods, and these possessed -exclusively one of the two characters, but in the subsequently -developed pods the normal proportions were maintained -nevertheless.</p> - -<p>As in separate pods, so did the distribution of the -characters vary in separate plants. By way of illustration -the first ten individuals from both series of experiments -may <span class="nowrap">serve<a id="FNanchor_34" href="#Footnote_34" class="fnanchor">34</a></span>.</p> - -<p><span class="pagenum" id="Page_53">53</span></p> - -<table id="tab1"> -<tr> -<td class="tac"></td> -<td class="tac" colspan="2"><div>Experiment 1.</div></td> -<td class="tac" colspan="2"><div>Experiment 2.</div></td> -</tr> -<tr> -<td class="tac"></td> -<td class="tac" colspan="2"><div>Form of Seed.</div></td> -<td class="tac" colspan="2"><div>Colour of Albumen.</div></td> -</tr> -<tr> -<td class="tac"><div>Plants.</div></td> -<td class="tac"><div>Round.</div></td> -<td class="tac"><div>Angular.</div></td> -<td class="tac"><div>Yellow.</div></td> -<td class="tac"><div>Green.</div></td> -</tr> -<tr> -<td class="tac"><div> 1</div></td> -<td class="tac"><div>45</div></td> -<td class="tac"><div>12</div></td> -<td class="tac"><div>25</div></td> -<td class="tac"><div>11</div></td> -</tr> -<tr> -<td class="tac"><div> 2</div></td> -<td class="tac"><div>27</div></td> -<td class="tac"><div> 8</div></td> -<td class="tac"><div>32</div></td> -<td class="tac"><div> 7</div></td> -</tr> -<tr> -<td class="tac"><div> 3</div></td> -<td class="tac"><div>24</div></td> -<td class="tac"><div> 7</div></td> -<td class="tac"><div>14</div></td> -<td class="tac"><div> 5</div></td> -</tr> -<tr> -<td class="tac"><div> 4</div></td> -<td class="tac"><div>19</div></td> -<td class="tac"><div>10</div></td> -<td class="tac"><div>70</div></td> -<td class="tac"><div>27</div></td> -</tr> -<tr> -<td class="tac"><div> 5</div></td> -<td class="tac"><div>32</div></td> -<td class="tac"><div>11</div></td> -<td class="tac"><div>24</div></td> -<td class="tac"><div>13</div></td> -</tr> -<tr> -<td class="tac"><div> 6</div></td> -<td class="tac"><div>26</div></td> -<td class="tac"><div> 6</div></td> -<td class="tac"><div>20</div></td> -<td class="tac"><div> 6</div></td> -</tr> -<tr> -<td class="tac"><div> 7</div></td> -<td class="tac"><div>88</div></td> -<td class="tac"><div>24</div></td> -<td class="tac"><div>32</div></td> -<td class="tac"><div>13</div></td> -</tr> -<tr> -<td class="tac"><div> 8</div></td> -<td class="tac"><div>22</div></td> -<td class="tac"><div>10</div></td> -<td class="tac"><div>44</div></td> -<td class="tac"><div> 9</div></td> -</tr> -<tr> -<td class="tac"><div> 9</div></td> -<td class="tac"><div>28</div></td> -<td class="tac"><div> 6</div></td> -<td class="tac"><div>50</div></td> -<td class="tac"><div>14</div></td> -</tr> -<tr> -<td class="tac"><div>10</div></td> -<td class="tac"><div>25</div></td> -<td class="tac"><div> 7</div></td> -<td class="tac"><div>44</div></td> -<td class="tac"><div>18</div></td> -</tr> -</table> - -<p>As extremes in the distribution of the two seed characters -in one plant, there were observed in Expt. 1 an instance -of 43 round and only 2 angular, and another of 14 round -and 15 angular seeds. In Expt. 2 there was a case of 32 -yellow and only 1 green seed, but also one of 20 yellow -and 19 green.</p> - -<p>These two experiments are important for the determination -of the average ratios, because with a smaller -number of experimental plants they show that very considerable -fluctuations may occur. In counting the seeds, -also, especially in Expt. 2, some care is requisite, since in -some of the seeds of many plants the green colour of the -albumen is less developed, and at first may be easily -overlooked. The cause of the partial disappearance of the -green colouring has no connection with the hybrid-character -of the plants, as it likewise occurs in the parental variety. -This peculiarity is also confined to the individual and is -not inherited by the offspring. In luxuriant plants this -appearance was frequently noted. Seeds which are damaged -by insects during their development often vary in colour -and form, but, with a little practice in sorting, errors are<span class="pagenum" id="Page_54">54</span> -easily avoided. It is almost superfluous to mention that the -pods must remain on the plants until they are thoroughly -ripened and have become dried, since it is only then that -the shape and colour of the seed are fully developed.</p> - -<p class="mt1em">Expt. 3. Colour of the seed-coats.—Among 929 plants -705 bore violet-red flowers and grey-brown seed-coats; 224 -had white flowers and white seed-coats, giving the proportion -3·15 to 1.</p> - -<p class="mt1em">Expt. 4. Form of pods.—Of 1,181 plants 882 had them -simply inflated, and in 299 they were constricted. Resulting -ratio, 2·95 to 1.</p> - -<p class="mt1em">Expt. 5. Colour of the unripe pods.—The number of -trial plants was 580, of which 428 had green pods and 152 -yellow ones. Consequently these stand in the ratio 2·82 to 1.</p> - -<p class="mt1em">Expt. 6. Position of flowers.—Among 858 cases 651 -blossoms were axial and 207 terminal. Ratio, 3·14 to 1.</p> - -<p class="mt1em">Expt. 7. Length of stem.—Out of 1,064 plants, in 787 -cases the stem was long, and in 277 short. Hence a mutual -ratio of 2·84 to 1. In this experiment the dwarfed plants -were carefully lifted and transferred to a special bed. This -precaution was necessary, as otherwise they would have -perished through being overgrown by their tall relatives. -Even in their quite young state they can be easily picked -out by their compact growth and thick dark-green foliage.</p> - -<p class="mt1em">If now the results of the whole of the experiments be -brought together, there is found, as between the number -of forms with the dominant and recessive characters, an -average ratio of 2·98 to 1, or 3 to 1.</p> - -<p>The dominant character can have here a <i>double signification</i>—viz. -that of a parental-character, or a hybrid-<span class="nowrap">character<a id="FNanchor_35" href="#Footnote_35" class="fnanchor">35</a></span>.<span class="pagenum" id="Page_55">55</span> -In which of the two significations it appears -in each separate case can only be determined by the following -generation. As a parental character it must pass over -unchanged to the whole of the offspring; as a hybrid-character, -on the other hand, it must observe the same -behaviour as in the first generation.</p> - - -<h3><span class="smcap">The Second Generation [Bred] from the Hybrids.</span></h3> - -<p>Those forms which in the first generation maintain the -recessive character do not further vary in the second -generation as regards this character; they remain constant -in their offspring.</p> - -<p>It is otherwise with those which possess the dominant -character in the first generation [bred from the hybrids]. -Of these <i>two</i>-thirds yield offspring which display the -dominant and recessive characters in the proportion of -3 to 1, and thereby show exactly the same ratio as the -hybrid forms, while only <i>one</i>-third remains with the dominant -character constant.</p> - -<p>The separate experiments yielded the following results:—</p> - -<p class="mt1em">Expt. 1.—Among 565 plants which were raised from -round seeds of the first generation, 193 yielded round seeds -only, and remained therefore constant in this character; -372, however, gave both round and angular seeds, in the -proportion of 3 to 1. The number of the hybrids, therefore, -as compared with the constants is 1·93 to 1.</p> - -<p class="mt1em">Expt. 2.—Of 519 plants which were raised from seeds -whose albumen was of yellow colour in the first generation, -166 yielded exclusively yellow, while 353 yielded yellow<span class="pagenum" id="Page_56">56</span> -and green seeds in the proportion of 3 to 1. There resulted, -therefore, a division into hybrid and constant forms in the -proportion of 2·13 to 1.</p> - -<p>For each separate trial in the following experiments -100 plants were selected which displayed the dominant -character in the first generation, and in order to ascertain -the significance of this, ten seeds of each were cultivated.</p> - -<p class="mt1em">Expt. 3.—The offspring of 36 plants yielded exclusively -grey-brown seed-coats, while of the offspring of 64 plants -some had grey-brown and some had white.</p> - -<p class="mt1em">Expt. 4.—The offspring of 29 plants had only simply -inflated pods; of the offspring of 71, on the other hand, -some had inflated and some constricted.</p> - -<p class="mt1em">Expt. 5.—The offspring of 40 plants had only green -pods; of the offspring of 60 plants some had green, some -yellow ones.</p> - -<p class="mt1em">Expt. 6.—The offspring of 33 plants had only axial -flowers; of the offspring of 67, on the other hand, some -had axial and some terminal flowers.</p> - -<p class="mt1em">Expt. 7.—The offspring of 28 plants inherited the long -axis, and those of 72 plants some the long and some the -short axis.</p> - -<p>In each of these experiments a certain number of the -plants came constant with the dominant character. For -the determination of the proportion in which the separation -of the forms with the constantly persistent character results, -the two first experiments are of especial importance, since -in these a larger number of plants can be compared. The -ratios 1·93 to 1 and 2·13 to 1 gave together almost exactly -the average ratio of 2 to 1. The sixth experiment has a -quite concordant result; in the others the ratio varies more -or less, as was only to be expected in view of the smaller<span class="pagenum" id="Page_57">57</span> -number of 100 trial plants. Experiment 5, which shows -the greatest departure, was repeated, and then in lieu of -the ratio of 60 and 40 that of 65 and 35 resulted. <i>The -average ratio of 2 to 1 appears, therefore, as fixed with -certainty.</i> It is therefore demonstrated that, of those forms -which possess the dominant character in the first generation, -in two-thirds the hybrid character is embodied, while one-third -remains constant with the dominant character.</p> - -<p>The ratio of 3 to 1, in accordance with which the -distribution of the dominant and recessive characters -results in the first generation, resolves itself therefore in -all experiments into the ratio of 2 : 1 : 1 if the dominant -character be differentiated according to its significance as -a hybrid character or a parental one. Since the members -of the first generation spring directly from the seed of the -hybrids, <i>it is now clear that the hybrids form seeds having -one or other of the two differentiating characters, and of -these one-half develop again the hybrid form, while the other -half yield plants which remain constant and receive the dominant -or recessive characters [respectively] in equal numbers</i>.</p> - - -<h3><span class="smcap">The Subsequent Generations [Bred] from the Hybrids.</span></h3> - -<p>The proportions in which the descendants of the hybrids -develop and split up in the first and second generations -presumably hold good for all subsequent progeny. Experiments -1 and 2 have already been carried through six -generations, 3 and 7 through five, and 4, 5, and 6 through -four, these experiments being continued from the third -generation with a small number of plants, and no departure -from the rule has been perceptible. The offspring of the -hybrids separated in each generation in the ratio of 2 : 1 : 1 -into hybrids and constant forms.</p> - -<p><span class="pagenum" id="Page_58">58</span></p> - -<p>If <i>A</i> be taken as denoting one of the two constant -characters, for instance the dominant, <i>a</i>, the recessive, -and <i>Aa</i> the hybrid form in which both are conjoined, the -expression</p> - -<p class="tac"><i>A</i> + 2<i>Aa</i> + <i>a</i></p> - -<p>shows the terms in the series for the progeny of the hybrids -of two differentiating characters.</p> - -<p>The observation made by Gärtner, Kölreuter, and others, -that hybrids are inclined to revert to the parental forms, is -also confirmed by the experiments described. It is seen -that the number of the hybrids which arise from one -fertilisation, as compared with the number of forms which -become constant, and their progeny from generation to -generation, is continually diminishing, but that nevertheless -they could not entirely disappear. If an average -equality of fertility in all plants in all generations be -assumed, and if, furthermore, each hybrid forms seed of -which one-half yields hybrids again, while the other half -is constant to both characters in equal proportions, the -ratio of numbers for the offspring in each generation is -seen by the following summary, in which <i>A</i> and <i>a</i> denote -again the two parental characters, and <i>Aa</i> the hybrid -forms. For brevity’s sake it may be assumed that each -plant in each generation furnishes only 4 seeds.</p> - -<div class="center"> -<table id="tab2"> -<tr> -<td class="tac"></td> -<td class="tac"></td> -<td class="tac"></td> -<td class="tac"></td> -<td class="tac" colspan="5"><div><span class="smcap">Ratios.</span></div></td> -</tr> -<tr> -<td class="tac"><div>Generation</div></td> -<td class="tac"><div><i>A</i></div></td> -<td class="tac"><div> <i>Aa</i> </div></td> -<td class="tac"><div><i>a</i> </div></td> -<td class="tac"><div><i>A</i></div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div> <i>Aa</i> </div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div> <i>a</i></div></td> -</tr> -<tr> -<td class="tac"><div>1</div></td> -<td class="tac"><div> 1</div></td> -<td class="tac"><div> 2</div></td> -<td class="tac"><div> 1 </div></td> -<td class="tac"><div> 1</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div>2</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div> 1</div></td> -</tr> -<tr> -<td class="tac"><div>2</div></td> -<td class="tac"><div> 6</div></td> -<td class="tac"><div> 4</div></td> -<td class="tac"><div> 6 </div></td> -<td class="tac"><div> 3</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div>2</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div> 3</div></td> -</tr> -<tr> -<td class="tac"><div>3</div></td> -<td class="tac"><div> 28</div></td> -<td class="tac"><div> 8</div></td> -<td class="tac"><div> 28 </div></td> -<td class="tac"><div> 7</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div>2</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div> 7</div></td> -</tr> -<tr> -<td class="tac"><div>4</div></td> -<td class="tac"><div>120</div></td> -<td class="tac"><div>16</div></td> -<td class="tac"><div>120 </div></td> -<td class="tac"><div>15</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div>2</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div>15</div></td> -</tr> -<tr> -<td class="tac"><div>5</div></td> -<td class="tac"><div> 496 </div></td> -<td class="tac"><div>32</div></td> -<td class="tac"><div> 496  </div></td> -<td class="tac"><div>31</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div>2</div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div>31</div></td> -</tr> -<tr> -<td class="tac"><div><i>n</i></div></td> -<td class="tac"></td> -<td class="tac"></td> -<td class="tac"><div> </div></td> -<td class="tac"><div>2<sup><i>n</i></sup>-1  </div></td> -<td class="tac"><div>:</div></td> -<td class="tac"><div>2</div></td> -<td class="tac"><div>:</div></td> -<td class="tar"><div><span class="ilb">      2<sup><i>n</i></sup>-1</span></div><span class="pagenum" id="Page_59">59</span></td> -</tr> -</table> -</div> - -<p>In the tenth generation, for instance, 2<sup><i>n</i></sup>-1 = 1023. -There result, therefore, in each 2,048 plants which arise in -this generation 1,023 with the constant dominant character, -1,023 with the recessive character, and only two hybrids.</p> - - -<h3><span class="smcap">The Offspring of Hybrids in which Several -Differentiating Characters are Associated.</span></h3> - -<p>In the experiments above described plants were used -which differed only in one essential <span class="nowrap">character<a id="FNanchor_36" href="#Footnote_36" class="fnanchor">36</a></span>. The next -task consisted in ascertaining whether the law of development -discovered in these applied to each pair of differentiating -characters when several diverse characters are united -in the hybrid by crossing. As regards the form of the -hybrids in these cases, the experiments showed throughout -that this invariably more nearly approaches to that one of -the two parental plants which possesses the greater number -of dominant characters. If, for instance, the seed plant has -a short stem, terminal white flowers, and simply inflated -pods; the pollen plant, on the other hand, a long stem, -violet-red flowers distributed along the stem, and constricted -pods; the hybrid resembles the seed parent only in -the form of the pod; in the other characters it agrees with -the pollen parent. Should one of the two parental types -possess only dominant characters, then the hybrid is -scarcely or not at all distinguishable from it.</p> -<p><span class="pagenum" id="Page_60">60</span></p> -<p>Two experiments were made with a larger number of -plants. In the first experiment the parental plants differed -in the form of the seed and in the colour of the albumen; -in the second in the form of the seed, in the colour of the -albumen, and in the colour of the seed-coats. Experiments -with seed characters give the result in the simplest and -most certain way.</p> - -<p>In order to facilitate study of the data in these experiments, -the different characters of the seed plant will be -indicated by <i>A</i>, <i>B</i>, <i>C</i>, those of the pollen plant by <i>a</i>, <i>b</i>, <i>c</i>, -and the hybrid forms of the characters by <i>Aa</i>, <i>Bb</i>, and <i>Cc</i>.</p> - -<table class="fs100 ml2em"> -<tr> -<td class="tar" colspan="2"><div>Expt. 1.—<i>AB</i>,</div></td> -<td class="tal"> seed parents;</td> -<td class="tar"><div><i>ab</i>,</div></td> -<td class="tal"> pollen parents;</td> -</tr> -<tr> -<td class="tar"></td> -<td class="tar"><div><i>A</i>,</div></td> -<td class="tal"> form round;</td> -<td class="tar"><div><i>a</i>,</div></td> -<td class="tal"> form angular;</td> -</tr> -<tr> -<td class="tar"></td> -<td class="tar"><div><i>B</i>,</div></td> -<td class="tal"> albumen yellow. </td> -<td class="tar"><div><i>b</i>,</div></td> -<td class="tal"> albumen green.</td> -</tr> -</table> - -<p>The fertilised seeds appeared round and yellow like those -of the seed parents. The plants raised therefrom yielded -seeds of four sorts, which frequently presented themselves -in one pod. In all 556 seeds were yielded by 15 plants, -and of these there were:—</p> - -<p class="ml2em"> -315 round and yellow,<br /> -101 angular and yellow,<br /> -108 round and green,<br /> - 32 angular and green. -</p> - -<p>All were sown the following year. Eleven of the round -yellow seeds did not yield plants, and three plants did not -form seeds. Among the rest:</p> - - -<table class="ml2em fs100"> -<tr> -<td class="tal"> 38 had round yellow seeds</td> -<td class="tal pl1"><i>AB</i></td> -</tr> -<tr> -<td class="tal"> 65 round yellow and green seeds</td> -<td class="tal pl1"><i>ABb</i></td> -</tr> -<tr> -<td class="tal"> 60 round yellow and angular yellow seeds</td> -<td class="tal pl1"><i>AaB</i></td> -</tr> -<tr> -<td class="tal">138 round yellow and green, angular yellow<br /> -  and green seeds</td> -<td class="tal pl1 vab"><span class="ilb"><i>AaBb</i>.</span><span class="pagenum" id="Page_61">61</span></td> -</tr> -</table> - -<p>From the angular yellow seeds 96 resulting plants bore -seed, of which:</p> - -<table class="ml2em fs100"> -<tr><td>28 had only angular yellow seeds</td><td class="pl1"><i>aB</i></td></tr> -<tr><td>68 angular yellow and green seeds</td><td class="pl1"><span class="ilb"><i>aBb</i>.</span></td></tr> -</table> - -<p>From 108 round green seeds 102 resulting plants fruited, -of which:</p> - -<table class="ml2em fs100"> -<tr><td>35 had only round green seeds</td><td class="pl1"><i>Ab</i></td></tr> -<tr><td>67 round and angular green seeds</td><td class="pl1"><span class="ilb"><i>Aab</i>.</span></td></tr> -</table> - -<p>The angular green seeds yielded 30 plants which bore seeds -all of like character; they remained constant <i>ab</i>.</p> - -<p>The offspring of the hybrids appeared therefore under -nine different forms, some of them in very unequal numbers. -When these are collected and co-ordinated we find:</p> - -<div class="table ml2em"> -<div class="row fs100"><div class="cell"> 38</div><div class="cell"> plants</div><div class="cell"> with</div><div class="cell"> the sign</div><div class="cell tal"> <i>AB</i></div></div> -<div class="row fs100"><div class="cell"> 35</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tal"> <i>Ab</i></div></div> -<div class="row fs100"><div class="cell"> 28</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tal"> <i>aB</i></div></div> -<div class="row fs100"><div class="cell"> 30</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tal"> <i>ab</i></div></div> -<div class="row fs100"><div class="cell"> 65</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tal"> <i>ABb</i></div></div> -<div class="row fs100"><div class="cell"> 68</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tal"> <i>aBb</i></div></div> -<div class="row fs100"><div class="cell"> 60</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tal"> <i>AaB</i></div></div> -<div class="row fs100"><div class="cell"> 67</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tal"> <i>Aab</i></div></div> -<div class="row fs100"><div class="cell">138</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tal"> <i>AaBb</i>.</div></div> -</div> - -<p>The whole of the forms may be classed into three -essentially different groups. The first embraces those with -the signs <i>AB</i>, <i>Ab</i>, <i>aB</i>, and <i>ab</i>: they possess only constant -characters and do not vary again in the next generation. -Each of these forms is represented on the average thirty-three -times. The second group embraces the signs <i>ABb</i>, -<i>aBb</i>, <i>AaB</i>, <i>Aab</i>: these are constant in one character and -hybrid in another, and vary in the next generation only -as regards the hybrid character. Each of these appears on<span class="pagenum" id="Page_62">62</span> -an average sixty-five times. The form <i>AaBb</i> occurs 138 -times: it is hybrid in both characters, and behaves exactly -as do the hybrids from which it is derived.</p> - -<p>If the numbers in which the forms belonging to these -classes appear be compared, the ratios of 1, 2, 4 are unmistakably -evident. The numbers 32, 65, 138 present very -fair approximations to the ratio numbers of 33, 66, 132.</p> - -<p>The developmental series consists, therefore, of nine -classes, of which four appear therein always once and are -constant in both characters; the forms <i>AB</i>, <i>ab</i>, resemble -the parental forms, the two others present combinations -between the conjoined characters <i>A</i>, <i>a</i>, <i>B</i>, <i>b</i>, which combinations -are likewise possibly constant. Four classes -appear always twice, and are constant in one character -and hybrid in the other. One class appears four times, -and is hybrid in both characters. Consequently the -offspring of the hybrids, if two kinds of differentiating -characters are combined therein, are represented by the -expression</p> - -<p class="ml2em"> -<i>AB</i> + <i>Ab</i> + <i>aB</i> + <i>ab</i> + 2<i>ABb</i> + 2<i>aBb</i> + 2<i>AaB</i> + 2<i>Aab</i> + 4<i>AaBb</i>. -</p> - -<p>This expression is indisputably a combination series in -which the two expressions for the characters <i>A</i> and <i>a</i>, <i>B</i> -and <i>b</i>, are combined. We arrive at the full number of the -classes of the series by the combination of the expressions:</p> - -<p class="ml4em"> -<i>A</i> + 2 <i>Aa</i> + <i>a</i><br /> -<i>B</i> + 2 <i>Bb</i> + <i>b</i>. -</p> - -<p class="ml2em">Second Expt.</p> - -<table id="tab3"> -<tr> -<td class="tar"><div><i>ABC</i>,</div></td> -<td class="tal"> seed parents;</td> -<td class="tar"><div><i>abc</i>,</div></td> -<td class="tal"> pollen parents;</td> -</tr> -<tr> -<td class="tar"><div><i>A</i>,</div></td> -<td class="tal"> form round;</td> -<td class="tar"><div><i>a</i>,</div></td> -<td class="tal"> form angular;</td> -</tr> -<tr> -<td class="tar"><div><i>B</i>,</div></td> -<td class="tal"> albumen yellow;</td> -<td class="tar"><div><i>b</i>,</div></td> -<td class="tal"> albumen green;</td> -</tr> -<tr> -<td class="tar"><div><i>C</i>,</div></td> -<td class="tal"> seed-coat grey-brown.</td> -<td class="tar"><div><i>c</i>,</div></td> -<td class="tal"> seed-coat white.<span class="pagenum" id="Page_63">63</span></td> -</tr> -</table> - - -<p>This experiment was made in precisely the same way as -the previous one. Among all the experiments it demanded -the most time and trouble. From 24 hybrids 687 seeds -were obtained in all: these were all either spotted, grey-brown -or grey-green, round or <span class="nowrap">angular<a id="FNanchor_37" href="#Footnote_37" class="fnanchor">37</a></span>. From these in -the following year 639 plants fruited, and, as further -investigation showed, there were among them:</p> - -<table id="tab4"> -<tr> -<td class="tac"><div> 8</div></td> -<td class="tac"><div> plants</div></td> -<td class="tal"> <i>ABC</i>.</td> -<td class="tac pl1"><div>22</div></td> -<td class="tac"><div> plants</div></td> -<td class="tal"> <i>ABCc</i>.</td> -<td class="tac pl1"><div>45</div></td> -<td class="tac"><div> plants</div></td> -<td class="tal"> <i>ABbCc</i>.</td> -</tr> -<tr> -<td class="tac"><div>14</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>ABc</i>.</td> -<td class="tac pl1"><div>17</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>AbCc</i>.</td> -<td class="tac pl1"><div>36</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>aBbCc</i>.</td> -</tr> -<tr> -<td class="tac"><div> 9</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>AbC</i>.</td> -<td class="tac pl1"><div>25</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>aBCc</i>.</td> -<td class="tac pl1"><div>38</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>AaBCc</i>.</td> -</tr> -<tr> -<td class="tac"><div>11</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>Abc</i>.</td> -<td class="tac pl1"><div>20</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>abCc</i>.</td> -<td class="tac pl1"><div>40</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>AabCc</i>.</td> -</tr> -<tr> -<td class="tac"><div> 8</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>aBC</i>.</td> -<td class="tac pl1"><div>15</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>ABbC</i>.</td> -<td class="tac pl1"><div>49</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>AaBbC</i>.</td> -</tr> -<tr> -<td class="tac"><div>10</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>aBc</i>.</td> -<td class="tac pl1"><div>18</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>ABbc</i>.</td> -<td class="tac pl1"><div>48</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>AaBbc</i>.</td> -</tr> -<tr> -<td class="tac"><div>10</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>abC</i>.</td> -<td class="tac pl1"><div>19</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>aBbC</i>.</td> -<td colspan="3" rowspan="2"></td> -</tr> -<tr> -<td class="tac"><div> 7</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>abc</i>.</td> -<td class="tac pl1"><div>24</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>aBbc</i>.</td> -</tr> -<tr> -<td class="tac"></td> -<td class="tac"></td> -<td class="tal"></td> -<td class="tac pl1"><div>14</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>AaBC</i>.</td> -<td class="tac pl1"><div>78</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"><span class="ilb"> <i>AaBbCc</i>.</span></td> -</tr> -<tr> -<td class="tac"></td> -<td class="tac"></td> -<td class="tal"></td> -<td class="tac pl1"><div><div>18</div></div></td> -<td class="tac"><div><div>"</div></div></td> -<td class="tal"> <i>AaBc</i>.</td> -<td colspan="3" rowspan="3"></td> -</tr> -<tr> -<td class="tac"></td> -<td class="tac"></td> -<td class="tal"></td> -<td class="tac pl1"><div>20</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>AabC</i>.</td> -</tr> -<tr> -<td class="tac"></td> -<td class="tac"></td> -<td class="tal"></td> -<td class="tac pl1"><div>16</div></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>Aabc</i>.</td> -</tr> -</table> - -<p>The whole expression contains 27 terms. Of these 8 -are constant in all characters, and each appears on the -average 10 times; 12 are constant in two characters, and -hybrid in the third; each appears on the average 19 times; -6 are constant in one character and hybrid in the other -two; each appears on the average 43 times. One form -appears 78 times and is hybrid in all of the characters. -The ratios 10, 19, 43, 78 agree so closely with the ratios -10, 20, 40, 80, or 1, 2, 4, 8, that this last undoubtedly -represents the true value.</p> - -<p>The development of the hybrids when the original<span class="pagenum" id="Page_64">64</span> -parents differ in three characters results therefore according -to the following expression:</p> - -<p class="ml2em"><i>ABC</i> + <i>ABc</i> + <i>AbC</i> + <i>Abc</i> + <i>aBC</i> + <i>aBc</i> + <i>abC</i> + <i>abc</i> +<br /> -2 <i>ABCc</i> + 2 <i>AbCc</i> + 2 <i>aBCc</i> + 2 <i>abCc</i> + 2 <i>ABbC</i> + 2 <i>ABbc</i> +<br /> -2 <i>aBbC</i> + 2 <i>aBbc</i> + 2 <i>AaBC</i> + 2 <i>AaBc</i> + 2 <i>AabC</i> + 2 <i>Aabc</i> +<br /> -4 <i>ABbCc</i> + 4 <i>aBbCc</i> + 4 <i>AaBCc</i> + 4 <i>AabCc</i> + 4 <i>AaBbC</i> +<br /> -4 <i>AaBbc</i> + 8 <i>AaBbCc</i>. -</p> - -<p>Here also is involved a combination series in which the -expressions for the characters <i>A</i> and <i>a</i>, <i>B</i> and <i>b</i>, <i>C</i> and <i>c</i>, -are united. The expressions</p> - -<p class="ml2em"> -<i>A</i> + 2 <i>Aa</i> + <i>a</i><br /> -<i>B</i> + 2 <i>Bb</i> + <i>b</i><br /> -<i>C</i> + 2 <i>Cc</i> + <i>c</i> -</p> - -<p>give all the classes of the series. The constant combinations -which occur therein agree with all combinations which are -possible between the characters <i>A</i>, <i>B</i>, <i>C</i>, <i>a</i>, <i>b</i>, <i>c</i>; two thereof, -<i>ABC</i> and <i>abc</i>, resemble the two original parental stocks.</p> - -<p>In addition, further experiments were made with a -smaller number of experimental plants in which the remaining -characters by twos and threes were united as -hybrids: all yielded approximately the same results. There -is therefore no doubt that for the whole of the characters -involved in the experiments the principle applies that <i>the -offspring of the hybrids in which several essentially different -characters are combined represent the terms of a series of -combinations, in which the developmental series for each pair -of differentiating characters are associated</i>. It is demonstrated -at the same time that <i>the relation of each pair of -different characters in hybrid union is independent of the -other differences in the two original parental stocks</i>.</p> - -<p>If <i>n</i> represent the number of the differentiating characters -<span class="pagenum" id="Page_65">65</span>in the two original stocks, 3<sup><i>n</i></sup> gives the number of terms -of the combination series, 4<sup><i>n</i></sup> the number of individuals -which belong to the series, and 2<sup><i>n</i></sup> the number of unions -which remain constant. The series therefore embraces, if -the original stocks differ in four characters, 3<sup>4</sup> = 81 of classes, -4<sup>4</sup> = 256 individuals, and 2<sup>4</sup> = 16 constant forms; or, which -is the same, among each 256 offspring of the hybrids there -are 81 different combinations, 16 of which are constant.</p> - -<p>All constant combinations which in Peas are possible by -the combination of the said seven differentiating characters -were actually obtained by repeated crossing. Their number -is given by 2<sup>7</sup> = 128. Thereby is simultaneously given the -practical proof <i>that the constant characters which appear in -the several varieties of a group of plants may be obtained in -all the associations which are possible according to the -[mathematical] laws of combination, by means of repeated -artificial fertilisation</i>.</p> - -<p>As regards the flowering time of the hybrids, the experiments -are not yet concluded. It can, however, already -be stated that the period stands almost exactly between -those of the seed and pollen parents, and that the constitution -of the hybrids with respect to this character -probably happens in the same way as in the case of the -other characters. The forms which are selected for experiments -of this class must have a difference of at least twenty -days from the middle flowering period of one to that of the -other; furthermore, the seeds when sown must all be placed -at the same depth in the earth, so that they may germinate -simultaneously. Also, during the whole flowering period, -the more important variations in temperature must be taken -into account, and the partial hastening or delaying of the -flowering which may result therefrom. It is clear that this -experiment presents many difficulties to be overcome and -necessitates great attention.</p> - -<p><span class="pagenum" id="Page_66">66</span></p> - -<p>If we endeavour to collate in a brief form the results -arrived at, we find that those differentiating characters -which admit of easy and certain recognition in the -experimental plants, all behave exactly alike in their -hybrid associations. The offspring of the hybrids of each -pair of differentiating characters are, one-half, hybrid again, -while the other half are constant in equal proportions having -the characters of the seed and pollen parents respectively. -If several differentiating characters are combined by cross-fertilisation -in a hybrid, the resulting offspring form the -terms of a combination series in which the permutation -series for each pair of differentiating characters are united.</p> - -<p>The uniformity of behaviour shown by the whole of the -characters submitted to experiment permits, and fully -justifies, the acceptance of the principle that a similar -relation exists in the other characters which appear less -sharply defined in plants, and therefore could not be -included in the separate experiments. An experiment -with peduncles of different lengths gave on the whole a -fairly satisfactory result, although the differentiation and -serial arrangement of the forms could not be effected with -that certainty which is indispensable for correct experiment.</p> - - -<h3><span class="smcap">The Reproductive Cells of Hybrids.</span></h3> - -<p>The results of the previously described experiments -induced further experiments, the results of which appear -fitted to afford some conclusions as regards the composition -of the egg and pollen cells of hybrids. An important matter -for consideration is afforded in <i>Pisum</i> by the circumstance -that among the progeny of the hybrids constant forms -appear, and that this occurs, too, in all combinations of the -associated characters. So far as experience goes, we find<span class="pagenum" id="Page_67">67</span> -it in every case confirmed that constant progeny can only -be formed when the egg cells and the fertilising pollen are -of like character, so that both are provided with the material -for creating quite similar individuals, as is the case with the -normal fertilisation of pure <span class="nowrap">species<a id="FNanchor_38" href="#Footnote_38" class="fnanchor">38</a></span>. We must therefore -regard it as essential that exactly similar factors are at work -also in the production of the constant forms in the hybrid -plants. Since the various constant forms are produced in -<i>one</i> plant, or even in <i>one</i> flower of a plant, the conclusion -appears logical that in the ovaries of the hybrids there are -formed as many sorts of egg cells, and in the anthers as -many sorts of pollen cells, as there are possible constant -combination forms, and that these egg and pollen cells -agree in their internal composition with those of the -separate forms.</p> - -<p>In point of fact it is possible to demonstrate theoretically -that this hypothesis would fully suffice to account for the -development of the hybrids in the separate generations, if -we might at the same time assume that the various kinds -of egg and pollen cells were formed in the hybrids on the -average in equal <span class="nowrap">numbers<a id="FNanchor_39" href="#Footnote_39" class="fnanchor">39</a></span>.</p> - -<p>In order to bring these assumptions to an experimental -proof, the following experiments were designed. Two forms -which were constantly different in the form of the seed and -the colour of the albumen were united by fertilisation.</p> - -<p>If the differentiating characters are again indicated as -<i>A</i>, <i>B</i>, <i>a</i>, <i>b</i>, we have:</p> - -<table class="fs100 ml2em"> -<tr> -<td class="tar"><div><i>AB</i>,</div></td> -<td class="tal"> seed parent;</td> -<td class="tar pl2"><div><i>ab</i>,</div></td> -<td class="tal"> pollen parent;</td> -</tr> -<tr> -<td class="tar"><div> <i>A</i>,</div></td> -<td class="tal"> form round;</td> -<td class="tar pl2"><div><i>a</i>,</div></td> -<td class="tal"> form angular;</td> -</tr> -<tr> -<td class="tar"><div> <i>B</i>,</div></td> -<td class="tal"> albumen yellow.</td> -<td class="tar pl2"><div><i>b</i>,</div></td> -<td class="tal"> albumen green.<span class="pagenum" id="Page_68">68</span></td> -</tr> -</table> - -<p>The artificially fertilised seeds were sown together with -several seeds of both original stocks, and the most vigorous -examples were chosen for the reciprocal crossing. There -were fertilised:</p> - -<div class="table ml2em"> -<div class="row fs110"><div class="cell">1. The hybrids</div><div class="cell tac"><div>with the</div></div><div class="cell tac"><div>pollen of</div></div><div class="cell"><i>AB</i>.</div></div> -<div class="row fs110"><div class="cell">2. The hybrids</div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell"><i>ab</i>.</div></div> -<div class="row fs110"><div class="cell">3. <i>AB</i></div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell"><span class="ilb">the hybrids.</span></div></div> -<div class="row fs110"><div class="cell">4. <i>ab</i></div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell"><span class="ilb">the hybrids.</span></div></div> -</div> - -<p>For each of these four experiments the whole of the -flowers on three plants were fertilised. If the above theory -be correct, there must be developed on the hybrids egg and -pollen cells of the forms <i>AB</i>, <i>Ab</i>, <i>aB</i>, <i>ab</i>, and there would -be combined:—</p> - -<p class="ml2em lh1em">1. The egg cells <i>AB</i>, <i>Ab</i>, <i>aB</i>, <i>ab</i> with the pollen -cells <i>AB</i>.</p> - -<p class="ml2em lh1em">2. The egg cells <i>AB</i>, <i>Ab</i>, <i>aB</i>, <i>ab</i> with the pollen -cells <i>ab</i>.</p> - -<p class="ml2em lh1em">3. The egg cells <i>AB</i> with the pollen cells <i>AB</i>, <i>Ab</i>, -<i>aB</i>, <i>ab</i>.</p> - -<p class="ml2em lh1em">4. The egg cells <i>ab</i> with the pollen cells <i>AB</i>, <i>Ab</i>, -<i>aB</i>, <i>ab</i>.</p> - -<p>From each of these experiments there could then result -only the following forms:—</p> - -<p class="ml2em"> -1. <i>AB</i>, <i>ABb</i>, <i>AaB</i>, <i>AaBb</i>.<br /> -2. <i>AaBb</i>, <i>Aab</i>, <i>aBb</i>, <i>ab</i>.<br /> -3. <i>AB</i>, <i>ABb</i>, <i>AaB</i>, <i>AaBb</i>.<br /> -4. <i>AaBb</i>, <i>Aab</i>, <i>aBb</i>, <i>ab</i>. -</p> - -<p>If, furthermore, the several forms of the egg and pollen -cells of the hybrids were produced on an average in equal -numbers, then in each experiment the said four combinations<span class="pagenum" id="Page_69">69</span> -should stand in the same ratio to each other. A perfect -agreement in the numerical relations was, however, not to -be expected, since in each fertilisation, even in normal -cases, some egg cells remain undeveloped or subsequently -die, and many even of the well-formed seeds fail to -germinate when sown. The above assumption is also -limited in so far that, while it demands the formation of -an equal number of the various sorts of egg and pollen -cells, it does not require that this should apply to each -separate hybrid with mathematical exactness.</p> - -<p>The first and second experiments had primarily the -object of proving the composition of the hybrid egg cells, -while the third and fourth experiments were to decide that of -the pollen <span class="nowrap">cells<a id="FNanchor_40" href="#Footnote_40" class="fnanchor">40</a></span>. As is shown by the above demonstration -the first and second experiments and the third and fourth -experiments should produce precisely the same combinations, -and even in the second year the result should be partially -visible in the form and colour of the artificially fertilised -seed. In the first and third experiments the dominant -characters of form and colour, <i>A</i> and <i>B</i>, appear in each -union, and are also partly constant and partly in hybrid -union with the recessive characters <i>a</i> and <i>b</i>, for which -reason they must impress their peculiarity upon the whole -of the seeds. All seeds should therefore appear round and -yellow, if the theory be justified. In the second and fourth -experiments, on the other hand, one union is hybrid in -form and in colour, and consequently the seeds are round -and yellow; another is hybrid in form, but constant in the -recessive character of colour, whence the seeds are round -and green; the third is constant in the recessive character -of form but hybrid in colour, consequently the seeds are<span class="pagenum" id="Page_70">70</span> -angular and yellow; the fourth is constant in both recessive -characters, so that the seeds are angular and green. In -both these experiments there were consequently four sorts -of seed to be expected—viz. round and yellow, round and -green, angular and yellow, angular and green.</p> - -<p>The crop fulfilled these expectations perfectly. There -were obtained in the</p> - -<div class="table ml2em"> -<div class="row fs110"><div class="cell">1st</div><div class="cell tac"><div>Experiment,</div></div><div class="cell tac"><div>98</div></div><div class="cell tac"><div>exclusively</div></div><div class="cell tac"><div>round</div></div><div class="cell tac"><div>yellow</div></div><div class="cell tac"><div><span class="ilb">seeds;</span></div></div></div> -<div class="row fs110"><div class="cell">3rd</div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>94</div></div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div><div class="cell tac"><div>"</div></div></div> -</div> - -<p>In the 2nd Experiment, 31 round and yellow, 26 round -and green, 27 angular and yellow, 26 angular and green seeds.</p> - -<p>In the 4th Experiment, 24 round and yellow, 25 round -and green, 22 angular and yellow, 27 angular and green -seeds.</p> - -<p>A favourable result could now scarcely be doubted; the -next generation must afford the final proof. From the seed -sown there resulted for the first experiment 90 plants, and -for the third 87 plants which fruited: these yielded for the—</p> - -<table class="ml2em"> -<tr> -<td class="tac"><div>1st Exp.</div></td> -<td class="tac"><div> 3rd Exp.</div></td> -<td class="tal"></td> -<td class="tar"></td> -</tr> -<tr> -<td class="tac"><div>20</div></td> -<td class="tac"><div>25</div></td> -<td class="tal">round yellow seeds</td> -<td class="tar"><div><i>AB</i></div></td> -</tr> -<tr> -<td class="tac"><div>23</div></td> -<td class="tac"><div>19</div></td> -<td class="tal">round yellow and green seeds</td> -<td class="tar"><div><i>ABb</i></div></td> -</tr> -<tr> -<td class="tac"><div>25</div></td> -<td class="tac"><div>22</div></td> -<td class="tal">round and angular yellow seeds</td> -<td class="tar"><div><i>AaB</i></div></td> -</tr> -<tr> -<td class="tac"><div>22</div></td> -<td class="tac"><div>21</div></td> -<td class="tal">round and angular green and yellow seeds</td> -<td class="tar"><div> <i>AaBb</i></div></td> -</tr> -</table> - -<p>In the second and fourth experiments the round and -yellow seeds yielded plants with round and angular yellow -and green seeds, <i>AaBb</i>.</p> - -<p>From the round green seeds plants resulted with round -and angular green seeds, <i>Aab</i>.</p> - -<p>The angular yellow seeds gave plants with angular -yellow and green seeds, <i>aBb</i>.</p> - -<p>From the angular green seeds plants were raised which -yielded again only angular and green seeds, <i>ab</i>.</p> - -<p><span class="pagenum" id="Page_71">71</span></p> - -<p>Although in these two experiments likewise some seeds -did not germinate, the figures arrived at already in the -previous year were not affected thereby, since each kind of -seed gave plants which, as regards their seed, were like each -other and different from the others. There resulted therefore -from the</p> - -<table class="ml2em"> -<tr> -<td class="tac"><div>2nd Exp.</div></td> -<td class="tac" colspan="5"><div> 4th Exp.</div></td> -</tr> -<tr> -<td class="tac"><div>31</div></td> -<td class="tac"><div>24</div></td> -<td class="tac"><div>plants</div></td> -<td class="tac"><div> of</div></td> -<td class="tac"><div> the form</div></td> -<td class="tal"> <i>AaBb</i></td> -</tr> -<tr> -<td class="tac"><div>26</div></td> -<td class="tac"><div>25</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>Aab</i></td> -</tr> -<tr> -<td class="tac"><div>27</div></td> -<td class="tac"><div>22</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>aBb</i></td> -</tr> -<tr> -<td class="tac"><div>26</div></td> -<td class="tac"><div>27</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"></td> -<td class="tac"><div>"</div></td> -<td class="tal"> <i>ab</i></td> -</tr> -</table> - -<p>In all the experiments, therefore, there appeared all the -forms which the proposed theory demands, and also in -nearly equal numbers.