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diff --git a/30701.txt b/30701.txt new file mode 100644 index 0000000..ce78e49 --- /dev/null +++ b/30701.txt @@ -0,0 +1,3661 @@ +The Project Gutenberg EBook of A Critique of the Theory of Evolution, by +Thomas Hunt Morgan + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: A Critique of the Theory of Evolution + +Author: Thomas Hunt Morgan + +Release Date: December 17, 2009 [EBook #30701] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK CRITIQUE OF THEORY OF EVOLUTION *** + + + + +Produced by Bryan Ness, Keith Edkins, and the Online +Distributed Proofreading Team at https://www.pgdp.net. Pages +scanned by Bryan Ness. + + + + + +Transcriber's note: A few typographical errors have been corrected: they +are listed at the end of the text. + + * * * * * + + +Princeton University + +THE LOUIS CLARK VANUXEM FOUNDATION +LECTURES FOR 1915-1916 + + * * * * * + +The Louis Clark Vanuxem Foundation of Princeton University + +was established in 1912 with a bequest of $25,000 under the will of Louis +Clark Vanuxem, of the Class of 1879. By direction of the executors of Mr. +Vanuxem's estate, the income of the foundation is to be used for a series +of public lectures delivered in Princeton annually, at least one half of +which shall be on subjects of current scientific interest. The lectures are +to be published and distributed among schools and libraries generally. + +The following lectures have already been published or are in press: + + 1912-13 The Theory of Permutable Functions, by Vito Volterra + + 1913-14 Lectures delivered in connection with the dedication of the + Graduate College of Princeton University by Emile Boutroux, Alois + Riehl, A. D. Godley, and Arthur Shipley + + 1914-15 Romance, by Sir Walter Raleigh + + 1915-16 A Critique of the Theory of Evolution, by Thomas Hunt Morgan + + * * * * * + +LOUIS CLARK VANUXEM FOUNDATION + +A CRITIQUE + +OF THE + +THEORY OF EVOLUTION + +BY + +THOMAS HUNT MORGAN + +PROFESSOR OF EXPERIMENTAL ZOOLOGY IN +COLUMBIA UNIVERSITY + +LECTURES DELIVERED AT PRINCETON UNIVERSITY +FEBRUARY 24, MARCH 1, 8, 15, 1916 + +PRINCETON UNIVERSITY PRESS +PRINCETON +LONDON: HUMPHREY MILFORD +OXFORD UNIVERSITY PRESS +1916 + + * * * * * + +Copyright, 1916, by +PRINCETON UNIVERSITY PRESS +Published October, 1916 + +[Illustration] + + * * * * * + + +PREFACE + +Occasionally one hears today the statement that we have come to realize +that we know nothing about evolution. This point of view is a healthy +reaction to the over-confident belief that we knew everything about +evolution. There are even those rash enough to think that in the last few +years we have learned more about evolution than we might have hoped to know +a few years ago. A _critique_ therefore not only becomes a criticism of the +older evidence but an appreciation of the new evidence. + +In the first lecture an attempt is made to put a new valuation on the +traditional evidence for evolution. In the second lecture the most recent +work on heredity is dealt with, for only characters that are inherited can +become a part of the evolutionary process. In the third lecture the +physical basis of heredity and the composition of the germ plasm stream are +examined in the light of new observations; while in the fourth lecture the +thesis is developed that chance variation combined with a property of +living things to manifold themselves is the key note of modern evolutionary +thought. + +T. H. MORGAN + +_July, 1916_ + + * * * * * + + + TABLE OF CONTENTS + + CHAPTER I + A REVALUATION OF THE EVIDENCE ON + WHICH THE THEORY OF EVOLUTION WAS BASED + + PAGE + PREFACE v + + 1. THREE KINDS OF EVOLUTION 1-7 + + 2. THE EVIDENCE FOR ORGANIC EVOLUTION 7-27 + a. The Evidence from Comparative Anatomy 7-14 + b. The Evidence from Embryology 14-23 + c. The Evidence from Paleontology 24-27 + + 3. THE FOUR GREAT HISTORICAL SPECULATIONS 27-39 + a. The Environment 27-31 + Geoffroy St. Hilaire + b. Use and Disuse 31-34 + From Lamarck to Weismann + c. The Unfolding Principle 34-36 + Naegeli and Bateson + d. Natural Selection 36-39 + Darwin + + CHAPTER II + THE BEARING OF MENDEL'S DISCOVERY ON + THE ORIGIN OF HEREDITY CHARACTERS + + 1. Mendel's First Discovery--Segregation 41-52 + + 2. Mendel's Second Discovery--Independent + Assortment 52-59 + + 3. The Characters of Wild Animals and Plants + Follow the Same Laws of Inheritance as do + the Characters of Domesticated Animals and + Plants 59-84 + a. Sexual Dimorphism 61-64 + Eosin eye color of Drosophila 61-62 + Color of the Clover Butterfly, Colias + philodice 62-63 + Color of Papilio turnus 63 + Color pattern of Papilio polytes 63-64 + b. Duplication of parts 65-66 + Thorax of Drosophila 65 + Legs of Drosophila 65-66 + c. Loss of characters 66-68 + "Eyeless" of Drosophila 66-67 + Vestigial wings of Drosophila 67 + Bar eye of Drosophila 67-68 + d. Small changes of characters 68-70 + "Speck" 68 + Bristles of "club" 70 + e. Manifold effects of same factor 71 + f. Constant but trivial effects may be the + product of factors having other vital + aspect 73 + g. Sex-linked inheritance 75-80 + in Drosophila ampelophila 75-76 + in the wild species D. repleta 76 + in man 77 + in domesticated Fowls 77-78 + in the wild moth, Abraxas 78-80 + h. Multiple allelomorphs 81-84 + in the wild Grouse Locust 81-83 + in domesticated mice and rabbits 83 + in Drosophila ampelophila 84 + + 4. MUTATION AND EVOLUTION 84-88 + + CHAPTER III + THE FACTORIAL THEORY OF HEREDITY + AND THE COMPOSITION OF THE GERM PLASM + + 1. THE CELLULAR BASIS OF ORGANIC EVOLUTION + AND HEREDITY 89-98 + + 2. THE MECHANISM OF MENDELIAN HEREDITY + DISCOVERED IN THE BEHAVIOR OF THE + CHROMOSOMES 98-102 + + 3. THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA + AMPELOPHILA 103-118 + a. Group I. 104-109 + b. Group II. 109-112 + c. Group III. 112-115 + d. Group IV. 115-118 + + 4. LOCALIZATION OF FACTORS IN THE CHROMOSOMES 118-142 + a. The Evidence from Sex Linked Inheritance 118-137 + b. The Evidence from Interference 137-138 + c. The Evidence from Non-Disjunction 139-142 + + 5. HOW MANY GENETIC FACTORS ARE THERE IN + THE GERM-PLASM OF A SINGLE INDIVIDUAL? 142-143 + + 6. CONCLUSIONS 144 + + CHAPTER IV + SELECTION AND EVOLUTION + + 1. THE THEORY OF NATURAL SELECTION 145-161 + + 2. HOW HAS SELECTION IN DOMESTICATED ANIMALS + AND PLANTS BROUGHT ABOUT ITS RESULTS? 161-165 + + 3. ARE FACTORS CHANGED THROUGH SELECTION? 165-187 + + 4. HOW DOES NATURAL SELECTION INFLUENCE + THE COURSE OF EVOLUTION? 187-193 + + 5. CONCLUSIONS 193-194 + + INDEX 195-197 + + * * * * * + + +CHAPTER I + +A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED + +We use the word evolution in many ways--to include many different kinds of +changes. There is hardly any other scientific term that is used so +carelessly--to imply so much, to mean so little. + +THREE KINDS OF EVOLUTION + +We speak of the evolution of the stars, of the evolution of the horse, of +the evolution of the steam engine, as though they were all part of the same +process. What have they in common? Only this, that each concerns itself +with the _history_ of something. When the astronomer thinks of the +_evolution_ of the earth, the moon, the sun and the stars, he has a picture +of diffuse matter that has slowly condensed. With condensation came heat; +with heat, action and reaction within the mass until the chemical +substances that we know today were produced. This is the nebular hypothesis +of the astronomer. The astronomer explains, or tries to explain, how this +evolution took place, by an appeal to the physical processes that have been +worked out in the laboratory, processes which he thinks have existed +through all the eons during which this evolution was going on and which +were its immediate causes. + +When the biologist thinks of the evolution of animals and plants, a +different picture presents itself. He thinks of series of animals that have +lived in the past, whose bones (fig. 1) and shells have been preserved in +the rocks. He thinks of these animals as having in the past given birth, +through an unbroken succession of individuals, to the living inhabitants of +the earth today. He thinks that the old, simpler types of the past have in +part changed over into the more complex forms of today. + +He is thinking as the historian thinks, but he sometimes gets confused and +thinks that he is explaining evolution when he is only describing it. + +[Illustration: FIG. 1. A series of skulls and feet. Eohippus, Mesohippus, +Meryhippus, Hipparion and Equus. (American Museum of Natural History. After +Matthews.)] + +A third kind of evolution is one for which man himself is responsible, in +the sense that he has brought it about, often with a definite end in view. + +His mind has worked slowly from stage to stage. We can often trace the +history of the stages through which his psychic processes have passed. The +evolution of the steam-boat, the steam engine, paintings, clothing, +instruments of agriculture, of manufacture, or of warfare (fig. 2) +illustrates the history of human progress. There is an obvious and striking +similarity between the evolution of man's inventions and the