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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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