</p> - -<p>In a further experiment the characters of floral colour -and length of stem were experimented upon, and selection -so made that in the third year of the experiment each -character ought to appear in half of all the plants if the -above theory were correct. <i>A</i>, <i>B</i>, <i>a</i>, <i>b</i> serve again as -indicating the various characters.</p> - -<table class="fs100 ml2em"> -<tr> -<td class="tar"><div><i>A</i>,</div></td> -<td class="tal"> violet-red flowers.</td> -<td class="tar pl2"><div><i>a</i>,</div></td> -<td class="tal"><span class="ilb"> white flowers.</span></td> -</tr> -<tr> -<td class="tar"><div><i>B</i>,</div></td> -<td class="tal"> axis long.</td> -<td class="tar pl2"><div><i>b</i>,</div></td> -<td class="tal"><span class="ilb"> axis short.</span></td> -</tr> -</table> - -<p>The form <i>Ab</i> was fertilised with <i>ab</i>, which produced the -hybrid <i>Aab</i>. Furthermore, <i>aB</i> was also fertilised with <i>ab</i>, -whence the hybrid <i>aBb</i>. In the second year, for further -fertilisation, the hybrid <i>Aab</i> was used as seed parent, and -hybrid <i>aBb</i> as pollen parent.</p> - -<table class="fs100 ml2em "> -<tr> -<td class="tal">Seed parent, <i>Aab</i>.</td> -<td class="tal pl2">Pollen parent, <i>aBb</i>.</td> -</tr> -<tr> -<td class="tal">Possible egg cells, <i>Abab</i>.</td> -<td class="tal pl2">Pollen cells, <i>aBab</i>.</td> -</tr> -</table> - -<p>From the fertilisation between the possible egg and -pollen cells four combinations should result, viz.:—</p> - -<p class="ml2em"> -<i>AaBb</i> + <i>aBb</i> + <i>Aab</i> + <i>ab</i>. -</p> - -<p><span class="pagenum" id="Page_72">72</span></p> - -<p>From this it is perceived that, according to the above -theory, in the third year of the experiment out of all the -plants</p> - -<div class="table ml2em"> -<div class="row fs110"><div class="cell tac">Half</div><div class="cell tac">should</div><div class="cell">have</div><div class="cell tal">violet-red flowers (<i>Aa</i>),</div><div class="cell tac">Classes</div><div class="cell tal"><span class="ilb"> 1, 3</span></div></div> -<div class="row fs110"><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell">white flowers (<i>a</i>)</div><div class="cell tac">"</div><div class="cell tal"><span class="ilb"> 2, 4</span></div></div> -<div class="row fs110"><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell">a long axis (<i>Bb</i>)</div><div class="cell tac">"</div><div class="cell tal"><span class="ilb"> 1, 2</span></div></div> -<div class="row fs110"><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell">a short axis (<i>b</i>)</div><div class="cell tac">"</div><div class="cell tal"><span class="ilb"> 3, 4</span></div></div> -</div> - -<p>From 45 fertilisations of the second year 187 seeds -resulted, of which only 166 reached the flowering stage in -the third year. Among these the separate classes appeared -in the numbers following:—</p> - -<table class="ml2em"> -<tr> -<td class="tac"><div>Class.</div></td> -<td class="tac"><div> Colour of flower. </div></td> -<td class="tal" colspan="3">Stem.</td> -</tr> -<tr> -<td class="tac"><div>1</div></td> -<td class="tal pl25">violet-red</td> -<td class="tal">long</td> -<td class="tac pl2"><div>47</div></td> -<td class="tac"><div>times</div></td> -</tr> -<tr> -<td class="tac"><div>2</div></td> -<td class="tal pl25">white</td> -<td class="tal">long</td> -<td class="tac pl2"><div>40</div></td> -<td class="tac"><div>"</div></td> -</tr> -<tr> -<td class="tac"><div>3</div></td> -<td class="tal pl25">violet-red</td> -<td class="tal">short</td> -<td class="tac pl2"><div>38</div></td> -<td class="tac"><div>"</div></td> -</tr> -<tr> -<td class="tac"><div>4</div></td> -<td class="tal pl25">white</td> -<td class="tal">short</td> -<td class="tac pl2"><div>41</div></td> -<td class="tac"><div>"</div></td> -</tr> -</table> - -<p>There consequently appeared—</p> - -<div class="table ml2em"> -<div class="row fs110"><div class="cell tac">The</div><div class="cell">violet-red</div><div class="cell">flower</div><div class="cell">colour</div><div class="cell tal">(<i>Aa</i>)</div><div class="cell">in 85</div><div class="cell">plants.</div></div> -<div class="row fs110"><div class="cell tac">"</div><div class="cell">white</div><div class="cell tac">"</div><div class="cell tac">"</div><div class="cell tal">(<i>a</i>)</div><div class="cell">in 81</div><div class="cell tac">"</div></div> -<div class="row fs110"><div class="cell tac">"</div><div class="cell">long stem</div><div class="cell"></div><div class="cell"></div><div class="cell tal">(<i>Bb</i>)</div><div class="cell">in 87</div><div class="cell tac">"</div></div> -<div class="row fs110"><div class="cell tac">"</div><div class="cell">short  "</div><div class="cell"></div><div class="cell"></div><div class="cell tal">(<i>b</i>)</div><div class="cell">in 79</div><div class="cell tac">"</div></div> -</div> - -<p>The theory adduced is therefore satisfactorily confirmed in -this experiment also.</p> - -<p>For the characters of form of pod, colour of pod, and -position of flowers experiments were also made on a small -scale, and results obtained in perfect agreement. All -combinations which were possible through the union of the -differentiating characters duly appeared, and in nearly -equal numbers.</p> - -<p>Experimentally, therefore, the theory is justified <i>that -the pea hybrids form egg and pollen cells which, in their<span class="pagenum" id="Page_73">73</span> -constitution, represent in equal numbers all constant forms -which result from the combination of the characters when -united in fertilisation</i>.</p> - -<p>The difference of the forms among the progeny of the -hybrids, as well as the respective ratios of the numbers in -which they are observed, find a sufficient explanation in the -principle above deduced. The simplest case is afforded by -the developmental series of each pair of differentiating -characters. This series is represented by the expression -<i>A</i> + 2<i>Aa</i> + <i>a</i>, in which <i>A</i> and <i>a</i> signify the forms with -constant differentiating characters, and <i>Aa</i> the hybrid -form of both. It includes in three different classes four -individuals. In the formation of these, pollen and egg -cells of the form <i>A</i> and <i>a</i> take part on the average equally -in the fertilisation; hence each form [occurs] twice, since -four individuals are formed. There participate consequently -in the fertilisation—</p> - -<p class="ml2em"> -The pollen cells <i>A</i> + <i>A</i> + <i>a</i> + <i>a</i><br /> -The egg cells <i>A</i> + <i>A</i> + <i>a</i> + <i>a</i>. -</p> - -<p>It remains, therefore, purely a matter of chance which -of the two sorts of pollen will become united with each -separate egg cell. According, however, to the law of -probability, it will always happen, on the average of many -cases, that each pollen form <i>A</i> and <i>a</i> will unite equally -often with each egg cell form <i>A</i> and <i>a</i>, consequently one -of the two pollen cells <i>A</i> in the fertilisation will meet with -the egg cell <i>A</i> and the other with an egg cell <i>a</i>, and so -likewise one pollen cell <i>a</i> will unite with an egg cell <i>A</i>, -and the other with egg cell <i>a</i>.</p> - -<div class="center"> -<table> - <tr> - <td class="tal lh13em">Pollen cells - <br /> - <br /> - <br /> - Egg cells - </td> - - <td class="pl3"> - <div class="figcenter illowp100" id="pollination" style="max-width: 13.125em;"> - <img class="w100" src="images/pollination.jpg" alt="" /> - </div> - </td> - </tr> -</table> -</div> - -<p><span class="pagenum" id="Page_74">74</span></p> - -<p>The result of the fertilisation may be made clear by -putting the signs for the conjoined egg and pollen cells in -the form of fractions, those for the pollen cells above and -those for the egg cells below the line. We then have</p> - -<p class="ml2em"><span class="nowrap"><span class="fraction2"><span class="fnum"><i>A</i></span><span class="bar">/</span><span class="fden2">A</span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>A</i></span><span class="bar">/</span><span class="fden2">A</span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>a</i></span><span class="bar">/</span><span class="fden2">a</span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>a</i></span><span class="bar">/</span><span class="fden2">a</span></span></span>.</p> - -<p>In the first and fourth term the egg and pollen cells are of -like kind, consequently the product of their union must be -constant, viz. <i>A</i> and <i>a</i>; in the second and third, on the -other hand, there again results a union of the two differentiating -characters of the stocks, consequently the forms -resulting from these fertilisations are identical with those -of the hybrid from which they sprang. <i>There occurs -accordingly a repeated hybridisation.</i> This explains the -striking fact that the hybrids are able to produce, besides -the two parental forms, offspring which are like themselves; -<span class="nowrap"><span class="fraction2"><span class="fnum"><i>A</i></span><span class="bar">/</span><span class="fden2">A</span></span></span> and <span class="nowrap"><span class="fraction2"><span class="fnum"><i>a</i></span><span class="bar">/</span><span class="fden2">a</span></span></span> both give the same union <i>Aa</i>, since, as already -remarked above, it makes no difference in the result of -fertilisation to which of the two characters the pollen or -egg cells belong. We may write then—</p> - -<p class="ml2em"><span class="nowrap"><span class="fraction2"><span class="fnum"><i>A</i></span><span class="bar">/</span><span class="fden2">A</span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>A</i></span><span class="bar">/</span><span class="fden2">A</span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>a</i></span><span class="bar">/</span><span class="fden2">a</span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>a</i></span><span class="bar">/</span><span class="fden2">a</span></span></span> = <i>A</i> + 2<i>Aa</i> + <i>a</i>.</p> - -<p>This represents the average result of the self-fertilisation -of the hybrids when two differentiating characters are -united in them. In solitary flowers and in solitary plants, -however, the ratios in which the forms of the series are produced -may suffer not inconsiderable <span class="nowrap">fluctuations<a id="FNanchor_41" href="#Footnote_41" class="fnanchor">41</a></span>. Apart -from the fact that the numbers in which both sorts of egg -cells occur in the seed vessels can only be regarded as equal -on the average, it remains purely a matter of chance which<span class="pagenum" id="Page_75">75</span> -of the two sorts of pollen may fertilise each separate egg -cell. For this reason the separate values must necessarily -be subject to fluctuations, and there are even extreme cases -possible, as were described earlier in connection with the -experiments on the form of the seed and the colour of the -albumen. The true ratios of the numbers can only be -ascertained by an average deduced from the sum of as -many single values as possible; the greater the number -the more are merely chance elements eliminated.</p> - -<p>The developmental series for hybrids in which two -kinds of differentiating characters are united contains -among sixteen individuals nine different forms, viz., -<i>AB</i> + <i>Ab</i> + <i>aB</i> + <i>ab</i> + 2<i>ABb</i> + 2<i>aBb</i> + 2<i>AaB</i> + 2<i>Aab</i> + 4<i>AaBb</i>. -Between the differentiating characters of the original stocks -<i>Aa</i> and <i>Bb</i> four constant combinations are possible, and -consequently the hybrids produce the corresponding four -forms of egg and pollen cells <i>AB</i>, <i>Ab</i>, <i>aB</i>, <i>ab</i>, and each -of these will on the average figure four times in the -fertilisation, since sixteen individuals are included in the -series. Therefore the participators in the fertilisation are—</p> - -<div class="table"> -<div class="row fs110"><div class="cell">Pollen cells </div><div class="cell"><i>AB</i> + <i>AB</i> + <i>AB</i> + <i>AB</i> + <i>Ab</i> + <i>Ab</i> + <i>Ab</i> + <i>Ab</i> +</div></div> -<div class="row fs110"><div class="cell"></div><div class="cell"><i>aB</i> + <i>aB</i> + <i>aB</i> + <i>aB</i> + <i>ab</i> + <i>ab</i> + <i>ab</i> + <i>ab</i>.</div></div> -</div> -<div class="table"> -<div class="row fs110"><div class="cell">Egg cells  </div><div class="cell"><i>AB</i> + <i>AB</i> + <i>AB</i> + <i>AB</i> + <i>Ab</i> + <i>Ab</i> + <i>Ab</i> + <i>Ab</i> +</div></div> -<div class="row fs110"><div class="cell"></div><div class="cell"><i>aB</i> + <i>aB</i> + <i>aB</i> + <i>aB</i> + <i>ab</i> + <i>ab</i> + <i>ab</i> + <i>ab</i>.</div></div> -</div> - -<p>In the process of fertilisation each pollen form unites on an -average equally often with each egg cell form, so that each -of the four pollen cells <i>AB</i> unites once with one of the -forms of egg cell <i>AB</i>, <i>Ab</i>, <i>aB</i>, <i>ab</i>. In precisely the same -way the rest of the pollen cells of the forms <i>Ab</i>, <i>aB</i>, <i>ab</i> -unite with all the other egg cells. We obtain therefore—</p> - -<p class="ml2em lh3em"><span class="nowrap"><span class="fraction2"><span class="fnum"><i>AB</i></span><span class="bar">/</span><span class="fden2"><i>AB</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>AB</i></span><span class="bar">/</span><span class="fden2"><i>Ab</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>AB</i></span><span class="bar">/</span><span class="fden2"><i>aB</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>AB</i></span><span class="bar">/</span><span class="fden2"><i>ab</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>Ab</i></span><span class="bar">/</span><span class="fden2"><i>AB</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>Ab</i></span><span class="bar">/</span><span class="fden2"><i>Ab</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>Ab</i></span><span class="bar">/</span><span class="fden2"><i>aB</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>Ab</i></span><span class="bar">/</span><span class="fden2"><i>ab</i></span></span></span> +<br /> - <span class="nowrap"><span class="fraction2"><span class="fnum"><i>aB</i></span><span class="bar">/</span><span class="fden2"><i>AB</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>aB</i></span><span class="bar">/</span><span class="fden2"><i>Ab</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>aB</i></span><span class="bar">/</span><span class="fden2"><i>aB</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>aB</i></span><span class="bar">/</span><span class="fden2"><i>ab</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>ab</i></span><span class="bar">/</span><span class="fden2"><i>AB</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>ab</i></span><span class="bar">/</span><span class="fden2"><i>Ab</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>ab</i></span><span class="bar">/</span><span class="fden2"><i>aB</i></span></span></span> + <span class="nowrap"><span class="fraction2"><span class="fnum"><i>ab</i></span><span class="bar">/</span><span class="fden2"><i>ab</i></span></span></span>,</p> - -<p><span class="pagenum" id="Page_76">76</span></p> - -<p>or</p> - -<p class="ml2em"><i>AB</i> + <i>ABb</i> + <i>AaB</i> + <i>AaBb</i> + <i>ABb</i> + <i>Ab</i> + <i>AaBb</i> + <i>Aab</i> +<br /> -<i>AaB</i> + <i>AaBb</i> + <i>aB</i> + <i>aBb</i> + <i>AaBb</i> + <i>Aab</i> + <i>aBb</i> + <i>ab</i> = <i>AB</i> +<br /> -<i>Ab</i> + <i>aB</i> + <i>ab</i> + 2<i>ABb</i> + 2<i>aBb</i> + 2<i>AaB</i> + 2<i>Aab</i> + <span class="nowrap">4<i>AaBb</i><a id="FNanchor_42" href="#Footnote_42" class="fnanchor">42</a></span>.</p> - -<p>In precisely similar fashion is the developmental series -of hybrids exhibited when three kinds of differentiating -characters are conjoined in them. The hybrids form eight -various kinds of egg and pollen cells—<i>ABC</i>, <i>ABc</i>, <i>AbC</i>, -<i>Abc</i>, <i>aBC</i>, <i>aBc</i>, <i>abC</i>, <i>abc</i>—and each pollen form unites -itself again on the average once with each form of egg cell.</p> - -<p>The law of combination of different characters which -governs the development of the hybrids finds therefore its -foundation and explanation in the principle enunciated, -that the hybrids produce egg cells and pollen cells which -in equal numbers represent all constant forms which result -from the combinations of the characters brought together -in fertilisation.</p> - - -<h3><span class="smcap">Experiments with Hybrids of other Species of Plants.</span></h3> - -<p>It must be the object of further experiments to ascertain -whether the law of development discovered for <i>Pisum</i> -applies also to the hybrids of other plants. To this end -several experiments were recently commenced. Two minor -experiments with species of <i>Phaseolus</i> have been completed, -and may be here mentioned.</p> - -<p>An experiment with <i>Phaseolus vulgaris</i> and <i>Phaseolus -nanus</i> gave results in perfect agreement. <i>Ph. nanus</i> had -together with the dwarf axis simply inflated green pods. -<i>Ph. vulgaris</i> had, on the other hand, an axis 10 feet to<span class="pagenum" id="Page_77">77</span> -12 feet high, and yellow coloured pods, constricted when -ripe. The ratios of the numbers in which the different -forms appeared in the separate generations were the same -as with <i>Pisum</i>. Also the development of the constant -combinations resulted according to the law of simple combination -of characters, exactly as in the case of <i>Pisum</i>. -There were obtained—</p> - - -<table id="tab5"> -<colgroup> -<col> -<col class="colw12"> -<col> -<col> -</colgroup> -<tr> -<td class="tac"><div>Constant<br />combinations</div></td> -<td class="tac vat"><div>Axis</div></td> -<td class="tac"><div>Colour of<br />the unripe pods.</div></td> -<td class="tac"><div>Form of<br />the ripe pods.</div></td> -</tr> -<tr> -<td class="tac"><div>1</div></td> -<td class="tac"><div>long</div></td> -<td class="tac"><div>green</div></td> -<td class="tac"><div>inflated</div></td> -</tr> -<tr> -<td class="tac"><div>2</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>constricted</div></td> -</tr> -<tr> -<td class="tac"><div>3</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>yellow</div></td> -<td class="tac"><div>inflated</div></td> -</tr> -<tr> -<td class="tac"><div>4</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>constricted</div></td> -</tr> -<tr> -<td class="tac"><div>5</div></td> -<td class="tac"><div>short</div></td> -<td class="tac"><div>green</div></td> -<td class="tac"><div>inflated</div></td> -</tr> -<tr> -<td class="tac"><div>6</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>constricted</div></td> -</tr> -<tr> -<td class="tac"><div>7</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>yellow</div></td> -<td class="tac"><div>inflated</div></td> -</tr> -<tr> -<td class="tac"><div>8</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>constricted</div></td> -</tr> -</table> - - -<p>The green colour of the pod, the inflated forms, and the -long axis were, as in <i>Pisum</i>, dominant characters.</p> - -<p>Another experiment with two very different species of -<i>Phaseolus</i> had only a partial result. <i>Phaseolus nanus</i>, L., -served as seed parent, a perfectly constant species, with -white flowers in short racemes and small white seeds in -straight, inflated, smooth pods; as pollen parent was used -<i>Ph. multiflorus</i>, W., with tall winding stem, purple-red -flowers in very long racemes, rough, sickle-shaped crooked -pods, and large seeds which bore black flecks and splashes -on a peach-blood-red ground.</p> - -<p>The hybrids had the greatest similarity to the pollen -parent, but the flowers appeared less intensely coloured. -Their fertility was very limited; from seventeen plants, -which together developed many hundreds of flowers, only -forty-nine seeds in all were obtained. These were of<span class="pagenum" id="Page_78">78</span> -medium size, and were flecked and splashed similarly to -those of <i>Ph. multiflorus</i>, while the ground colour was not -materially different. The next year forty-four plants were -raised from these seeds, of which only thirty-one reached -the flowering stage. The characters of <i>Ph. nanus</i>, which -had been altogether latent in the hybrids, reappeared in -various combinations; their ratio, however, with relation -to the dominant characters was necessarily very fluctuating -owing to the small number of trial plants. With certain -characters, as in those of the axis and the form of pod, it -was, however, as in the case of <i>Pisum</i>, almost exactly 1 : 3.</p> - -<p>Insignificant as the results of this experiment may be as -regards the determination of the relative numbers in which -the various forms appeared, it presents, on the other hand, -the phenomenon of a remarkable change of colour in the -flowers and seed of the hybrids. In <i>Pisum</i> it is known -that the characters of the flower- and seed-colour present -themselves unchanged in the first and further generations, -and that the offspring of the hybrids display exclusively the -one or the other of the characters of the original <span class="nowrap">stocks<a id="FNanchor_43" href="#Footnote_43" class="fnanchor">43</a></span>. -It is otherwise in the experiment we are considering. The -white flowers and the seed-colour of <i>Ph. nanus</i> appeared, it -is true, at once in the first generation [<i>from</i> the hybrids] -in one fairly fertile example, but the remaining thirty -plants developed flower colours which were of various -grades of purple-red to pale violet. The colouring of the -seed-coat was no less varied than that of the flowers. No<span class="pagenum" id="Page_79">79</span> -plant could rank as fully fertile; many produced no fruit -at all; others only yielded fruits from the flowers last produced, -which did not ripen. From fifteen plants only were -well-developed seeds obtained. The greatest disposition -to infertility was seen in the forms with preponderantly -red flowers, since out of sixteen of these only four yielded -ripe seed. Three of these had a similar seed pattern to -<i>Ph. multiflorus</i>, but with a more or less pale ground colour; -the fourth plant yielded only one seed of plain brown tint. -The forms with preponderantly violet coloured flowers had -dark brown, black-brown, and quite black seeds.</p> - -<p>The experiment was continued through two more generations -under similar unfavourable circumstances, since even -among the offspring of fairly fertile plants there were still -some which were less fertile or even quite sterile. Other -flower- and seed-colours than those cited did not subsequently -present themselves. The forms which in the -first generation [bred from the hybrids] contained one or -more of the recessive characters remained, as regards these, -constant without exception. Also of those plants which -possessed violet flowers and brown or black seed, some did -not vary again in these respects in the next generation; -the majority, however, yielded, together with offspring -exactly like themselves, some which displayed white flowers -and white seed-coats. The red flowering plants remained -so slightly fertile that nothing can be said with certainty -as regards their further development.</p> - -<p>Despite the many disturbing factors with which the -observations had to contend, it is nevertheless seen by this -experiment that the development of the hybrids, with -regard to those characters which concern the form of the -plants, follows the same laws as does <i>Pisum</i>. With regard -to the colour characters, it certainly appears difficult to<span class="pagenum" id="Page_80">80</span> -perceive a substantial agreement. Apart from the fact -that from the union of a white and a purple-red colouring -a whole series of colours results, from purple to pale violet -and white, the circumstance is a striking one that among -thirty-one flowering plants only one received the recessive -character of the white colour, while in <i>Pisum</i> this occurs -on the average in every fourth plant.</p> - -<p>Even these enigmatical results, however, might probably -be explained by the law governing <i>Pisum</i> if we might -assume that the colour of the flowers and seeds of <i>Ph. -multiflorus</i> is a combination of two or more entirely -independent colours, which individually act like any other -constant character in the plant. If the flower colour A -were a combination of the individual characters <i>A</i><sub>1</sub> + <i>A</i><sub>2</sub> + . . . -which produce the total impression of a purple colouration, -then by fertilisation with the differentiating character, -white colour, <i>a</i>, there would be produced the hybrid unions -<i>A</i><sub>1</sub><i>a</i> + <i>A</i><sub>2</sub><i>a</i> + . . . and so would it be with the corresponding -colouring of the seed-<span class="nowrap">coats<a id="FNanchor_44" href="#Footnote_44" class="fnanchor">44</a></span>. According to the above -assumption, each of these hybrid colour unions would be -independent, and would consequently develop quite independently -from the others. It is then easily seen that -from the combination of the separate developmental series -<span class="pagenum" id="Page_81">81</span>a perfect colour-series must result. If, for instance, -<i>A</i> = <i>A</i><sub>1</sub> + <i>A</i><sub>2</sub>, then the hybrids <i>A</i><sub>1</sub><i>a</i> and <i>A</i><sub>2</sub><i>a</i> form the -developmental series—</p> - -<p class="ml2em"> -<i>A</i><sub>1</sub> + 2<i>A</i><sub>1</sub><i>a</i> + <i>a</i><br /> -<i>A</i><sub>2</sub> + 2<i>A</i><sub>2</sub><i>a</i> + <i>a</i>. -</p> - -<p>The members of this series can enter into nine different -combinations, and each of these denotes another <span class="nowrap">colour<a id="FNanchor_45" href="#Footnote_45" class="fnanchor">45</a></span>—</p> - - -<table id="tab6"> -<tr> -<td class="tal">1 <i>A</i><sub>1</sub><i>A</i><sub>2</sub></td> -<td class="tal">2 <i>A</i><sub>1</sub><i>aA</i><sub>2</sub></td> -<td class="tal">1 <i>A</i><sub>2</sub><i>a</i></td> -</tr> -<tr> -<td class="tal">2 <i>A</i><sub>1</sub><i>A</i><sub>2</sub><i>a</i></td> -<td class="tal">4 <i>A</i><sub>1</sub><i>aA</i><sub>2</sub><i>a</i></td> -<td class="tal">2 <i>A</i><sub>2</sub><i>aa</i></td> -</tr> -<tr> -<td class="tal">1 <i>A</i><sub>1</sub><i>a</i></td> -<td class="tal">2 <i>A</i><sub>1</sub><i>aa</i></td> -<td class="tal">1 <i>aa</i>.</td> -</tr> -</table> - - -<p>The figures prescribed for the separate combinations -also indicate how many plants with the corresponding -colouring belong to the series. Since the total is sixteen, -the whole of the colours are on the average distributed -over each sixteen plants, but, as the series itself indicates, -in unequal proportions.</p> - -<p>Should the colour development really happen in this -way, we could offer an explanation of the case above -described, viz. that the white flowers and seed-coat colour -only appeared once among thirty-one plants of the first -generation. This colouring appears only once in the series, -and could therefore also only be developed once in the -average in each sixteen, and with three colour characters -only once even in sixty-four plants.</p> - -<p>It must, however, not be forgotten that the explanation -here attempted is based on a mere hypothesis, only supported -by the very imperfect result of the experiment just described. -It would, however, be well worth while to follow -up the development of colour in hybrids by similar experiments,<span class="pagenum" id="Page_82">82</span> -since it is probable that in this way we might learn -the significance of the extraordinary variety in the colouring -of our ornamental flowers.</p> - -<p>So far, little at present is known with certainty beyond -the fact that the colour of the flowers in most ornamental -plants is an extremely variable character. The opinion has -often been expressed that the stability of the species is -greatly disturbed or entirely upset by cultivation, and -consequently there is an inclination to regard the development -of cultivated forms as a matter of chance devoid of -rules; the colouring of ornamental plants is indeed usually -cited as an example of great instability. It is, however, -not clear why the simple transference into garden soil -should result in such a thorough and persistent revolution -in the plant organism. No one will seriously maintain -that in the open country the development of plants is ruled -by other laws than in the garden bed. Here, as there, -changes of type must take place if the conditions of life be -altered, and the species possesses the capacity of fitting -itself to its new environment. It is willingly granted that -by cultivation the origination of new varieties is favoured, -and that by man’s labour many varieties are acquired -which, under natural conditions, would be lost; but nothing -justifies the assumption that the tendency to the formation -of varieties is so extraordinarily increased that the species -speedily lose all stability, and their offspring diverge into -an endless series of extremely variable forms. Were the -change in the conditions of vegetation the sole cause of -variability we might expect that those cultivated plants -which are grown for centuries under almost identical conditions -would again attain constancy. That, as is well -known, is not the case, since it is precisely under such -circumstances that not only the most varied but also the<span class="pagenum" id="Page_83">83</span> -most variable forms are found. It is only the <i>Leguminosæ</i>, -like <i>Pisum</i>, <i>Phaseolus</i>, <i>Lens</i>, whose organs of fertilisation -are protected by the keel, which constitute a noteworthy -exception. Even here there have arisen numerous varieties -during a cultural period of more than 1000 years; these -maintain, however, under unchanging environments a stability -as great as that of species growing wild.</p> - -<p>It is more than probable that as regards the variability -of cultivated plants there exists a factor which so far has -received little attention. Various experiments force us to -the conclusion that our cultivated plants, with few exceptions, -are <i>members of various hybrid series</i>, whose -further development in conformity with law is changed and -hindered by frequent crossings <i>inter se</i>. The circumstance -must not be overlooked that cultivated plants are mostly -grown in great numbers and close together, affording -the most favourable conditions for reciprocal fertilisation -between the varieties present and the species itself. The -probability of this is supported by the fact that among the -great array of variable forms solitary examples are always -found, which in one character or another remain constant, -if only foreign influence be carefully excluded. These forms -develop precisely as do those which are known to be members -of the compound hybrid series. Also with the most susceptible -of all characters, that of colour, it cannot escape -the careful observer that in the separate forms the inclination -to vary is displayed in very different degrees. Among -plants which arise from <i>one</i> spontaneous fertilisation there -are often some whose offspring vary widely in the constitution -and arrangement of the colours, while others furnish forms of -little deviation, and among a greater number solitary examples -occur which transmit the colour of the flowers unchanged -to their offspring. The cultivated species of <i>Dianthus</i><span class="pagenum" id="Page_84">84</span> -afford an instructive example of this. A white-flowered -example of <i>Dianthus caryophyllus</i>, which itself was derived -from a white-flowered variety, was shut up during its -blooming period in a greenhouse; the numerous seeds -obtained therefrom yielded plants entirely white-flowered -like itself. A similar result was obtained from a subspecies, -with red flowers somewhat flushed with violet, and one -with flowers white, striped with red. Many others, on the -other hand, which were similarly protected, yielded progeny -which were more or less variously coloured and marked.</p> - -<p>Whoever studies the colouration which results in ornamental -plants from similar fertilisation can hardly escape -the conviction that here also the development follows a -definite law which possibly finds its expression <i>in the -combination of several independent colour characters</i>.</p> - - -<h3><span class="smcap">Concluding Remarks.</span></h3> - -<p>It can hardly fail to be of interest to compare the -observations made regarding <i>Pisum</i> with the results arrived -at by the two authorities in this branch of knowledge, -Kölreuter and Gärtner, in their investigations. According -to the opinion of both, the hybrids in outer appearance -present either a form intermediate between the original -species, or they closely resemble either the one or the other -type, and sometimes can hardly be discriminated from it. -From their seeds usually arise, if the fertilisation was -effected by their own pollen, various forms which differ -from the normal type. As a rule, the majority of individuals -obtained by one fertilisation maintain the hybrid form, -while some few others come more like the seed parent, -and one or other individual approaches the pollen parent. -This, however, is not the case with all hybrids without -exception. With some the offspring have more nearly<span class="pagenum" id="Page_85">85</span> -approached, some the one and some the other, original -stock, or they all incline more to one or the other side; -while with others <i>they remain perfectly like the hybrid</i> and -continue constant in their offspring. The hybrids of varieties -behave like hybrids of species, but they possess greater variability -of form and a more pronounced tendency to revert to -the original type.</p> - -<p>With regard to the form of the hybrids and their -development, as a rule an agreement with the observations -made in <i>Pisum</i> is unmistakable. It is otherwise with the -exceptional cases cited. Gärtner confesses even that the -exact determination whether a form bears a greater resemblance -to one or to the other of the two original species -often involved great difficulty, so much depending upon -the subjective point of view of the observer. Another -circumstance could, however, contribute to render the -results fluctuating and uncertain, despite the most careful -observation and differentiation; for the experiments plants -were mostly used which rank as good species and are -differentiated by a large number of characters. In addition -to the sharply defined characters, where it is a question of -greater or less similarity, those characters must also be -taken into account which are often difficult to define in -words, but yet suffice, as every plant specialist knows, to -give the forms a strange appearance. If it be accepted -that the development of hybrids follows the law which is -valid for <i>Pisum</i>, the series in each separate experiment -must embrace very many forms, since the number of the -components, as is known, increases with the number of -the differentiating characters in <i>cubic ratio</i>. With a -relatively small number of experimental-plants the result -therefore could only be approximately right, and in single -cases might fluctuate considerably. If, for instance, the<span class="pagenum" id="Page_86">86</span> -two original stocks differ in seven characters, and 100 and -200 plants were raised from the seeds of their hybrids to -determine the grade of relationship of the offspring, we can -easily see how uncertain the decision must become, since -for seven differentiating characters the combination series -contains 16,384 individuals under 2187 various forms; -now one and then another relationship could assert its -predominance, just according as chance presented this or -that form to the observer in a majority of cases.</p> - -<p>If, furthermore, there appear among the differentiating -characters at the same time dominant characters, which -are transferred entire or nearly unchanged to the hybrids, -then in the terms of the developmental series that one of -the two original stocks which possesses the majority of -dominant characters must always be predominant. In the -experiment described relative to <i>Pisum</i>, in which three -kinds of differentiating characters were concerned, all the -dominant characters belonged to the seed parent. Although -the terms of the series in their internal composition -approach both original stock plants equally, in this experiment -the type of the seed parent obtained so great a -preponderance that out of each sixty-four plants of the -first generation fifty-four exactly resembled it, or only -differed in one character. It is seen how rash it may be -under such circumstances to draw from the external resemblances -of hybrids conclusions as to their internal nature.</p> - -<p>Gärtner mentions that in those cases where the development -was regular among the offspring of the hybrids the -two original species were not reproduced, but only a few -closely approximating individuals. With very extended -developmental series it could not in fact be otherwise. -For seven differentiating characters, for instance, among -more than 16,000 individuals—offspring of the hybrids—each<span class="pagenum" id="Page_87">87</span> -of the two original species would occur only once. It -is therefore hardly possible that these should appear at all -among a small number of experimental plants; with some -probability, however, we might reckon upon the appearance -in the series of a few forms which approach them.</p> - -<p>We meet with an <i>essential difference</i> in those hybrids -which remain constant in their progeny and propagate -themselves as truly as the pure species. According to -Gärtner, to this class belong the <i>remarkably fertile hybrids</i> -<i>Aquilegia atropurpurea canadensis</i>, <i>Lavatera pseudolbia -thuringiaca</i>, <i>Geum urbano-rivale</i>, and some <i>Dianthus</i> -hybrids; and, according to Wichura, the hybrids of the -Willow species. For the history of the evolution of plants -this circumstance is of special importance, since constant -hybrids acquire the status of new species. The correctness -of this is evidenced by most excellent observers, and cannot -be doubted. Gärtner had opportunity to follow up <i>Dianthus -Armeria deltoides</i> to the tenth generation, since it regularly -propagated itself in the garden.</p> - -<p>With <i>Pisum</i> it was shown by experiment that the -hybrids form egg and pollen cells of <i>different</i> kinds, and that -herein lies the reason of the variability of their offspring. -In other hybrids, likewise, whose offspring behave similarly -we may assume a like cause; for those, on the other hand, -which remain constant the assumption appears justifiable -that their fertilising cells are all alike and agree with the -foundation-cell [fertilised ovum] of the hybrid. In the -opinion of renowned physiologists, for the purpose of -propagation one pollen cell and one egg cell unite in -<span class="nowrap">Phanerogams<a id="FNanchor_46" href="#Footnote_46" class="fnanchor">46</a></span> into a single cell, which is capable by<span class="pagenum" id="Page_88">88</span> -assimilation and formation of new cells to become an -independent organism. This development follows a constant -law, which is founded on the material composition -and arrangement of the elements which meet in the cell -in a vivifying union. If the reproductive cells be of the -same kind and agree with the foundation cell [fertilised -ovum] of the mother plant, then the development of the -new individual will follow the same law which rules the -mother plant. If it chance that an egg cell unites with a -<i>dissimilar</i> pollen cell, we must then assume that between -those elements of both cells, which determine the mutual -differences, some sort of compromise is effected. The -resulting compound cell becomes the foundation of the -hybrid organism, the development of which necessarily -follows a different scheme from that obtaining in each of the -two original species. If the compromise be taken to be a -complete one, in the sense, namely, that the hybrid embryo -is formed from cells of like kind, in which the differences -are <i>entirely and permanently accommodated</i> together, the -further result follows that the hybrids, like any other stable -plant species, remain true to themselves in their offspring. -The reproductive cells which are formed in their seed<span class="pagenum" id="Page_89">89</span> -vessels and anthers are of one kind, and agree with the -fundamental compound cell [fertilised ovum].</p> - -<p>With regard to those hybrids whose progeny is <i>variable</i> -we may perhaps assume that between the differentiating -elements of the egg and pollen cells there also occurs a -compromise, in so far that the formation of a cell as -foundation of the hybrid becomes possible; but, nevertheless, -the arrangement between the conflicting elements -is only temporary and does not endure throughout the life -of the hybrid plant. Since in the habit of the plant no -changes are perceptible during the whole period of vegetation, -we must further assume that it is only possible for -the differentiating elements to liberate themselves from the -enforced union when the fertilising cells are developed. In -the formation of these cells all existing elements participate -in an entirely free and equal arrangement, in which it -is only the differentiating ones which mutually separate -themselves. In this way the production would be rendered -possible of as many sorts of egg and pollen cells as there -are combinations possible of the formative elements.</p> - -<p>The attribution attempted here of the essential difference -in the development of hybrids to <i>a permanent or temporary -union</i> of the differing cell elements can, of course, only -claim the value of an hypothesis for which the lack of -definite data offers a wide field. Some justification of the -opinion expressed lies in the evidence afforded by <i>Pisum</i> -that the behaviour of each pair of differentiating characters -in hybrid union is independent of the other differences -between the two original plants, and, further, that the -hybrid produces just so many kinds of egg and pollen -cells as there are possible constant combination forms. -The differentiating characters of two plants can finally, -however, only depend upon differences in the composition<span class="pagenum" id="Page_90">90</span> -and grouping of the elements which exist in the foundation-cells -[fertilised ova] of the same in vital <span class="nowrap">interaction<a id="FNanchor_47" href="#Footnote_47" class="fnanchor">47</a></span>.</p> - -<p>Even the validity of the law formulated for <i>Pisum</i> -requires still to be confirmed, and a repetition of the more -important experiments is consequently much to be desired, -that, for instance, relating to the composition of the hybrid -fertilising cells. A differential [element] may easily escape -the single <span class="nowrap">observer<a id="FNanchor_48" href="#Footnote_48" class="fnanchor">48</a></span>, which although at the outset may -appear to be unimportant, may yet accumulate to such -an extent that it must not be ignored in the total result. -Whether the variable hybrids of other plant species observe -an entire agreement must also be first decided experimentally. -In the meantime we may assume that in material -points a difference in principle can scarcely occur, since the -unity in the developmental plan of organic life is beyond -question.</p> - -<p>In conclusion, the experiments carried out by Kölreuter, -Gärtner, and others with respect to <i>the transformation of -one species into another by artificial fertilisation</i> merit -special mention. A special importance has been attached -to these experiments, and Gärtner reckons them among -“the most difficult of all in hybridisation.”</p> - -<p>If a species <i>A</i> is to be transformed into a species <i>B</i>, -both must be united by fertilisation and the resulting -hybrids then be fertilised with the pollen of <i>B</i>; then, out -of the various offspring resulting, that form would be -selected which stood in nearest relation to <i>B</i> and once -more be fertilised with <i>B</i> pollen, and so continuously until -finally a form is arrived at which is like <i>B</i> and constant in<span class="pagenum" id="Page_91">91</span> -its progeny. By this process the species <i>A</i> would change -into the species <i>B</i>. Gärtner alone has effected thirty such -experiments with plants of genera <i>Aquilegia</i>, <i>Dianthus</i>, -<i>Geum</i>, <i>Lavatera</i>, <i>Lychnis</i>, <i>Malva</i>, <i>Nicotiana</i>, and <i>Œnothera</i>. -The period of transformation was not alike for all species. -While with some a triple fertilisation sufficed, with others -this had to be repeated five or six times, and even in the -same species fluctuations were observed in various experiments. -Gärtner ascribes this difference to the circumstance -that “the specific [<i>typische</i>] force by which a species, during -reproduction, effects the change and transformation of the -maternal type varies considerably in different plants, and -that, consequently, the periods within which the one species -is changed into the other must also vary, as also the number -of generations, so that the transformation in some species -is perfected in more, and in others in fewer generations.” -Further, the same observer remarks “that in these transformation -experiments a good deal depends upon which type -and which individual be chosen for further transformation.”