evolution of +the shells of molluscs and of the bones of mammals, yet in neither case +does a knowledge of the order in which these things arose explain them. If +we appeal to the psychologist he will probably tell us that human +inventions are either the result of happy accidents, that have led to an +unforeseen, but discovered use; or else the use of the invention was +foreseen. It is to the latter process more especially that the idea of +_purpose_ is applied. When we come to review the four great lines of +evolutionary thought we shall see that this human idea of purpose recurs in +many forms, suggesting that man has often tried to explain how organic +evolution has taken place by an appeal to the method which he believes he +makes use of himself in the inorganic world. + +[Illustration: FIG. 2. Evolution of pole arms. (Metropolitan Museum. After +Dean.)] + +What has the evolution of the stars, of the horse and of human inventions +in common? Only this, that in each case from a simple beginning through a +series of changes something more complex, or at least different, has come +into being. To lump all these kinds of changes into one and call them +evolution is no more than asserting that you believe in consecutive series +of events (which is history) causally connected (which is science); that +is, that you believe in history and that you believe in science. But let us +not forget that we may have complete faith in both without thereby offering +any explanation of either. It is the business of science to find out +_specifically_ what kinds of events were involved when the stars evolved in +the sky, when the horse evolved on the earth, and the steam engine was +evolved from the mind of man. + +Is it not rather an empty generalization to say that any kind of change is +a process of evolution? At most it means little more than that you want to +intimate that miraculous intervention is not necessary to account for such +kinds of histories. + +We are concerned here more particularly with the biologists' ideas of +evolution. My intention is to review the evidence on which the old theory +rested its case, in the light of some of the newer evidence of recent +years. + +Four great branches of study have furnished the evidence of organic +evolution. They are: + + Comparative anatomy. + Embryology. + Paleontology. + Experimental Breeding or Genetics. + +_The Evidence from Comparative Anatomy_ + +When we study animals and plants we find that they can be arranged in +groups according to their resemblances. This is the basis of comparative +anatomy, which is only an accurate study of facts that are superficially +obvious to everyone. + +The groups are based not on a single difference, but on a very large number +of resemblances. Let us take for example the group of vertebrates. + +[Illustration: FIG. 3. Limb skeletons of extinct and living animals, +showing the homologous bones: 1, salamander; 2, frog; 3, turtle; 4, +Aetosaurus; 5, Pleisiosaurus; 6, Ichthyosaurus; 7, Mesosaurus; 8, duck. +(After Jordan and Kellogg.)] + +The hand and the arm of man are similar to the hand and arm of the ape. We +find the same plan in the forefoot of the rat, the elephant, the horse and +the opossum. We can identify the same parts in the forefoot of the lizard, +the frog (fig. 3), and even, though less certainly, in the pectoral fins of +fishes. Comparison does not end here. We find similarities in the skull and +back bones of these same animals; in the brain; in the digestive system; in +the heart and blood vessels; in the muscles. + +Each of these systems is very complex, but the same general arrangement is +found in all. Anyone familiar with the evidence will, I think, probably +reach the conclusion either that these animals have been created on some +preconceived plan, or else that they have some other bond that unites them; +for we find it difficult to believe that such complex, yet similar things +could have arisen independently. But we try to convince our students of the +truth of the theory of evolution not so much by calling their attention to +this relation as by tracing each organ from a simple to a complex +structure. + +I have never known such a course to fail in its intention. In fact, I know +that the student often becomes so thoroughly convinced that he resents any +such attempt as that which I am about to make to point out that the +evidence for his conviction is not above criticism. + +[Illustration: FIG. 4. Drosophila ampelophila. a, Female and b, male.] + +Because we can often arrange the series of structures in a line