</p> - -<p>If it may be assumed that in these experiments the -constitution of the forms resulted in a similar way to that -of <i>Pisum</i>, the entire process of transformation would find -a fairly simple explanation. The hybrid forms as many -kinds of egg cells as there are constant combinations -possible of the characters conjoined therein, and one of -these is always of the same kind as the fertilising pollen -cells. Consequently there always exists the possibility with -all such experiments that even from the second fertilisation -there may result a constant form identical with that of the -pollen parent. Whether this really be obtained depends in -each separate case upon the number of the experimental -plants, as well as upon the number of differentiating -characters which are united by the fertilisation. Let us,<span class="pagenum" id="Page_92">92</span> -for instance, assume that the plants selected for experiment -differed in three characters, and the species <i>ABC</i> is to -be transformed into the other species <i>abc</i> by repeated -fertilisation with the pollen of the latter; the hybrids -resulting from the first cross form eight different kinds of -egg cells, viz.:</p> - -<p class="ml2em"><i>ABC</i>, <i>ABc</i>, <i>AbC</i>, <i>aBC</i>, <i>Abc</i>, <i>aBc</i>, <i>abC</i>, <i>abc</i>.</p> - -<p>These in the second year of experiment are united again -with the pollen cells <i>abc</i>, and we obtain the series</p> - -<p class="ml2em"><i>AaBbCc</i> + <i>AaBbc</i> + <i>AabCc</i> + <i>aBbCc</i> -+ <i>Aabc</i> + <i>aBbc</i> + <i>abCc</i> + <i>abc</i>.</p> - -<p>Since the form <i>abc</i> occurs once in the series of eight -components, it is consequently little likely that it would be -missing among the experimental plants, even were these -raised in a smaller number, and the transformation would -be perfected already by a second fertilisation. If by chance -it did not appear, then the fertilisation must be repeated -with one of those forms nearest akin, <i>Aabc</i>, <i>aBbc</i>, <i>abCc</i>. -It is perceived that such an experiment must extend the -farther <i>the smaller the number of experimental plants and -the larger the number of differentiating characters</i> in the -two original species; and that, furthermore, in the same -species there can easily occur a delay of one or even of two -generations such as Gärtner observed. The transformation -of widely divergent species could generally only be completed -in five or six years of experiment, since the number of -different egg cells which are formed in the hybrid increases -in square ratio with the number of differentiating characters.</p> - -<p>Gärtner found by repeated experiments that the respective -period of transformation varies in many species, so that -frequently a species <i>A</i> can be transformed into a species <i>B</i><span class="pagenum" id="Page_93">93</span> -a generation sooner than can species <i>B</i> into species <i>A</i>. He -deduces therefrom that Kölreuter’s opinion can hardly be -maintained that “the two natures in hybrids are perfectly -in equilibrium.” It appears, however, that Kölreuter does -not merit this criticism, but that Gärtner rather has overlooked -a material point, to which he himself elsewhere -draws attention, viz. that “it depends which individual is -chosen for further transformation.” Experiments which in -this connection were carried out with two species of <i>Pisum</i> -demonstrated that as regards the choice of the fittest -individuals for the purpose of further fertilisation it may -make a great difference which of two species is transformed -into the other. The two experimental plants differed in -five characters, while at the same time those of species <i>A</i> -were all dominant and those of species <i>B</i> all recessive. -For mutual transformation <i>A</i> was fertilised with pollen of -<i>B</i>, and <i>B</i> with pollen of <i>A</i>, and this was repeated with -both hybrids the following year. With the first experiment -<span class="nowrap"><span class="fraction2"><span class="fnum"><i>B</i></span><span class="bar">/</span><span class="fden2"><i>A</i></span></span></span> there were eighty-seven plants available in the third -year of experiment for the selections of individuals for -further crossing, and these were of the possible thirty-two -forms; with the second experiment <span class="nowrap"><span class="fraction2"><span class="fnum"><i>A</i></span><span class="bar">/</span><span class="fden2"><i>B</i></span></span></span> seventy-three plants -resulted, which <i>agreed throughout perfectly in habit with -the pollen parent</i>; in their internal composition, however, -they must have been just as varied as the forms of the -other experiment. A definite selection was consequently -only possible with the first experiment; with the second -some plants selected at random had to be excluded. Of -the latter only a portion of the flowers were crossed with -the <i>A</i> pollen, the others were left to fertilise themselves. -Among each five plants which were selected in both<span class="pagenum" id="Page_94">94</span> -experiments for fertilisation there agreed, as the following -year’s culture showed, with the pollen parent:—</p> - -<table class="ml2em"> -<tr> -<td class="tac"><div>1st Experiment.</div></td> -<td class="tac"><div> </div></td> -<td class="tac"><div>2nd Experiment.</div></td> -<td class="tac"><div> </div></td> -<td class="tac"><div> </div></td> -<td colspan="2"></td> -</tr> -<tr> -<td class="tac"><div>2 plants</div></td> -<td class="tac"></td> -<td class="tac"><div>—</div></td> -<td class="tac"></td> -<td class="tac"><div>in </div></td> -<td class="tac"><div>all </div></td> -<td class="tac"><div><span class="ilb">characters</span></div></td> -</tr> -<tr> -<td class="tac"><div>3  "  </div></td> -<td class="tac"></td> -<td class="tac"><div>—</div></td> -<td class="tac"></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>4</div></td> -<td class="tac"><div>"</div></td> -</tr> -<tr> -<td class="tac"><div>—   </div></td> -<td class="tac"></td> -<td class="tac"><div>2 plants</div></td> -<td class="tac"></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>3</div></td> -<td class="tac"><div>"</div></td> -</tr> -<tr> -<td class="tac"><div>—   </div></td> -<td class="tac"></td> -<td class="tac"><div>2  "  </div></td> -<td class="tac"></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>2</div></td> -<td class="tac"><div>"</div></td> -</tr> -<tr> -<td class="tac"><div>—   </div></td> -<td class="tac"></td> -<td class="tac"><div>1 plant </div></td> -<td class="tac"></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>1</div></td><td class="tal">character</td> -</tr> -</table> - -<p>In the first experiment, therefore, the transformation -was completed; in the second, which was not continued -further, two more fertilisations would probably have been -required.</p> - -<p>Although the case may not frequently occur that the -dominant characters belong exclusively to one or the other -of the original parent plants, it will always make a difference -which of the two possesses the majority. If the pollen parent -shows the majority, then the selection of forms for further -crossing will afford a less degree of security than in the -reverse case, which must imply a delay in the period of -transformation, provided that the experiment is only -considered as completed when a form is arrived at which -not only exactly resembles the pollen plant in form, but -also remains as constant in its progeny.</p> - -<p>Gärtner, by the results of these transformation experiments, -was led to oppose the opinion of those naturalists -who dispute the stability of plant species and believe in a -continuous evolution of vegetation. He perceives in the -complete transformation of one species into another an -indubitable proof that species are fixed within limits -beyond which they cannot change. Although this opinion -cannot be unconditionally accepted we find on the other -hand in Gärtner’s experiments a noteworthy confirmation<span class="pagenum" id="Page_95">95</span> -of that supposition regarding variability of cultivated -plants which has already been expressed.</p> - -<p>Among the experimental species there were cultivated -plants, such as <i>Aquilegia atropurpurea</i> and <i>canadensis</i>, -<i>Dianthus caryophyllus</i>, <i>chinensis</i>, and <i>japonicus</i>, <i>Nicotiana -rustica</i> and <i>paniculata</i>, and hybrids between these species -lost none of their stability after four or five <span class="nowrap">generations<a id="FNanchor_49" href="#Footnote_49" class="fnanchor">49</a></span>.</p> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_96">96</span></p> -<h2 class="nobreak" id="ON_HIERACIUM-HYBRIDS_OBTAINED_BY">ON HIERACIUM-HYBRIDS OBTAINED BY -ARTIFICIAL FERTILISATION</h2> -</div> - -<p class="tac"><span class="smcap">By G. Mendel.</span></p> - -<p class="tac fs90">(<i>Communicated to the Meeting 9 June, 1869<a id="FNanchor_50" href="#Footnote_50" class="fnanchor">50</a>.</i>)</p> - - -<p>Although I have already undertaken many experiments -in fertilisation between species of <i>Hieracium</i>, I have only -succeeded in obtaining the following 6 hybrids, and only -from one to three specimens of them.</p> - -<div class="table ml2em"> -<div class="row fs110"><div class="cell tar"><i>H. Auricula</i></div><div class="cell">♀ ×</div><div class="cell"><i>H. aurantiacum</i> ♂</div></div> -<div class="row fs110"><div class="cell tar"><i>H. Auricula</i></div><div class="cell">♀ ×</div><div class="cell"><i>H. Pilosella</i> ♂</div></div> -<div class="row fs110"><div class="cell tar"><i>H. Auricula</i></div><div class="cell">♀ ×</div><div class="cell"><i>H. pratense</i> ♂</div></div> -<div class="row fs110"><div class="cell tar"><span class="nowrap"><i>H. echioides</i><a id="FNanchor_51" href="#Footnote_51" class="fnanchor">51</a></span></div><div class="cell">♀ ×</div><div class="cell"><i>H. aurantiacum</i> ♂</div></div> -<div class="row fs110"><div class="cell tar"><i>H. præaltum</i></div><div class="cell">♀ ×</div><div class="cell"><i>H. flagellare</i> Rchb. ♂</div></div> -<div class="row fs110"><div class="cell tar"><i>H. præaltum</i></div><div class="cell">♀ ×</div><div class="cell"><i>H. aurantiacum</i> ♂</div></div> -</div> - -<p>The difficulty of obtaining a larger number of hybrids -is due to the minuteness of the flowers and their peculiar -structure. On account of this circumstance it was seldom -possible to remove the anthers from the flowers chosen for<span class="pagenum" id="Page_97">97</span> -fertilisation without either letting pollen get on to the -stigma or injuring the pistil so that it withered away. -As is well known, the anthers are united to form a tube, -which closely embraces the pistil. As soon as the flower -opens, the stigma, already covered with pollen, protrudes. -In order to prevent self-fertilisation the anther-tube must -be taken out before the flower opens, and for this purpose -the bud must be slit up with a fine needle. If this -operation is attempted at a time when the pollen is mature, -which is the case two or three days before the flower opens, -it is seldom possible to prevent self-fertilisation; for with -every care it is not easily possible to prevent a few pollen -grains getting scattered and communicated to the stigma. -No better result has been obtained hitherto by removing -the anthers at an earlier stage of development. Before the -approach of maturity the tender pistil and stigma are exceedingly -sensitive to injury, and even if they are not actually -injured, they generally wither and dry up after a little -time if deprived of their protecting investments. I hope -to obviate this last misfortune by placing the plants after -the operation for two or three days in the damp atmosphere -of a greenhouse. An experiment lately made with <i>H. -Auricula</i> treated in this way gave a good result.</p> - -<p>To indicate the object with which these fertilisation -experiments were undertaken, I venture to make some -preliminary remarks respecting the genus <i>Hieracium</i>. This -genus possesses such an extraordinary profusion of distinct -forms that no other genus of plants can compare with it. -Some of these forms are distinguished by special peculiarities -and may be taken as type-forms of species, while all the -rest represent intermediate and transitional forms by which -the type-forms are connected together. The difficulty in -the separation and delimitation of these forms has demanded<span class="pagenum" id="Page_98">98</span> -the close attention of the experts. Regarding no other -genus has so much been written or have so many and such -fierce controversies arisen, without as yet coming to a -definite conclusion. It is obvious that no general understanding -can be arrived at, so long as the value and -significance of the intermediate and transitional forms is -unknown.</p> - -<p>Regarding the question whether and to what extent -hybridisation plays a part in the production of this wealth -of forms, we find very various and conflicting views held -by leading botanists. While some of them maintain that -this phenomenon has a far-reaching influence, others, for -example, Fries, will have nothing to do with hybrids in -<i>Hieracia</i>. Others take up an intermediate position; and -while granting that hybrids are not rarely formed between -the species in a wild state, still maintain that no great -importance is to be attached to the fact, on the ground -that they are only of short duration. The [suggested] -causes of this are partly their restricted fertility or complete -sterility; partly also the knowledge, obtained by experiment, -that in hybrids self-fertilisation is always prevented if -pollen of one of the parent-forms reaches the stigma. On -these grounds it is regarded as inconceivable that <i>Hieracium</i> -hybrids can constitute and maintain themselves as fully -fertile and constant forms when growing near their progenitors.</p> - -<p>The question of the origin of the numerous and constant -intermediate forms has recently acquired no small interest -since a famous <i>Hieracium</i> specialist has, in the spirit of -the Darwinian teaching, defended the view that these -forms are to be regarded as [arising] from the transmutation -of lost or still existing species.</p> - -<p>From the nature of the subject it is clear that without<span class="pagenum" id="Page_99">99</span> -an exact knowledge of the structure and fertility of the -hybrids and the condition of their offspring through several -generations no one can undertake to determine the possible -influence exercised by hybridisation over the multiplicity -of intermediate forms in <i>Hieracium</i>. The condition of -the <i>Hieracium</i> hybrids in the range we are concerned with -must necessarily be determined by experiments; for we do -not possess a complete theory of hybridisation, and we may -be led into erroneous conclusions if we take rules deduced -from observation of certain other hybrids to be Laws of -hybridisation, and try to apply them to <i>Hieracium</i> without -further consideration. If by the experimental method we -can obtain a sufficient insight into the phenomenon of -hybridisation in <i>Hieracium</i>, then by the help of the experience -which has been collected respecting the structural -relations of the wild forms, a satisfactory judgment in -regard to this question may become possible.</p> - -<p>Thus we may express the object which was sought after -in these experiments. I venture now to relate the very -slight results which I have as yet obtained with reference -to this object.</p> - - -<p class="mt15em">1. Respecting the structure of the hybrids, we have -to record the striking phenomenon that the forms hitherto -obtained by similar fertilisation are not identical. The -hybrids <i>H. præaltum</i> ♀ x <i>H. aurantiacum</i> ♂ and <i>H. Auricula</i> -♀ x <i>H. aurantiacum</i> ♂ are each represented by two, -and <i>H. Auricula</i> ♀ x <i>H. pratense</i> ♂ by three individuals, -while as to the remainder only one of each has been -obtained.</p> - -<p>If we compare the individual characters of the hybrids -with the corresponding characters of the two parent types, -we find that they sometimes present intermediate structures,<span class="pagenum" id="Page_100">100</span> -but are sometimes so near to one of the parent characters -that the [corresponding] character of the other has receded -considerably or almost evades observation. So, for instance, -we see in one of the two forms of <i>H. Auricula</i> ♀ x <i>H. -aurantiacum</i> ♂ pure yellow disc-florets; only the petals -of the marginal florets are on the outside tinged with red -to a scarcely noticeable degree: in the other on the contrary -the colour of these florets comes very near to <i>H. aurantiacum</i>, -only in the centre of the disc the orange red passes into a -deep golden-yellow. This difference is noteworthy, for the -flower-colour in <i>Hieracium</i> has the value of a constant -character. Other similar cases are to be found in the -leaves, the peduncles, &c.</p> - -<p>If the hybrids are compared with the parent types as -regards the sum total of their characters, then the two -forms of <i>H. præaltum</i> ♀ x <i>H. aurantiacum</i> ♂ constitute -approximately intermediate forms which do not agree in -certain characters. On the contrary in <i>H. Auricula</i> ♀ x <i>H. -aurantiacum</i> ♂ and in <i>H. Auricula</i> ♀ x <i>H. pratense</i> ♂ we -see the forms widely divergent, so that one of them is -nearer to the one and the other to the other parental type, -while in the case of the last-named hybrid there is still a -third which is almost precisely intermediate between them.</p> - -<p>The conviction is then forced on us that we have here -only single terms in an unknown series which may be -formed by the direct action of the pollen of one species on -the egg-cells of another.</p> - - -<p class="mt15em">2. With a single exception the hybrids in question -form seeds capable of germination. <i>H. echioides</i> ♀ x <i>H. -aurantiacum</i> ♂ may be described as fully fertile; <i>H. præaltum</i> -♀ x <i>H. flagellare</i> ♂ as fertile; <i>H. præaltum</i> ♀ x <i>H. -aurantiacum</i> ♂ and <i>H. Auricula</i> ♀ x <i>H. pratense</i> ♂ as<span class="pagenum" id="Page_101">101</span> -partially fertile; <i>H. Auricula</i> ♀ x <i>H. Pilosella</i> ♂ as slightly -fertile, and <i>H. Auricula</i> ♀ x <i>H. aurantiacum</i> ♂ as unfertile. -Of the two forms of the last named hybrid, the red-flowered -one was completely sterile, but from the yellow-flowered -one a single well-formed seed was obtained. Moreover it -must not pass unmentioned that among the seedlings of the -partially fertile hybrid <i>H. præaltum</i> ♀ x <i>H. aurantiacum</i> ♂ -there was one plant which possessed full fertility.</p> - - -<p class="mt15em">[3.] As yet the offspring produced by self-fertilisation -of the hybrids have not varied, but agree in their characters -both with each other and with the hybrid plant from which -they were derived.</p> - -<p>From <i>H. præaltum</i> ♀ x <i>H. flagellare</i> ♂ two generations -have flowered; from <i>H. echioides</i> ♀ x <i>H. aurantiacum</i> ♂, -<i>H. præaltum</i> ♀ x <i>H. aurantiacum</i> ♂, <i>H. Auricula</i> ♀ x <i>H. -Pilosella</i> ♂ one generation in each case has flowered.</p> - - -<p class="mt15em">4. The fact must be declared that in the case of the -fully fertile hybrid <i>H. echioides</i> ♀ x <i>H. aurantiacum</i> ♂ the -pollen of the parent types was not able to prevent self-fertilisation, -though it was applied in great quantity to the -stigmas protruding through the anther-tubes when the -flowers opened.</p> - -<p>From two flower-heads treated in this way seedlings -were produced resembling this hybrid plant. A very -similar experiment, carried out this summer with the -partially fertile <i>H. præaltum</i> ♀ x <i>H. aurantiacum</i> ♂ led to -the conclusion that those flower-heads in which pollen -of the parent type or of some other species had been -applied to the stigmas, developed a notably larger number -of seeds than those which had been left to self-fertilisation -alone. The explanation of this result must only be sought -in the circumstance that as a large part of the pollen-grains<span class="pagenum" id="Page_102">102</span> -of the hybrid, examined microscopically, show a defective -structure, a number of egg-cells capable of fertilisation do -not become fertilised by their own pollen in the ordinary -course of self-fertilisation.</p> - -<p>It not rarely happens that in fully fertile species in the -wild state the formation of the pollen fails, and in many -anthers not a single good grain is developed. If in these -cases seeds are nevertheless formed, such fertilisation must -have been effected by foreign pollen. In this way hybrids -may easily arise by reason of the fact that many forms -of insects, notably the industrial Hymenoptera, visit the -flowers of <i>Hieracia</i> with great zeal and are responsible for -the pollen which easily sticks to their hairy bodies reaching -the stigmas of neighbouring plants.</p> - -<p>From the few facts that I am able to contribute it -will be evident the work scarcely extends beyond its first -inception. I must express some scruple in describing in -this place an account of experiments just begun. But the -conviction that the prosecution of the proposed experiments -will demand a whole series of years, and the uncertainty -whether it will be granted to me to bring the same to a -conclusion have determined me to make the present -communication. By the kindness of Dr Nägeli, the -Munich Director, who was good enough to send me species -which were wanting, especially from the Alps, I am in a -position to include a larger number of forms in my -experiments. I venture to hope even next year to be able -to contribute something more by way of extension and confirmation -of the present account.</p> - -<p>If finally we compare the described result, still very -uncertain, with those obtained by crosses made between -forms of <i>Pisum</i>, which I had the honour of communicating -in the year 1865, we find a very real distinction.<span class="pagenum" id="Page_103">103</span> -In <i>Pisum</i> the hybrids, obtained from the immediate -crossing of two forms, have in all cases the same type, -but their posterity, on the contrary, are variable and -follow a definite law in their variations. In <i>Hieracium</i> -according to the present experiments the exactly opposite -phenomenon seems to be exhibited. Already in describing -the <i>Pisum</i> experiments it was remarked that there are -also hybrids whose posterity do not vary, and that, for -example, according to Wichura the hybrids of <i>Salix</i> -reproduce themselves like pure species. In <i>Hieracium</i> -we may take it we have a similar case. Whether from -this circumstance we may venture to draw the conclusion -that the polymorphism of the genera <i>Salix</i> and <i>Hieracium</i> -is connected with the special condition of their hybrids is -still an open question, which may well be raised but not -as yet answered.</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_104">104</span></p> - -<h2 class="nobreak" id="A_DEFENCE_OF_MENDELS_PRINCIPLES">A DEFENCE OF MENDEL’S PRINCIPLES -OF HEREDITY.</h2> -</div> - -<p class="fs90 pl2hi2 mrl10">“<i>The most fertile men of science have made blunders, and their -consciousness of such slips has been retribution enough; it is -only their more sterile critics who delight to dwell too often -and too long on such mistakes.</i>” <span class="smcap">Biometrika</span>, 1901.</p> - - -<h3><span class="smcap">Introductory.</span></h3> - -<p>On the rediscovery and confirmation of Mendel’s Law by -de Vries, Correns, and Tschermak two years ago, it became -clear to many naturalists, as it certainly is to me, that we -had found a principle which is destined to play a part in -the Study of Evolution comparable only with the achievement -of Darwin—that after the weary halt of forty years -we have at last begun to march.</p> - -<p>If we look back on the post-Darwinian period we -recognize one notable effort to advance. This effort—fruitful -as it proved, memorable as it must ever be—was -that made by Galton when he enuntiated his Law of -Ancestral Heredity, subsequently modified and restated -by Karl Pearson. Formulated after long and laborious -inquiry, this principle beyond question gives us an -expression including and denoting many phenomena in -which previously no regularity had been detected. But<span class="pagenum" id="Page_105">105</span> -to practical naturalists it was evident from the first that -there are great groups of facts which could not on any -interpretation be brought within the scope of Galton’s -Law, and that by no emendation could that Law be -extended to reach them. The existence of these phenomena -pointed to a different physiological conception of -heredity. Now it is precisely this conception that Mendel’s -Law enables us to form. Whether the Mendelian principle -can be extended so as to include some apparently Galtonian -cases is another question, respecting which we have as yet -no facts to guide us, but we have certainly no warrant for -declaring such an extension to be impossible.</p> - -<p>Whatever answer the future may give to that question, -it is clear from this moment that every case which obeys -the Mendelian principle is removed finally and irretrievably -from the operations of the Law of Ancestral Heredity.</p> - -<p>At this juncture Professor Weldon intervenes as a -professed exponent of Mendel’s work. It is not perhaps -to a devoted partisan of the Law of Ancestral Heredity -that we should look for the most appreciative exposition of -Mendel, but some bare measure of care and accuracy in -representation is demanded no less in justice to fine work, -than by the gravity of the issue.</p> - -<p>Professor Weldon’s article appears in the current number -of <i>Biometrika</i>, Vol. I. Pt. <span class="lowercase smcap">II.</span> which reached me on Saturday, -Feb. 8. The paper opens with what purports to be a -restatement of Mendel’s experiments and results. In this -“restatement” a large part of Mendel’s experiments—perhaps -the most significant—are not referred to at all. -The perfect simplicity and precision of Mendel’s own -account are destroyed; with the result that the reader of -Professor Weldon’s paper, unfamiliar with Mendel’s own -memoir, can scarcely be blamed if he fail to learn the<span class="pagenum" id="Page_106">106</span> -essence of the discovery. Of Mendel’s conception of the -hybrid as a distinct entity with characters proper to itself, -apart from inheritance—the most novel thing in the -whole paper—Professor Weldon gives no word. Upon this -is poured an undigested mass of miscellaneous “facts” -and statements from which the reader is asked to conclude, -first, that a proposition attributed to Mendel regarding -dominance of one character is not of “general<span class="nowrap">”<a id="FNanchor_52" href="#Footnote_52" class="fnanchor">52</a></span> application, -and finally that “all work based on Mendel’s method” is -“vitiated” by a “fundamental mistake,” namely “the -neglect of <span class="nowrap">ancestry<a id="FNanchor_53" href="#Footnote_53" class="fnanchor">53</a></span>.”</p> - -<p>To find a parallel for such treatment of a great theme -in biology we must go back to those writings of the orthodox -which followed the appearance of the “Origin of Species.”</p> - -<p>On 17th December 1900 I delivered a Report to the -Evolution Committee of the Royal Society on the experiments -in Heredity undertaken by Miss E. R. Saunders and -myself. This report has been offered to the Society for -publication and will I understand shortly appear. In it we -have attempted to show the extraordinary significance of -Mendel’s principle, to point out what in his results is -essential and what subordinate, the ways in which the -principle can be extended to apply to a diversity of more -complex phenomena—of which some are incautiously cited<span class="pagenum" id="Page_107">107</span> -by Professor Weldon as conflicting facts—and lastly to -suggest a few simple terms without which (or some equivalents) -the discussion of such phenomena is difficult. -Though it is impossible here to give an outline of facts and -reasoning there set out at length, I feel that his article -needs an immediate reply. Professor Weldon is credited -with exceptional familiarity with these topics, and his paper -is likely to be accepted as a sufficient statement of the case. -Its value will only be known to those who have either -worked in these fields themselves or have been at the -trouble of thoughtfully studying the original materials.</p> - -<p>The nature of Professor Weldon’s article may be most -readily indicated if I quote the summary of it issued in a -paper of abstracts sent out with Review copies of the Part. -This paper was most courteously sent to me by an editor -of <i>Biometrika</i> in order to call my attention to the article -on Mendel, a subject in which he knew me to be interested. -The abstract is as follows.</p> - - -<div class="blockquot"> -<p>“Few subjects have excited so much interest in the last -year or two as the laws of inheritance in hybrids. Professor -W. F. R. Weldon describes the results obtained by Mendel by -crossing races of Peas which differed in one or more of seven -characters. From a study of the work of other observers, and -from examination of the ‘Telephone’ group of hybrids, the -conclusion is drawn that Mendel’s results do not justify any -general statement concerning inheritance in cross-bred Peas. A -few striking cases of other cross-bred plants and animals are -quoted to show that the results of crossing cannot, as Mendel -and his followers suggest, be predicted from a knowledge of the -characters of the two parents crossed without knowledge of the -more remote ancestry.”</p> -</div> - -<p>Such is the judgment a fellow-student passes on this -mind</p> - -<p class="ml2em fs90"> -“<i>Voyaging through strange seas of thought alone.</i>”<br /> -</p> - -<p><span class="pagenum" id="Page_108">108</span></p> - -<p>The only conclusion which most readers could draw -from this abstract and indeed from the article it epitomizes, -is that Mendel’s discovery so far from being of -paramount importance, rests on a basis which Professor -Weldon has shown to be insecure, and that an error has -come in through disregard of the law of Ancestral Heredity. -On examining the paper it is perfectly true that Professor -Weldon is careful nowhere directly to question Mendel’s -facts or his interpretation of them, for which indeed in -some places he even expresses a mild enthusiasm, but there -is no mistaking the general purpose of the paper. It must -inevitably produce the impression that the importance of -the work has been greatly exaggerated and that supporters -of current views on Ancestry may reassure themselves. -That this is Professor Weldon’s own conclusion in the -matter is obvious. After close study of his article it is -evident to me that Professor Weldon’s criticism is baseless -and for the most part irrelevant, and I am strong in the -conviction that the cause which will sustain damage from -this debate is not that of Mendel.</p> - - -<h3>I. <span class="smcap">The Mendelian Principle of Purity of Germ-Cells -and the Laws of Heredity Based on Ancestry.</span></h3> - -<p>Professor Weldon’s article is entitled “Mendel’s Laws -of Alternative Inheritance in Peas.” This title expresses -the scope of Mendel’s work and discovery none too -precisely and even exposes him to distinct misconception.</p> - -<p>To begin with, it says both too little and too much. -Mendel did certainly determine Laws of Inheritance in<span class="pagenum" id="Page_109">109</span> -peas—not precisely the laws Professor Weldon has been -at the pains of drafting, but of that anon. Having done -so, he knew what his discovery was worth. He saw, and -rightly, that he had found a principle which <i>must</i> govern -a wide area of phenomena. He entitles his paper therefore -“<i>Versuche über Pflanzen-Hybriden</i>,” or, Experiments in -Plant-Hybridisation.</p> - -<p>Nor did Mendel start at first with any particular -intention respecting Peas. He tells us himself that he -wanted to find the laws of inheritance in <i>hybrids</i>, which -he suspected were definite, and that after casting about -for a suitable subject, he found one in peas, for the reasons -he sets out.</p> - -<p>In another respect the question of title is much more -important. By the introduction of the word “Alternative” -the suggestion is made that the Mendelian principle applies -peculiarly to cases of “alternative” inheritance. Mendel -himself makes no such limitation in his earlier paper, -though perhaps by rather remote implication in the second, -to which the reader should have been referred. On the -contrary, he wisely abstains from prejudicial consideration -of unexplored phenomena.</p> - -<p class="mt15em">To understand the significance of the word “alternative” -as introduced by Professor Weldon we must go back a -little in the history of these studies. In the year 1897 -Galton formally announced the Law of Ancestral Heredity -referred to in the <i>Introduction</i>, having previously “stated -it briefly and with hesitation” in <i>Natural Inheritance</i>, -p. 134. In 1898 Professor Pearson published his modification -and generalisation of Galton’s Law, introducing a -correction of admitted theoretical importance, though it is -not in question that the principle thus restated is fundamentally<span class="pagenum" id="Page_110">110</span> -not very different from Galton’<span class="nowrap">s<a id="FNanchor_54" href="#Footnote_54" class="fnanchor">54</a></span>. <i>It is an -essential part of the Galton-Pearson Law of Ancestral -Heredity that in calculating the probable structure of each -descendant the structure of each several ancestor must be -brought to account.</i></p> - -<p>Professor Weldon now tells us that these two papers -of Galton and of Professor Pearson have “given us an -expression for the effects of <i>blended</i> inheritance which -seems likely to prove generally applicable, though the -constants of the equations which express the relation -between divergence from the mean in one generation, and -that in another, may require modification in special cases. -Our knowledge of <i>particulate</i> or mosaic inheritance, and of -<i>alternative</i> inheritance, is however still rudimentary, and -there is so much contradiction between the results obtained -by different observers, that the evidence available is difficult -to appreciate.”</p> - -<p>But Galton stated (p. 401) in 1897 that his statistical -law of heredity “appears to be universally applicable to -bi-sexual descent.” Pearson in re-formulating the principle -in 1898 made no reservation in regard to “alternative” -inheritance. On the contrary he writes (p. 393) that “if -Mr Galton’s law can be firmly established, <i>it is a complete -solution, at any rate to a first approximation, of the whole -problem of heredity</i>,” and again (p. 412) that “it is highly -probable that it [this law] is the simple descriptive statement<span class="pagenum" id="Page_111">111</span> -which brings into a single focus all the complex -lines of hereditary influence. If Darwinian evolution be -natural selection combined with <i>heredity</i>, then the single -statement which embraces the whole field of heredity must -prove almost as epoch-making as the law of gravitation -to the <span class="nowrap">astronomer<a id="FNanchor_55" href="#Footnote_55" class="fnanchor">55</a></span>.”</p> - -<p>As I read there comes into my mind that other fine -passage where Professor Pearson warns us</p> - -<div class="blockquot"> -<p>“There is an insatiable desire in the human breast -to resume in some short formula, some brief -statement, the facts of human experience. It leads -the savage to ‘account’ for all natural phenomena -by deifying the wind and the stream and the tree. -It leads civilized man, on the other hand, to express -his emotional experience in works of art, and his -physical and mental experience in the formulae or -so-called laws of <span class="nowrap">science<a id="FNanchor_56" href="#Footnote_56" class="fnanchor">56</a></span>.”</p> -</div> - -<p>No naturalist who had read Galton’s paper and had -tried to apply it to the facts he knew could fail to see -that here was a definite advance. We could all perceive -phenomena that were in accord with it and there was no -reasonable doubt that closer study would prove that accord -to be close. It was indeed an occasion for enthusiasm, -though no one acquainted with the facts of experimental -breeding could consider the suggestion of universal application -for an instant.</p> - -<p><span class="pagenum" id="Page_112">112</span></p> - -<p>But two years have gone by, and in 1900 Pearson -<span class="nowrap">writes<a id="FNanchor_57" href="#Footnote_57" class="fnanchor">57</a></span> that the values obtained from the Law of Ancestral -Heredity</p> - -<div class="blockquot"> -<p>“seem to fit the observed facts fairly well in the case of -<i>blended</i> inheritance. In other words we have a -certain amount of evidence in favour of the -conclusion: <i>That whenever the sexes are equipotent, -blend their characters and mate pangamously, all -characters will be inherited at the same rate</i>,”</p> -</div> - -<p>or, again in other words, that the Law of Ancestral Heredity -after the glorious launch in 1898 has been home for a -complete refit. The top-hamper is cut down and the vessel -altogether more manageable; indeed she looks trimmed -for most weathers. Each of the qualifications now introduced -wards off whole classes of dangers. Later on (pp. -487–8) Pearson recites a further list of cases regarded as -exceptional. “All characters will be inherited at the same -rate” might indeed almost be taken to cover the results in -Mendelian cases, though the mode by which those results -are arrived at is of course wholly different.</p> - -<p>Clearly we cannot speak of the Law of Gravitation now. -Our Tycho Brahe and our Kepler, with the yet more distant -Newton, are appropriately named as yet to <span class="nowrap">come<a id="FNanchor_58" href="#Footnote_58" class="fnanchor">58</a></span>.</p> - -<p>But the truth is that even in 1898 such a comparison -was scarcely happy. Not to mention moderns, these high -hopes had been finally disposed of by the work of the -experimental breeders such as Kölreuter, Knight, Herbert, -Gärtner, Wichura, Godron, Naudin, and many more. To -have treated as non-existent the work of this group of -naturalists, who alone have attempted to solve the problems<span class="pagenum" id="Page_113">113</span> -of heredity and species—Evolution, as we should now say—by -the only sound method—<i>experimental breeding</i>—to -leave out of consideration almost the whole block of -evidence collected in <i>Animals and Plants</i>—Darwin’s finest -legacy as I venture to declare—was unfortunate on the -part of any exponent of Heredity, and in the writings of a -professed naturalist would have been unpardonable. But -even as modified in 1900 the Law of Ancestral Heredity -is heavily over-sparred, and any experimental breeder could -have increased Pearson’s list of unconformable cases by as -many again.</p> - -<p>But to return to Professor Weldon. He now repeats -that the Law of Ancestral Heredity seems likely to prove -generally applicable to <i>blended</i> inheritance, but that the -case of <i>alternative</i> inheritance is for the present reserved. -We should feel more confidence in Professor Weldon’s -exposition if he had here reminded us that the special -case which fitted Galton’s Law so well that it emboldened -him to announce that principle as apparently “universally -applicable to bi-sexual descent” was one of <i>alternative</i> -inheritance—namely the coat-colour of Basset-hounds. -Such a fact is, to say the least, ominous. Pearson, in -speaking (1900) of this famous case of Galton’s, says that -these phenomena of alternative inheritance must be treated -separately (from those of blended inheritance<span class="nowrap">)<a id="FNanchor_59" href="#Footnote_59" class="fnanchor">59</a></span>, and for -them he deduces a proposed “<i>law of reversion</i>,” based of -course on ancestry. He writes, “In both cases we may -speak of a law of ancestral heredity, but the first predicts -the probable character of the individual produced by a<span class="pagenum" id="Page_114">114</span> -given ancestry, while the second tells us the percentages -of the total offspring which on the average revert to each -ancestral <span class="nowrap">type<a id="FNanchor_60" href="#Footnote_60" class="fnanchor">60</a></span>.”</p> - -<p>With the distinctions between the original Law of -Ancestral Heredity, the modified form of the same law, -and the Law of Reversion, important as all these considerations -are, we are not at present concerned.</p> - -<p>For the Mendelian principle of heredity asserts a -proposition absolutely at variance with all the laws of -ancestral heredity, however formulated. In those cases to -which it applies strictly, this principle declares that the -cross-breeding of parents <i>need</i> not diminish the purity of -their germ-cells or consequently the purity of their offspring. -When in such cases individuals bearing opposite -characters, <i>A</i> and <i>B</i>, are crossed, the germ-cells of the -resulting cross-bred, <i>AB</i>, are each to be bearers either -of character A or of character <i>B</i>, not both.</p> - -<p>Consequently when the cross-breds breed either together -or with the pure forms, individuals will result of the forms -<i>AA</i>, <i>AB</i>, <i>BA</i>, <span class="nowrap"><i>BB</i><a id="FNanchor_61" href="#Footnote_61" class="fnanchor">61</a></span>. Of these the forms <i>AA</i> and <i>BB</i>, -formed by the union of similar germs, are stated to be as -pure as if they had had no cross in their pedigree, and -henceforth their offspring will be no more likely to depart -from the <i>A</i> type or the <i>B</i> type respectively, than those of -any other originally pure specimens of these types.</p> - -<p>Consequently in such examples it is <i>not</i> the fact that -each ancestor must be brought to account as the Galton-Pearson -Law asserts, and we are clearly dealing with a -physiological phenomenon not contemplated by that Law -at all.</p> - -<p><span class="pagenum" id="Page_115">115</span></p> - -<p>Every case therefore which obeys the Mendelian principle -is in direct contradiction to the proposition to which Professor -Weldon’s school is committed, and it is natural that -he should be disposed to consider the Mendelian principle -as applying especially to “alternative” inheritance, while -the law of Galton and Pearson is to include the phenomenon -of blended inheritance. The latter, he tells us, is “the -most usual case,” a view which, if supported by evidence, -might not be without value.</p> - -<p>It is difficult to blame those who on first acquaintance -concluded Mendel’s principle can have no strict application -save to alternative inheritance. Whatever blame there is -in this I share with Professor Weldon and those whom he -follows. Mendel’s own cases were almost all alternative; -also the fact of dominance is very dazzling at first. But -that was two years ago, and when one begins to see clearly -again, it does not look so certain that the real essence of -Mendel’s discovery, the purity of germ-cells in respect of -certain characters, may not apply also to some phenomena -of blended inheritance. The analysis of this possibility -would take us to too great length, but I commend to those -who are more familiar with statistical method, the consideration -of this question: whether dominance being absent, -indefinite, or suppressed, the phenomena of heritages -completely blended in the zygote, may not be produced -by gametes presenting Mendelian purity of characters. -A brief discussion of this possibility is given in the -Introduction, p. <a href="#Page_31">31</a>.</p> - -<p>Very careful inquiry would be needed before such a -possibility could be negatived. For example, we know -that the Laws based on Ancestry can apply to <i>alternative</i> -inheritance; witness the case of the Basset-hounds. Here -there is no simple Mendelian dominance; but are we sure<span class="pagenum" id="Page_116">116</span> -there is no purity of germ-cells? The new conception goes -a long way and it may well reach to such facts as these.</p> - -<p>But for the present we will assume that Mendel’s -principle applies only to <i>certain phenomena of alternative -inheritance</i>, which is as far as our warrant yet runs.</p> - -<p>No close student of the recent history of evolutionary -thought needs to be told what the attitude of Professor -Weldon and his followers has been towards these same -disquieting and unwelcome phenomena of alternative -inheritance and discontinuity in variation. Holding at -first each such fact for suspect, then treating them as rare -and negligible occurrences, he and his followers have of -late come slowly to accede to the facts of discontinuity a -bare and grudging recognition in their scheme of <span class="nowrap">evolution<a id="FNanchor_62" href="#Footnote_62" class="fnanchor">62</a></span>.</p> - -<p>Therefore on the announcement of that discovery which -once and for all ratifies and consolidates the conception of -discontinuous variation, and goes far to define that of -alternative inheritance, giving a finite body to what before -was vague and tentative, it is small wonder if Professor -Weldon is disposed to criticism rather than to cordiality.</p> - - -<p class="mt15em">We have now seen what is the essence of Mendel’s -discovery based on a series of experiments of unequalled -simplicity which Professor Weldon does not venture to -dispute.</p> - -<p><span class="pagenum" id="Page_117">117</span></p> - - -<h3>II. <span class="smcap">Mendel and the Critic’s Version of him.</span></h3> - -<p class="tac mtb1em"><i>The “Law of Dominance.”</i></p> - -<p>I proceed to the question of dominance which Professor -Weldon treats as a prime issue, almost to the virtual concealment -of the great fact of gametic purity.