extending +from the very simple to the more complex, we are apt to become unduly +impressed by this fact and conclude that if we found the complete series we +should find all the intermediate steps and that they have arisen in the +order of their complexity. This conclusion is not necessarily correct. Let +me give some examples that have come under my own observation. We have bred +for five years the wild fruit fly Drosophila ampelophila (fig. 4) and we +have found over a hundred and twenty-five new types that breed true. Each +has arisen independently and suddenly. Every part of the body has been +affected by one or another of these mutations. For instance many different +kinds of changes have taken place in the wings and several of these involve +the size of the wings. If we arrange the latter arbitrarily in the order of +their size there will be an almost complete series beginning with the +normal wings and ending with those of apterous flies. Several of these +types are represented in figure 5. The order in which these mutations +occurred bears no relation to their size; each originated independently +from the wild type. + +[Illustration: FIG. 5. Mutants of Drosophila ampelophila arranged in order +of size of wings: (a) cut; (b) beaded; (c) stumpy; (d) another individual +of stumpy; (f) vestigial (g) apterous.] + +The wings of the wild fly are straight (fig. 4). Several types have arisen +in which the wings are bent upwards and in the most extreme type the wings +are curled over the back, as seen in figure 54 (g), yet there is no +historical connection between these stages. + +Mutations have occurred involving the pigmentation of the body and wings. +The head and thorax of the wild Drosophila ampelophila are grayish yellow, +the abdomen is banded with yellow and black, and the wings are gray. There +have appeared in our cultures several kinds of darker types ranging to +almost black flies (fig. 20) and to lighter types that are quite yellow. If +put in line a series may be made from the darkest flies at one end to the +light yellow flies at the other. These types, with the fluctuations that +occur within each type, furnish a complete series of gradations; yet +historically they have arisen independently of each other. + +Many changes in eye color have appeared. As many as thirty or more races +differing in eye color are now maintained in our cultures. Some of them are +so similar that they can scarcely be separated from each other. It is +easily possible beginning with the darkest eye color, sepia, which is deep +brown, to pick out a perfectly graded series ending with pure white eyes. +But such a serial arrangement would give a totally false idea of the way +the different types have arisen; and any conclusion based on the existence +of such a series might very well be entirely erroneous, for the fact that +such a series exists bears no relation to the order in which its members +have appeared. + +Suppose that evolution "in the open" had taken place in the same way, by +means of _discontinuous_ variation. What value then would the evidence from +comparative anatomy have in so far as it is based on a continuous series of +variants of any organ? + +No one familiar with the entire evidence will doubt for a moment that these +125 races of Drosophila ampelophila belong to the same species and have had +a common origin, for while they may differ mainly in one thing they are +extremely alike in a hundred other things, and in the general relation of +the parts to each other. + +It is in this sense that the evidence from comparative anatomy can be used +I think as an argument for evolution. It is the resemblances that the +animals or plants in any group have in common that is the basis for such a +conclusion; it is not because we can arrange in a continuous series any +particular variations. In other words, our inference concerning the common +descent of two or more species is based on the totality of such +resemblances that still remain in large part after each change has taken +place. In this sense the argument from comparative anatomy, while not a +demonstration, carries with it, I think, a high degree of probability. + +_The Evidence from Embryology_ + +In passing from the egg to the adult the individual goes through a series +of changes. In the course of this development we see not only the +beginnings of the organs that gradually enlarge and change into those of +the adult animal, but also see that organs appear and later disappear +before the adult stage is reached. We find, moreover, that the young +sometimes resemble in a most striking way the adult stage of groups that we +place lower in the scale of evolution. + +Many years before Darwin advanced his theory of evolution through natural +selection, the resemblance of the young of higher animals to the adults of +lower animals had attracted the attention of zoologists and various views, +often very naive, had been advanced to account for the resemblance. Among +these speculations there was one practically identical with that adopted by +Darwin and the post-Darwinians, namely that the higher animals repeat in +their development the _adult stages_ of lower animals. Later this view +became one of the cornerstones of the theory of organic evolution. It +reached its climax in the writings of Haeckel, and I think I may add +without exaggeration that for twenty-five years it furnished the chief +inspiration of the school of descriptive embryology. Today it is taught in +practically all textbooks of biology. Haeckel called this interpretation +the Biogenetic Law. + +[Illustration: FIG. 6. Young trout (Trutta fario) six days after hatching. +(After Ziegler.)] + +It was recognized, of course, that many embryonic stages could not possibly +represent ancestral animals. A young fish with a huge yolk sac attached +(fig. 6) could scarcely ever have led a happy, free life as an adult +individual. Such stages were interpreted, however, as _embryonic_ additions +to the original ancestral type. The embryo had done something on its own +account. + +In some animals the young have structures that attach them to the mother, +as does the placenta of the mammals. In other cases the young develop +membranes about themselves--like the amnion of the chick (fig. 7) and +mammal--that would have shut off an adult animal from all intercourse with +the outside world. Hundreds of such embryonic adaptations are known to +embryologists. These were explained as adaptations and as falsifications of +the ancestral records. + +[Illustration: FIG. 7. Diagram of chick showing relations of amnion, +allantois and yolk. (After Lillie.)] + +At the end of the last century Weismann injected a new idea into our views +concerning the origin of variations. He urged that variations are germinal, +i.e. they first appear in the egg and the sperm as changes that later bring +about modifications in the individual. The idea has been fruitful and is +generally accepted by most biologists today. It means that the offspring of +a pair of animals are not affected by the structure or the activities of +their parents, but the germ plasm is the unmodified stream from which both +the parent and the young have arisen. Hence their resemblance. Now, it has +been found that a variation arising in the germ plasm, no matter what its +cause, may affect any stage in the development of the next individuals that +arise from it. There is no reason to suppose that such a change produces a +new character that always sticks itself, as it were, on to the end of the +old series. This idea of germinal variation therefore carried with it the +death of the older conception of evolution by superposition. + +In more recent times another idea has become current, mainly due to the +work of Bateson and of de Vries--the idea that variations are +discontinuous. Such a conception does not fall easily into line with the +statement of the biogenetic "law"; for actual experience with discontinuous +variation has taught us that new characters that arise do not add +themselves to the end of the line of already existing characters but if +they affect the adult characters they change them without, as it were, +passing through and beyond them. + +[Illustration: FIG. 8. Diagram of head of chick A and B, showing gill +slits, and aortic arches; and head of fish C showing aortic arches. (After +Hesse.)] + +[Illustration: FIG. 9. Human embryo showing gill slits and aortic arches. +(After His; from Marshall.)] + +I venture to think that these new ideas and this new evidence have played +havoc with the biogenetic "law". Nevertheless, there is an interpretation +of the facts that is entirely compatible with the theory of evolution. Let +me illustrate this by an example. + +[Illustration: FIG. 10. Young fish, dorsal view, and side view, showing +gill slits. (After Kopsch.)] + +The embryos of the chick (fig. 8) and of man (fig. 9) possess at an early +stage in their development gill-slits on the sides of the neck like those +of fishes. No one familiar with the relations of the parts will for a +moment doubt that the gill slits of these embryos and of the fish represent +the