</p> - -<p>Cross-breds in general, <i>AB</i> and <i>BA</i>, named above, -may present many appearances. They may all be indistinguishable -from <i>A</i>, or from <i>B</i>; some may appear <i>A</i>’s -and some <i>B</i>’s; they may be patchworks of both; they may -be blends presenting one or many grades between the two; -and lastly they <i>may have an appearance special to themselves</i> -(<i>being in the latter case, as it often happens, “reversionary”</i>), -a possibility which Professor Weldon does not stop to -consider, though it is the clue that may unravel many -of the facts which mystify him now.</p> - -<p>Mendel’s discovery became possible because he worked -with regular cases of the first category, in which he was able -to recognize that <i>one</i> of each of the pairs of characters -he studied <i>did</i> thus prevail and <i>was</i> “dominant” in the -cross-bred to the exclusion of the other character. This -fact, which is still an accident of particular cases, Professor -Weldon, following some of Mendel’s interpreters, dignifies -by the name of the “Law of Dominance,” though he -omits to warn his reader that Mendel states no “Law of -Dominance” whatever. The whole question whether one or -other character of the antagonistic pair is dominant though -of great importance is logically a subordinate one. It -depends on the specific nature of the varieties and individuals -used, sometimes probably on the influence of<span class="pagenum" id="Page_118">118</span> -external conditions and on other factors we cannot now -discuss. There is as yet no universal law here perceived -or declared.</p> - -<p>Professor Weldon passes over the proof of the purity -of the germ-cells lightly enough, but this proposition of -dominance, suspecting its weakness, he puts prominently -forward. Briefest equipment will suffice. Facing, as he -supposes, some new pretender—some local Theudas—offering -the last crazy prophecy,—any argument will do -for such an one. An eager gathering in an unfamiliar -literature, a scrutiny of samples, and he will prove to -us with small difficulty that dominance of yellow over -green, and round over wrinkled, is irregular even in peas -after all; that in the sharpness of the discontinuity exhibited -by the variations of peas there are many grades; -that many of these grades co-exist in the same variety; -that some varieties may perhaps be normally intermediate. -All these propositions are supported by the production -of a collection of evidence, the quality of which we -shall hereafter consider. “Enough has been said,” he -writes (p. 240), “to show the grave discrepancy between the -evidence afforded by Mendel’s own experiments and that -obtained by other observers, equally competent and trustworthy.”</p> - -<p>We are asked to believe that Professor Weldon has -thus discovered “a fundamental mistake” vitiating all that -work, the importance of which, he elsewhere tells us, he -has “no wish to belittle.”</p> - -<p><span class="pagenum" id="Page_119">119</span></p> - - -<h3>III. <span class="smcap">The Facts in regard to Dominance of -Characters in Peas.</span></h3> - -<p>Professor Weldon refers to no experiments of his own -and presumably has made none. Had he done so he would -have learnt many things about dominance in peas, whether -of the yellow cotyledon-colour or of the round form, that -might have pointed him to caution.</p> - -<p>In the year 1900 Messrs Vilmorin-Andrieux & Co. were -kind enough to send to the Cambridge Botanic Garden on -my behalf a set of samples of the varieties of <i>Pisum</i> and -<i>Phaseolus</i>, an exhibit of which had greatly interested me -at the Paris Exhibition of that year. In the past summer -I grew a number of these and made some preliminary -cross-fertilizations among them (about 80 being available -for these deductions) with a view to a future study of -certain problems, Mendelian and others. In this work -I had the benefit of the assistance of Miss Killby of -Newnham College. Her cultivations and crosses were -made independently of my own, but our results are almost -identical. The experience showed me, what a naturalist -would expect and practical men know already, that <i>a great -deal turns on the variety used</i>; that some varieties are -very sensitive to conditions while others maintain their -type sturdily; that in using certain varieties Mendel’s -experience as to dominance is regularly fulfilled, while in -the case of other varieties irregularities and even some -contradictions occur. That the dominance of yellow -cotyledon-colour over green, and the dominance of the -smooth form over the wrinkled, is a <i>general</i> truth for -<i>Pisum sativum</i> appears at once; that it is a universal -truth I cannot believe any competent naturalist would -imagine, still less assert. Mendel certainly never did.<span class="pagenum" id="Page_120">120</span> -When he speaks of the “law” or “laws” that he has -established for <i>Pisum</i> he is referring to his own discovery -of the purity of the germ-cells, that of the statistical -distribution of characters among them, and the statistical -grouping of the different germ-cells in fertilization, and -not to the “Law of Dominance” which he never drafted -and does not propound.</p> - -<p>The issue will be clearer if I here state briefly what, as -far as my experience goes, are the facts in regard to the -characters <i>cotyledon-colour</i> and <i>seed-shapes</i> in peas. I have -not opportunity for more than a passing consideration of -the <i>seed-coats</i> of pure <span class="nowrap">forms<a id="FNanchor_63" href="#Footnote_63" class="fnanchor">63</a></span>; that is a maternal character, -a fact I am not sure Professor Weldon fully appreciates. -Though that may be incredible, it is evident from many -passages that he has not, in quoting authorities, considered -the consequences of this circumstance.</p> - - -<p class="tac mtb1em"><i>The normal characters: colour of cotyledons -and seed-coats.</i></p> - -<p>Culinary peas (<i>P. sativum</i>, omitting purple sorts) can -primarily be classified on colour into two groups, yellow -and green. In the green certain pigmentary matters -persist in the ripe seed which disappear or are decomposed -in the yellow as the seed ripens. But it may be observed<span class="pagenum" id="Page_121">121</span> -that the “green” class itself is treated as of two -divisions, <i>green</i> and <i>blue</i>. In the seedsmen’s lists the -classification is made on the <i>external appearance</i> of the -seed, without regard to whether the colour is due to the -seed-coat, the cotyledons, or both. As a rule perhaps -yellow coats contain yellow cotyledons, and green coats -green cotyledons, though yellow cotyledons in green coats -are common, e.g. <i>Gradus</i>, of which the cotyledons are yellow -while the seed-coats are about as often green as yellow (or -“white,” as it is called technically). Those called “blue” -consist mostly of seeds which have green cotyledons seen -through transparent skins, or yellow cotyledons combined -with green skins. The skins may be roughly classified into -thin and transparent, or thick and generally at some stage -pigmented. In numerous varieties the colour of the cotyledon -is wholly yellow, or wholly green. Next there are -many varieties which are constant in habit and other -properties but have seeds belonging to these two colour -categories in various proportions. How far these proportions -are known to be constant I cannot ascertain.</p> - -<p>Of such varieties showing mixture of <i>cotyledon</i>-colours -nearly all can be described as dimorphic in colour. For -example in Sutton’s <i>Nonpareil Marrowfat</i> the cotyledons -are almost always <i>either</i> yellow <i>or</i> green, with some piebalds, -and the colours of the seed-coats are scarcely less distinctly -dimorphic. In some varieties which exist in both colours -intermediates are so common that one cannot assert any -regular <span class="nowrap">dimorphism<a id="FNanchor_64" href="#Footnote_64" class="fnanchor">64</a></span>.</p> -<p><span class="pagenum" id="Page_122">122</span></p> -<p>There are some varieties which have cotyledons green -and intermediate shading to greenish yellow, like <i>Stratagem</i> -quoted by Professor Weldon. Others have yellow and -intermediate shading to yellowish green, such as McLean’s -<i>Best of all</i><a id="FNanchor_65" href="#Footnote_65" class="fnanchor">65</a>. I am quite disposed to think there may be -truly monomorphic varieties with cotyledons permanently -of intermediate colour only, but so far I have not seen -<span class="nowrap">one<a id="FNanchor_66" href="#Footnote_66" class="fnanchor">66</a></span>. The variety with greatest <i>irregularity</i> (apart from -regular dimorphism) in cotyledon-colour I have seen is a -sample of “<i>mange-tout à rames, à grain vert</i>,” but it was a -good deal injured by weevils (<i>Bruchus</i>), which always cause -irregularity or change of colour.</p> - -<p>Lastly in some varieties there are many piebalds or -mosaics.</p> - -<p>From what has been said it will be evident that the -description of a pea in an old book as having been green, -blue, white, and so forth, unless the cotyledon-colour is -distinguished from seed-coat colour, needs careful consideration -before inferences are drawn from it.</p> - -<p class="tac mtb1em"><i>Shape.</i></p> - -<p>In regard to shape, if we keep to ordinary shelling peas, -the facts are somewhat similar, but as shape is probably -more sensitive to conditions than cotyledon-colour (not -than <i>seed-coat</i> colour) there are irregularities to be perhaps -ascribed to this cause. Broadly, however, there are two -main divisions, round and wrinkled. It is unquestioned -that between these two types every intermediate occurs.<span class="pagenum" id="Page_123">123</span> -Here again a vast number of varieties can be at once -classified into round and wrinkled (the classification -commonly used), others are intermediate normally. Here -also I suspect some fairly clear sub-divisions might be -made in the wrinkled group and in the round group too, -but I would not assert this as a fact.</p> - -<p>I cannot ascertain from botanists what is the nature of -the difference between round and wrinkled peas, though no -doubt it will be easily discovered. In maize the round -seeds contain much unconverted starch, while in the -wrinkled or sugar-maize this seems to be converted in -great measure as the seed ripens; with the result that, -on drying, the walls collapse. In such seeds we may -perhaps suppose that the process of conversion, which in -round seeds takes place on germination, is begun earlier, -and perhaps the variation essentially consists in the premature -appearance of the converting ferment. It would be -most rash to suggest that such a process may be operating -in the pea, for the phenomenon may have many causes; -but however that may be, there is evidently a difference of -such a nature that when the water dries out of the seed on -ripening, its walls <span class="nowrap">collapse<a id="FNanchor_67" href="#Footnote_67" class="fnanchor">67</a></span>; and this collapse may occur -in varying degrees.</p> - -<p><span class="pagenum" id="Page_124">124</span></p> - -<p>In respect of <i>shape</i> the seeds of a variety otherwise -stable are as a rule fairly uniform, the co-existence of -both shapes and of intermediates between them in the -same variety is not infrequent. As Professor Weldon has -said, <i>Telephone</i> is a good example of an extreme case of -mixture of both colours and shapes. <i>William I.</i> is another. -It may be mentioned that regular dimorphism in respect -of shape is not so common as dimorphism in respect -of colour. Of great numbers of varieties seen at Messrs -Suttons’ I saw none so distinctly dimorphic in shape as -<i>William I.</i> which nevertheless contains all grades commonly.</p> - -<p>So far I have spoken of the shapes of ordinary English -culinary peas. But if we extend our observations to the -shapes of <i>large-seeded</i> peas, which occur for the most part -among the sugar-peas (<i>mange-touts</i>), of the “grey” peas -with coloured flowers, etc., there are fresh complications -to be considered.</p> - -<p>Professor Weldon does not wholly avoid these (as -Mendel did in regard to shape) and we will follow him -through his difficulties hereafter. For the present let me -say that the classes <i>round</i> and <i>wrinkled</i> are not readily -applicable to those other varieties and are not so applied -either by Mendel or other practical writers on these -subjects. To use the terms indicated in the Introduction, -<i>seed-shape</i> depends on more than one pair of allelomorphs—possibly -on several.</p> - - -<p class="tac mtb1em"><i>Stability and Variability.</i></p> - -<p>Generally speaking peas which when seen in bulk are -monomorphic in colour and shape, will give fairly true and -uniform offspring (but such strict monomorphism is rather -exceptional). Instances to the contrary occur, and in my -own brief experience I have seen some. In a row of <i>Fill-basket</i><span class="pagenum" id="Page_125">125</span> -grown from selected seed there were two plants of -different habit, seed-shape, etc. Each bore pods with seeds -few though large and round. Again <i>Blue Peter</i> (blue and -round) and <i>Laxton’s Alpha</i> (blue and wrinkled), grown in -my garden and left to nature uncovered, have each given -a considerable proportion of seeds with <i>yellow</i> cotyledons, -about 20% in the case of <i>Laxton’s Alpha</i>. The distribution -of these on the plants I cannot state. The plants bearing -them in each case sprang from green-cotyledoned seeds -taken from samples containing presumably unselected green -seeds only. A part of this exceptional result may be due -to crossing, but heterogeneity of <span class="nowrap">conditions<a id="FNanchor_68" href="#Footnote_68" class="fnanchor">68</a></span> especially in -or after ripening is a more likely cause, hypotheses I hope -to investigate next season. Hitherto I had supposed the -crossing, if any, to be done by <i>Bruchus</i> or Thrips, but -Tschermak also suspects <i>Megachile</i>, the leaf-cutter bee, -which abounds in my garden.</p> - -<p>Whatever the cause, these irregularities may undoubtedly -occur; and if they be proved to be largely independent of -crossing and conditions, this will in nowise vitiate the truth -of the Mendelian principle. For in that case it may simply -be variability. Such true variation, or sporting, in the -pea is referred to by many observers. Upon this subject I -have received most valuable facts from Mr Arthur Sutton, -who has very kindly interested himself in these inquiries.<span class="pagenum" id="Page_126">126</span> -He tells me that several highly bred varieties, selected with -every possible care, commonly throw a small but constant -proportion of poor and almost vetch-like plants, with short -pods and small round seeds, which are hoed out by experienced -men each year before ripening. Other high-class -varieties always, wherever grown, and when far from other -sorts, produce a small percentage of some one or more -definite “sports.” Of these peculiar sports he has sent me -a collection of twelve, taken from as many standard varieties, -each “sport” being represented by eight seeds, which though -quite distinct from the type agree with each other in almost -all cases.</p> - -<p>In two cases, he tells me, these seed-sports sown -separately have been found to give plants identical with -the standard type and must therefore be regarded as sports -in <i>seed characters</i> only; in other cases change of plant-type -is associated with the change of seed-type.</p> - -<p>In most standard varieties these definite sports are not -very common, but in a few they are common enough to -require continual removal by <span class="nowrap">selection<a id="FNanchor_69" href="#Footnote_69" class="fnanchor">69</a></span>.</p> - -<p>I hope before long to be able to give statistical details<span class="pagenum" id="Page_127">127</span> -and experiments relating to this extraordinarily interesting -subject. As de Vries writes in his fine work <i>Die Mutationstheorie</i> -(<span class="lowercase smcap">I.</span> p. 580), “a study of the seed-differences of -inconstant, or as they are called, ‘still’ unfixed varieties, is -a perfect treasure-house of new discoveries.”</p> - -<p>Let us consider briefly the possible significance of these -facts in the light of Mendelian teaching. First, then, it is -clear that as regards most of such cases the hypothesis is -not excluded that these recurring sports may be due to the -fortuitous concurrence of certain scarcer hypallelomorphs, -which may either have been free in the original parent -varieties from which the modern standard forms were -raised, or may have been freed in the crossing to which the -latter owe their origin (see p. <a href="#Page_28">28</a>). This possibility raises -the question whether, if we could make “<i>pure</i> cultures” of -the gametes, any variations of this nature would ever occur. -This may be regarded as an unwarrantable speculation, but -it is not wholly unamenable to the test of experiments.</p> - -<p>But variability, in the sense of division of gonads into -heterogeneous gametes, may surely be due to causes other -than crossing. This we cannot doubt. Cross-fertilization -of the zygote producing those gametes is <i>one</i> of the causes -of such heterogeneity among them. We cannot suppose it -to be the sole cause of this phenomenon.</p> - -<p>When Mendel asserts the purity of the germ-cells of -cross-breds he cannot be understood to mean that they are -<i>more pure</i> than those of the original parental races. These -must have varied in the past. The wrinkled seed arose -from the round, the green from the yellow (or <i>vice versâ</i>, -if preferred), and probably numerous intermediate forms -from both.</p> - -<p>The variations, or as I provisionally conceive it, that -differentiant division among the gametes of which variation<span class="pagenum" id="Page_128">128</span> -(neglecting environment) is the visible expression, has arisen -and can arise at one or more points of time, and we have -no difficulty in believing it to occur now. In many cases -we have clear evidence that it does. Crossing,—dare we -call it asymmetrical fertilization?—is <i>one</i> of the causes of -the production of heterogeneous gametes—the result of -divisions qualitatively differentiant and perhaps <span class="nowrap">asymmetrical<a id="FNanchor_70" href="#Footnote_70" class="fnanchor">70</a></span>.</p> - -<p>There are other causes and we have to find them. -Some years ago I wrote that consideration of the causes -of variation was in my judgment <span class="nowrap">premature<a id="FNanchor_71" href="#Footnote_71" class="fnanchor">71</a></span>. Now that -through Mendel’s work we are clearing our minds as to the -fundamental nature of “gametic” variation, the time is -approaching when an investigation of such causes may be -not unfruitful.</p> - -<p>Of <i>variation</i> as distinct from <i>transmission</i> why does -Professor Weldon take no heed? He writes (p. 244):</p> - -<div class="blockquot"> -<p>“If Mendel’s statements were universally valid, even among -Peas, the characters of the seeds in the numerous hybrid races -now existing should fall into one or other of a few definite -categories, which should not be connected by intermediate -forms.”</p> -</div> - -<p class="mt15em">Now, as I have already pointed out, Mendel made no -pretence of universal statement: but had he done so, the -conclusion, which Professor Weldon here suggests should -follow from such a universal statement, is incorrectly -drawn. Mendel is concerned with the laws of <i>transmission<span class="pagenum" id="Page_129">129</span> -of existing characters</i>, not with <i>variation</i>, which he does -not discuss.</p> - -<p>Nevertheless Professor Weldon has some acquaintance -with the general fact of variability in certain peas, which -he mentions (p. 236), but the bearing of this fact on the -difficulty he enuntiates escapes him.</p> - - -<p class="tac mtb1em"><i>Results of crossing in regard to seed characters: -normal and exceptional.</i></p> - -<p>The conditions being the same, the question of the -characters of the cross-bred zygotes which we will call -<i>AB</i>’s depends primarily on the specific nature of the -varieties which are crossed to produce them. It is unnecessary -to point out that if all <i>AB</i>’s are to look alike, -both the varieties <i>A</i> and <i>B</i> must be <i>pure</i>—not in the -common sense of descended, as far as can be traced, -through individuals identical with themselves, but pure in -the Mendelian sense, that is to say that each must be at that -moment producing only homogeneous gametes bearing the -same characters <i>A</i> and <i>B</i> respectively. Purity of pedigree -in the breeder’s sense is a distinct matter altogether. The -length of time—or if preferred—the number of generations -through which a character of a variety has remained pure, -alters the probability of its <i>dominance</i>, i.e. its appearance -when a gamete bearing it meets another bearing an antagonistic -character, no more, so far as we are yet aware, than -the length of time a stable element has been isolated alters -the properties of the chemical compound which may be -prepared from it.</p> - -<p>Now when individuals (bearing contrary characters), -pure in the sense indicated, are crossed together, the -question arises, What will be the appearance of the first<span class="pagenum" id="Page_130">130</span> -cross individuals? Here again, <i>generally speaking</i>, when -thoroughly green cotyledons are crossed with thoroughly -yellow cotyledons, the first-cross seeds will have yellow -cotyledons; when fully round peas are crossed with fully -wrinkled the first result will <i>generally speaking</i> be <i>round</i>, -often with slight pitting as Mendel has stated. This has -been the usual experience of Correns, Tschermak, Mendel, -and <span class="nowrap">myself<a id="FNanchor_72" href="#Footnote_72" class="fnanchor">72</a></span> and, as we shall see, the amount of clear -and substantial evidence to the contrary is still exceedingly -small. But as any experienced naturalist would -venture to predict, there is no <i>universal</i> rule in the -matter. As Professor Weldon himself declares, had there -been such a universal rule it would surely have been -notorious. He might further have reflected that in -Mendel’s day, when hybridisation was not the <i>terra -incognita</i> it has since become, the assertion of such universal -propositions would have been peculiarly foolish. -Mendel does not make it; but Professor Weldon perceiving -the inherent improbability of the assertion conceives at -once that Mendel <i>must</i> have made it, and if Mendel -doesn’t say so in words then he must have implied it. -As a matter of fact Mendel never treats dominance as -more than an incident in his results, merely using it as -a means to an end, and I see no reason to suppose he -troubled to consider to what extent the phenomenon is or -is not universal—a matter with which he had no concern.</p> -<p><span class="pagenum" id="Page_131">131</span></p> -<p>Of course there may be exceptions. As yet we cannot -detect the causes which control them, though injury, -impurity, accidental crossing, mistakes of various kinds, -account for many. Mendel himself says, for instance, that -unhealthy or badly grown plants give uncertain results. -Nevertheless there seems to be a true residuum of exceptions -not to be explained away. I will recite some -that I have seen. In my own crosses I have seen green × -green give yellow four times. This I incline to attribute -to conditions or other disturbance, for the natural pods of -these plants gave several yellows. At Messrs Suttons’ I saw -second-generation seeds got by allowing a cross of <i>Sutton’s -Centenary</i> (gr. wr.) × <i>Eclipse</i> (gr. rd.) to go to seed; the -resulting seeds were both green and <i>yellow</i>, wrinkled and -round. But in looking at a sample of <i>Eclipse</i> I found -a few <i>yellow</i> seeds, say two per cent., which may perhaps -be the explanation. Green wrinkled × green round <i>may</i> -give all wrinkled, and again wrinkled × wrinkled may give -<span class="nowrap"><i>round</i><a id="FNanchor_73" href="#Footnote_73" class="fnanchor">73</a></span>. Of this I saw a clear case—supposing no mistake -to have occurred—at Messrs Suttons’. Lastly we have -the fact that in exceptional cases crossing two forms—apparently -pure in the strict sense—may give a mixture -in the <i>first</i> generation. There are doubtless examples also -of unlikeness between reciprocals, and of this too I have -seen one putative <span class="nowrap">case<a id="FNanchor_74" href="#Footnote_74" class="fnanchor">74</a></span>.</p> - -<p>Such facts thus set out for the first cross-bred -generation may without doubt be predicated for subsequent -generations.</p> - -<p>What then is the significance of the facts?</p> - -<p><span class="pagenum" id="Page_132">132</span></p> - - -<p class="tac mtb1em"><i>Analysis of exceptions.</i></p> - -<p>Assuming that all these “contradictory” phenomena -happened truly as alleged, and were not pathological or -due to error—an explanation which seems quite inadequate—there -are at least four possible accounts of such diverse -results—each valid, without any appeal to ancestry.</p> - -<p>1. That dominance may exceptionally fail—or in other -words be created on the side which is elsewhere recessive. -For this exceptional failure we have to seek exceptional -causes. The artificial <i>creation</i> of dominance (in a character -usually recessive) has not yet to my knowledge been demonstrated -experimentally, but experiments are begun by which -such evidence may conceivably be obtained.</p> - -<p>2. There may be what is known to practical students -of evolution as the <i>false hybridism of Millardet</i>, or in other -words, fertilisation with—from unknown causes—transmission -of none or of only some of the characters of one pure -parent. The applicability of this hypothesis to the colours -and shapes of peas is perhaps remote, but we may notice that -it is one possible account of those rare cases where two -pure forms give a <i>mixed</i> result in the first generation, even -assuming the gametes of each pure parent to be truly -monomorphic as regards the character they bear. The -applicability of this suggestion can of course be tested by -study of the subsequent generations, self-fertilised or fertilised -by similar forms produced in the same way. In the -case of a <i>genuine</i> false-hybrid the lost characters will not -reappear in the posterity.</p> - -<p>3. The result may not be a case of transmission at all -as it is at present conceived, but of the creation on crossing<span class="pagenum" id="Page_133">133</span> -of something <i>new</i>. Our <i>AB</i>’s may have one or more -characters <i>peculiar to themselves</i>. We may in fact have -made a distinct “mule” or heterozygote form. Where this -is the case, there are several subordinate possibilities we -need not at present pursue.</p> - -<p>4. There may be definite <i>variation</i> (distinct from that -proper to the “mule”) consequent on causes we cannot -yet surmise (see pp. <a href="#Page_125">125</a> and 128).</p> - -<p>The above possibilities are I believe at the present time -the only ones that need to be considered in connexion with -these exceptional <span class="nowrap">cases<a id="FNanchor_75" href="#Footnote_75" class="fnanchor">75</a></span>. They are all of them capable -of experimental test and in certain instances we are -beginning to expect the conclusion.</p> - - -<p class="tac mtb1em"><i>The “mule” or heterozygote.</i></p> - -<p>There can be little doubt that in many cases it is to -the third category that the phenomena belong. An indication -of the applicability of this reasoning will generally be found -in the fact that in such “mule” forms the colour or the -shape of the seeds will be recognizably peculiar and proper -to the specimens themselves, as distinct from their parents, -and we may safely anticipate that when those seeds are -grown the plants will show some character which is -recognizable as novel. The <i>proof</i> that the reasoning may -apply can as yet only be got by finding that the forms in<span class="pagenum" id="Page_134">134</span> -question cannot breed true even after successive selections, -but constantly break up into the same series of <span class="nowrap">forms<a id="FNanchor_76" href="#Footnote_76" class="fnanchor">76</a></span>.</p> - -<p>This conception of the “mule” form, or “hybrid-character” -as Mendel called it, though undeveloped, is -perfectly clear in his work. He says that the dominant -character may have two significations, it may be either a -parental character or a hybrid-character, and it must be -differentiated according as it appears in the one capacity -or the other. He does not regard the character displayed -by the hybrid, whether dominant or other, <i>as a thing -inherited from or transmitted by the pure parent at all, but -as the peculiar function or property of the hybrid</i>. When -this conception has been fully understood and appreciated -in all its bearings it will be found to be hardly less fruitful -than that of the purity of the germ-cells.</p> - -<p>The two parents are two—let us say—<span class="nowrap">substances<a id="FNanchor_77" href="#Footnote_77" class="fnanchor">77</a></span> -represented by corresponding gametes. These gametes -unite to form a new “substance”—the cross-bred zygote. -This has its own properties and structure, just as a chemical -compound has, and the properties of this new “substance” -are <i>not more strictly</i> traceable to, or “inherited” from, -those of the two parents than are those of a new chemical -compound “inherited” from those of the component -elements. If the case be one in which the gametes are -pure, the new “substance” is not represented by them, -but the compound is again dissociated into its components, -each of which is separately represented by gametes.</p> - -<p><span class="pagenum" id="Page_135">135</span></p> - -<p>The character of the cross-bred zygote may be anything. -It may be something we have seen before in one or other of -the parents, it may be intermediate between the two, or it -may be something new. All these possibilities were known -to Mendel and he is perfectly aware that his principle is -equally applicable to all. The first case is his “dominance.” -That he is ready for the second is sufficiently shown by his -brief reference to time of flowering considered as a character -(p. <a href="#Page_65">65</a>). The hybrids, he says, flower at a time <i>almost -exactly intermediate</i> between the flowering times of the -parents, and he remarks that the development of the -hybrids in this case probably happens in the same way as -it does in the case of the other <span class="nowrap">characters<a id="FNanchor_78" href="#Footnote_78" class="fnanchor">78</a></span>.</p> - -<p>That he was thoroughly prepared for the third possibility -appears constantly through the paper, notably in the -argument based on the <i>Phaseolus</i> hybrids, and in the -statement that the hybrid between talls and dwarfs is -generally taller than the tall parent, having increased -height as its “hybrid-character.”</p> - -<p>All this Professor Weldon has missed. In place of it -he offers us the <i>sententia</i> that no one can expect to -understand these phenomena if he neglect ancestry. This -is the idle gloss of the scribe, which, if we erase it not -thoroughly, may pass into the text.</p> - -<p>Enough has been said to show how greatly Mendel’s -conception of heredity was in advance of those which -pass current at the present day; I have here attempted<span class="pagenum" id="Page_136">136</span> -the barest outline of the nature of the “hybrid-character,” -and I have not sought to indicate the conclusions that we -reach when the reasoning so clear in the case of the hybrid -is applied to the pure forms and their own characters.</p> - -<p>In these considerations we reach the very base on which -all conceptions of heredity and variation must henceforth -rest, and that it is now possible for us to attempt any such -analysis is one of the most far-reaching consequences of -Mendel’s principle. Till two years ago no one had made -more than random soundings of this abyss.</p> - -<p>I have briefly discussed these possibilities to assist the -reader in getting an insight into Mendel’s conceptions. -But in dealing with Professor Weldon we need not make -this excursion; for his objection arising from the absence of -uniform regularity in dominance is not in point.</p> - -<p>The soundness of Mendel’s work and conclusions would -be just as complete if dominance be found to fail often -instead of rarely. For it is perfectly certain that varieties -<i>can</i> be chosen in such a way that the dominance of one -character over its antagonist is so regular a phenomenon -that it <i>can</i> be used in the way Mendel indicates. He chose -varieties, in fact, in which a known character <i>was</i> regularly -dominant and it is because he did so that he made his -<span class="nowrap">discovery<a id="FNanchor_79" href="#Footnote_79" class="fnanchor">79</a></span>. When Professor Weldon speaks of the existence -of fluctuation and diversity in regard to dominance as -proof of a “grave discrepancy” between Mendel’s facts and -those of other <span class="nowrap">observers<a id="FNanchor_80" href="#Footnote_80" class="fnanchor">80</a></span>, he merely indicates the point at -which his own misconceptions began.</p> - -<p><span class="pagenum" id="Page_137">137</span></p> - -<p>From Mendel’s style it may be inferred that if he had -meant to state universal dominance in peas he would -have done so in unequivocal language. Let me point out -further that of the 34 varieties he collected for study, he -discarded 12 as not amenable to his <span class="nowrap">purposes<a id="FNanchor_81" href="#Footnote_81" class="fnanchor">81</a></span>. He tells -us he would have nothing to do with characters which -were not sharp, but of a “more or less” description. As -the 34 varieties are said to have all come true from seed, -we may fairly suppose that the reason he discarded twelve -was that they were unsuitable for his calculations, having -either ill-defined and intermediate characters, or possibly -defective and irregular dominance.</p> - - -<h3>IV. <span class="smcap">Professor Weldon’s collection of “Other -Evidence concerning Dominance in Peas.”</span></h3> - - -<p class="tac mtb1em"><i>A. In regard to cotyledon colour: Preliminary.</i></p> - -<p>I have been at some pains to show how the contradictory -results, no doubt sometimes occurring, on which Professor -Weldon lays such stress, may be comprehended without -any injury to Mendel’s main conclusions. This excursion -was made to save trouble with future discoverers of -exceptions, though the existence of such facts need -scarcely disturb many minds. As regards the dominance -of yellow cotyledon-colour over green the whole number of -genuine unconformable cases is likely to prove very small -indeed, though in regard to the dominance of round shape -over wrinkled we may be prepared for more discrepancies. -Indeed my own crosses alone are sufficient to show that -in using some varieties irregularities are to be expected.<span class="pagenum" id="Page_138">138</span> -Considering also that the shapes of peas depend unquestionably -on more than one pair of allelomorphs I -fully expect regular blending in some cases.</p> - -<p>As however it may be more satisfactory to the reader -and to Professor Weldon if I follow him through his -“contradictory” evidence I will endeavour to do so. Those -who have even a slight practical acquaintance with the -phenomena of heredity will sympathize with me in the -difficulty I feel in treating this section of his arguments -with that gravity he conceives the occasion to demand.</p> - -<p>In following the path of the critic it will be necessary -for me to trouble the reader with a number of details of a -humble order, but the journey will not prove devoid of -entertainment.</p> - -<p>Now exceptions are always interesting and suggestive -things, and sometimes hold a key to great mysteries. Still -when a few exceptions are found disobeying rules elsewhere -conformed to by large classes of phenomena it is not an -unsafe course to consider, with such care as the case permits, -whether the exceptions may not be due to exceptional -causes, or failing such causes whether there may be any -possibility of error. But to Professor Weldon, an exception -is an exception—and as such may prove a very serviceable -missile; so he gathers them as they were “smooth stones -from the brook.”</p> - -<p>Before examining the quality of this rather miscellaneous -ammunition I would wish to draw the non-botanical reader’s -attention to one or two facts of a general nature.</p> - -<p>For our present purpose the seed of a pea may be -considered as consisting of two parts, the <i>embryo with its -cotyledons</i>, enclosed in a <i>seed-coat</i>. It has been known for -about a century that this coat or skin is a <i>maternal</i> structure, -being part of the mother plant just as much as the pods<span class="pagenum" id="Page_139">139</span> -are, and consequently not belonging to the next generation -at all. If then any changes take place in it consequent on -fertilisation, they are to be regarded not as in any sense a -transmission of character by heredity, but rather as of the -nature of an “infection.” If on the other hand it is desired -to study the influence of hereditary transmission on seed-coat -characters, then the crossed seeds must be sown and -the seed-coats of their seeds studied. Such infective changes -in maternal tissues have been known from early times, a -notable collection of them having been made especially by -Darwin; and for these cases Focke suggested the convenient -word <i>Xenia</i>. With this familiar fact I would not for a -moment suppose Professor Weldon unacquainted, though it -was with some surprise that I found in his paper no reference -to the phenomenon.</p> - -<p>For as it happens, xenia is not at all a rare occurrence -with <i>certain varieties</i> of peas; though in them, as I believe -is generally the case with this phenomenon, it is highly -irregular in its manifestations, being doubtless dependent -on slight differences of conditions during ripening.</p> - -<p>The coats of peas differ greatly in different varieties, -being sometimes thick and white or yellow, sometimes -thick and highly pigmented with green or other colours, -in both of which cases it may be impossible to judge the -cotyledon-colour without peeling off the opaque coat; or -the coats may be very thin, colourless and transparent, so -that the cotyledon-colour is seen at once. It was such a -transparent form that Mendel says he used for his experiments -with cotyledon-colour. In order to see xenia a pea -with a <i>pigmented</i> seed-coat should be taken as seed-parent, -and crossed with a variety having a different cotyledon-colour. -There is then a fair chance of seeing this -phenomenon, but much still depends on the variety. For<span class="pagenum" id="Page_140">140</span> -example, <i>Fillbasket</i> has green cotyledons and seed-coat -green except near the hilar surface. Crossed with <i>Serpette -nain blanc</i> (yellow cotyledons and yellow coat) this variety -gave three pods with 17 seeds in which the seed-coats were -almost full yellow (xenia). Three other pods (25 seeds), -similarly produced, showed slight xenia, and one pod with -eight seeds showed little or none.</p> - -<p>On the other hand <i>Fillbasket</i> fertilised with <i>nain de -Bretagne</i> (yellow cotyledons, seed-coats yellow to yellowish -green) gave six pods with 39 seeds showing slight xenia, -distinct in a few seeds but absent in most.</p> - -<p>Examples of xenia produced by the contrary proceeding, -namely fertilising a yellow pea with a green, may indubitably -occur and I have seen doubtful cases; but as by the nature -of the case these are <i>negative</i> phenomena, i.e. the seed-coat -remaining greenish and <i>not</i> going through its normal -maturation changes, they must always be equivocal, and -would require special confirmation before other causes were -excluded.</p> - -<p>Lastly, the special change (xenia) Mendel saw in “grey” -peas, appearance or increase of purple pigment in the thick -coats, following crossing, is common but also irregular.</p> - -<p>If a <i>transparent</i> coated form be taken as seed-parent -there is no appreciable xenia, so far as I know, and such a -phenomenon would certainly be <span class="nowrap">paradoxical<a id="FNanchor_82" href="#Footnote_82" class="fnanchor">82</a></span>.</p> - -<p>In this connection it is interesting to observe that -Giltay, whom Professor Weldon quotes as having obtained -purely Mendelian results, got no xenia though searching -for it. If the reader goes carefully through Giltay’s -numerous cases, he will find, <i>almost</i> without doubt, that -none of them were such as produce it. <i>Reading Giant</i>, as<span class="pagenum" id="Page_141">141</span> -Giltay states, has a <i>transparent</i> skin, and the only xenia -likely to occur in the other cases would be of the peculiar -and uncertain kind seen in using “grey” peas. Professor -Weldon notes that Giltay, who evidently worked with extreme -care, <i>peeled</i> his seeds before describing them, a course -which Professor Weldon, not recognizing the distinction -between the varieties with opaque and transparent coats, -himself wisely recommends. The coincidence of the peeled -seeds giving simple Mendelian results is one which might -have alarmed a critic less intrepid than Professor Weldon.</p> - -<p>Bearing in mind, then, that the coats of peas may be -transparent or opaque; and in the latter case may be -variously pigmented, green, grey, reddish, purplish, etc.; -that in any of the latter cases there may or may not be -xenia; the reader will perceive that to use the statements -of an author, whether scientific or lay, to the effect that on -crossing varieties he obtained peas of such and such colours -<i>without specifying at all whether the coats were transparent -or whether the colours he saw were coat- or cotyledon-colours</i> -is a proceeding fraught with peculiar and special risks.</p> - - -<p>(1) <i>Gärtner’s cases.</i> Professor Weldon gives, as exceptions, -a series of Gärtner’s observations. Using several -varieties, amongst them <i>Pisum sativum macrospermum</i>, -a “grey” pea, with coloured flowers and seed-<span class="nowrap">coats<a id="FNanchor_83" href="#Footnote_83" class="fnanchor">83</a></span>, -he obtained results partly Mendelian and partly, as -now alleged, contradictory. The latter consist of seeds -“dirty yellow” and “yellowish green,” whereas it is -suggested they should have been simply yellow.</p> - -<p>Now students of this department of natural history will -know that these same observations of Gärtner’s, whether -rightly or wrongly, have been doing duty for more than -half a century as stock illustrations of xenia. In this<span class="pagenum" id="Page_142">142</span> -capacity they have served two generations of naturalists. -The ground nowadays may be unfamiliar, but others have -travelled it before and recorded their impressions. Darwin, -for example, has the following <span class="nowrap">passage<a id="FNanchor_84" href="#Footnote_84" class="fnanchor">84</a></span>:</p> - -<div class="blockquot"> -<p>“These statements led Gärtner, who was highly sceptical on -the subject, carefully to try a long series of experiments; he -selected the most constant varieties, and the results conclusively -showed <i>that the colour of the skin of the pea</i> is modified when -pollen of a differently coloured variety is used.” (The italics are -mine.)</p> -</div> - -<p>In the true spirit of inquiry Professor Weldon doubtless -reflected,</p> - -<p class="ml2em"> -“’Tis not <i>Antiquity</i> nor <i>Author</i>,<br /> -That makes <i>Truth Truth</i>, altho’ <i>Time’s Daughter</i>”; -</p> - -<p>but perhaps a word of caution to the reader that another -interpretation exists would have been in place. It cannot -be without amazement therefore that we find him appropriating -these examples as referring to cotyledon-colour, -with never a hint that the point is doubtful.</p> - -<p>Giltay, without going into details, points out the -<span class="nowrap">ambiguity<a id="FNanchor_85" href="#Footnote_85" class="fnanchor">85</a></span>. As Professor Weldon refers to the writings -both of Darwin and Giltay, it is still more remarkable -that he should regard the phenomenon as clearly one of -cotyledon-colour and not coat-colour as Darwin and many -other writers have supposed.</p> -<p><span class="pagenum" id="Page_143">143</span></p> -<p>Without going further it would be highly improbable -that Gärtner is speaking solely or even chiefly of the -cotyledons, from the circumstance that these observations -are given as evidence of “<i>the influence of foreign pollen on -the female organs</i>”; and that Gärtner was perfectly aware of -the fact that the coat of the seed was a maternal structure -is evident from his statement to that effect on p. 80.</p> - -<p>To go into the whole question in detail would require -considerable space; but indeed it is unnecessary to labour -the point. The reader who examines Gärtner’s account -with care, especially the peculiar phenomena obtained in -the case of the “grey” pea (<i>macrospermum</i>), with specimens -before him, will have no difficulty in recognizing that -Gärtner is simply describing the seeds <i>as they looked in -their coats</i>, and is not attempting to distinguish cotyledon-characters -and coat-characters. If he had peeled them, -which in the case of “grey” peas would be <i>absolutely -necessary</i> to see cotyledon-colour, he must surely have -said so.