same structures. When we look further into the matter we find that +young fish also possess gill slits (fig. 10 and 11)--even in young stages +in their development. Is it not then more probable that the mammal and bird +possess this stage in their development simply because it has never been +lost? Is not this a more reasonable view than to suppose that the gill +slits of the embryos of the higher forms represent the adult gill slits of +the fish that in some mysterious way have been pushed back into the embryo +of the bird? + +[Illustration: FIG. 11. Side views of head of embryo sharks, showing gill +slits.] + +I could give many similar examples. All can be interpreted as embryonic +survivals rather than as phyletic contractions. Not one of them calls for +the latter interpretation. + +The study of the cleavage pattern of the segmenting egg furnishes the most +convincing evidence that a different explanation from the one stated in the +biogenetic law is the more probable explanation. + +[Illustration: FIG. 12. Cleavage stages of four types of eggs, showing the +origin of the mesenchyme cells (stippled) and mesoderm cells (darker); a, +Planarian; b, Annelid (Podarke); c, Mollusc (Crepidula), d, Mollusc +(Unio).] + +It has been found that the cleavage pattern has the same general +arrangement in the early stages of flat worms, annelids and molluscs (fig. +12). Obviously these stages have never been adult ancestors, and obviously +if their resemblance has any meaning at all, it is that each group has +retained the same general plan of cleavage, possessed by their common +ancestor. + +Accepting this view, let us ask, does the evidence from embryology favor +the theory of evolution? I think that it does very strongly. The embryos of +the mammal, bird, and lizard have gill slits today because gill slits were +present in the embryos of their ancestors. There is no other view that +explains so well their presence in the higher forms. + +Perhaps someone will say, Well! is not this all that we have contended for! +Have you not reached the old conclusion in a roundabout way? I think not. +To my mind there is a wide difference between the old statement that the +higher animals living today have the original adult stages telescoped into +their embryos, and the statement that the resemblance between certain +characters in the embryos of higher animals and corresponding stages in the +embryos of lower animals is most plausibly explained by the assumption that +they have descended from the same ancestors, and that their common +structures are embryonic survivals. + +_The Evidence from Paleontology_ + +The direct evidence furnished by fossil remains is by all odds the +strongest evidence that we have in favor of organic evolution. Paleontology +holds the incomparable position of being able to point directly to the +evidence showing that the animals and plants living in past times are +connected with those living at the present time, often through an unbroken +series of stages. Paleontology has triumphed over the weakness of the +evidence, which Darwin admitted was serious, by filling in many of the +missing links. + +Paleontology has been criticised on the ground that she cannot pretend to +show the actual ancestors of living forms because, if in the past genera +and species were as abundant and as diverse as we find them at present, it +is very improbable that the bones of any individual that happened to be +preserved are the bones of just that species that took part in the +evolution. Paleontologists will freely admit that in many cases this is +probably true, but even then the evidence is, I think, still just as +valuable and in exactly the same sense as is the evidence from comparative +anatomy. It suffices to know that there lived in the past a particular +"group" of animals that had many points in common with those that preceded +them and with those that came later. Whether these are the actual ancestors +or not does not so much matter, for the view that from such a group of +species the later species have been derived is far more probable than any +other view that has been proposed. + +With this unrivalled material and splendid series of gradations, +paleontology has constructed many stages in the past history of the globe. +But paleontologists have sometimes gone beyond this descriptive phase of +the subject and have attempted to formulate the "causes", "laws" and +"principles" that have led to the development of their series. It has even +been claimed that paleontologists are in an incomparably better position +than