</p> - -<p>Had he done so, he would have found the cotyledons -full yellow in every ripe seed; for I venture to assert that -anyone who tries, as we have, crosses between a yellow-cotyledoned -“grey” pea, such as Gärtner’s was, with any -pure green variety will see that there is no question -whatever as to absolute dominance of the yellow cotyledon-character -here, more striking than in any other case. -If exceptions are to be looked for, they will not be found -<i>there</i>; and, except in so far as they show simple dominance -of yellow, Gärtner’s observations cannot be cited in this -connection at all.</p> - - -<p>(2) <i>Seton’s case.</i> Another exception given by Professor -Weldon is much more interesting and instructive.<span class="pagenum" id="Page_144">144</span> -It is the curious case of <span class="nowrap">Seton<a id="FNanchor_86" href="#Footnote_86" class="fnanchor">86</a></span>. Told in the words of -the critic it is as follows:—</p> - -<div class="blockquot"> -<p>“Mr Alexander Seton crossed the flowers of <i>Dwarf Imperial</i>, -‘a well-known green variety of the Pea,’ with the pollen of -‘a white free-growing variety.’ Four hybrid seeds were obtained, -‘which did not differ in appearance from the others -of the female parent.’ These seeds therefore did <i>not</i> obey the -law of dominance, or if the statement be preferred, greenness -became dominant in this case. The seeds were sown, and -produced plants bearing ‘green’ and ‘white’ seeds side by -side in the same pod. An excellent coloured figure of one of -these pods is given (<i>loc. cit.</i> Plate 9, Fig. 1), and is the only -figure I have found which illustrates segregation of colours in -hybrid Peas of the second generation.”</p> -</div> - -<p>Now if Professor Weldon had applied to this case the -same independence of judgment he evinced in dismissing -Darwin’s interpretation of Gärtner’s observations, he might -have reached a valuable result. Knowing how difficult it -is to give all the points in a brief citation, I turned up the -original passage, where I find it stated that the mixed -seeds of the second generation “were all completely either -of one colour or the other, none of them having an intermediate -tint, as Mr Seton had expected.” The utility of -this observation of the absence of intermediates, is that it -goes some way to dispose of the suggestion of xenia as a -cause contributing to the result.</p> - -<p>Moreover, feeling perfectly clear, from the fact of the -absence of intermediates, that the case must be one of -simple dominance in spite of first appearances, I suggest -the following account with every confidence that it is -the true one. There have been several “<i>Imperials</i>,”<span class="pagenum" id="Page_145">145</span> -though <i>Dwarf Imperial</i>, in a form which I can feel sure -is Seton’s form, I have not succeeded in seeing; but -from Vilmorin’s description that the peas when ripe are -“<i>franchement verts</i>” I feel no doubt it was a green pea -<i>with a green skin</i>. If it had had a transparent skin this -description would be inapplicable. Having then a green -skin, which may be assumed with every probability of truth, -the seeds, even though the cotyledons were yellow, might, -especially if examined fresh, be indistinguishable from those -of the maternal type. Next from the fact of the mixture -in the second generation we learn that the <i>semi-transparent -seed-coat of the paternal form was dominant</i> as a plant-character, -and indeed the coloured plate makes this fairly -evident. It will be understood that this explanation is -as yet suggestive, but from the facts of the second generation, -any supposition that there was real irregularity in -dominance in this case is out of the <span class="nowrap">question<a id="FNanchor_87" href="#Footnote_87" class="fnanchor">87</a></span>.</p> - - -<p>(3) <i>Tschermak’s exceptions.</i> These are a much more -acceptable lot than those we have been considering. -Tschermak was thoroughly alive to the seed-coat question -and consequently any exception stated as an unqualified -fact on his authority must be accepted. The nature of these -cases we shall see. Among the many varieties he used, -some being <i>not</i> monomorphic, it would have been surprising -if he had not found true irregularities in dominance.</p> - - -<p>(3 <i>a</i>) <i>Buchsbaum case.</i> This variety, growing in the -open, gave once a pod in which <i>every seed but one was green</i>. -In stating this case Professor Weldon refers to <i>Buchsbaum</i><span class="pagenum" id="Page_146">146</span> -as “a yellow-seeded variety.” <span class="nowrap">Tschermak<a id="FNanchor_88" href="#Footnote_88" class="fnanchor">88</a></span>, however, describes -it as having “<i>gelbes, öfters gelblich-grünes Speichergewebe</i>” -(cotyledons); and again says the cotyledon-colour -is “<i>allerdings gerade bei Buchsbaum zur Spontanvariation -nach gelb-grün neigend!</i>” The (!) is Tschermak’s. Therefore -Professor Weldon can hardly claim <i>Buchsbaum</i> as -“yellow-seeded” without qualification.</p> - -<p><i>Buchsbaum</i> in fact is in all probability a blend-form -and certainly not a true, stable yellow. One of the green -seeds mentioned above grew and gave 15 <i>yellows</i> and three -<i>greens</i>, and the result showed pretty clearly, as Tschermak -says, that there had been an accidental cross with a tall -green.</p> - -<p>On another occasion <i>Telephone</i> ♀ (another impure -green) × <i>Buchsbaum</i> gave four <i>yellow smooth and</i> two <i>green -wrinkled</i>, but one [? both: the grammar is obscure] of the -greens did not <span class="nowrap">germinate<a id="FNanchor_89" href="#Footnote_89" class="fnanchor">89</a></span>.</p> - - -<p>(3 <i>b</i>) <i>Telephone cases.</i> <i>Telephone</i>, crossed with at least -one yellow variety (<i>Auvergne</i>) gave all or some green or -greenish. These I have no doubt are good cases of -“defective dominance” of yellow. But it must be noted -that <i>Telephone is an impure green</i>. Nominally a green, it -is as Professor Weldon has satisfied himself, very irregular -in colour, having many intermediates shading to pure yellow -and many piebalds. It is the variety from which alone -Professor Weldon made his colour-scale. <i>I desire therefore -to call special attention to the fact that Telephone, though<span class="pagenum" id="Page_147">147</span> -not a pure green, Tschermak’s sample being as he says -“gelblichweiss grün,” a yellowish-white-green in cotyledon-colour, -is the variety which has so far contributed the -clearest evidence of the green colour dominating in its -crosses with a yellow</i>; and that <i>Buchsbaum</i> is probably a -similar case. To this point we shall return. It may not -be superfluous to mention also that one cross between -<i>Fillbasket</i> (a thorough <i>green</i>) and <i>Telephone</i> gave three -<i>yellowish</i> green seeds (Tschermak, (36), p. 501).</p> - - -<p class="mt1em">(3 <i>c</i>) <i>Couturier cases.</i> This fully yellow variety in -crosses with two fully green sorts gave seeds either yellow -or greenish yellow. In one case <i>Fillbasket</i> ♀ fertilised by -<i>Couturier</i> gave mixed seeds, green and yellow. For any -evidence to the contrary, the green in this case may have -been self-fertilised. Nevertheless, taking the evidence -together, I think it is most likely that <i>Couturier</i> is a -genuine case of imperfect dominance of yellow. If so, it is -the only true “exception” in crosses between stable forms.</p> - -<p class="mt1em">We have now narrowed down Professor Weldon’s -exceptions to dominance of cotyledon-colour to two varieties, -one yellow (<i>Couturier</i>), and one yellow “tending to green” -(<i>Buchsbaum</i>), which show imperfect dominance of yellow; -and one variety, <i>Telephone</i>, an impure and irregular green, -which shows occasional but uncertain dominance of <i>green</i>.</p> - -<p>What may be the meaning of the phenomenon shown -by the unstable or mosaic varieties we cannot tell; but I -venture to suggest that when we more fully appreciate the -nature and genesis of the gametes, it will be found that -the peculiarities of heredity seen in these cases have more -in common with those of “false hybridism” (see p. <a href="#Page_34">34</a>) -than with any true failure of dominance.</p> - -<p>Before, however, feeling quite satisfied in regard even<span class="pagenum" id="Page_148">148</span> -to this residuum of exceptions, one would wish to learn -the subsequent fate of these aberrant seeds and how their -offspring differed from that of their sisters. One only of -them can I yet trace, viz. the green seed from <i>Telephone</i> ♀ -× <i>Buchsbaum</i> ♂, which proved a veritable “green dominant.” -As for the remainder, Tschermak promises in his first -paper to watch them. But in his second paper the only -passage I can find relating to them declares that perhaps -some of the questionable cases he mentioned in his first -paper “<i>are attributable to similar isolated anomalies in -dominance; some proved themselves by subsequent cultivation -to be cases of accidental self-fertilisation; others failed to -germinate</i><a id="FNanchor_90" href="#Footnote_90" class="fnanchor">90</a>.” I may warn those interested in these questions, -that in estimating changes due to ripening, <i>dead</i> -seeds are not available.</p> - - -<p class="tac mtb1em"><i>B. Seed-coats and shapes.</i></p> - - -<p>1. <i>Seed-coats.</i> Professor Weldon lays some stress on -the results obtained by <span class="nowrap">Correns<a id="FNanchor_91" href="#Footnote_91" class="fnanchor">91</a></span> in crossing a pea having -green cotyledons and a thin almost colourless coat (<i>grüne -späte Erfurter Folger-erbse</i>) with two purple-flowered -varieties. The latter are what are known in England -as “grey” peas, though the term grey is not generally -appropriate.</p> - -<p>In these varieties the cotyledon-colour is yellow and<span class="pagenum" id="Page_149">149</span> -the coats are usually highly coloured or orange-brown. -In reciprocal crosses Correns found no change from the -maternal seed-coat-colour or seed-shape. On sowing these -peas he obtained plants bearing peas which, using the -terminology of Mendel and others, he speaks of as the “first -generation.”</p> - -<p>These peas varied in the colour of their seed-coats -from an almost colourless form slightly tinged with green -like the one parent to the orange-brown of the other -parent. The seeds varied in this respect not only from -plant to plant, but from pod to pod, and from seed to seed, -as Professor Correns has informed me.</p> - -<p>The peas with more highly-coloured coats were sown and -gave rise to plants with seeds showing the whole range of -seed-coat-colours again.</p> - -<p>Professor Weldon states that in this case neither the -law of dominance nor the law of segregation was observed; -and the same is the opinion of Correns, who, as I understand, -inclines to regard the colour-distribution as indicating -a “mosaic” formation. This is perhaps conceivable; -and in that case the statement that there was no -dominance would be true, and it would also be true that -the unit of segregation, if any, was smaller than the individual -plant and may in fact be the individual seed.</p> - -<p>A final decision of this question is as yet impossible. -Nevertheless from Professor Correns I have learnt one -point of importance, namely, that the coats of all these -seeds were <i>thick</i>, like that of the coloured and as usual -dominant form. There is no “mosaic” of coats like one -parent and coats like the other, though there may be a -mosaic of colours. In regard to the distribution of <i>colour</i> -however the possibility does not seem to me excluded that -we are here dealing with changes influenced by conditions.<span class="pagenum" id="Page_150">150</span> -I have grown a “grey” pea and noticed that the seed-coats -ripened in my garden differ considerably and not quite -uniformly from those received from and probably ripened -in France, mine being mostly pale and greyish, instead -of reddish-brown. We have elsewhere seen (p. <a href="#Page_120">120</a>) that -pigments of the seed-coat-colour may be very sensitive to -conditions, and slight differences of moisture, for example, -may in some measure account for the differences in colour. -Among my crosses I have a pod of such “grey” peas fertilised -by <i>Laxton’s Alpha</i> (green cotyledons, coat transparent). -It contained five seeds, of which four were <i>red-brown on -one side</i> and grey with purple specks on the other. The -fifth was of the grey colour on both sides. I regard this -difference not as indicating segregation of character but -merely as comparable with the difference between the two -sides of a ripe apple, and I have little doubt that Correns’ -case may be of the same <span class="nowrap">nature<a id="FNanchor_92" href="#Footnote_92" class="fnanchor">92</a></span>. Phenomena somewhat -similar to these will be met with in Laxton’s case of the -“maple” seeded peas (see p. <a href="#Page_161">161</a>).</p> - - -<p>2. <i>Seed-shapes.</i> Here Professor Weldon has three sets -of alleged exceptions to the rule of dominance of round -shape over wrinkled. The first are Rimpau’s cases, the -second are Tschermak’s cases, the third group are cases of -“grey” peas, which we will treat in a separate section (see -pp. <a href="#Page_153">153</a> and <a href="#Page_158">158</a>).</p> - - -<p>(<i>a</i>) <i>Rimpau’s cases.</i> Professor Weldon quotes Rimpau -as having crossed wrinkled and round <span class="nowrap">peas<a id="FNanchor_93" href="#Footnote_93" class="fnanchor">93</a></span> and found<span class="pagenum" id="Page_151">151</span> -the second hybrid generation dimorphic as usual. The -wrinkled peas were selected and sown and gave wrinkled -peas <i>and round</i> peas, becoming “true” to the wrinkled -character in one case only in the fifth year, while in the -second case—that of a <i>Telephone</i> cross—there was a mixture -of round and wrinkled similarly resulting from <i>wrinkled</i> -seed for two years, but the experiment was not continued.</p> - -<p>These at first sight look like genuine exceptions. In -reality, however, they are capable of a simple explanation. It -must be remembered that Rimpau was working in ignorance -of Mendel’s results, was not testing any rule, and was not -on the look out for irregularities. Now all who have -crossed wrinkled and round peas on even a moderate scale -will have met with the fact that there is frequently <i>some</i> -wrinkling in the cross-bred seeds. Though round when compared -with the true wrinkled, these are often somewhat more -wrinkled than the round type, and in irregular degrees. -For my own part I fully anticipate that we may find rare -cases of complete blending in this respect though I do not -as yet know one.</p> - -<p>Rimpau gives a photograph of eight peas (Fig. 146) -which he says represent the wrinkled form derived from -this cross. It is evident that these are not from <i>one pod</i> -but a miscellaneous selection. On close inspection it will -be seen that while the remainder are shown with their -<i>cotyledon</i>-surfaces upwards, the two peas at the lower end -of the row are represented with their <i>hilar</i>-surfaces -upwards. Remembering this it will be recognized that -these two lower peas are in fact <i>not</i> fully wrinkled peas -but almost certainly <i>round</i> “hybrids,” and the depression -is merely that which is often seen in round peas (such as -<i>Fillbasket</i>), squared by mutual pressure. Such peas, when -sown, might of course give some round.</p> - -<p><span class="pagenum" id="Page_152">152</span></p> - -<p>As Tschermak writes ((37), p. 658), experience has -shown him that cross-bred seeds with character transitional -between “round” and “wrinkled” behave as hybrids, and -have both wrinkled and round offspring, and he now reckons -them accordingly with the round dominants.</p> - -<p>Note further the fact that Rimpau found the wrinkled -form came true in the <i>fifth</i> year, while the round gave at -first more, later fewer, wrinkleds, not coming true till the -<i>ninth</i> year. This makes it quite clear that there <i>was</i> -dominance of the round form, but that the heterozygotes -were not so sharply distinguishable from the two pure -forms as to be separated at once by a person not on the -look-out for the distinctions. Nevertheless there <i>was</i> -sufficient difference to lead to a practical distinction of -the cross-breds both from the pure dominants and from -the pure recessives.</p> - -<p>The <i>Telephone</i> case may have been of the same nature; -though, as we have seen above, this pea is peculiar in its -colour-heredity and may quite well have followed a different -rule in shape also. As stated before, the wrinkled offspring -were not cultivated after the third year, but the -<i>round</i> seeds are said to have still given some wrinkleds in -the eighth year after the cross, as would be expected in a -simple Mendelian case.</p> - -<p>(<i>b</i>) <i>Tschermak’s cases.</i> The cases Professor Weldon -quotes from Tschermak all relate to crosses with <i>Telephone</i> -again, and this fact taken with the certainty that the -colour-heredity of <i>Telephone</i> is abnormal makes it fairly -clear that there is here something of a really exceptional -character. What the real nature of the exception is, and -how far it is to be taken as contradicting the “law of -dominance,” is quite another matter.</p> - -<p><span class="pagenum" id="Page_153">153</span></p> - - -<p>3. <i>Other phenomena, especially regarding seed-shapes, -in the case of “grey” peas. Modern evidence.</i> Professor -Weldon quotes from Tschermak the interesting facts about -the “grey” pea, <i>Graue Riesen</i>, but does not attempt to -elucidate them. He is not on very safe ground in adducing -these phenomena as conflicting with the “law of dominance.” -Let us see whither we are led if we consider these cases. -On p. <a href="#Page_124">124</a> I mentioned that the classes round and wrinkled -do not properly hold if we try to extend them to large-seeded -sorts, and that these cases require separate consideration. -In many of such peas, which usually belong -either to the classes of sugar-peas (<i>mange-touts</i>) or “grey” -peas (with coloured flowers), the seeds would be rather -described as irregularly indented, lumpy or <span class="nowrap">stony<a id="FNanchor_94" href="#Footnote_94" class="fnanchor">94</a></span>, than by -any use of the terms round or wrinkled. One sugar-pea -(<i>Debarbieux</i>) which I have used has large flattish, smooth, -yellow seeds with white skins, and this also in its crossings -follows the rules about to be described for the large-seeded -“grey” peas.</p> - -<p>In the large “grey” peas the most conspicuous feature -is the seed-coat, which is grey, brownish, or of a bright -reddish colour. Such seed-coats are often speckled with -purple, and on boiling these seed-coats turn dark brown. -They are in fact the very peas used by Mendel in making -up his third pair of characters. Regarding them Professor<span class="pagenum" id="Page_154">154</span> -Weldon, stating they may be considered separately, writes -as follows:—</p> - -<div class="blockquot"> -<p>“Tschermak has crossed <i>Graue Riesen</i> with five races of -<i>P. sativum</i>, and he finds that the form of the first hybrid seeds -<i>follows the female parent</i>, so that if races of <i>P. sativum</i> with -round smooth seeds be crossed with <i>Graue Riesen</i> (which has -flattened, feebly wrinkled seeds) the hybrids will be round and -smooth or flattened and wrinkled, as the <i>P. sativum</i> or the -<i>Graue Riesen</i> is used as female <span class="nowrap">parent<a id="FNanchor_95" href="#Footnote_95" class="fnanchor">95</a></span>. There is here a more -complex phenomenon than at first sight appears; because if the -flowers of the first hybrid generation are self-fertilised, the -resulting seeds of the second generation invariably resemble -those of the <i>Graue Riesen</i> in shape, although in colour they -follow Mendel’s law of segregation!”</p> -</div> - -<p>From this account who would not infer that we have -here some mystery which does not accord with the -Mendelian principles? As a matter of fact the case is -dominance in a perfectly obvious if distinct form.</p> - -<p><i>Graue Riesen</i>, a large grey sugar-pea, the <i>pois sans -parchemin géant</i> of the French seedsmen, has full-yellow -cotyledons and a highly coloured seed-coat of varying tints. -In shape the seed is somewhat flattened with irregular -slight indentations, lightly wrinkled if the term be preferred. -Tschermak speaks of it in his first paper as “<i>Same flach, -zusammengedrückt</i>”—a flat, compressed seed; in his second -paper as “<i>flache, oft schwach gerunzelte Cotyledonen-form</i>,” -or cotyledon-shape, flat, often feebly wrinkled, as Professor -Weldon translates.</p> - -<p>First-crosses made from this variety, each with a different -form of <i>P. sativum</i>, are stated on the authority of -Tschermak’s five cases, to follow exclusively the maternal -seed-shape. From “<i>schwach gerunzelte</i>,” “feebly wrinkled,” -Professor Weldon easily passes to “wrinkled,” and tells us<span class="pagenum" id="Page_155">155</span> -that according as a round <i>sativum</i> or the <i>Graue Riesen</i> is -used as mother, the first-cross seeds “will be round and -smooth or flattened and wrinkled.”</p> - -<p>As a matter of fact, however, the seeds of <i>Graue Riesen</i> -though <i>slightly</i> wrinkled do not belong to the “wrinkled” -class; but if the classification “wrinkled” and “round” is -to be extended to such peas at all, they belong to the <i>round</i>. -Mendel is careful to state that his <i>round</i> class are “either -spherical or roundish, the depressions on the surface, when -there are any, always slight”; while the “wrinkled” class -are “irregularly angular, deeply <span class="nowrap">wrinkled<a id="FNanchor_96" href="#Footnote_96" class="fnanchor">96</a></span>.”</p> - -<p>On this description alone it would be very likely that -<i>Graue Riesen</i> should fall into the <i>round</i> class, and as such -it behaves in its crosses, <i>being dominant over wrinkled</i> -(see Nos. 3 and 6, below). I can see that in this case -Professor Weldon has been partly misled by expressions -of Tschermak’s, but the facts of the second generation -should have aroused suspicion. Neither author notices -that as all five varieties crossed by Tschermak with <i>Graue -Riesen</i> were <i>round</i>, the possibilities are not exhausted. -Had Tschermak tried a really wrinkled <i>sativum</i> with <i>Graue -Riesen</i> he would have seen this obvious explanation.</p> - -<p>As some of my own few observations of first-crosses bear -on this point I may quote them, imperfect though they are.</p> - -<p>I grew the purple-flowered sugar-pea “<i>Pois sans parchemin -géant à très large cosse</i>,” a soft-podded “<i>mange-tout</i>” -pea, flowers and seed-coats coloured, from Vilmorin’s, -probably identical with <i>Graue Riesen</i>.</p> - -<div class="fs95"> - -<p>1. One flower of this variety fertilised with <i>Pois très -nain de Bretagne</i> (very small seed; yellow cotyledons; very<span class="pagenum" id="Page_156">156</span> -round) gave seven seeds indistinguishable (in their coats) -from those of the mother, save for a doubtful increase in -purple pigmentation of coats.</p> - -<p>2. Fertilised by <i>Laxton’s Alpha</i> (green; wrinkled; coats -transparent), two flowers gave 11 seeds exactly as above, -the purple being in this case clearly increased.</p> - -<p>In the following the purple sugar-pea was <i>father</i>.</p> - -<p>3. <i>Laxton’s Alpha</i> (green; wrinkled; coats transparent) -fertilised by the purple sugar-pea gave one pod of four -seeds with yellow cotyledons and <i>round</i> form.</p> - -<p>4. <i>Fillbasket</i> (green; smooth but squared; coats -green) fertilised by the <i>purple</i> sugar-pea gave one pod -with six seeds, yellow <span class="nowrap">cotyledons<a id="FNanchor_97" href="#Footnote_97" class="fnanchor">97</a></span>; <i>Fillbasket</i> size and -shape; but the normally green coat yellowed near <i>the hilum</i> -by xenia.</p> - -<p>5. <i>Express</i> (“blue”-green cotyledons and transparent -skins; round) fertilised with <i>purple sugar-pea</i> gave one -pod with four seeds, yellow cotyledons, shape round, much -as in <i>Fillbasket</i>.</p> - -<p>6. <i>British Queen</i> (yellow cotyledons, wrinkled, white -coats) ♀ × purple sugar-pea gave two pods with seven seeds, -cotyledons yellow, coats <i>tinged greenish</i> (xenia?), all <i>round</i>.</p> - -<p>So much for the “<i>Purple</i>” sugar-pea.</p> - -<p>I got similar results with <i>Mange-tout Debarbieux</i>. This -is a soft-podded <i>Mange-tout</i> or sugar-pea, with white flowers, -large, flattish, smooth seeds, scarcely dimpled; yellow cotyledons.</p> - -<p><span class="pagenum" id="Page_157">157</span></p> - -<p>7. <i>Debarbieux</i> fertilised by <i>Serpette nain blanc</i> (yellow -cotyledons; wrinkled; white skin; dwarf) gave one pod -with six seeds, size and shape of <i>Debarbieux</i>, with slight -dimpling.</p> - -<p>8. <i>Debarbieux</i> by <i>nain de Bretagne</i> (very small; yellow -cotyledons; very round) gave three pods, 12 seeds, all -yellow cotyledons, of which two pods had eight seeds identical -in shape with <i>Debarbieux</i>, while the third had four -seeds like <i>Debarbieux</i> but more dimpled. The reciprocal -cross gave two seeds exactly like <i>nain de Bretagne</i>.</p> -</div> - -<p>But it may be objected that the shape of this large -grey pea is very <span class="nowrap">peculiar<a id="FNanchor_98" href="#Footnote_98" class="fnanchor">98</a></span>; and that it maintains its type -remarkably when fertilised by many distinct varieties -though its pollen effects little or no change in them; for, -so long as round varieties of <i>sativum</i> are used as mothers, -this is true as we have seen. But when once it is understood -that in <i>Graue Riesen</i> there is no question of wrinkling, -seeing that the variety behaves as a <i>round</i> variety, the -shape and especially the size of the seed must be treated -as a maternal property.</p> - -<p><i>Why</i> the distinction between the shape of <i>Graue -Riesen</i> and that of ordinary round peas should be a matter -of maternal physiology we do not know. The question is -one for the botanical chemist. But there is evidently very -considerable regularity, the seeds borne by the <i>cross-breds</i> -exhibiting the form of the “grey” pea, which is then a -dominant character as much as the seed-coat characters<span class="pagenum" id="Page_158">158</span> -are. And that is what Tschermak’s <i>Graue Riesen</i> crosses -actually did, thereby exhibiting dominance in a very clear -form. To interject these cases as a mystery without pointing -out how easily they can be reconciled with the “law of -dominance” may throw an unskilled reader into gratuitous -doubt.</p> - -<p>Finally, since <i>the wrinkled peas</i>, <i>Laxton’s Alpha</i> and -<i>British Queen</i>, <i>pollinated by a large flat mange-tout, witness -Nos. 3 and 6 above</i>, became round in both cases where this -experiment was made, we here merely see the usual dominance -of the non-wrinkled character; though of course if a -<i>round</i>-seeded mother be used there can be no departure -from the maternal shape, as far as roundness is concerned.</p> - -<p>Correns’ observations on the shapes of a “grey” pea -crossed with a round shelling pea, also quoted by Professor -Weldon as showing no dominance of roundness, are of -course of the same nature as those just discussed.</p> - - -<p class="tac mtb1em"><i>C. Evidence of Knight and Laxton.</i></p> - -<p>In the last two sections we have seen that in using -peas of the “grey” class, i.e. with brown, red, or purplish -coats, special phenomena are to be looked for, and also -that in the case of large “indented” peas, the phenomena -of size and shape may show some divergence from that -simple form of the phenomenon of dominance seen when -ordinary round and wrinkled are crossed. Here the fuller -discussion of these phenomena must have been left to await -further experiment, were it not that we have other evidence -bearing on the same questions.</p> - -<p>The first is that of Knight’s well-known experiments, -long familiar but until now hopelessly mysterious. I have -not space to quote the various interpretations which Knight -and others have put upon them, but as the Mendelian<span class="pagenum" id="Page_159">159</span> -principle at once gives a complete account of the whole, -this is scarcely necessary, though the matter is full of -historical interest.</p> - -<p>Crossing a white pea with a very large grey purple-flowered -form Knight (21) found that the peas so produced -“were not in any sensible degree different from those -afforded by other plants of the same [white] variety; -owing, I imagine, to the external covering of the seed (as -I have found in other plants) being furnished entirely by -the <span class="nowrap">female<a id="FNanchor_99" href="#Footnote_99" class="fnanchor">99</a></span>.” All grew very <span class="nowrap">tall<a id="FNanchor_100" href="#Footnote_100" class="fnanchor">100</a></span>, and had colours of -male <span class="nowrap">parent<a id="FNanchor_101" href="#Footnote_101" class="fnanchor">101</a></span>. The seeds they produced were dark <span class="nowrap">grey<a id="FNanchor_102" href="#Footnote_102" class="fnanchor">102</a></span>.</p> - -<p>“I had frequent occasion to observe, in this plant [the -hybrid], a stronger tendency to produce purple blossoms, -and coloured seeds, than white ones; for when I introduced -the farina of a purple blossom into a white one, the whole -of the seeds in the succeeding year became coloured [viz. -<i>DR</i> × <i>D</i> giving <i>DD</i> and <i>DR</i>]; but, when I endeavoured -to discharge this colour, by reversing the process, a part -only of them afforded plants with white blossoms; this -part sometimes occupying one end of the pod, and being at -times irregularly intermixed with those which, when sown, -retained their colour” [viz. <i>DR</i> × <i>R</i> giving <i>DR</i> and <i>RR</i>] -(draws conclusions, now obviously <span class="nowrap">erroneous<a id="FNanchor_103" href="#Footnote_103" class="fnanchor">103</a></span>).</p> - -<p>In this account we have nothing not readily intelligible -in the light of Mendel’s hypothesis.</p> - -<p>The next evidence is supplied by an exceptionally -complete record of a most valuable experiment made by<span class="pagenum" id="Page_160">160</span> -<span class="nowrap">Laxton<a id="FNanchor_104" href="#Footnote_104" class="fnanchor">104</a></span>. The whole story is replete with interest, and as -it not only carries us on somewhat beyond the point -reached by Mendel, but furnishes an excellent illustration -of how his principles may be applied, I give the whole -account in Laxton’s words, only altering the paragraphing -for clearness, and adding a commentary. The paper appears -in <i>Jour. Hort. Soc.</i> N.S. <span class="lowercase smcap">III.</span> 1872, p. 10, and very -slightly abbreviated in <i>Jour. of Hort.</i> <span class="lowercase smcap">XVIII.</span> 1870, p. 86. -Some points in the same article do not specially relate to -this section, but for simplicity I treat the whole together. -It is not too much to say that two years ago the -whole of this story would have been a maze of bewildering -confusion. There are still some points in it -that we cannot fully comprehend, for the case is one of far -more than ordinary complexity, but the general outlines -are now clear. In attempting to elucidate the phenomena -it will be remembered that there are no statistics (those -given being inapplicable), and the several offspring are -only imperfectly referred to the several classes of seeds. -This being so, our rationale cannot hope to be complete. -Laxton states that as the seeds of peas are liable to change -colour with keeping, for this and other reasons he sent to -the Society a part of the seeds resulting from his experiment -before it was brought to a conclusion.</p> - -<div class="blockquot"> - -<p>“The seeds exhibited were derived from a single experiment. -Amongst these seeds will be observed some of several remarkable -colours, including black, violet, purple-streaked and spotted, -maple, grey, greenish, white, and almost every intermediate tint, -the varied colours being apparently produced on the outer coat -or envelope of the cotyledons only.</p> - -<p><span class="pagenum" id="Page_161">161</span></p> - -<p>The peas were selected for their colours, &c., from the third -year’s sowing in 1869 of the produce of a cross in 1866 of the -early round white-seeded and white-flowered garden variety -“Ringleader,” which is about <span class="nowrap">2 <span class="fraction"><span class="fnum">1</span><span class="bar">/</span><span class="fden">2</span></span></span> ft. in height, fertilised by the -pollen of the common purple-flowered “maple” pea, which is -taller than “Ringleader,” and has slightly indented seeds. -I effected impregnation by removing the anthers of the seed-bearer, -and applying the pollen at an early stage. This cross -produced a pod containing five round white peas, exactly like -the ordinary “Ringleader” <span class="nowrap">seeds<a id="FNanchor_105" href="#Footnote_105" class="fnanchor">105</a></span>.</p> - -<p>In 1867 I sowed these seeds, and all five produced tall -purple-flowered purplish-stemmed <span class="nowrap">plants<a id="FNanchor_106" href="#Footnote_106" class="fnanchor">106</a></span>, and the seeds, with -few exceptions, had all maple or brownish-streaked envelopes -of various shades; the remainder had entirely violet or deep -purple-coloured <span class="nowrap">envelopes<a id="FNanchor_107" href="#Footnote_107" class="fnanchor">107</a></span>: in shape the peas were partly indented;<span class="pagenum" id="Page_162">162</span> -but a few were <span class="nowrap">round<a id="FNanchor_108" href="#Footnote_108" class="fnanchor">108</a></span>. Some of the plants ripened off -earlier than the “maple,” which, in comparison with “Ringleader,” -is a late variety; and although the pods were in many -instances partially abortive, the produce was very <span class="nowrap">large<a id="FNanchor_109" href="#Footnote_109" class="fnanchor">109</a></span>.</p> - -<p>In 1868 I sowed the peas of the preceding year’s growth, and -selected various plants for earliness, productiveness, &c. Some -of the plants had light-coloured stems and leaves; these all -showed white flowers, and produced round white <span class="nowrap">seeds<a id="FNanchor_110" href="#Footnote_110" class="fnanchor">110</a></span>. Others -had purple flowers, showed the purple on the stems and at the -axils of the stipules, and produced seeds with maple, grey, -purple-streaked, or mottled, and a few only, again, with violet-coloured -envelopes. Some of the seeds were round, some partially -<span class="nowrap">indented<a id="FNanchor_111" href="#Footnote_111" class="fnanchor">111</a></span>. The pods on each plant, in the majority of instances, -contained peas of like characters; but in a few cases the peas in -the same pod varied slightly, and in some instances a pod or -two on the same plant contained seeds all distinct from the -<span class="nowrap">remainder<a id="FNanchor_112" href="#Footnote_112" class="fnanchor">112</a></span>. The white-flowered plants were generally dwarfish,<span class="pagenum" id="Page_163">163</span> -of about the height of “Ringleader”; but the coloured-flowered -sorts varied altogether as to height, period of ripening, and -colour and shape of <span class="nowrap">seed<a id="FNanchor_113" href="#Footnote_113" class="fnanchor">113</a></span>. Those seeds with violet-coloured -envelopes produced nearly all maple- or parti-coloured seeds, -and only here and there one with a violet-coloured envelope; -that colour, again, appeared only incidentally, and in a like -degree in the produce of the maple-coloured <span class="nowrap">seeds<a id="FNanchor_114" href="#Footnote_114" class="fnanchor">114</a></span>.</p> - -<p>In 1869 the seeds of various selections of the previous year -were again sown separately; and the white-seeded peas again -produced only plants with white flowers and round white <span class="nowrap">seeds<a id="FNanchor_115" href="#Footnote_115" class="fnanchor">115</a></span>. -Some of the coloured seeds, which I had expected would produce -purple-flowered plants, produced plants with white flowers and -round white seeds <span class="nowrap">only<a id="FNanchor_116" href="#Footnote_116" class="fnanchor">116</a></span>; the majority, however, brought plants -with purple flowers and with seeds principally marked with -purple or grey, the maple- or brown-streaked being in the -<span class="nowrap">minority<a id="FNanchor_117" href="#Footnote_117" class="fnanchor">117</a></span>. On some of the purple-flowered plants were again -a few pods with peas differing entirely from the remainder on -the same plant. In some pods the seeds were all white, in -others all black, and in a few, again, all <span class="nowrap">violet<a id="FNanchor_118" href="#Footnote_118" class="fnanchor">118</a></span>; but those plants -which bore maple-coloured seeds seemed the most constant and -fixed in character of the purple-flowered <span class="nowrap">seedlings<a id="FNanchor_119" href="#Footnote_119" class="fnanchor">119</a></span>, and the -purplish and grey peas, being of intermediate characters, appeared<span class="pagenum" id="Page_164">164</span> -to vary <span class="nowrap">most<a id="FNanchor_120" href="#Footnote_120" class="fnanchor">120</a></span>. The violet-coloured seeds again produced -almost invariably purplish, grey, or maple peas, the clear violet -colour only now and then appearing, either wholly in one pod or -on a single pea or two in a pod. All the seeds of the purple-flowered -plants were again either round or only partially indented; -and the plants varied as to height and earliness. In -no case, however, does there seem to have been an intermediate-coloured -flower; for although in some flowers I thought I found -the purple of a lighter shade, I believe this was owing to light, -temperature, or other circumstances, and applied equally to the -parent maple. I have never noticed a single tinted white flower -nor an indented white seed in either of the three years’ produce. -The whole produce of the third sowing consisted of seeds of the -colours and in the approximate quantities in order as follows,—viz.: -1st, white, about half; 2nd, purplish, grey, and violet -(intermediate colours), about three-eighths; and, 3rd, maple, -about one-eighth.</p> - -<p>From the above I gather that the white-flowered white-seeded -pea is (if I may use the term) an original variety well -fixed and distinct entirely from the maple, that the two do not -thoroughly intermingle (for whenever the white flower crops out, -the plant and its parts all appear to follow exactly the characters -of the white pea), and that the maple is a cross-bred variety -which has become somewhat permanent and would seem to -include amongst its ancestors one or more bearing seeds either -altogether or partly violet- or purple-coloured; for although -this colour does not appear on the seed of the “maple,” it is -very potent in the variety, and appears in many parts of the -plant and its offspring from cross-fertilised flowers, sometimes -on the external surface or at the sutures of the pods of the -latter, at others on the seeds and stems, and very frequently on -the seeds; and whenever it shows itself on any part of the -plant, the flowers are invariably purple. My deductions have -been confirmed by intercrosses effected between the various -white-, blue-, some singularly bright green-seeded peas which I -have selected, and the maple- and purple-podded and the purple-flowered -sugar peas, and by reversing those crosses.</p> - -<p><span class="pagenum" id="Page_165">165</span></p> - -<p>I have also deduced from my experiments, in accordance -with the conclusions of the late Mr Knight and others, that the -colours of the envelopes of the seeds of peas immediately -resulting from a cross are never <span class="nowrap">changed<a id="FNanchor_121" href="#Footnote_121" class="fnanchor">121</a></span>. I find, however, -that the colour and probably the substance of the cotyledons -are sometimes, but not always, changed by the cross fertilisation -of two different varieties; and I do not agree with Mr Knight -that the form and size of the seeds produced are <span class="nowrap">unaltered<a id="FNanchor_122" href="#Footnote_122" class="fnanchor">122</a></span>; -for I have on more than one occasion observed that the cotyledons -in the seeds directly resulting from a cross of a blue -wrinkled pea fertilised by the pollen of a white round variety -have been of a greenish-white <span class="nowrap">colour<a id="FNanchor_123" href="#Footnote_123" class="fnanchor">123</a></span>, and the seeds nearly -<span class="nowrap">round<a id="FNanchor_124" href="#Footnote_124" class="fnanchor">124</a></span> and larger or smaller according as there may have been -a difference in the size of the seeds of the two <span class="nowrap">varieties<a id="FNanchor_125" href="#Footnote_125" class="fnanchor">125</a></span>.</p> - -<p>I have also noticed that a cross between a round white and -a blue wrinkled pea will in the third and fourth generations -(second and third years’ produce) at times bring forth blue -round, blue wrinkled, white round and white wrinkled peas in -the same pods, that the white round seeds, when again sown, -will produce only white round seeds, that the white wrinkled -seeds will, up to the fourth or fifth generation, produce both -blue and white wrinkled and round peas, that the blue round -peas will produce blue wrinkled and round peas, but that the -blue wrinkled peas will bear only blue wrinkled <span class="nowrap">seeds<a id="FNanchor_126" href="#Footnote_126" class="fnanchor">126</a></span>. This<span class="pagenum" id="Page_166">166</span> -would seem to indicate that the white round and the blue -wrinkled peas are distinct varieties derived from ancestors -respectively possessing one only of those marked qualities; and, -in my opinion, the white round peas trace their origin to a -dwarfish pea having white flowers and round white seeds, and -the blue wrinkled varieties to a tall variety, having also white -flowers but blue wrinkled seeds. It is also noticeable, that from -a single cross between two different peas many hundreds of -varieties, not only like one or both parents and intermediate, -but apparently differing from either, may be produced in the<span class="pagenum" id="Page_167">167</span> -course of three or four years (the shortest time which I have -ascertained it takes to attain the climax of variation in the -produce of cross-fertilised peas, and until which time it would -seem useless to expect a fixed seedling variety to be <span class="nowrap">produced<a id="FNanchor_127" href="#Footnote_127" class="fnanchor">127</a></span>), -although a reversion to the characters of either parent, or of -any one of the ancestors, may take place at an earlier period.</p> - -<p>These circumstances do not appear to have been known to -Mr Knight, as he seems to have carried on his experiments by -continuing to cross his seedlings in the year succeeding their -production from a cross and treating the results as reliable; -whereas it is probable that the results might have been materially -affected by the disturbing causes then in existence arising from -the previous cross fertilisation, and which, I consider, would, in -all cases where either parent has not become fixed or permanent, -lead to results positively perplexing and uncertain, and to variations -almost innumerable. I have again selected, and intend -to sow, watch, and report; but as the usual climax of variation -is nearly reached in the recorded experiment, I do not anticipate -much further deviation, except in height and period of ripening—characters -which are always very unstable in the pea. There -are also important botanical and other variations and changes -occurring in cross-fertilised peas to which it is not my -province here to allude; but in conclusion I may, perhaps, in -furtherance of the objects of this paper, be permitted to inquire -whether any light can, from these observations or other means, -be thrown upon the origin of the cultivated kinds of peas, -especially the “maple” variety, and also as to the source whence -the violet and other colours which appear at intervals on the -seeds and in the offspring of cross-fertilised purple-flowered peas -are derived.”</p> -</div> - -<p>The reader who has closely followed the preceding -passage will begin to appreciate the way in which the new -principles help us to interpret these hitherto paradoxical -phenomena. Even in this case, imperfectly recorded as it -is, we can form a fairly clear idea of what was taking place.<span class="pagenum" id="Page_168">168</span> -If the “round” seeds really occurred as a distinct class, on -the heterozygotes as described, it is just possible that the -fact may be of great use hereafter.</p> - -<p>We are still far from understanding maternal seed-form—and -perhaps size—as a dominant character. So far, -as Miss Saunders has pointed out to me, it appears to be -correlated with a thick and coloured seed-coat.</p> - -<hr class="tb" /> - -<p>We have now seen the nature of Professor Weldon’s -collection of contradictory evidence concerning dominance -in peas. He tells us: “Enough has been said to show the -grave discrepancy between the evidence afforded by Mendel’s -experiments and that obtained by observers equally trustworthy.”</p> - -<p>He proceeds to a discussion of the <i>Telephone</i> and -<i>Telegraph</i> group and recites facts, which I do not doubt -for a moment, showing that in this group of peas—which -have unquestionably been more or less “blend” or “mosaic” -forms from their beginning—the “laws of dominance and -segregation” do not hold. Professor Weldon’s collection -of the facts relating to <i>Telephone</i>, &c. has distinct value, -and it is the chief addition he makes to our knowledge -of these phenomena. The merit however of this addition -is diminished by the erroneous conclusion drawn from it, as -will be shown hereafter. Meanwhile the reader who has -studied what has been written above on the general questions -of stability, “purity,” and “universal” dominance, will easily -be able to estimate the significance of these phenomena and -their applicability to Mendel’s hypotheses.</p> - -<p><span class="pagenum" id="Page_169">169</span></p> - - -<p class="tac mtb1em"><i>D. Miscellaneous cases in other plants and animals</i>.</p> - -<p>Professor Weldon proceeds:</p> - -<div class="blockquot"> -<p>“In order to emphasize the need that the ancestry of the -parents, used in crossing, should be considered in discussing the -results of a cross, it may be well to give one or two more examples -of fundamental inconsistency between different competent -observers.”</p> -</div> - -<p>The “one or two” run to three, viz. Stocks (hoariness -and colour); <i>Datura</i> (character of fruits and colour of -flowers); and lastly colours of Rats and Mice. Each of -these subjects, as it happens, has been referred to in the -forthcoming paper by Miss Saunders and myself. <i>Datura</i> -and <i>Matthiola</i> have been subjected to several years’ experiment -and I venture to refer the reader who desires to see -whether the facts are or are not in accord with Mendel’s -expectation and how far there is “fundamental inconsistency” -amongst them to a perusal of our work.</p> - -<p>But as Professor Weldon refers to some points that -have not been explicitly dealt with there, it will be safer -to make each clear as we proceed.</p> - - -<p class="mt15em">1. <i>Stocks</i> (<i>Matthiola</i>). Professor Weldon quotes -Correns’ observation that glabrous Stocks crossed with -hoary gave offspring all hoary, while Trevor Clarke thus -obtained some hoary and some glabrous. As there are -some twenty different sorts of <span class="nowrap">Stocks<a id="FNanchor_128" href="#Footnote_128" class="fnanchor">128</a></span> it is not surprising -that different observers should have chanced on different -materials and obtained different results. Miss Saunders<span class="pagenum" id="Page_170">170</span> -has investigated laws of heredity in Stocks on a large -scale and an account of her results is included in our -forthcoming Report. Here it must suffice to say that the -cross hoary ♀ × glabrous ♂ always gave offspring all hoary -except once: that the cross glabrous ♀ × hoary ♂ of several -types gave all hoary; <i>but</i> the same cross using other -hoary types did frequently give a mixture, some of the -offspring being hoary, others glabrous. Professor Weldon -might immediately decide that here was the hoped for -phenomenon of “reversed” dominance, due to ancestry, -but here again that hypothesis is excluded. For the -glabrous (recessive) cross-breds were <i>pure</i>, and produced -on self-fertilisation glabrous plants only, being in fact, -almost beyond question, “false hybrids” (see p. <a href="#Page_34">34</a>), a -specific phenomenon which has nothing to do with the -question of dominance.</p> - -<p>Professor Weldon next suggests that there is discrepancy -between the observations as to flower-colour. He tells us -that Correns found <i>violet</i> Stocks crossed with “<i>yellowish -white</i>” gave violet or shades of violet flaked together. -According to Professor Weldon</p> - -<div class="blockquot"> -<p>“On the other hand Nobbe crossed a number of varieties of -<i>M. annua</i> in which the flowers were white, violet, carmine-coloured, -crimson or dark blue. These were crossed in various -ways, and before a cross was made the colour of each parent was -matched by a mixture of dry powdered colours which was preserved. -In every case the hybrid flower was of an intermediate -colour, which could be matched by mixing the powders which -recorded the parental colours. The proportions in which the -powders were mixed are not given in each [any] case, but it is -clear that the colours <span class="nowrap">blended<a id="FNanchor_129" href="#Footnote_129" class="fnanchor">129</a></span>.”</p> -</div> - -<p><span class="pagenum" id="Page_171">171</span></p> - -<p>On comparing Professor Weldon’s version with the -originals we find the missing explanations. Having served -some apprenticeship to the breeding of Stocks, we, here, -are perhaps in a better position to take the points, but -it is to me perfectly inexplicable how in such a simple -matter as this he can have gone wrong.</p> - -<p class="mt1em">Note then</p> - -<p>(1) That Nobbe does <i>not</i> specify <i>which</i> colours he -crossed together, beyond the fact that <i>white</i> was crossed -with each fertile form. The <i>crimson</i> form (<i>Karmoisinfarbe</i>), -being double to the point of sterility, was not used. There -remain then, white, carmine, and two purples (violet, “dark -blue”). When <i>white</i> was crossed with either of these, -Nobbe says the colour becomes <i>paler</i>, whichever sort gave -the pollen. Nobbe does not state that he crossed <i>carmine</i> -with the purples.</p> - -<p>(2) Professor Weldon gives no qualification in his -version. Nobbe however states that he found it very -difficult to distinguish the result of crossing <i>carmine with -white</i> from that obtained by crossing <i>dark blue or violet -with white</i><a id="FNanchor_130" href="#Footnote_130" class="fnanchor">130</a>, thereby nullifying Professor Weldon’s statement -that in every case the cross was a simple mixture of -the parental colours—a proposition sufficiently disproved by -Miss Saunders’ elaborate experiments.</p> - -<p>(3) Lately the champion of the “importance of small -variations,” Professor Weldon now prefers to treat the -distinctions between established varieties as negligible<span class="pagenum" id="Page_172">172</span> -fluctuations instead of specific <span class="nowrap">phenomena<a id="FNanchor_131" href="#Footnote_131" class="fnanchor">131</a></span>. Therefore -when Correns using “<i>yellowish white</i>” obtained one result -and Nobbe using “<i>white</i>” obtained another, Professor -Weldon hurries to the conclusion that the results are -comparable and therefore contradictory. Correns however -though calling his flowers <i>gelblich-weiss</i> is careful to state -that they are described by Haage and Schmidt (the seed-men) -as “<i>schwefel-gelb</i>” or sulphur-yellow. The topics -Professor Weldon treats are so numerous that we cannot -fairly expect him to be personally acquainted with all; -still had he <i>looked</i> at Stocks before writing, or even at the -literature relating to them, he would have easily seen that -these yellow Stocks are a thoroughly distinct <span class="nowrap">form<a id="FNanchor_132" href="#Footnote_132" class="fnanchor">132</a></span>; and -in accordance with this fact it would be surprising if they -had not a distinctive behaviour in their crosses. To use -our own terminology their colour character depends almost -certainly on a <i>compound</i> allelomorph. Consequently there -is no evidence of contradiction in the results, and appeal to -ancestry is as unnecessary as futile.</p> - - -<p class="mt15em">2. <i>Datura.</i> As for the evidence on <i>Datura</i>, I must -refer the reader again to the experiments set forth in our -Report.</p> - -<p>The phenomena obey the ordinary Mendelian rules with -accuracy. There are (as almost always where discontinuous<span class="pagenum" id="Page_173">173</span> -variation is concerned) occasional cases of “mosaics,” a -phenomenon which has nothing to do with “ancestry.”</p> - - -<p class="mt15em">3. <i>Colours of Rats and Mice.</i> Professor Weldon -reserves his collection of evidence on this subject for the -last. In it we reach an indisputable contribution to the -discussion—a reference to Crampe’s papers, which together -constitute without doubt the best evidence yet published, -respecting colour-heredity in an animal. So far as I have -discovered, the only previous reference to these memoirs is -that of Ritzema <span class="nowrap">Bos<a id="FNanchor_133" href="#Footnote_133" class="fnanchor">133</a></span>, who alludes to them in a consideration -of the alleged deterioration due to in-breeding.</p> - -<p>Now Crampe through a long period of years made an -exhaustive study of the peculiarities of the colour-forms of -Rats, white, black, grey and their piebalds, as exhibited in -Heredity.</p> - -<p>Till the appearance of Professor Weldon’s article -Crampe’s work was unknown to me, and all students of -Heredity owe him a debt for putting it into general -circulation. My attention had however been called by -Dr Correns to the interesting results obtained by von -Guaita, experimenting with crosses originally made between -albino <i>mice</i> and piebald Japanese waltzing mice. This -paper also gives full details of an elaborate investigation -admirably carried out and recorded.</p> - -<p>In the light of modern knowledge both these two -researches furnish material of the most convincing character -demonstrating the Mendelian principles. It would be a useful -task to go over the evidence they contain and rearrange -it in illustration of the laws now perceived. To do this here -is manifestly impossible, and it must suffice to point out -that the albino is a simple recessive in both cases (the<span class="pagenum" id="Page_174">174</span> -waltzing character in mice being also a recessive), and that -the “wild grey” form is one of the commonest heterozygotes—there -appearing, like the yellow cotyledon-colour of peas, -<i>in either of two capacities</i>, i.e. as a pure form, or as the -heterozygote form of one or more <span class="nowrap">combinations<a id="FNanchor_134" href="#Footnote_134" class="fnanchor">134</a></span>.</p> - -<p>Professor Weldon refers to both Crampe and von -Guaita, whose results show an essential harmony in the -fact that both found <i>albino</i> an obvious recessive, pure -almost without exception, while the coloured forms show -various phenomena of dominance. Both found heterozygous -colour-types. He then searches for something that -looks like a contradiction. Of this there is no lack in the -works of Johann von Fischer (11)—an authority of a very -different character—whom he quotes in the following -few words:</p> - -<div class="blockquot"> -<p>“In both rats and mice von Fischer says that piebald rats -crossed with albino varieties of their species, give piebald young -if the father only is piebald, white young if the mother only is -piebald.”</p> -</div> - -<p>But this is doing small justice to the completeness of -Johann von Fischer’s statement, which is indeed a proposition -of much more amazing import.</p> - -<p>That investigator in fact began by a study of the cross -between the albino Ferret and the Polecat, as a means of -testing whether they were two species or merely varieties. -The cross, he found, was in colour and form a blend of the -parental types. Therefore, he declares, the Ferret and the<span class="pagenum" id="Page_175">175</span> -Polecat are two distinct species, because, “as everybody -ought to know,”</p> - -<div class="blockquot"> -<p>“<i>The result of a cross between albino and normal [of -one species] is always a constant one, namely an offspring -like the father at least in colour</i><a id="FNanchor_135" href="#Footnote_135" class="fnanchor">135</a>,”</p> -</div> - -<p>whereas in <i>crosses</i> (between species) this is <i>not</i> the case.</p> - -<p>And again, after reciting that the Ferret-Polecat crosses -gave intermediates, he states:</p> - -<div class="blockquot"> -<p>“But all this is <i>not</i> the case in crosses between albinos and -normal animals within the species, in which always and without -any exception the young resemble the father in <span class="nowrap">colour<a id="FNanchor_136" href="#Footnote_136" class="fnanchor">136</a></span>.”</p> -</div> - -<p>These are admirable illustrations of what is meant by -a “<i>universal</i>” proposition. But von Fischer doesn’t stop -here. He proceeds to give a collection of evidence in proof -of this truth which he says “ought to be known to everyone.” -He has observed the fact in regard to albino mole, -albino shrew (<i>Sorex araneus</i>), melanic squirrel (<i>Sciurus -vulgaris</i>), albino ground-squirrel (<i>Hypudaeus terrestris</i>), -albino hamster, albino rats, albino mice, piebald (grey-and-white -or black-and-white) mice and rats, partially -albino sparrow, and we are even presented with two cases -in Man. No single exception was known to von <span class="nowrap">Fischer<a id="FNanchor_137" href="#Footnote_137" class="fnanchor">137</a></span>.</p> -<p><span class="pagenum" id="Page_176">176</span></p> -<p>In his subsequent paper von Fischer declares that from -matings of rats in which the mothers were grey and the -fathers albino he bred 2017 pure albinos; and from albino -mothers and grey fathers 3830 normal greys. “Not a -single individual varied in any respect, or was in any way -intermediate.”</p> - -<p>With piebalds the same result is asserted, save that -certain melanic forms appeared. Finally von Fischer -repeats his laws already reached, giving them now in this -form: <i>that if the offspring of a cross show only the colour -of the father, then the parents are varieties of one species; -but if the colour of the offspring be intermediate or different -from that of the father, then the parents belong to distinct -species</i>.</p> - -<p>The reader may have already gathered that we have -here that bane of the advocate—the witness who proves -too much. But why does Professor Weldon confine von -Fischer to the few modest words recited above? That -author has—so far as colour is concerned—a complete -law of heredity supported by copious “observations.” -Why go further?</p> - -<p>Professor Weldon “brings forth these strong reasons” -of the rats and mice with the introductory sentence:</p> - -<div class="blockquot"> -<p>“Examples might easily be multiplied, but as before, I have -chosen rather to cite a few cases which rest on excellent authority, -than to quote examples which may be doubted. I would only -add one case among animals, in which the evidence concerning -the inheritance of colour is affected by the ancestry of the -varieties used.”</p> -</div> - -<p>So once again Professor Weldon suggests that his laws -of ancestry will explain even the discrepancies between -von Fischer on the one hand and Crampe and von Guaita<span class="pagenum" id="Page_177">177</span> -on the other but he does not tell us how he proposes to -apply them.</p> - -<p>In the cross between the albino and the grey von Fischer -tells us that both colours appear in the offspring, but always, -without exception or variation, that of the father only, in -5847 individuals.</p> - -<p>Surely, the law of ancestry, if he had a moment’s -confidence in it, might rather have warned Professor -Weldon that von Fischer’s results were wrong somewhere, -of which there cannot be any serious doubt. The precise -source of error is not easy to specify, but probably carelessness -and strong preconception of the expected result were -largely responsible, though von Fischer says he did all the -recording most carefully himself.</p> - -<p>Such then is the evidence resting “on excellent -authority”: may we some day be privileged to see the -“examples which may be doubted”?</p> - -<p>The case of mice, invoked by Professor Weldon, has -also been referred to in our Report. Its extraordinary -value as illustrating Mendel’s principles and the beautiful -way in which that case may lead on to extensions of those -principles are also there set forth (see the present -Introduction, p. <a href="#Page_25">25</a>). Most if not all of such “conflicting” -evidence can be reconciled by the steady application of -the Mendelian principle that the progeny will be constant -when—and only <span class="nowrap">when<a id="FNanchor_138" href="#Footnote_138" class="fnanchor">138</a></span>—<i>similar</i> gametes meet in fertilisation, -apart from any question of the characters of the -parent which produces those gametes.</p> -<p><span class="pagenum" id="Page_178">178</span></p> - -<h3>V. <span class="smcap">Professor Weldon’s quotations from Laxton.</span></h3> - -<p>In support of his conclusions Professor Weldon adduces -two passages from Laxton, some of whose testimony we -have just considered. This further evidence of Laxton -is so important that I reproduce it in full. The first -passage, published in 1866, is as follows:—</p> - -<div class="blockquot"> -<p>“The results of experiments in crossing the Pea tend to show -that the colour of the immediate offspring or second generation -sometimes follows that of the female parent, is sometimes -intermediate between that and the male parent, and is sometimes -distinct from both; and although at times it partakes of the -colour of the male, it has not been ascertained by the experimenter -ever to follow the exact colour of the male <span class="nowrap">parent<a id="FNanchor_139" href="#Footnote_139" class="fnanchor">139</a></span>. In shape, -the seed frequently has an intermediate character, but as often -follows that of either parent. In the second generation, in a -single pod, the result of a cross of Peas different in shape and -colour, the seeds are sometimes all intermediate, sometimes -represent either or both parents in shape or colour, and -sometimes both colours and characters, with their intermediates, -appear. The results also seem to show that the third generation -or the immediate offspring of a cross, frequently varies from its -parents in a limited manner—usually in one direction only, -but that the fourth generation produces numerous and wider -<span class="nowrap">variations<a id="FNanchor_140" href="#Footnote_140" class="fnanchor">140</a></span>; the seed often reverting partly to the colour and -character of its ancestors of the first generation, partly partaking -of the various intermediate colours and characters, and partly -sporting quite away from any of its ancestry.”</p> -</div> - -<p><span class="pagenum" id="Page_179">179</span></p> - -<p>Here Professor Weldon’s quotation ceases. It is unfortunate -he did not read on into the very next sentence -with which the paragraph concludes:—</p> - -<div class="blockquot"> -<p class="ti5">“These sports appear to become -fixed and permanent in the next and succeeding generations; -and the tendency to revert and sport thenceforth seems to -become checked if not absolutely <span class="nowrap">stopped<a id="FNanchor_141" href="#Footnote_141" class="fnanchor">141</a></span>.”</p> -</div> - -<p>Now if Professor Weldon instead of leaving off on the -word “ancestry” had noticed this passage, I think his article -would never have been written.</p> - -<p>Laxton proceeds:—</p> - -<div class="blockquot"> - -<p>“The experiments also tend to show that the height of the -plant is singularly influenced by crossing; a cross between two -dwarf peas, commonly producing some dwarf and some tall -[? in the second generation]; but on the other hand, a cross -between two tall peas does not exhibit a tendency to diminution -in height.</p> - -<p>“No perceptible difference appears to result from reversing -the parents; the influence of the pollen of each parent at the -climax or fourth generation producing similar <span class="nowrap">results<a id="FNanchor_142" href="#Footnote_142" class="fnanchor">142</a></span>.”</p> -</div> - -<p>The significance of this latter testimony I will presently -discuss.</p> - -<p>Professor Weldon next appeals to a later paper of -Laxton’s published in 1890. From it he quotes this passage:</p> - -<div class="blockquot"> - -<p>“By means, however, of cross-fertilisation alone, and unless it -be followed by careful and continuous selection, the labours of -the cross-breeder, instead of benefiting the gardener, may lead -to utter confusion,”</p> -</div> - -<p><span class="pagenum" id="Page_180">180</span></p> - -<p>Here again the reader would have gained had Professor -Weldon, instead of leaving off at the comma, gone on to -the end of the paragraph, which proceeds thus:—</p> - -<div class="blockquot"> -<p class="ti5">“because, as I have previously stated, -the Pea under ordinary conditions is much given to sporting -and reversion, for when two dissimilar old or fixed varieties -have been cross-fertilised, three or four generations at least -must, under the most favourable circumstances, elapse before -the progeny will become fixed or settled; and from one such -cross I have no doubt that, by sowing every individual Pea -produced during the three or four generations, hundreds of -different varieties may be obtained; but as might be expected, -I have found that where the two varieties desired to be -intercrossed are unfixed, confusion will become <span class="nowrap">confounded<a id="FNanchor_143" href="#Footnote_143" class="fnanchor">143</a></span>, -and the variations continue through many generations, the -number at length being utterly incalculable.”</p> -</div> - -<p>Professor Weldon declares that Laxton’s “experience -was altogether different from that of Mendel.” The reader -will bear in mind that when Laxton speaks of fixing a -variety he is not thinking particularly of seed-characters, -but of all the complex characters, fertility, size, flavour, -season of maturity, hardiness, etc., which go to make a -serviceable pea. Considered carefully, Laxton’s testimony -is so closely in accord with Mendelian expectation that -I can imagine no chance description in non-Mendelian -language more accurately stating the phenomena.</p> - -<p>Here we are told in unmistakable terms the breaking -up of the original combination of characters on crossing, -their re-arrangement, that at the fourth or fifth generation -the possibilities of sporting [sub-division of compound -allelomorphs and re-combinations of them?] are exhausted, -that there are then definite forms which if selected are<span class="pagenum" id="Page_181">181</span> -thenceforth fixed [produced by union of similar gametes?] -that it takes longer to select some forms [dominants?] -than others [recessives?], that there may be “mule” -<span class="nowrap">forms<a id="FNanchor_144" href="#Footnote_144" class="fnanchor">144</a></span> or forms which cannot be fixed at <span class="nowrap">all<a id="FNanchor_145" href="#Footnote_145" class="fnanchor">145</a></span> [produced -by union of dissimilar gametes?].</p> - -<p>But Laxton tells us more than this. He shows us that -numbers of varieties may be obtained—hundreds—“incalculable -numbers.” Here too if Professor Weldon had -followed Mendel with even moderate care he would have -found the secret. For in dealing with the crosses of -<i>Phaseolus</i> Mendel clearly forecasts the conception of -<i>compound characters themselves again consisting of definite -units</i>, all of which may be separated and re-combined in -the possible combinations, laying for us the foundation of -the new science of Analytical Biology.</p> - -<p>How did Professor Weldon, after reading Mendel, fail -to perceive these principles permeating Laxton’s facts? -Laxton must have seen the very things that Mendel saw, -and had he with his other gifts combined that penetration -which detects a great principle hidden in the thin mist of -“exceptions,” we should have been able to claim for him -that honour which must ever be Mendel’s in the history of -discovery.</p> - -<p>When Laxton speaks of selection and the need for it, -he means, what the raiser of new varieties almost always -means, the selection of <i>definite</i> forms, not impalpable -fluctuations. When he says that without selection there -will be utter confusion, he means—to use Mendelian terms—that<span class="pagenum" id="Page_182">182</span> -the plant which shows the desired combination of -characters must be chosen and bred from, and that if this -be not done the grower will have endless combinations -mixed together in his stock. If however such a selection -be made in the fourth or fifth generation the breeder may -very possibly have got a fixed form—namely, one that will -breed <span class="nowrap">true<a id="FNanchor_146" href="#Footnote_146" class="fnanchor">146</a></span>. On the other hand he may light on one -that does not breed true, and in the latter case it may be -that the particular type he has chosen is not represented -in the gametes and will <i>never</i> breed true, though selected -to the end of time. Of all this Mendel has given us the -simple and final account.</p> - -<p>At Messrs Sutton and Sons, to whom I am most -grateful for unlimited opportunities of study, I have seen -exactly such a case as this. For many years Messrs Sutton -have been engaged in developing new strains of the Chinese -Primrose (<i>Primula sinensis</i>, hort.). Some thirty thoroughly -distinct and striking varieties (not counting the <i>Stellata</i> -or “Star” section) have already been produced which -breed true or very nearly so. In 1899 Messrs Sutton -called my attention to a strain known as “Giant Lavender,” -a particularly fine form with pale magenta or lavender -flowers, telling me that it had never become fixed. On -examination it appeared that self-fertilised seed saved from -this variety gave some magenta-reds, some lavenders, and -some which are white on opening but tinge with very faint -pink as the flower matures.</p> - -<p>On counting these three forms in two successive years -the following figures appeared. Two separately bred -batches raised from “Giant Lavender” were counted in -each year.</p> - -<p><span class="pagenum" id="Page_183">183</span></p> - -<div class="tac"> -<table class=""> -<tr> -<td class="tac"></td> -<td class="tal"></td> -<td class="tac"></td> -<td class="tac prl1"><div>Magenta<br />red</div></td> -<td class="tac prl1"><div>Lavender</div></td> -<td class="tac prl1"><div>White<br /><span class="ilb">faintly tinged</span></div></td> -</tr> -<tr> -<td class="tac"><div>1901</div></td> -<td class="tal pl1">1st</td> -<td class="tac"><div>batch</div></td> -<td class="tac"><div>19</div></td> -<td class="tac"><div>27</div></td> -<td class="tac"><div>14</div></td> -</tr> -<tr> -<td class="tac"><div>"</div></td> -<td class="tal pl1"><div>2nd</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div> 9</div></td> -<td class="tac"><div>20</div></td> -<td class="tac"><div> 9</div></td> -</tr> -<tr> -<td class="tac"><div>1902</div></td> -<td class="tal pl1"><div>1st</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>12</div></td> -<td class="tac"><div>23</div></td> -<td class="tac"><div>11</div></td> -</tr> -<tr> -<td class="tac"><div>"</div></td> -<td class="tal pl1"><div>2nd</div></td> -<td class="tac"><div>"</div></td> -<td class="tac"><div>14</div></td> -<td class="tac"><div>26</div></td> -<td class="tac"><div>11</div></td> -</tr> -<tr> -<td class="tac"></td> -<td class="tal"></td> -<td class="tac"></td> -<td class="tac"><div>—</div></td> -<td class="tac"><div>—</div></td> -<td class="tac"><div>—</div></td> -</tr> -<tr> -<td class="tac"></td> -<td class="tal"></td> -<td class="tac"></td> -<td class="tac"><div>54</div></td> -<td class="tac"><div>96</div></td> -<td class="tac"><div>45</div></td> -</tr> -</table> -</div> - -<p>The numbers 54 : 96 : 45 approach the ratio 1 : 2 : 1 -so nearly that there can be no doubt we have here a simple -case of Mendelian laws, operating without definite dominance, -but rather with blending.</p> - -<p>When Laxton speaks of the “remarkably fine but -unfixable pea <i>Evolution</i>” we now know for the first time -exactly what the phenomenon meant. It, like the “Giant -Lavender,” was a “mule” form, not represented by germ-cells, -and in each year arose by “self-crossing.”</p> - -<p>This is only one case among many similar ones seen in -the Chinese Primrose. In others there is no doubt that -more complex factors are at work, the subdivision of -compound characters, and so on. The history of the -“Giant Lavender” goes back many years and is not -known with sufficient precision for our purposes, but -like all these forms it originated from crossings among -the old simple colour varieties of <i>sinensis</i>.</p> - - -<h3>VI. <span class="smcap">The Argument Built on Exceptions.</span></h3> - -<p>So much for the enormous advance that the Mendelian -principles already permit us to make. But what does -Professor Weldon offer to substitute for all this? Nothing.</p> - -<p>Professor Weldon suggests that a study of ancestry -will help us. Having recited Tschermak’s exceptions and<span class="pagenum" id="Page_184">184</span> -the great irregularities seen in the <i>Telephone</i> group, he -writes:</p> - -<div class="blockquot"> -<p>“Taking these results together with Laxton’s statements, -and with the evidence afforded by the <i>Telephone</i> group of -hybrids, I think we can only conclude that segregation of seed-characters -is not of universal occurrence among cross-bred peas, -and that when it does occur, it may or may not follow Mendel’s -law.”</p> -</div> - -<p>Premising that when pure types are used the exceptions -form but a small part of the whole, and that any supposed -absence of “segregation” may have been <i>variation</i>, this -statement is perfectly sound. He proceeds:—</p> - -<div class="blockquot"> -<p class="ti5">“The law of segregation, like the law of dominance, -appears therefore to hold only for races of <i>particular -ancestry</i> [my italics]. In special cases, other formulae expressing -segregation have been offered, especially by De Vries and by -Tschermak for other plants, but these seem as little likely to -prove generally valid as Mendel’s formula itself.</p> - -<p>“The fundamental mistake which vitiates all work based -upon Mendel’s method is the neglect of ancestry, and the -attempt to regard the whole effect upon offspring, produced by -a particular parent, as due to the existence in the parent of -particular structural characters; while the contradictory results -obtained by those who have observed the offspring of parents -identical in certain characters show clearly enough that not -only the parents themselves, but their race, that is their ancestry, -must be taken into account before the result of pairing them can -be predicted.”</p> -</div> - -<p>In this passage the Mendelian view is none too precisely -represented. I should rather have said that it was from -Mendel, first of all men, that we have learnt <i>not</i> to regard -the effects produced on offspring “as due to the existence -in the parent of particular structural characters.” We -have come rather to disregard the particular structure of<span class="pagenum" id="Page_185">185</span> -the parent except in so far as it may give us a guide as to -the nature of its gametes.</p> - -<p>This indication, if taken in the positive sense—as was -sufficiently shown in considering the significance of the -“mule” form or “hybrid-character”—we now know may -be absolutely worthless, and in any unfamiliar case is very -likely to be so. Mendel has proved that the inheritance -from individuals of <i>identical ancestry</i> may be entirely -different: that from identical ancestry, without new -variation, may be produced three kinds of individuals -(in respect of each pair of characters), namely, individuals -capable of transmitting one type, or another type, or both: -moreover that the statistical relations of these three classes -of individuals to each other will in a great number of cases -be a definite one: and of all this he shows a complete -account.</p> - -<p>Professor Weldon cannot deal with any part of this -phenomenon. He does little more than allude to it in -passing and point out exceptional cases. These he suggests -a study of ancestry will explain.</p> - -<p>As a matter of fact a study of ancestry will give little -guide—perhaps none—even as to the probability of the -phenomenon of dominance of a character, none as to the -probability of normal “purity” of germ-cells. Still less -will it help to account for fluctuations in dominance, or -irregularities in “purity.”</p> - - -<p class="tac mtb1em"><i>Ancestry and Dominance.</i></p> - -<p>In a series of astonishing paragraphs (pp. 241–2) Professor -Weldon rises by gradual steps, from the exceptional facts -regarding occasional dominance of green colour in <i>Telephone</i> -to suggest that the <i>whole phenomenon of dominance may be<span class="pagenum" id="Page_186">186</span> -attributable to ancestry</i>, and that in fact one character has no -natural dominance over another, apart from what has been -created by selection of ancestry. This piece of reasoning, -one of the most remarkable examples of special pleading to -be met with in scientific literature, must be read as a whole. -I reproduce it entire, that the reader may appreciate this -curious effort. The remarks between round parenthetical -marks are Professor Weldon’s, those between crotchets are -mine.</p> - -<div class="blockquot"> - -<p>“Mendel treats such characters as yellowness of cotyledons -and the like as if the condition of the character in two given -parents determined its condition in all their subsequent <span class="nowrap">offspring<a id="FNanchor_147" href="#Footnote_147" class="fnanchor">147</a></span>. -Now it is well known to breeders, and is clearly shown -in a number of cases by Galton and Pearson, that the condition -of an animal does not as a rule depend upon the condition of any -one pair of ancestors alone, but in varying degrees upon the -condition of all its ancestors in every past generation, the -condition in each of the half-dozen nearest generations having -a quite sensible effect. Mendel does not take the effect of -differences of ancestry into account, but considers that any -yellow-seeded pea, crossed with any green-seeded pea, will behave -in a certain definite way, whatever the ancestry of the green and -yellow peas may have been. (He does not say this in words, -but his attempt to treat his results as generally true of the -characters observed is unintelligible unless this hypothesis be -assumed.) The experiments afford no evidence which can be -held to justify this hypothesis. His observations on cotyledon -colour, for example, are based upon 58 cross-fertilised flowers, -all of which were borne upon ten plants; and we are not even -told whether these ten plants included individuals from more -than two races.</p> - -<p>“The many thousands of individuals raised from these ten<span class="pagenum" id="Page_187">187</span> -plants afford an admirable illustration of the effect produced -by crossing a few pairs of plants of known ancestry; but while -they show this perhaps better than any similar experiment, -they do not afford the data necessary for a statement as to the -behaviour of yellow-seeded peas in general, whatever their -ancestry, when crossed with green-seeded peas of any ancestry. -[Mendel of course makes no such statement.]</p> - -<p>“When this is remembered, the importance of the exceptions -to dominance of yellow cotyledon-colour, or of smooth and -rounded shape of seeds, observed by Tschermak, is much increased; -because although they form a small percentage of his -whole result, they form a very large percentage of the results -obtained with peas of certain races. [Certainly.] The fact that -<i>Telephone</i> behaved in crossing on the whole like a green-seeded -race of exceptional dominance shows that something other than -the mere character of the parental generation operated in this case. -Thus in eight out of 27 seeds from the yellow <span class="nowrap"><i>Pois d’Auvergne</i> ♀</span> -× <span class="nowrap"><i>Telephone</i> ♂</span> the cotyledons were yellow with green patches; -the reciprocal cross gave two green and one yellow-and-green -seed out of the whole ten obtained; and the cross <span class="nowrap"><i>Telephone</i> ♀</span> -× (yellow-seeded) <span class="nowrap"><i>Buchsbaum</i><a id="FNanchor_148" href="#Footnote_148" class="fnanchor">148</a> ♂</span> gave on one occasion two green -and four yellow seeds.</p> - -<p>“So the cross <i>Couturier</i> <span class="nowrap">(orange-yellow) ♀</span> × the green-seeded -<span class="nowrap"><i>Express</i> ♂</span> gave a number of seeds intermediate in colour. (It -is not clear from Tschermak’s paper whether <i>all</i> the seeds were -of this colour, but certainly some of them were.) The green -<i>Plein le Panier</i> <span class="nowrap">[<i>Fillbasket</i>] ♀</span> × <span class="nowrap"><i>Couturier</i> ♂</span> in three crosses -always gave either seeds of colour intermediate between green -and yellow, or some yellow and some green seeds in the same -pod. The cross reciprocal to this was not made; but <span class="nowrap"><i>Express</i> ♀</span> -× <span class="nowrap"><i>Couturier</i> ♂</span> gave 22 seeds of which four were yellowish -<span class="nowrap">green<a id="FNanchor_149" href="#Footnote_149" class="fnanchor">149</a></span>.</p> - -<p>“These facts show <i>first</i> that Mendel’s law of dominance -conspicuously fails for crosses between certain races, while it<span class="pagenum" id="Page_188">188</span> -appears to hold for others; and <i>secondly</i> that the intensity of a -character in one generation of a race is no trustworthy measure -of its dominance in hybrids. The obvious suggestion is that the -behaviour of an individual when crossed depends largely upon -the characters of its <span class="nowrap">ancestors<a id="FNanchor_150" href="#Footnote_150" class="fnanchor">150</a></span>. When it is remembered that -peas are normally self-fertilised, and that more than one named -variety may be selected out of the seeds of a single hybrid pod, -it is seen to be probable that Mendel worked with a very definite -combination of ancestral characters, and had no proper basis for -generalisation about yellow and green peas of any ancestry” -[which he never made].</p> -</div> - -<p>Let us pause a moment before proceeding to the climax. -Let the reader note we have been told of <i>two</i> groups of -cases in which dominance of yellow failed or was irregular. -(Why are not Gärtner’s and Seton’s “exceptions” -referred to here?) In one of these groups <i>Couturier</i> was -always one parent, either father or mother, and were it -not for Tschermak’s own obvious hesitation in regard to -his own exceptions (see p. <a href="#Page_148">148</a>), I would gladly believe -that <i>Couturier</i>—a form I do not know—may be an exceptional -variety. <i>How</i> Professor Weldon proposes to -explain its peculiarities by reference to ancestry he omits -to tell us. The <i>Buchsbaum</i> case is already disposed of, -for on Tschermak’s showing, it is an unstable form.</p> - -<p>Happily, thanks to Professor Weldon, we know rather -more of the third case, that of <i>Telephone</i>, which, whether -as father or mother, was frequently found by Tschermak to -give either green, greenish, or patchwork-seeds when crossed -with yellow varieties. It behaves, in short, “like a green-seeded -pea of exceptional dominance,” as we are now told. -For this dominant quality of <i>Telephone’s</i> greenness we are -asked to account <i>by appeal to its ancestry</i>. May we not<span class="pagenum" id="Page_189">189</span> -expect, then, this <i>Telephone</i> to be—if not a pure-bred green -pea from time immemorial—at least as pure-bred as other -green peas which do <i>not</i> exhibit dominance of green at all? -Now, what is <i>Telephone</i>? Do not let us ask too much. -Ancestry takes a lot of proving. We would not reject him -“<i>parce qu’il n’avait que soixante & onze quartiers, & que le -reste de son arbre généalogique avait été perdu par l’injure -du tems</i>.”</p> - -<p>But with stupefaction we learn from Professor Weldon -himself that <i>Telephone</i> is the very variety which he takes -<i>as his type of a permanent and incorrigible mongrel</i>, a -character it thoroughly deserves.</p> - -<p>From <i>Telephone</i> he made his colour scale! Tschermak -declares the cotyledons to be “yellowish or whitish green, -often entirely bright <span class="nowrap">yellow<a id="FNanchor_151" href="#Footnote_151" class="fnanchor">151</a></span>.” So little is it a thorough-bred -green pea, that it cannot always keep its own self-fertilised -offspring green. Not only is this pea a parti-coloured -mongrel, but Professor Weldon himself quotes -Culverwell that as late as 1882 both <i>Telegraph</i> and -<i>Telephone</i> “will always come from one sort, more especially -from the green variety”; and again regarding a supposed -good sample of <i>Telegraph</i> that “Strange to say, although -the peas were taken from one lot, those sown in January -produced a great proportion of the light variety known as -<i>Telephone</i>. These were of every shade of light green up to -white, and could have been shown for either variety,” <i>Gard. -Chron.</i> 1882 (2), p. 150. This is the variety whose green, -it is suggested, partially “dominates” over the yellow of -<i>Pois d’Auvergne</i>, a yellow variety which has a clear lineage -of about a century, and probably more. If, therefore, the -facts regarding <i>Telephone</i> have any bearing on the significance<span class="pagenum" id="Page_190">190</span> -of ancestry, they point the opposite way from that -in which Professor Weldon desires to proceed.</p> - -<p>In view of the evidence, the conclusion is forced upon -me that the suggestion that “ancestry” may explain the -facts regarding <i>Telephone</i> has no meaning behind it, but is -merely a verbal obstacle. Two words more on <i>Telephone</i>. -On p. <a href="#Page_147">147</a> I ventured to hint that if we try to understand -the nature of the appearance of green in the offspring of -<i>Telephone</i> bred with yellow varieties, we are more likely to -do so by comparing the facts with those of false hybridisation -than with fluctuations in dominance. In this -connection I would call the reader’s attention to a point -Professor Weldon misses, that Tschermak <i>also got yellowish-green -seeds from Fillbasket (green) crossed with Telephone</i>. -I suggest therefore that <i>Telephone’s</i> allelomorphs may be -in part transmitted to its offspring in a state which needs -no union with any corresponding allelomorph of the other -gamete, just as may the allelomorphs of “false hybrids.” -It would be quite out of place here to pursue this reasoning, -but the reader acquainted with special phenomena of -heredity will probably be able fruitfully to extend it. -It will be remembered that we have already seen the -further fact that the behaviour of <i>Telephone</i> in respect to -seed-shape was also peculiar (see p. <a href="#Page_152">152</a>).</p> - -<p>Whatever the future may decide on this interesting -question it is evident that with <i>Telephone</i> (and possibly -<i>Buchsbaum</i>) we are encountering a <i>specific</i> phenomenon, -which calls for specific elucidation and not a case simply -comparable with or contradicting the evidence of dominance -in general.</p> - -<p>In this excursion we have seen something more of the -“exceptions.” Many have fallen, but some still stand, -though even as to part of the remainder Tschermak entertains<span class="pagenum" id="Page_191">191</span> -some doubts, and, it will be remembered, cautions his -reader that of his exceptions some may be self-fertilisations, -and some did not <span class="nowrap">germinate<a id="FNanchor_152" href="#Footnote_152" class="fnanchor">152</a></span>. Truly a slender basis to -carry the coming structure!</p> - -<p>But Professor Weldon cannot be warned. He told us -the “law of dominance conspicuously fails for crosses -between certain races.” Thence the start. I venture to -give the steps in this impetuous argument. There are -<span class="nowrap">exceptions<a id="FNanchor_153" href="#Footnote_153" class="fnanchor">153</a></span>—a fair number if we count the bad ones—there -may be more—must be more—<i>are</i> more—no doubt many -more: so to the brink. Then the bold leap: may there -not be as many cases one way as the other? We have not -tried half the sorts of Peas yet. There is still hope. -True we know dominance of many characters in some -hundreds of crosses, using some twenty varieties—not to -speak of other plants and animals—but we <i>do</i> know some -exceptions, of which a few are still good. So dominance<span class="pagenum" id="Page_192">192</span> -may yet be all a myth, built up out of the petty facts those -purblind experimenters chanced to gather. Let us take -wider views. Let us look at fields more propitious—more -what we would have them be! Let us turn to eye-colour: -at least there is no dominance in that. Thus Professor -Weldon, telling us that Mendel “had no proper basis for -generalisation about yellow and green peas of any ancestry,” -proceeds to this lamentable passage:—</p> - -<div class="blockquot"> - -<p>“Now in such a case of alternative inheritance as that of -human eye-colour, it has been shown that a number of pairs of -parents, one of whom has dark and the other blue eyes, will -produce offspring of which nearly one half are dark-eyed, nearly -one half are blue-eyed, a small but sensible percentage being -children with mosaic eyes, the iris being a patch-work of -lighter and darker portions. But the dark-eyed and light-eyed -children are not equally distributed among all families; and it -would almost certainly be possible, by selecting cases of marriage -between men and women of appropriate ancestry, to demonstrate -for their families a law of dominance of dark over light eye-colour, -or of light over dark. Such a law might be as valid for the -families of selected ancestry as Mendel’s laws are for his peas -and for other peas of probably similar ancestral history, but it -would fail when applied to dark and light-eyed parents in -general,—that is, to parents of any ancestry who happen to -possess eyes of given colour.”</p> -</div> - -<p>The suggestion amounts to this: that because there -are exceptions to dominance in peas; and because by some -stupendous coincidence, or still more amazing incompetence, -a bungler might have thought he found dominance of -one eye-colour whereas really there was <span class="nowrap">none<a id="FNanchor_154" href="#Footnote_154" class="fnanchor">154</a></span>; therefore<span class="pagenum" id="Page_193">193</span> -Professor Weldon is at liberty to suggest there is a fair -chance that Mendel and all who have followed him have -either been the victims of this preposterous coincidence not -once, but again and again; or else persisted in the same -egregious and perfectly gratuitous blunder. Professor -Weldon is skilled in the Calculus of Chance: will he -compute the probabilities in favour of his hypothesis?</p> - - -<p class="tac mtb1em"><i>Ancestry and purity of germ-cells.</i></p> - -<p>To what extent ancestry is likely to elucidate dominance -we have now seen. We will briefly consider how laws -derived from ancestry stand in regard to segregation of -characters among the gametes.</p> - -<p>For Professor Weldon suggests that his view of ancestry -will explain the facts not only in regard to dominance and -its fluctuations but in regard to the purity of the germ-cells. -He does not apply this suggestion in detail, for its error -would be immediately exposed. In every strictly Mendelian -case the <i>ancestry</i> of the pure extracted recessives or -dominants, arising from the breeding of first crosses, is -identical with that of the impure dominants [or impure -recessives in cases where they exist]. Yet the posterity of -each is wholly different. The pure extracted forms, in -these simplest cases, are no more likely to produce the -form with which they have been crossed than was their -pure grandparent; while the impure forms break up again -into both grand-parental forms.</p> - -<p class="mt15em">Ancestry does not touch these facts in the least. They<span class="pagenum" id="Page_194">194</span> -and others like them have been a stumbling-block to all -naturalists. Of such paradoxical phenomena Mendel now -gives us the complete and final account. Will Professor -Weldon indicate how he proposes to regard them?</p> - -<p>Let me here call the reader’s particular attention to -that section of Mendel’s experiments to which Professor -Weldon does not so much as allude. Not only did Mendel -study the results of allowing his cross-breds (<i>DR</i>’s) to -fertilise themselves, giving the memorable ratio</p> - -<p class="tac"> -1 <i>DD</i> : 2 <i>DR</i> : 1 <i>RR</i>, -</p> - -<p>but he fertilised those cross-breds (<i>DR</i>’s) both with the -pure dominant (<i>D</i>) and with the pure recessive (<i>R</i>) -varieties reciprocally, obtaining in the former case the ratio</p> - -<p class="tac"> -1 <i>DD</i> : 1 <i>DR</i> -</p> - -<p>and in the latter the ratio</p> - -<p class="tac"> -1 <i>DR</i> : 1 <i>RR</i>. -</p> - -<p>The <i>DD</i> group and the <i>RR</i> group thus produced giving -on self-fertilisation pure <i>D</i> offspring and pure <i>R</i> offspring -respectively, while the <i>DR</i> groups gave again</p> - -<p class="tac"> -1 <i>DD</i> : 2 <i>DR</i> : 1 <i>RR</i>. -</p> - -<p>How does Professor Weldon propose to deal with these -results, and by what reasoning can he suggest that -considerations of ancestry are to be applied to them? -If I may venture to suggest what was in Mendel’s mind -when he applied this further test to his principles it -was perhaps some such considerations as the following. -Knowing that the cross-breds on self-fertilisation give</p> - -<p class="tac"> -1 <i>DD</i> : 2 <i>DR</i> : 1 <i>RR</i> -</p> - -<p>three explanations are possible:</p> - -<p><span class="pagenum" id="Page_195">195</span></p> - -<p class="ml2em">(<i>a</i>) These cross-breds may produce pure <i>D</i> germs of -both sexes and pure <i>R</i> germs of both sexes on an -average in equal numbers.</p> - -<p class="ml2em">(<i>b</i>) <i>Either</i> the female, <i>or</i> the male, gametes may be -<i>alone</i> differentiated according to the allelomorphs, -into pure <i>D</i>’s, pure <i>R</i>’s, and crosses <i>DR</i> or <i>RD</i>, the -gametes of the other sex being homogeneous and -neutral in regard to those allelomorphs.</p> - -<p class="ml2em">(<i>c</i>) There may be some neutralisation or cancelling -between characters in <i>fertilisation</i> occurring in such -a way that the well-known ratios resulted. The -absence of and inability to transmit the <i>D</i> character -in the <i>RR</i>’s, for instance, might have been due -not to the original purity of the germs constituting -them, but to some condition incidental to or connected -with fertilisation.</p> - -<p>It is clear that Mendel realized (<i>b</i>) as a possibility, for -he says <i>DR</i> was fertilised with the pure forms to test the -composition of its egg-cells, but the reciprocal crosses were -made to test the composition of the pollen of the hybrids. -Readers familiar with the literature will know that both -Gärtner and Wichura had in many instances shown that -the offspring of crosses in the form (<i>a</i> × <span class="nowrap"><i>b</i>) ♀</span> × <span class="nowrap"><i>c</i> ♂</span> were less -variable than those of crosses in the form <span class="nowrap"><i>a</i> ♀</span> × (<i>b</i> × <span class="nowrap"><i>c</i>) ♂</span>, -&c. This important fact in many cases is observed, and -points to differentiation of characters occurring frequently -among the male gametes when it does not occur or is much -less marked among the maternal gametes. Mendel of -course knew this, and proceeded to test for such a possibility, -finding by the result that differentiation was the -same in the gametes of both <span class="nowrap">sexes<a id="FNanchor_155" href="#Footnote_155" class="fnanchor">155</a></span>.</p> -<p><span class="pagenum" id="Page_196">196</span></p> -<p>Of hypotheses (<i>b</i>) and (<i>c</i>) the results of recrossing with -the two pure forms dispose; and we can suggest no -hypothesis but (<i>a</i>) which gives an acceptable account of the -facts.</p> - -<p>It is the purity of the “extracted” recessives and the -“extracted” dominants—primarily the former, as being -easier to recognize—that constitutes the real proof of the -validity of Mendel’s principle.</p> - -<p>Using this principle we reach immediately results of -the most far-reaching character. These theoretical deductions -cannot be further treated here—but of the -practical use of the principle a word may be said. Where-ever -there is marked dominance of one character the -breeder can at once get an indication of the amount of -trouble he will have in getting his cross-bred true to either -dominant or recessive character. He can only thus forecast -the future of the race in regard to each such pair of -characters taken severally, but this is an immeasurable -advance on anything we knew before. More than this, it -is certain that in some cases he will be able to detect the -“mule” or heterozygous forms by the statistical frequency -of their occurrence or by their structure, especially when -dominance is absent, and sometimes even in cases where -there is distinct dominance. With peas, the practical -seedsman cares, as it happens, little or nothing for those -simple characters of seed-structure, &c. that Mendel dealt -with. He is concerned with size, fertility, flavour, and -numerous similar characters. It is to these that Laxton -(invoked by Professor Weldon) primarily refers, when he -speaks of the elaborate selections which are needed to fix -his novelties.</p> - -<p>We may now point tentatively to the way in which -some even of these complex cases may be elucidated by an<span class="pagenum" id="Page_197">197</span> -extension of Mendel’s principle, though we cannot forget -that there are other undetected factors at work.</p> - - -<p class="tac mtb1em"><i>The value of the appeal to Ancestry.</i></p> - -<p>But it may be said that Professor Weldon’s appeal to -ancestry calls for more specific treatment. When he -suggests ancestry as “one great reason” for the different -properties displayed by different races or individuals, and -as providing an account of other special phenomena of -heredity, he is perhaps not to be taken to mean any -definite ancestry, known or hypothetical. He may, in -fact, be using the term “ancestry” merely as a brief -equivalent signifying the previous history of the race or -individual in question. But if such a plea be put forward, -the real utility and value of the appeal to ancestry is -even less evident than before.</p> - -<p>Ancestry, as used in the method of Galton and Pearson, -means a definite thing. The whole merit of that method -lies in the fact that by it a definite accord could be proved -to exist between the observed characters and behaviour -of specified descendants and the ascertained composition -of their pedigree. Professor Weldon in now attributing -the observed peculiarities of <i>Telephone</i> &c. to conjectural -peculiarities of pedigree—if this be his meaning—renounces -all that had positive value in the reference to ancestry. -His is simply an appeal to ignorance. The introduction of -the word “ancestry” in this sense contributes nothing. -The suggestion that ancestry might explain peculiarities -means no more than “we do not know how peculiarities are -to be explained.” So Professor Weldon’s phrase “peas of -probably similar ancestral <span class="nowrap">history<a id="FNanchor_156" href="#Footnote_156" class="fnanchor">156</a></span>” means “peas probably<span class="pagenum" id="Page_198">198</span> -similar”; when he speaks of Mendel having obtained his -results with “a few pairs of plants of known <span class="nowrap">ancestry<a id="FNanchor_157" href="#Footnote_157" class="fnanchor">157</a></span>,” he -means “a few pairs of known plants” and no more; when -he writes that “the law of segregation, like the law of -dominance appears to hold only for races of particular -<span class="nowrap">ancestry<a id="FNanchor_158" href="#Footnote_158" class="fnanchor">158</a></span>,” the statement loses nothing if we write simply -“for particular races.” We all know—the Mendelian, best -of all—that particular races and particular individuals -may, even though indistinguishable by any other test, -exhibit peculiarities in heredity.</p> - -<p>But though on analysis those introductions of the word -“ancestry” are found to add nothing, yet we can feel that -as used by Professor Weldon they are intended to mean a -great deal. Though the appeal may be confessedly to -ignorance, the suggestion is implied that if we did know -the pedigrees of these various forms we should then have -some real light on their present structure or their present -behaviour in breeding. Unfortunately there is not the -smallest ground for even this hope.</p> - -<p>As Professor Weldon himself tells <span class="nowrap">us<a id="FNanchor_159" href="#Footnote_159" class="fnanchor">159</a></span>, conclusions from -pedigree must be based on the conditions of the several -ancestors; and even more categorically (p. 244), “<i>The -degree to which a parental character affects offspring depends -not only upon its development in the individual parent, but -on its degree of development in the ancestors of that parent.</i>” -[My italics.] Having rehearsed this profession of an older -faith Professor Weldon proceeds to stultify it in his very -next paragraph. For there he once again reminds us that -<i>Telephone</i>, the mongrel pea of recent origin, which does not -breed true to seed characters, has yet manifested the peculiar -power of stamping the recessive characters on its cross-bred<span class="pagenum" id="Page_199">199</span> -offspring, though pure and stable varieties that have -exhibited the same characters in a high degree for -generations have <i>not</i> that power. As we now know, the -presence or absence of a character in a progenitor <i>may</i> be -no indication whatever as to the probable presence of the -character in the offspring; for the characters of the latter -depend on gametic and not on zygotic differentiation.</p> - -<p>The problem is of a different order of complexity from -that which Professor Weldon suggests, and facts like these -justify the affirmation that if we could at this moment -bring together the whole series of individuals forming the -pedigree of <i>Telephone</i>, or of any other plant or animal -known to be aberrant as regards heredity, we should have -no more knowledge of the nature of these aberrations; no -more prescience of the moment at which they would begin, -or of their probable modes of manifestation; no more -criterion in fact as to the behaviour such an individual -would exhibit in <span class="nowrap">crossing<a id="FNanchor_160" href="#Footnote_160" class="fnanchor">160</a></span>, or solid ground from which to -forecast its posterity, than we have already. We should -learn then—what we know already—that at some particular -point of time its peculiar constitution was created, -and that its peculiar properties then manifested themselves: -how or why this came about, we should no more comprehend -with the full ancestral series before us, than we can -in ignorance of the ancestry. Some cross-breds follow -Mendelian segregation; others do not. In some, palpable -dominance appears; in others it is absent.</p> - -<p>If there were no ancestry, there would be no posterity. -But to answer the question <i>why</i> certain of the posterity -depart from the rule which others follow, we must know, -not the ancestry, but how it came about <i>either</i> that at a<span class="pagenum" id="Page_200">200</span> -certain moment a certain gamete divided from its fellows in -a special and unwonted fashion; <i>or</i>, though the words are -in part tautological, the reason why the union of two particular -gametes in fertilisation took place in such a way that -gametes having new specific properties <span class="nowrap">resulted<a id="FNanchor_161" href="#Footnote_161" class="fnanchor">161</a></span>. No one -yet knows how to use the facts of ancestry for the elucidation -of these questions, or how to get from them a truth -more precise than that contained in the statement that a -diversity of specific consequences (in heredity) may follow -an apparently single specific disturbance. Rarely even can -we see so much. The appeal to ancestry, as introduced by -Professor Weldon, masks the difficulty he dare not face.</p> - -<p>In other words, it is the <i>cause of variation</i> we are here -seeking. To attack that problem no one has yet shown the -way. Knowledge of a different order is wanted for that -task; and a compilation of ancestry, valuable as the -exercise may be, does not provide that particular kind -of knowledge.</p> - -<p>Of course when once we have discovered by experiment -that—say, <i>Telephone</i>—manifests a peculiar behaviour in -heredity, we can perhaps make certain forecasts regarding -it with fair correctness; but that any given race or -individual will behave in such a way, is a fact not -deducible from its ancestry, for the simple reason that -organisms of identical ancestry may behave in wholly -distinct, though often definite, ways.</p> - -<p>It is from this hitherto hopeless paradox that Mendel -has begun at last to deliver us. The appeal to ancestry is -a substitution of darkness for light.</p> - -<p><span class="pagenum" id="Page_201">201</span></p> - - -<h3>VII. <span class="smcap">The question of absolute purity of germ-cells.</span></h3> - -<p>But let us go back to the cases of defective “purity” -and consider how the laws of ancestry stand in regard to -them. It appears from the facts almost certain that purity -may sometimes be wanting in a character which elsewhere -usually manifests it.</p> - -<p>Here we approach a question of greater theoretical -consequence to the right apprehension of the part borne -by Mendelian principles in the physiology of heredity. -We have to consider the question whether the purity of -the gametes in respect of one or other antagonistic character -is or is likely to be in case of <i>any</i> given character a -<i>universal</i> truth? The answer is unquestionably—No—but -for reasons in which “ancestry” plays no <span class="nowrap">part<a id="FNanchor_162" href="#Footnote_162" class="fnanchor">162</a></span>.</p> - -<p>Hoping to interest English men of science in the -Mendelian discoveries I offered in November 1900 a paper -on this subject to “Nature.” The article was of some -length and exceeded the space that the Editor could grant -without delay. I did not see my way to reduce it without -injury to clearness, and consequently it was returned to -me. At the time our own experiments were not ready for -publication and it seemed that all I had to say would -probably be common knowledge in the next few weeks, so -no further attempt at publication was made.</p> - -<p>In that article I discussed this particular question of -the absolute purity of the germ-cells, showing how, on -the analogy of other bud-variations, it is almost certain -that the germ-cells, even in respect to characters normally -Mendelian, may on occasion present the same mixture of -characters, whether apparently blended or mosaic, which<span class="pagenum" id="Page_202">202</span> -we know so well elsewhere. Such a fact would in nowise -diminish the importance of Mendel’s discovery. The fact -that mosaic peach-nectarines occur is no refutation of the -fact that the <i>total</i> variation is common. Just as there -may be trees with several such mosaic fruits, so there may -be units, whether varieties, individual plants, flowers or -gonads, or other structural units, bearing mosaic egg-cells -or pollen grains. Nothing is more likely or more in -accordance with analogy than that by selecting an individual -producing germs of blended or mosaic character, -a race could be established continuing to produce such -germs. Persistence of such blends or mosaics in <i>asexual</i> -reproduction is well-known to horticulturists; for example -“bizarre” carnations, oranges streaked with “blood”-orange -character, and many more. In the famous paper of -Naudin, who came nearer to the discovery of the Mendelian -principle than any other observer, a paper quoted by -Professor Weldon, other examples are given. These forms, -once obtained, can be multiplied <i>by division</i>; and there is -no reason why a zygote formed by the union of mosaic or -blended germs, once arisen, should not in the cell-divisions -by which its gametes are formed, continue to divide in a -similar manner and produce germs like those which united -to form that zygote. The irregularity, once begun, may -continue for an indefinite number of divisions.</p> - -<p>I am quite willing to suppose, with Professor Weldon -(p. 248), that the pea <i>Stratagem</i> may, as he suggests, be -such a case. I am even willing to accept provisionally as -probable that when two gametes, themselves of mosaic or -blended character, meet together in fertilisation, they are -more likely to produce gametes of mosaic or blended -character than of simply discontinuous character. Among -Messrs Sutton’s Primulas there are at least two striking<span class="pagenum" id="Page_203">203</span> -cases of “flaked” or “bizarre” unions of bright colours -and white which reproduce themselves by seed with fair -constancy, though Mendelian purity in respect of these -colours is elsewhere common in the varieties (I suspect -mosaics of “false hybridism” among allelomorphs in some -of these cases). Similarly Galton has shown that though -children having one light-eyed and one dark-eyed parent -generally have eyes either light or dark, the comparatively -rare medium eye-coloured persons when they mate together -frequently produce children with medium eye-colour.</p> - -<p>In this connection it may be worth while to allude to a -point of some practical consequence. We know that when -pure dominant—say yellow—is crossed with pure recessive—say -green—the dominance of yellow is seen; and we -have every reason to believe this rule generally (<i>not</i> -universally) true for pure varieties of peas. But we notice -that in the case of a form like the pea, depending on -human selection for its existence, it might be possible in -a few years for the races with pure seed characters to be -practically supplanted by the “mosaicized” races like the -<i>Telephone</i> group, if the market found in these latter some -specially serviceable quality. In the maincrop peas I -suspect this very process is taking <span class="nowrap">place<a id="FNanchor_163" href="#Footnote_163" class="fnanchor">163</a></span>. After such a<span class="pagenum" id="Page_204">204</span> -revolution it might be possible for a future experimenter to -conclude that <i>Pisum sativum</i> was by nature a “mosaicized” -species in these respects, though the mosaic character may -have arisen once in a seed or two as an exceptional -phenomenon. When the same reasoning is extended to -wild forms depending on other agencies for selection, some -interesting conclusions may be reached.</p> - -<p>But in Mendelian cases we are concerned primarily not -with the product of gametes of blended character, but with -the consequences of the union of gametes already discontinuously -dissimilar. The existence of pure Mendelian -gametes for given characters is perfectly compatible with -the existence of blended or mosaic gametes for similar -characters elsewhere, but this principle enables us to form -a comprehensive and fruitful conception of the relation of -the two phenomena to each other. As I also pointed -out, through the imperfection of our method which does -not yet permit us to <i>see</i> the differentiation among the -gametes though we know it exists, we cannot yet as a -rule obtain certain proof of the impurity of the gametes -(except perhaps in the case of mosaics) as distinct from -evidence of imperfect dominance. If however the case be -one of a “mule” form, distinct from either parent, and -not merely of dominance, there is no <i>a priori</i> reason why -even this may not be possible; for we should be able to<span class="pagenum" id="Page_205">205</span> -distinguish the results of breeding first crosses together -into <i>four</i> classes: two pure forms, one or more blend or -mosaic forms, and “mule” forms. Such a study could as -yet only be attempted in simplest cases: for where we are -concerned with a compound allelomorph capable of resolution, -the combinations of the integral components become -so numerous as to make this finer classification practically -inapplicable.</p> - -<p>But in many cases—perhaps a majority—though by -Mendel’s statistical method we can perceive the fluctuations -in the numbers of the several products of fertilisation, we -shall not know whether abnormalities in the distribution of -those products are due to a decline in dominance, or to -actual impurity of the gametes. We shall have further to -consider, as affecting the arithmetical results, the possibility -of departure from the rule that each kind of gamete is -produced in equal <span class="nowrap">numbers<a id="FNanchor_164" href="#Footnote_164" class="fnanchor">164</a></span>; also that there may be -the familiar difficulties in regard to possible selection and -assortative matings among the gametes.</p> - -<p>I have now shown how the mosaic and blend-forms are -to be regarded in the light of the Mendelian principle. -What has Professor Weldon to say in reference to them? -His suggestion is definite enough—that a study of ancestry -will explain the facts: <i>how</i>, we are not told.</p> - -<p>In speaking of the need of study of the characters of -the <i>race</i> he is much nearer the mark, but when he adds -“that is their ancestry,” he goes wide again. When -<i>Telephone</i> does not truly divide the antagonistic characters -among its germ-cells this fact is in nowise simply traceable -to its having originated in a cross—a history it shares with -almost all the peas in the market—but to its own peculiar<span class="pagenum" id="Page_206">206</span> -nature. In such a case imperfect dominance need not -surprise us.</p> - -<p>What we need in all these phenomena is a knowledge -of the properties of each race, or variety, as we call it in -peas. We must, as I have often pleaded, study the properties -of each form no otherwise than the chemist does the -properties of his substances, and thus only can we hope to -work our way through these phenomena. <i>Ancestry</i> holds -no key to these facts; for the same ancestry is common to -own brothers and sisters endowed with dissimilar properties -and producing dissimilar posterity. To the knowledge of -the properties of each form and the laws which it obeys -there are no short cuts. We have no periodic law to guide -us. Each case must as yet be separately worked out.</p> - -<p>We can scarcely avoid mention of a further category of -phenomena that are certain to be adduced in opposition to -the general truth of the purity of the extracted forms. It -is a fact well known to breeders that a highly-bred stock -may, unless selections be continued, “degenerate.” This -has often been insisted on in regard to peas. I have been -told of specific cases by Messrs Sutton and Sons, instances -which could be multiplied. Surely, will reply the supporters -of the theory of Ancestry, this is simply impurity in the -extracted stocks manifesting itself at last. Such a conclusion -by no means follows, and the proof that it is -inapplicable is obtained from the fact that the “degeneration,” -or variation as we should rather call it, need not -lead to the production of any proximate ancestor of the -selected stock at all, but immediately to a new form, or to -one much more remote—in the case of some high class peas, -<i>e.g.</i>, to the form which Mr Sutton describes as “vetch-like,” -with short pods, and a very few small round seeds, -two or three in a pod. Such plants are recognized by their<span class="pagenum" id="Page_207">207</span> -appearance and are rigorously hoed out every year before -seeding.</p> - -<p>To appreciate the meaning of these facts we must go -back to what was said above on the nature of compound -characters. We can perceive that, as Mendel showed, the -integral characters of the varieties can be dissociated and -re-combined in any combination. More than that; certain -integral characters can be resolved into further integral -components, by <i>analytical</i> variations. What is taking -place in this process of resolution we cannot surmise, but -we may liken the consequences of that process to various -phenomena of analysis seen elsewhere. To continue the -metaphor we may speak of return to the vetch-like type as -a <i>synthetical</i> variation: well remembering that we know -nothing of any <i>substance</i> being subtracted in the former -case or added in the latter, and that the phenomenon is -more likely to be primarily one of alteration in arrangement -than in substance.</p> - -<p>A final proof that nothing is to be looked for from an -appeal to ancestry is provided by the fact—of which the -literature of variation contains numerous illustrations—that -such newly synthesised forms, instead of themselves -producing a large proportion of the high class variety which -may have been their ancestor for a hundred generations, -may produce almost nothing but individuals like themselves. -A subject fraught with extraordinary interest will be the -determination whether by crossing these newly synthesised -forms with their parent, or another pure form, we may not -succeed in reproducing a great part of the known series of -components afresh. The pure parental form, produced, or -extracted, by “analytical” breeding, would not in ordinary -circumstances be capable of producing the other components -from which it has been separated; but by crossing it with<span class="pagenum" id="Page_208">208</span> -the “synthesised” variety it is not impossible that these -components would again reappear. If this can be shown -to be possible we shall have entirely new light on the nature -of variation and stability.</p> - - -<h3><span class="smcap">Conclusion.</span></h3> - -<p>I trust what I have written has convinced the reader that -we are, as was said in opening, at last beginning to move. -Professor Weldon declares he has “no wish to belittle the -importance of Mendel’s achievement”; he desires “simply -to call attention to a series of facts which seem to him to -suggest fruitful lines of inquiry.” In this purpose I venture -to assist him, for I am disposed to think that unaided he -is—to borrow Horace Walpole’s phrase—about as likely to -light a fire with a wet dish-clout as to kindle interest in -Mendel’s discoveries by his tempered appreciation. If I -have helped a little in this cause my time has not been -wasted.</p> - -<p>In these pages I have only touched the edge of that new -country which is stretching out before us, whence in ten -years’ time we shall look back on the present days of our -captivity. Soon every science that deals with animals and -plants will be teeming with discovery, made possible by -Mendel’s work. The breeder, whether of plants or of -animals, no longer trudging in the old paths of tradition, -will be second only to the chemist in resource and in -foresight. Each conception of life in which heredity bears -a part—and which of them is exempt?—must change before -the coming rush of facts.</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_209">209</span></p> - -<h2 class="nobreak" id="BIBLIOGRAPHY">BIBLIOGRAPHY.</h2> -</div> - - -<div class="blockquot"> - -<p>1. <span class="smcap">Correns, C. G.</span> Mendel’s Regel über das Verhalten der -Nachkommenschaft der Rassenbastarde, <i>Ber. deut. bot. -Ges.</i>, <span class="lowercase smcap">XVIII.</span>, 1900, p. 158.</p> - -<p>2. —— Gregor Mendel’s “Versuche über Pflanzen-Hybriden” -und die Bestätigung ihrer Ergebnisse durch die neuesten -Untersuchungen, <i>Bot. Ztg.</i>, 1900, p. 229.</p> - -<p>3. —— Ueber Levkoyenbastarde zur Kenntniss der Grenzen -der Mendel’schen Regeln, <i>Bot. Cblt.</i>, 1900, Vol. <span class="lowercase smcap">LXXXIV.</span>, -p. 97.</p> - -<p>4. —— Bastarde zwischen Maisrassen, mit besonderer Berücksichtigung -der Xenien, <i>Bibliotheca Botanica</i>, Hft. 53, -1901.</p> - -<p>5. <span class="smcap">Crampe.</span> Kreuzungen zwischen Wanderratten verschiedener -Farbe, <i>Landwirths. Jahrb.</i>, <span class="lowercase smcap">VI.</span>, 1877, p. 384.</p> - -<p>6. —— Zucht-Versuche mit zahmen Wanderratten. 1. Resultate -der Zucht in Verwandtschaft, <i>ibid.</i>, <span class="lowercase smcap">XII.</span>, 1883, -p. 389. 2. Resultate der Kreuzung der zahmen Ratten -mit wilden, <i>ibid.</i>, <span class="lowercase smcap">XIII.</span>, 1884, p. 699.</p> - -<p>7. —— Die Gesetze der Vererbung der Farbe, <i>ibid.</i>, <span class="lowercase smcap">XIV.</span>, -p. 539.</p> - -<p>8. <span class="smcap">Darwin, C.</span> <i>Variation of Animals and Plants under -Domestication</i>, ed. 2, <span class="lowercase smcap">I.</span>, pp. 348 and 428.</p> - -<p>9. <span class="smcap">Fischer, Johann von.</span> Die Säugethiere des S<sup>t</sup> Petersburger -Gouvernements, <i>Zool. Garten</i>, <span class="lowercase smcap">X.</span>, 1869, p. 336.</p> - -<p>10. —— Iltis (<i>Mustela putorius</i>) und Frett (<i>Mustela furo</i>), -<i>ibid.</i>, <span class="lowercase smcap">XIV.</span>, 1873, p. 108.</p> - -<p><span class="pagenum" id="Page_210">210</span></p> - -<p>11. <span class="smcap">Fischer, Johann von.</span> Beobachtungen über Kreuzungen -verschiedener Farbenspielarten innerhalb einer Species, -<i>ibid.</i>, <span class="lowercase smcap">XV.</span>, 1874, p. 361.</p> - -<p>12. <span class="smcap">Focke, W. O.</span> <i>Die Pflanzen-Mischlinge</i>, Bornträger, Berlin, -1881.</p> - -<p>13. —— Ueber dichotype Gewächse. <i>Oesterr. bot. Ztschr.</i>, -<span class="lowercase smcap">XVIII.</span>, 1868, p. 139.</p> - -<p>14. <span class="smcap">Galton, F.</span> <i>Natural Inheritance</i>, Macmillan and Co., -London, 1889.</p> - -<p>15. —— The Average Contribution of each several Ancestor -to the total Heritage of the Offspring, <i>Proc. Roy. Soc.</i>, -<span class="lowercase smcap">LXI.</span>, 1897, p. 401.</p> - -<p>16. <span class="smcap">Gärtner, C. F. von.</span> <i>Versuche und Beobachtungen über -die Bastarderzeugung im Pflanzenreich</i>, Stuttgart, -1849.</p> - -<p>17. <span class="smcap">Gitay, E.</span> Ueber den directen Einfluss des Pollens auf -Frucht- und Samenbildung, <i>Pringsheim’s JB. d. wiss. -Bot.</i>, <span class="lowercase smcap">XXV.</span>, 1893, p. 489.</p> - -<p>18. <span class="smcap">Godron, D. A.</span> Des Hybrides Végétaux, etc. <i>Ann. Sci. -Nat. Bot.</i>, Ser. 4, <span class="lowercase smcap">XIX.</span>, 1863, p. 135, and a series of -papers in <i>Mém. Acad. Stanislas</i>, Nancy, 1864, 1865, and -especially 1872.</p> - -<p>19. <span class="smcap">Guaita, G. von.</span> Versuche mit Kreuzungen von verschiedenen -Rassen der Hausmaus, <i>Ber. d. naturf. -Ges. Freiburg</i>, <span class="lowercase smcap">X.</span>, 1898, p. 317.</p> - -<p>20. —— Zweite Mittheilung, etc., <i>ibid.</i>, <span class="lowercase smcap">XI.</span>, 1900, p. 131.</p> - -<p>21. <span class="smcap">Knight, T. A.</span> An account of some experiments on the -Fecundation of Vegetables, <i>Phil. Trans.</i>, 1799, Pt. <span class="lowercase smcap">II.</span>, -p. 195.</p> - -<p>122. <span class="smcap">Küster, E.</span> Die Mendel’schen Regeln, ihre ursprüngliche -Fassung und ihre moderne Ergänzungen, <i>Biol. Cblt.</i>, -<span class="lowercase smcap">XXII.</span>, 1902, p. 129.</p> - -<p>23. <span class="smcap">Laxton, T.</span> Observations on the variations effected by -crossing in the colour and character of the seed of Peas, -<i>Internat. Hort. Exhib. and Bot. Congr.</i>, Report, 1866, -p. 156.</p> - -<p>24. —— Notes on some Changes and Variations in the<span class="pagenum" id="Page_211">211</span> -Offspring of Cross-fertilized Peas, <i>Jour. Hort. Soc.</i>, -N.S. <span class="lowercase smcap">III.</span>, 1872, p. 10.</p> - -<p>25. <span class="smcap">Laxton, T.</span> Improvement amongst Peas, <i>Jour. Hort. Soc.</i>, -1890, <span class="lowercase smcap">XII.</span>, 1, p. 29.</p> - -<p>26. <span class="smcap">Mendel, Gregor Johann.</span> Versuche über Pflanzen-Hybriden, -<i>Verh. naturf. Ver. in Brünn</i>, Band <span class="lowercase smcap">IV.</span>, 1865, -<i>Abhandlungen</i>, p. 1; reprinted in <i>Flora</i>, 1901, and -in Ostwald’s <i>Klassiker d. exakten Wiss.</i> English translation -in <i>Jour. R. Hort. Soc.</i>, 1901, <span class="lowercase smcap">XXVI.</span></p> - -<p>27. —— Ueber einige aus künstlicher Befruchtung gewonnenen -Hieracium-Bastarde, <i>ibid.</i>, <span class="lowercase smcap">VIII.</span>, 1869, <i>Abhandlungen</i>, -p. 26.</p> - -<p>28. <span class="smcap">Millardet.</span> Note sur l’hybridation sans croisement, ou -fausse hybridation, <i>Mém. Soc. Sci. Bordeaux</i>, Ser. 4, -<span class="lowercase smcap">IV.</span>, 1894, p. 347.</p> - -<p>29. <span class="smcap">C. Naudin.</span> Nouvelles recherches sur l’Hybridité dans les -Végétaux, <i>Nouv. Arch. Mus.</i>, <span class="lowercase smcap">I.</span>, 1865, p. 25.</p> - -<p>30. —— <i>Ann. sci. nat., Bot.</i>, Ser. 4, <span class="lowercase smcap">XIX.</span>, p. 180.</p> - -<p>31. <span class="smcap">Pearson, Karl.</span> On the Law of Ancestral Heredity, <i>Proc. -Roy. Soc.</i>, <span class="lowercase smcap">LXII.</span>, 1898, p. 386.</p> - -<p>32. —— On the Law of Reversion, <i>ibid.</i>, <span class="lowercase smcap">LXVI.</span>, 1900, p. -140.</p> - -<p>33. —— <i>The Grammar of Science</i>, second edition, London, -A. and Charles Black, 1900.</p> - -<p>34. —— Mathematical Contributions to the Theory of Evolution. -VIII. On the Inheritance of Characters not -capable of exact Measurement, <i>Phil. Trans. Roy. Soc.</i>, -1900, Vol. 195, p. 79.</p> - -<p>35. <span class="smcap">Rimpau.</span> Kreuzungsprodukte landw. Kulturpflanzen, -<i>Landw. Jahrb.</i>, <span class="lowercase smcap">XX.</span>, 1891.</p> - -<p>36. <span class="smcap">Tschermak, E.</span> Ueber künstliche Kreuzung bei <i>Pisum -sativum</i>, <i>Ztschrft. f. d. landwirths. Versuchswesen in -Oesterr.</i>, 1900, <span class="lowercase smcap">III.</span>, p. 465.</p> - -<p>37. —— Weitere Beiträge über Verschiedenwerthigkeit der -Merkmale bei Kreuzung von Erbsen and Bohnen, <i>ibid.</i>, -1901, <span class="lowercase smcap">IV.</span>, 641; <i>abstract in Ber. deut. bot. Ges.</i>, 1901, -<span class="lowercase smcap">XIX.</span>, p. 35.</p> - -<p><span class="pagenum" id="Page_212">212</span></p> - -<p>38. <span class="smcap">Tschermak, E.</span> Ueber Züchtung neuer Getreiderassen -mittelst künstlicher Kreuzung, <i>ibid.</i>, 1901, <span class="lowercase smcap">IV.</span>, p. 1029.</p> - -<p>39. <span class="smcap">Vilmorin-Andrieux and Co.</span> <i>Les Plantes Potagères</i>, 1st ed. -1883; 2nd ed. 1891.</p> - -<p>40. <span class="smcap">Vries, H. de.</span> Sur la loi de disjonction des hybrides, -<i>Comptes Rendus</i>, 26 March, 1900.</p> - -<p>41. —— Das Spaltungsgesetz der Bastarde, <i>Ber. deut. bot. -Ges.</i>, 1900, <span class="lowercase smcap">XVIII.</span>, p. 83.</p> - -<p>42. —— Ueber erbungleiche Kreuzungen, <i>ibid.</i>, p. 435.</p> - -<p>43. —— Sur les unités des caractères spécifiques et leur -application à l’étude des hybrides, <i>Rev. Gén. de Bot.</i>, -1900, <span class="lowercase smcap">XII.</span>, p. 257. See also by the same author, <i>Intracellulare -Pangenesis</i>, Jena, 1889, in which the conception -of unit-characters is clearly set forth.</p> - -<p>44. —— <i>Die Mutationstheorie</i>, Vol. <span class="lowercase smcap">I.</span>, Leipzig, 1901.</p> - -<p>45. <span class="smcap">Weldon, W. F. R.</span> Mendel’s Laws of Alternative Inheritance -in Peas, <i>Biometrika</i>, <span class="lowercase smcap">I.</span>, Pt. ii., 1902, p. 228.</p> - -<p>46. <span class="smcap">Wichura, Max.</span> Die Bastardbefruchtung im Pflanzenreich, -erläutert an den Bastarden der Weiden, Breslau, -1865.</p> -</div> - - -<p class="tac fs95 mtb1em"><i>Received as this sheet goes to press:—</i></p> - -<div class="blockquot"> - -<p><span class="smcap">Correns, C.</span> Die Ergebnisse der neuesten Bastardforschungen -für die Vererbungslehre, <i>Ber. deut. bot. Ges.</i>, <span class="lowercase smcap">XIX.</span>, Generalversammlungs-Heft -1.</p> - -<p>—— Ueber den Modus und den Zeitpunkt der Spaltung der -Anlagen bei den Bastarden vom Erbsen-Typus, <i>Bot. Ztg.</i>, -1902, p. 65.</p> -</div> - -<hr class="chap" /> - -<div class="footnotes"><h3>FOOTNOTES:</h3> - -<div class="footnote"> - -<p><a id="Footnote_1" href="#FNanchor_1" class="label">1</a> -<i>Biometrika</i>, <span class="lowercase smcap">I.</span>, 1902, Pt. <span class="lowercase smcap">II.</span></p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_2" href="#FNanchor_2" class="label">2</a> -<i>Biometrika</i>, <span class="lowercase smcap">I.</span> Pt. <span class="lowercase smcap">I.</span> p. 5.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_3" href="#FNanchor_3" class="label">3</a> -The first half of this paper is reprinted with additions and -modifications from the <i>Journal of the Royal Horticultural Society</i>, -1900, vol. <span class="lowercase smcap">XXV.</span>, parts 1 and 2. Written almost immediately after -the rediscovery of Mendel, it will be seen to be already in some -measure out of date, but it may thus serve to show the relation -of the new conceptions to the old.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_4" href="#FNanchor_4" class="label">4</a> -See later. Galton gave a simple diagrammatic representation of -his law in <i>Nature</i>, 1898, vol. <span class="lowercase smcap">LVII.</span> p. 293.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_5" href="#FNanchor_5" class="label">5</a> -These we now recognize as examples of Mendelian ‘dominance.’</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_6" href="#FNanchor_6" class="label">6</a> -<i>Comptes Rendus</i>, March 26, 1900, and <i>Ber. d. Deutsch. Bot. -Ges.</i> xviii. 1900, p. 83.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_7" href="#FNanchor_7" class="label">7</a> -This conception of discontinuity is of course pre-Mendelian.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_8" href="#FNanchor_8" class="label">8</a> -‘Versuche üb. Pflanzenhybriden’ in the <i>Verh. d. Naturf. Ver. -Brünn</i>, iv. 1865.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_9" href="#FNanchor_9" class="label">9</a> -Note that by these novel terms the complications involved by -use of the expression “prepotent” are avoided.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_10" href="#FNanchor_10" class="label">10</a> -Professor Weldon (p. 232) takes great exception to this statement, -which he considerately attributes to “some writers.” After -examining the conclusions he obtained by algebraical study of Mendel’s -figures I am disposed to think my statement not very far out.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_11" href="#FNanchor_11" class="label">11</a> -See later.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_12" href="#FNanchor_12" class="label">12</a> -Tschermak’s investigations were besides directed to a re-examination -of the question of the absence of beneficial results on cross-fertilising -<i>P. sativum</i>, a subject already much investigated by Darwin, -and upon this matter also important further evidence is given in -great detail.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_13" href="#FNanchor_13" class="label">13</a> -For simplicity the case of self-fertilisation is omitted from this -consideration.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_14" href="#FNanchor_14" class="label">14</a> -In all the cases discussed it is assumed that the gametes are -similar except in regard to the “heritage” they bear, and that no -<i>original</i> variation is taking place. The case of mosaics is also left -wholly out of account (see later).</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_15" href="#FNanchor_15" class="label">15</a> -The term “gamete” is now generally used as the equivalent of -“germ-cell,” whether male or female, and the term “zygote” is here -used for brevity to denote the organism resulting from fertilisation.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_16" href="#FNanchor_16" class="label">16</a> -In Pearson’s modification the parents contribute 0·3, the grandparents -0·15, the great-grandparents ·075.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_17" href="#FNanchor_17" class="label">17</a> -See the works referred to above.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_18" href="#FNanchor_18" class="label">18</a> -This conception was clearly formed by Naudin simultaneously -with Mendel, but it was not worked out by him and remained a mere -suggestion. In one place also Focke came very near to the same idea -(see Bibliography).</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_19" href="#FNanchor_19" class="label">19</a> -See von Guaita, <i>Ber. naturf. Ges. Freiburg</i> X. 1898 and XI. 1899, -quoted by Professor Weldon (see later).</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_20" href="#FNanchor_20" class="label">20</a> -This fact sufficiently indicates the difficulties involved in a -superficial treatment of the phenomenon of reversion. To call such -reversions as those named above “returns to ancestral type” would -be, if more than a descriptive phrase were intended, quite misleading. -It is not the ancestral <i>type</i> that has come back, but something else -has come in its guise, as the offspring presently prove. For the first -time we thus begin to get a rationale of “reversion.”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_21" href="#FNanchor_21" class="label">21</a> -It will be understood from what follows, that the existence of -mosaic zygotes is no <i>proof</i> that either component gamete was mosaic.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_22" href="#FNanchor_22" class="label">22</a> -A few additional particulars are given in Tschermak’s edition.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_23" href="#FNanchor_23" class="label">23</a> -[This translation was made by the Royal Horticultural Society, -and is reprinted with modifications and corrections, by permission. -The original paper was published in the <i>Verh. naturf. Ver. in Brünn, -Abhandlungen</i>, <span class="lowercase smcap">IV.</span> 1865, which appeared in 1866.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_24" href="#FNanchor_24" class="label">24</a> -[It is to the clear conception of these three primary necessities -that the whole success of Mendel’s work is due. So far as I know -this conception was absolutely new in his day.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_25" href="#FNanchor_25" class="label">25</a> -[Mendel uses the terms “albumen” and “endosperm” somewhat -loosely to denote the cotyledons, containing food-material, within the -seed.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_26" href="#FNanchor_26" class="label">26</a> -One species possesses a beautifully brownish-red coloured pod, -which when ripening turns to violet and blue. Trials with this -character were only begun last year. [Of these further experiments -it seems no account was published. Correns has since worked with -such a variety.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_27" href="#FNanchor_27" class="label">27</a> -[This is often called the Mummy Pea. It shows slight fasciation. -The form I know has white standard and salmon-red wings.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_28" href="#FNanchor_28" class="label">28</a> -[In my account of these experiments (<i>R.H.S. Journal</i>, vol. xxv. -p. 54) I misunderstood this paragraph and took “axis” to mean the -<i>floral</i> axis, instead of the main axis of the plant. The unit of -measurement, being indicated in the original by a dash (′), I carelessly -took to have been an <i>inch</i>, but the translation here given is -evidently correct.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_29" href="#FNanchor_29" class="label">29</a> -[It is somewhat surprising that no mention is made of Thrips, -which swarm in Pea flowers. I had come to the conclusion that this -is a real source of error and I see Laxton held the same opinion.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_30" href="#FNanchor_30" class="label">30</a> -[This also happens in Sweet Peas.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_31" href="#FNanchor_31" class="label">31</a> -[Mendel throughout speaks of his cross-bred Peas as “hybrids,” -a term which many restrict to the offspring of two distinct <i>species</i>. -He, as he explains, held this to be only a question of degree.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_32" href="#FNanchor_32" class="label">32</a> -[Note that Mendel, with true penetration, avoids speaking of -the hybrid-character as “transmitted” by either parent, thus escaping -the error pervading modern views of heredity.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_33" href="#FNanchor_33" class="label">33</a> -[Gärtner, p. 223.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_34" href="#FNanchor_34" class="label">34</a> -[It is much to be regretted that Mendel does not give the -complete series individually. No one who repeats such experiments -should fail to record the <i>individual</i> numbers, which on seriation are -sure to be full of interest.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_35" href="#FNanchor_35" class="label">35</a> -[This paragraph presents the view of the hybrid-character as -something incidental to the hybrid, and not “transmitted” to it—a -true and fundamental conception here expressed probably for the -first time.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_36" href="#FNanchor_36" class="label">36</a> -[This statement of Mendel’s in the light of present knowledge is -open to some misconception. Though his work makes it evident that -such varieties may exist, it is very unlikely that Mendel could have -had seven pairs of varieties such that the members of each pair -differed from each other in <i>only</i> one considerable character (<i>wesentliches -Merkmal</i>). The point is probably of little theoretical or practical -consequence, but a rather heavy stress is thrown on “<i>wesentlich</i>.”]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_37" href="#FNanchor_37" class="label">37</a> -[Note that Mendel does not state the cotyledon-colour of the -first crosses in this case; for as the coats were thick, it could not -have been seen without opening or peeling the seeds.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_38" href="#FNanchor_38" class="label">38</a> -[“False hybridism” was of course unknown to Mendel.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_39" href="#FNanchor_39" class="label">39</a> -[This and the preceding paragraph contain the essence of the -Mendelian principles of heredity.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_40" href="#FNanchor_40" class="label">40</a> -[To prove, namely, that both were similarly differentiated, and -not one or other only.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_41" href="#FNanchor_41" class="label">41</a> -[Whether segregation by such units is more than purely fortuitous -could probably be determined by seriation.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_42" href="#FNanchor_42" class="label">42</a> -[In the original the sign of equality (=) is here represented -by +, evidently a misprint.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_43" href="#FNanchor_43" class="label">43</a> -[This is the only passage where Mendel can be construed as -asserting universal dominance for <i>Pisum</i>; and even here, having -regard to the rest of the paper, it is clearly unfair to represent him as -predicating more than he had seen in his own experiments. Moreover -in flower and seed-coat colour (which is here meant), using his -characters dominance must be almost universal, if not quite.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_44" href="#FNanchor_44" class="label">44</a> -[It appears to me clear that this expression is incorrectly given, -and the argument regarding compound characters is consequently not -legitimately developed. The original compound character should be -represented as <i>A</i><sub>1</sub><i>A</i><sub>2</sub><i>A</i><sub>3</sub> . . . which when fertilised by <i>a</i><sub>1</sub> gives <i>A</i><sub>1</sub><i>A</i><sub>2</sub><i>A</i><sub>3</sub> . . . a -as the hybrid of the first generation. Mendel practically tells us -these were all alike, and there is nothing to suggest that they were -diverse. When on self-fertilisation, they break up, they will produce -the gametes he specifies; but they may also produce <i>A</i><sub>1</sub><i>A</i><sub>1</sub> and <i>A</i><sub>2</sub><i>A</i><sub>2</sub>, -<i>A</i><sub>1</sub><i>A</i><sub>2</sub><i>a</i>, &c., thereby introducing terms of a nature different from any -indicated by him. That this point is one of the highest significance, -both practical and theoretical, is evident at once.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_45" href="#FNanchor_45" class="label">45</a> -[It seems very doubtful if the zygotes are correctly represented by -the terms <i>A</i><sub>1</sub><i>aA</i><sub>2</sub><i>a</i>, <i>A</i><sub>2</sub><i>aa</i>, <i>A</i><sub>1</sub><i>aa</i>; for in the hybrids <i>A</i><sub>1</sub><i>a</i>, &c. the allelomorphs -<i>A</i><sub>1</sub> and <i>a</i>, &c. should by hypothesis be separated in the gametes.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_46" href="#FNanchor_46" class="label">46</a> -In <i>Pisum</i> it is placed beyond doubt that for the formation of the -new embryo a perfect union of the elements of both fertilising cells -must take place. How could we otherwise explain that among the -offspring of the hybrids both original types reappear in equal numbers -and with all their peculiarities? If the influence of the egg cell upon -the pollen cell were only external, if it fulfilled the <i>rôle</i> of a nurse -only, then the result of each artificial fertilisation could be no other -than that the developed hybrid should exactly resemble the pollen -parent, or at any rate do so very closely. This the experiments so far -have in no wise confirmed. An evident proof of the complete union -of the contents of both cells is afforded by the experience gained on -all sides that it is immaterial, as regards the form of the hybrid, -which of the original species is the seed parent or which the pollen -parent.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_47" href="#FNanchor_47" class="label">47</a> -“<i>Welche in den Grundzellen derselben in lebendiger Wechselwirkung -stehen.</i>”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_48" href="#FNanchor_48" class="label">48</a> -“<i>Dem einzelnen Beobachter kann leicht ein Differenziale entgehen.</i>”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_49" href="#FNanchor_49" class="label">49</a> -[The argument of these two last paragraphs appears to be that -though the general mutability of natural species might be doubtful, -yet among cultivated plants the transference of characters may be -accomplished, and may occur by integral steps until one species is -definitely “transformed” into the other.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_50" href="#FNanchor_50" class="label">50</a> -[Published in <i>Verh. naturf. Ver. Brünn, Abhandlungen</i>, <span class="lowercase smcap">VIII</span>. 1869, -p. 26, which appeared in 1870.]</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_51" href="#FNanchor_51" class="label">51</a> -The plant used in this experiment is not exactly the typical -<i>H. echioides</i>. It appears to belong to the series transitional to -<i>H. præaltum</i>, but approaches more nearly to <i>H. echioides</i> and for -this reason was reckoned as belonging to the latter.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_52" href="#FNanchor_52" class="label">52</a> -The words “general” and “universal” appear to be used by -Professor Weldon as interchangeable. Cp. Weldon, p. 235 and -elsewhere, with Abstract given below.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_53" href="#FNanchor_53" class="label">53</a> -These words occur p. 252: “The fundamental mistake which -vitiates all work based upon Mendel’s method is the neglect of -ancestry, and the attempt to regard the whole effect upon offspring produced -by a particular parent, as due to the existence in the parent of -particular structural characters, &c.” As a matter of fact the view -indicated in these last words is especially repugnant to the Mendelian -principle, as will be seen.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_54" href="#FNanchor_54" class="label">54</a> -I greatly regret that I have not a precise understanding of the -basis of the modification proposed by Pearson. His treatment is in -algebraical form and beyond me. Nevertheless I have every confidence -that the arguments are good and the conclusion sound. I trust it -may not be impossible for him to provide the non-mathematical reader -with a paraphrase of his memoir. The arithmetical differences between -the original and the modified law are of course clear.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_55" href="#FNanchor_55" class="label">55</a> -I have searched Professor Pearson’s paper in vain for any considerable -reservation regarding or modification of this general statement. -Professor Pearson enuntiates the law as “only correct on -certain limiting hypotheses,” but he declares that of these the most -important is “the absence of reproductive selection, i.e. the negligible -correlation of fertility with the inherited character, and the absence -of sexual selection.” The case of in-and-in breeding is also reserved.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_56" href="#FNanchor_56" class="label">56</a> -K. Pearson, <i>Grammar of Science</i>, 2nd ed. 1900, p. 36.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_57" href="#FNanchor_57" class="label">57</a> -<i>Grammar of Science</i>, 2nd ed. 1900, p. 480.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_58" href="#FNanchor_58" class="label">58</a> -<i>Phil. Trans.</i> 1900, vol. 195, A, p. 121.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_59" href="#FNanchor_59" class="label">59</a> -“If this be done, we shall, I venture to think, keep not only our -minds, but our points for observation, clearer; and further, the failure -of Mr Galton’s statement in the one case will not in the least affect -its validity in the other.” Pearson (32), p. 143.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_60" href="#FNanchor_60" class="label">60</a> -<i>Grammar of Science</i>, 1900, p. 494. See also Pearson, <i>Proc. Roy. -Soc.</i> 1900, <span class="lowercase smcap">LXVI.</span> pp. 142–3.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_61" href="#FNanchor_61" class="label">61</a> -On an average of cases, in equal numbers, as Mendel found.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_62" href="#FNanchor_62" class="label">62</a> -Read in this connexion Pearson, K., <i>Grammar of Science</i>, 2nd -ed. 1900, pp. 390–2.</p> - -<p>Professor Weldon even now opens his essay with the statement—or -perhaps reminiscence—that “it is perfectly possible and indeed -probable that the difference between these forms of inheritance -[blended, mosaic, and alternative] is only one of degree.” This may be -true; but reasoning favourable to this proposition could equally be -used to prove the difference between mechanical mixture and chemical -combination to be a difference of degree.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_63" href="#FNanchor_63" class="label">63</a> -The whole question as to seed-coat colour is most complex. -Conditions of growth and ripening have a great effect on it. Mr -Arthur Sutton has shown me samples of <i>Ne Plus Ultra</i> grown in -England and abroad. This pea has yellow cotyledons with seed-coats -either yellow or “blue.” The foreign sample contained a much -greater proportion of the former. He told me that generally speaking -this is the case with samples ripened in a hot, dry climate.</p> - -<p>Unquestionable Xenia appears occasionally, and will be spoken of -later. Moreover to experiment with such a <i>plant</i>-character an extra -generation has to be sown and cultivated. Consequently the evidence -is meagre.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_64" href="#FNanchor_64" class="label">64</a> -Knowing my interest in this subject Professor Weldon was -so good as to forward to me a series of his peas arranged to -form a scale of colours and shapes, as represented in his Plate I. -I have no doubt that the use of such colour-scales will much facilitate -future study of these problems.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_65" href="#FNanchor_65" class="label">65</a> -I notice that Vilmorin in the well-known <i>Plantes Potagères</i>, -1883, classifies the intermediate-coloured peas with the <i>green</i>.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_66" href="#FNanchor_66" class="label">66</a> -Similarly though <i>tall</i> and <i>dwarf</i> are Mendelian characters, peas -occur of all heights and are usually classified as tall, half-dwarfs, and -dwarfs.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_67" href="#FNanchor_67" class="label">67</a> -Wrinkling must of course be distinguished further from the -squaring due to the peas pressing against each other in the pod.</p> - -<p>In connexion with these considerations I may mention that -Vilmorin makes the interesting statement that most peas retain their -vitality three years, dying as a rule rapidly after that time is passed, -though occasionally seeds seven or eight years old are alive; but -that <i>wrinkled</i> peas germinate as a rule less well than round, and -do not retain their vitality so long as the round. Vilmorin-Andrieux, -<i>Plantes Potagères</i>, 1883, p. 423. Similar statements regarding the -behaviour of wrinkled peas in India are made by Firminger, <i>Gardening -for India</i>, 3rd ed. 1874, p. 146.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_68" href="#FNanchor_68" class="label">68</a> -Cotyledon-colour is not nearly so sensitive to ordinary changes -in conditions as coat-colour, provided the coat be uninjured. But -even in monomorphic <i>green</i> varieties, a seed which for any cause has -burst on ripening, has the exposed parts of its cotyledons <i>yellow</i>. -The same may be the case in seeds of green varieties injured by -<i>Bruchus</i> or birds. These facts make one hesitate before denying the -effects of conditions on the cotyledon-colour even of uninjured -seeds, and the variation described above may have been simply -weathering. The seeds were gathered very late and many were -burst in <i>Laxton’s Alpha</i>. I do not yet know they are alive.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_69" href="#FNanchor_69" class="label">69</a> -It is interesting to see that in at least one case the same—or -practically the same—variety has been independently produced by -different raisers, as we now perceive, by the fortuitous combination -of similar allelomorphs. <i>Sutton’s Ringleader</i> and <i>Carter’s First Crop</i> -(and two others) are cases in point, and it is peculiarly instructive to -see that in the discussion of these varieties when they were new, one -of the points indicating their identity was taken to be the fact that -they produced <i>the same “rogues.”</i> See <i>Gard. Chron.</i> 1865, pp. 482 and -603; 1866, p. 221; 1867, pp. 546 and 712.</p> - -<p>Rimpau quotes Blomeyer (<i>Kultur der Landw. Nutzpflanzen</i>, Leipzig, -1889, pp. 357 and 380) to the effect that <i>purple</i>-flowered plants with -<i>wrinkled</i> seeds may spring as direct sports from peas with <i>white</i> -flowers and <i>round</i> seeds. I have not seen a copy of Blomeyer’s -work. Probably this “wrinkling” was “indentation.”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_70" href="#FNanchor_70" class="label">70</a> -The asymmetries here conceived may of course be combined in -an inclusive symmetry. Till the differentiation can be optically -recognized in the gametes we shall probably get no further with this -part of the problem.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_71" href="#FNanchor_71" class="label">71</a> -<i>Materials for the Study of Variation</i>, 1894, p. 78.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_72" href="#FNanchor_72" class="label">72</a> -The varieties used were <i>Express</i>, <i>Laxton’s Alpha</i>, <i>Fillbasket</i>, -<i>McLean’s Blue Peter</i>, <i>Serpette nain blanc</i>, <i>British Queen</i>, <i>très nain -de Bretagne</i>, Sabre, <i>mange-tout</i> Debarbieux, and a large “grey” -sugar-pea, <i>pois sans parchemin géant à très large cosse</i>. Not counting -the last two, five are round and three are wrinkled. As to cotyledons, -six have yellow and four have green. In about 80 crosses I saw no -exception to dominance of yellow; but one apparently clear case of -dominance of wrinkled and some doubtful ones.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_73" href="#FNanchor_73" class="label">73</a> -Professor Weldon may take this as a famous blow for Mendel, -till he realizes what is meant by Mendel’s “Hybrid-character.”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_74" href="#FNanchor_74" class="label">74</a> -In addition to those spoken of later, where the great difference -between reciprocals is due to the <i>maternal</i> characters of the seeds.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_75" href="#FNanchor_75" class="label">75</a> -I have not here considered the case in which male and female -elements of a pure variety are not homologous and the variety is a -<i>permanent</i> monomorphic “mule.” Such a phenomenon, when present, -will prove itself in reciprocal crossing. I know no such case in -peas for certain.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_76" href="#FNanchor_76" class="label">76</a> -It will be understood that a “mule” form is quite distinct from -what is generally described as a “blend.” One certain criterion of -the “mule” form is the fact that it cannot be fixed, see p. 25. -There is little doubt that Laxton had such a “mule” form when he -speaks of “the remarkably fine but unfixable pea, Evolution.” <i>J. R. -Hort. Soc.</i> <span class="lowercase smcap">XII.</span> 1890, p. 37 (<i>v. infra</i>).</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_77" href="#FNanchor_77" class="label">77</a> -Using the word metaphorically.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_78" href="#FNanchor_78" class="label">78</a> -“<i>Ueber die Blüthezeit der Hybriden sind die Versuche noch nicht -abgeschlossen. So viel kann indessen schon angegeben werden, dass -dieselbe fast genau in der Mitte zwischen jener der Samen- und -Pollenpflanze steht, und die Entwicklung der Hybriden bezüglich -dieses Merkmales wahrscheinlich in der nämlichen Weise erfolgt, wie es -für die übrigen Merkmale der Fall ist.</i>” Mendel, p. 23.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_79" href="#FNanchor_79" class="label">79</a> -As has been already shown the discovery could have been -made equally well and possibly with greater rapidity in a case in -which the hybrid had a character distinct from either parent. The -cases that would <i>not</i> have given a clear result are those where there -is irregular dominance of one or other parent.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_80" href="#FNanchor_80" class="label">80</a> -Weldon, p. 240.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_81" href="#FNanchor_81" class="label">81</a> -See p. 43.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_82" href="#FNanchor_82" class="label">82</a> -In some transparent coats there is pigment, but so little as a -rule that xenia would be scarcely noticeable.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_83" href="#FNanchor_83" class="label">83</a> -Usually correlated characters, as Mendel knew.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_84" href="#FNanchor_84" class="label">84</a> -<i>Animals and Plants</i>, 2nd ed. 1885, p. 428.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_85" href="#FNanchor_85" class="label">85</a> -“<i>Eine andere Frage ist jedoch, ob der Einfluss des Pollens auf -den Keim schon äusserlich an diesen letzteren sichtbar sein kann. -Darwin führt mehrere hierher gehörige Fälle an, und wahrscheinlich -sind auch die Resultate der von Gärtner über diesen Gegenstand ausgeführten -Experimente hier zu erwähnen, wenn es auch nicht ganz -deutlich ist, ob der von Gärtner erwähnte directe Einfluss des Pollens -sich nur innerhalb der Grenzen des Keimes merklich macht oder nicht.</i>” -p. 490.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_86" href="#FNanchor_86" class="label">86</a> -Appendix to paper of Goss, <i>Trans. Hort. Soc.</i> v. 1822, pub. -1824 (<i>not</i> 1848, as given by Professor Weldon), p. 236.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_87" href="#FNanchor_87" class="label">87</a> -Since the above passage was written I find the “<i>Imperials</i>” -described in “Report of Chiswick Trials,” <i>Proc. R. Hort. Soc.</i> 1860, -<span class="lowercase smcap">I.</span> p. 340, as “skin thick”; and on p. 360 “skin thick, blue”; which -finally disposes of this “exception.”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_88" href="#FNanchor_88" class="label">88</a> -(36), p. 502 and (37), p. 663.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_89" href="#FNanchor_89" class="label">89</a> -Professor Weldon should have alluded to this. <i>Dead</i> seeds -have no bearing on these questions, seeing that their characters may -be pathological. The same seeds are later described as “<i>wie -Telephone selbst</i>,” so, apart from the possibility of death, they may -also have been self-fertilised.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_90" href="#FNanchor_90" class="label">90</a> -“<i>Vielleicht sind einige der l.c. 507 bis 508 erwähnten fraglichen -Fälle auf ähnliche vereinzelte Anomalien der Merkmalswerthigkeit -zu beziehen; einige erwiesen sich allerdings beim Anbau als Producte -ungewollter Selbstbefruchtung, andere keimten nicht.</i>”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_91" href="#FNanchor_91" class="label">91</a> -Regarding this case I have to thank Professor Correns for a -good deal of information which he kindly sent me in response to my -inquiry. I am thus able to supplement the published account in -some particulars.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_92" href="#FNanchor_92" class="label">92</a> -Mr Hurst, of Burbage, tells me that in varieties having coats -green or white, e.g. <i>American Wonder</i>, the white coats are mostly -from early, the green from later pods, the tints depending on -conditions and exposure.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_93" href="#FNanchor_93" class="label">93</a> -In the first case <i>Knight’s Marrow</i> with <i>Victoria</i>, both ways; in -the second <i>Victoria</i> with <i>Telephone</i>, both ways.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_94" href="#FNanchor_94" class="label">94</a> -Gärtner’s <i>macrospermum</i> was evidently one of these, though -from the further account (p. 498) it was probably more wrinkled. -There are of course <i>mange-touts</i> which have perfectly round seeds. -Mendel himself showed that the <i>mange-tout</i> character, the soft -constricted pod, was transferable. There are also <i>mange-touts</i> with -fully wrinkled seeds and “grey” peas with small seeds (see Vilmorin-Andrieux, -<i>Plantes Potagères</i>, 1883).</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_95" href="#FNanchor_95" class="label">95</a> -Correns found a similar result.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_96" href="#FNanchor_96" class="label">96</a> -“<i>Entweder kugelrund oder rundlich, die Einsenkungen, wenn -welche an der Oberfläche vorkommen, immer nur seicht, oder sie sind -unregelmässig kantig, tief runzlig</i> (<i>P. quadratum</i>).”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_97" href="#FNanchor_97" class="label">97</a> -The colour is the peculiarly deep yellow of the “grey” <i>mange-tout</i>.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_98" href="#FNanchor_98" class="label">98</a> -It is certainly subject to considerable changes according to -conditions. Those ripened in my garden are without exception much -larger and flatter than Vilmorin’s seeds (now two years old) from -which they grew. The colour of the coats is also much duller. These -changes are just what is to be expected from the English climate—taken -with the fact that my sample of this variety was late sown.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_99" href="#FNanchor_99" class="label">99</a> -Thus avoiding the error of Seton, see p. 144. There is no xenia -perhaps because the seed-coat of mother was a transparent coat.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_100" href="#FNanchor_100" class="label">100</a> -As heterozygotes often do.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_101" href="#FNanchor_101" class="label">101</a> -Dominance of the purple form.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_102" href="#FNanchor_102" class="label">102</a> -Dominance of the grey coat as a maternal character.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_103" href="#FNanchor_103" class="label">103</a> -Sherwood’s view (<i>J. R. Hort. Soc.</i> <span class="lowercase smcap">XXII.</span> p. 252) that this was the -origin of the “Wrinkled” pea, seems very dubious.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_104" href="#FNanchor_104" class="label">104</a> -It will be well known to all practical horticulturalists that -Laxton, originally of Stamford, made and brought out a large number -of the best known modern peas. The firm is now in Bedford.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_105" href="#FNanchor_105" class="label">105</a> -A round white ♀ × grey ♂ giving the usual result, round, “white” -(yellow) seeds.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_106" href="#FNanchor_106" class="label">106</a> -Tall heterozygotes, with normal dominance of purple flowers.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_107" href="#FNanchor_107" class="label">107</a> -Here we see dominance of the <i>pigmented</i> seed-coat as a maternal -character over <i>white</i> seed-coat. The colours of the seed-coats are -described as essentially two: maple or brown-streaked, and violet, the -latter being a small minority. As the sequel shows, the latter are -heterozygotes, not breeding true. Now Mendel found, and the fact -has been confirmed both by Correns and myself, that crossing a grey -pea which is capable of producing purple leads to such production as -a form of xenia.</p> - -<p>We have here therefore in the purple seeds the union of dissimilar -gametes, with production of xenia. But as the brown-streaked seeds -are also in part heterozygous, the splitting of a compound allelomorph -has probably taken place, though without precise statistics and -allotment of offspring among the several seeds the point is uncertain. -The colour of seed-coats in “grey” peas and probably “maples” also -is, as was stated on p. 150, sensitive to conditions, but the whole -difference between “maples” and purple is too much to attribute -safely to such irregularity. “Maple” is the word used to describe -certain seed-coats which are pigmented with intricate brown mottlings -on a paler buff ground. In French they are <i>perdrix</i>.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_108" href="#FNanchor_108" class="label">108</a> -This is not, as it stands, explicable. It seems from this point -and also from what follows that if the account is truly given, some -of the plants may have been mosaic with segregation of characters in -particular flowers; but see subsequent note.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_109" href="#FNanchor_109" class="label">109</a> -As, commonly, in heterozygotes when fertile.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_110" href="#FNanchor_110" class="label">110</a> -Recessive in flower-colour, seed-coat colour, and in seed-shape -as a maternal character: pure recessives as the sequel proved.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_111" href="#FNanchor_111" class="label">111</a> -These are then a mixture of pure dominants and cross-bred -dominants, and are now inextricably confused. This time the round -seeds may have been all on particular plants—showing recessive seed-shape -as a maternal character. It seems just possible that this -fact suggested the idea of “round” seeds on the <i>coloured</i> plants in -the last generation. Till that result is confirmed it should be -regarded as very doubtful on the evidence. But we cannot at the -present time be sure how much difference there was between these -round seeds and the <i>normal</i> maples in point of shape; and on the -whole it seems most probable that the roundness was a mere fluctuation, -such as commonly occurs among the peas with large indented -seeds.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_112" href="#FNanchor_112" class="label">112</a> -Is this really evidence of segregation of characters, the flower -being the unit? In any case the possibility makes the experiment -well worth repeating, especially as Correns has seen a phenomenon -conceivably similar.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_113" href="#FNanchor_113" class="label">113</a> -Being a mixture of heterozygotes (probably involving several -pairs of allelomorphs) and homozygotes.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_114" href="#FNanchor_114" class="label">114</a> -This looks as if the violet colour was merely due to irregularity -of xenia.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_115" href="#FNanchor_115" class="label">115</a> -Pure recessives.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_116" href="#FNanchor_116" class="label">116</a> -Pure recessives in coats showing maternal dominant character.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_117" href="#FNanchor_117" class="label">117</a> -Now recognized as pure homozygotes.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_118" href="#FNanchor_118" class="label">118</a> -This seems almost certainly segregation by flower-units, and is -as yet inexplicable on any other hypothesis. Especially paradoxical -is the presence of “white” seeds on these plants. The impression is -scarcely resistible that some remarkable phenomenon of segregation -was really seen here.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_119" href="#FNanchor_119" class="label">119</a> -Being now homozygotes.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_120" href="#FNanchor_120" class="label">120</a> -Being heterozygotes exclusively.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_121" href="#FNanchor_121" class="label">121</a> -The nature of this mistake is now clear; for as stated above -xenia is only likely to occur when the maternal seed-coat is pigmented. -The violet coats in this experiment are themselves cases of xenia.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_122" href="#FNanchor_122" class="label">122</a> -Knight, it was seen, crossed round ♀ × indented ♂ and consequently -got no change of form.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_123" href="#FNanchor_123" class="label">123</a> -Cotyledons seen through coat.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_124" href="#FNanchor_124" class="label">124</a> -Ordinary dominance of round.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_125" href="#FNanchor_125" class="label">125</a> -This is an extraordinary statement to be given as a general -truth. There are sometimes indications of this kind, but certainly -the facts are not usually as here stated.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_126" href="#FNanchor_126" class="label">126</a> -If we were obliged to suppose that this is a matured conclusion -based on detailed observation it would of course constitute the most -serious “exception” yet recorded. But it is clear that the five -statements are not mutually consistent. We have dominance of -round white in first cross.</p> - -<p>In the second generation blue wrinkled give only blue wrinkled, -and blue round give blue wrinkled and round, in accordance with -general experience. But we are told that white round give <i>only</i> -white round. This would be true of some white rounds, but not, -according to general experience, of all. Lastly we are told <i>white -wrinkled give all four classes</i>. If we had not been just told by -Laxton that the first cross showed dominance of white round, and -that blue wrinkled and blue round give the Mendelian result, I should -hesitate in face of this positive statement, but as it is inconsistent with -the rest of the story I think it is unquestionably an error of statement. -The context, and the argument based on the maple crosses show -clearly also what was in Laxton’s mind. He plainly expected the -characters of the original pure varieties to separate out according to -their original combinations, and this expectation confused his -memory and general impressions. This, at least, until any such -result is got by a fresh observer, using strict methods, is the only -acceptable account.</p> - -<p>Of the same nature is the statement given by the late Mr Masters -to Darwin (<i>Animals and Plants</i>, <span class="lowercase smcap">I.</span> p. 318) that blue round, white round, -blue wrinkled, and white wrinkled, all reproduced all four sorts during -successive years. Seeing that one sort would give all four, and two -would give two kinds, without special counting such an impression -might easily be produced. There are the further difficulties due to -seed-coat colour, and the fact that the distinction between round and -wrinkled may need some discrimination. The sorts are not named, -and the case cannot be further tested.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_127" href="#FNanchor_127" class="label">127</a> -See later.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_128" href="#FNanchor_128" class="label">128</a> -The number in Haage and Schmidt’s list exceeds 200, counting -colour-varieties.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_129" href="#FNanchor_129" class="label">129</a> -The original passage is in <i>Landwirths. Versuchstationen</i>, 1888, -<span class="lowercase smcap">XXXV.</span> [<i>not</i> <span class="lowercase smcap">XXXIV.</span>], p. 151.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_130" href="#FNanchor_130" class="label">130</a> -“<i>Es ist sogar sehr schwierig, einen Unterschied in der Farbe der -Kreuzungsprodukte von Karmin und Weiss gegenüber Dunkelblau oder -Violett und Weiss zu erkennen.</i>”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_131" href="#FNanchor_131" class="label">131</a> -See also the case of <i>Buchsbaum</i>, p. 146, which received similar -treatment.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_132" href="#FNanchor_132" class="label">132</a> -One of the peculiarities of most <i>double</i> “sulphur” races is that -the singles they throw are <i>white</i>. See Vilmorin, <i>Fleurs de pleine -Terre</i>, 1866, p. 354, <i>note</i>. In <i>Wien. Ill. Gartenztg.</i> 1891, p. 74, -mention is made of a new race with singles also “sulphur,” cp. -<i>Gartenztg.</i> 1884, p. 46. Messrs Haage and Schmidt have kindly -written to me that this new race has the alleged property, but that -six other yellow races (two distinct colours) throw their singles white.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_133" href="#FNanchor_133" class="label">133</a> -<i>Biol. Cblt.</i> <span class="lowercase smcap">XIV.</span> 1894, p. 79.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_134" href="#FNanchor_134" class="label">134</a> -The various “contradictions” which Professor Weldon suggests -exist between Crampe, von Guaita and Colladon can almost certainly -be explained by this circumstance. For Professor Weldon “wild-coloured” -mice, however produced, are “wild-coloured” mice and -no more (see Introduction).</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_135" href="#FNanchor_135" class="label">135</a> -“Das Resultat einer Kreuzung zwischen Albino- und Normal-form -ist stets, also, constant, ein dem Vater mindestens in der -Färbung gleiches Junge.” This law is predicated for the case in -which both parents belong to the same species.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_136" href="#FNanchor_136" class="label">136</a> -“Dieses Alles ist aber <i>nie</i> der Fall bei Kreuzungen unter -Leucismen und normalen Thieren innerhalb der Species, bei denen -<i>stets und ohne jede Ausnahme die Jungen in Färbung dem Vater -gleichen</i>.”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_137" href="#FNanchor_137" class="label">137</a> -He even withdraws two cases of his own previously published, -in which grey and albino mice were alleged to have given mixtures, -saying that this result must have been due to the broods having -been accidentally mixed by the servants in his absence.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_138" href="#FNanchor_138" class="label">138</a> -Excluding “false hybridisations.”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_139" href="#FNanchor_139" class="label">139</a> -This is of course on account of the maternal seed characters. -Unless the coat-characters are treated separately from the cotyledon-characters -Laxton’s description is very accurate. Both this and the -statements respecting the “shape” of the seeds, a term which as used -by Laxton means much more than merely “wrinkled” and “smooth,” -are recognizably true as general statements.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_140" href="#FNanchor_140" class="label">140</a> -Separation of hypallelomorphs.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_141" href="#FNanchor_141" class="label">141</a> -The combinations being exhausted. Perhaps Professor Weldon -thought his authority was here lapsing into palpable nonsense!</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_142" href="#FNanchor_142" class="label">142</a> -Laxton constantly refers to this conception of the “climax” of—as -we now perceive—analytical variation and recombination. Many -citations could be given respecting his views on this “climax” (cp. -p. 167).</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_143" href="#FNanchor_143" class="label">143</a> -Further subdivision and recombination of hypallelomorphs.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_144" href="#FNanchor_144" class="label">144</a> -For instance the <i>talls</i> produced by crossing <i>dwarfs</i> are such -“mules.” Tschermak found in certain cases distinct increase in -height in such a case, though not always (p. 531).</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_145" href="#FNanchor_145" class="label">145</a> -“The remarkably fine but unfixable pea <i>Evolution</i>.” Laxton, -p. 37.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_146" href="#FNanchor_146" class="label">146</a> -Apart from fresh original variations, and perhaps in some cases -imperfect homozygosis of some hypallelomorphs.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_147" href="#FNanchor_147" class="label">147</a> -Mendel, on the contrary, disregards the “condition of the -character” in the parent altogether; but is solely concerned with the -nature of the characters of the <i>gametes</i>.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_148" href="#FNanchor_148" class="label">148</a> -Regarding this “exception” see p. 146.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_149" href="#FNanchor_149" class="label">149</a> -See p. 148.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_150" href="#FNanchor_150" class="label">150</a> -Where was that “logician,” the “consulting-partner,” when -this piece of reasoning passed the firm?</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_151" href="#FNanchor_151" class="label">151</a> -“<i>Speichergewebe gelblich—oder weisslich—grün, manchmal auch -vollständig hellgelb.</i>” Tschermak (36), p. 480.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_152" href="#FNanchor_152" class="label">152</a> -In his latest publication on this subject, the notes to the -edition of Mendel in Ostwald’s <i>Klassiker</i> (pp. 60–61), Tschermak, -who has seen more true exceptions than any other observer, thus -refers to them. As to dominance:—“<i>Immerhin kommen vereinzelt -auch zweifellose Fälle von Merkmalmischung, d. h. Uebergangsformen -zwischen gelber und grüner Farbe, runder und runzeliger Form vor, -die sich in weiteren Generationen wie dominantmerkmalige Mischlinge -verhalten.</i>” As to purity of the extracted recessives:—<i>Ganz vereinzelt -scheinen Ausnahmsfälle vorzukommen.</i>"</p> - -<p>Küster (22) also in a recent note on Mendelism points out, with -reason, that the number of “exceptions” to dominance that we -shall find, depends simply on the stringency with which the supposed -“law” is drawn. The same writer remarks further that Mendel -makes no such rigid definition of dominance as his followers have -done.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_153" href="#FNanchor_153" class="label">153</a> -If the “logician-consulting-partner” will successfully apply this -<i>Fallacia acervalis</i>, the “method of the vanishing heap,” to dominant -peas, he will need considerable leisure.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_154" href="#FNanchor_154" class="label">154</a> -I have no doubt there is no universal dominance in eye-colour. -Is it <i>quite</i> certain there is no dominance at all? I have searched -the works of Galton and Pearson relating to this subject without -finding a clear proof. If there is in them material for this decision -may perhaps be pardoned for failing to discover it, since the tabulations -are not prepared with this point in view. Reference to the -original records would soon clear up the point.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_155" href="#FNanchor_155" class="label">155</a> -See Wichura (46), pp. 55–6.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_156" href="#FNanchor_156" class="label">156</a> -See above, p. 192.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_157" href="#FNanchor_157" class="label">157</a> -See above, p. 187.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_158" href="#FNanchor_158" class="label">158</a> -See above, p. 184.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_159" href="#FNanchor_159" class="label">159</a> -See above, p. 186.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_160" href="#FNanchor_160" class="label">160</a> -Beyond an indication as to the homogeneity or “purity” of its -gametes at a given time.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_161" href="#FNanchor_161" class="label">161</a> -May there be a connection between the extraordinary fertility -and success of the <i>Telephone</i> group of peas, and the peculiar frequency -of a blended or mosaic condition of their allelomorphs? The conjecture -may be wild, but it is not impossible that the two phenomena -may be interdependent.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_162" href="#FNanchor_162" class="label">162</a> -This discussion leaves “false hybridism” for separate consideration.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_163" href="#FNanchor_163" class="label">163</a> -Another practical point of the same nature arises from the great -variability which these peas manifest in plant- as well as seed-characters. -Mr Hurst of Burbage tells me that in <i>e.g.</i> <i>William the -First</i>, a pea very variable in seed-characters also, tall plants may be -so common that they have to be rogued out even when the variety is -grown for the vegetable market, and that the same is true of several -such varieties. It seems by no means improbable that it is by such -roguing that the unstable mosaic or blend-form is preserved. In a -thoroughly stable variety such as <i>Ne Plus Ultra</i> roguing is hardly -necessary even for the seed-market.</p> - -<p>Mr N. N. Sherwood in his useful account of the origin and races -of peas (<i>Jour. R. Hort. Soc.</i> <span class="lowercase smcap">XXII.</span> 1899, p. 254) alludes to the great -instability of this class of pea. To Laxton, he says, “we are indebted -for a peculiar type of Pea, a round seed with a very slight indent, the -first of this class sent out being <i>William the First</i>, the object being to -get a very early blue-seeded indented Pea of the same earliness as the -Sangster type with a blue seed, or in other words with a Wrinkled Pea -flavour. This type of Pea is most difficult to keep true on account of -the slight taint of the Wrinkled Pea in the breed, which causes it to -run back to the Round variety.”</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_164" href="#FNanchor_164" class="label">164</a> -In dealing with cases of decomposition or resolution of compound -characters this consideration is of highest importance.</p> - -</div> -</div> - -<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK MENDEL'S PRINCIPLES OF HEREDITY ***</div> -<div style='text-align:left'> - -<div style='display:block; margin:1em 0'> -Updated editions will replace the previous one—the old editions will -be renamed. -</div> - -<div style='display:block; margin:1em 0'> -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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