zoologists to discover such principles, because they know both the +beginning and the end of the evolutionary series. The retort is obvious. In +his sweeping and poetic vision the paleontologist may fail completely to +find out the nature of the pigments that have gone into the painting of his +picture, and he may confuse a familiarity with the different views he has +enjoyed of the canvas with a knowledge of how the painting is being done. + +My good friend the paleontologist is in greater danger than he realizes, +when he leaves descriptions and attempts explanation. He has no way to +check up his speculations and it is notorious that the human mind without +control has a bad habit of wandering. + +When the modern student of variation and heredity--the geneticist--looks +over the different "continuous" series, from which certain "laws" and +"principles" have been deduced, he is struck by two facts: that the gaps, +in some cases, are enormous as compared with the single changes with which +he is familiar, and (what is more important) that they involve numerous +parts in many ways. The geneticist says to the paleontologist, since you do +not know, and from the nature of your case can never know, whether your +differences are due to one change or to a thousand, you can not with +certainty tell us anything about the hereditary units which have made the +process of evolution possible. And without this knowledge there can be no +understanding of the causes of evolution. + +THE FOUR GREAT HISTORICAL SPECULATIONS + +Looking backward over the history of the evolution theory we recognize that +during the hundred and odd years that have elapsed since Buffon, there have +been four main lines of _speculation_ concerning evolution. We might call +them the four great cosmogonies or the four modern epics of evolution. + +THE ENVIRONMENT + +_Geoffroy St. Hilaire_ + +About the beginning of the last century Geoffroy St. Hilaire, protege, and +in some respects a disciple of Buffon, was interested as to how living +species are related to the animals and plants that had preceded them. He +was familiar with the kind of change that takes place in the embryo if it +is put into new or changed surroundings, and from this knowledge he +concluded that as the surface of the earth slowly changed--as the carbon +dioxide contents in the air altered--as land appeared--and as marine +animals left the water to inhabit it, they or their embryos responded to +the new conditions and those that responded favorably gave rise to new +creations. As the environment changed the fauna and flora changed--change +for change. Here we have a picture of progressive evolution that carries +with it an idea of mechanical necessity. If there is anything mystical or +even improbable in St. Hilaire's argument it does not appear on the +surface; for he did not assume that the response to the new environment was +always a favorable one or, as we say, an adaptation. He expressly stated +that _if_ the response was unfavorable the individual or the race died out. +He assumed that _sometimes_ the change might be favorable, i.e., that +certain species, entire groups, would respond in a direction favorable to +their existence in a new environment and these would come to inherit the +earth. In this sense he anticipated certain phases of the natural selection +theory of Darwin, but only in part; for his picture is not one of strife +within and without the species, but rather the escape of the species from +the old into a new world. + +If then we recognize the intimate bond in chemical constitution of living +things and of the world in which they develop, what is there improbable in +St. Hilaire's hypothesis? Why, in a word is not more credit given to St. +Hilaire in modern evolutionary thought? The reasons are to be found, I +think, first, in that the evidence to which he appealed was meagre and +inconclusive; and, second, in that much of his special evidence does not +seem to us to be applicable. For example the monstrous forms that +development often assumes in a strange environment, and with which every +embryologist is only too familiar, rarely if ever furnish combinations, as +he supposed, that are capable of living. On the contrary, they lead rather +to the final catastrophe of the organism. And lastly, St. Hilaire's appeal +to sudden and great transformations, such as a crocodile's egg hatching +into a bird, has exposed his view to too easy ridicule. + +But when all is said, St. Hilaire's conception of evolution contains +elements that form the background of our thinking to-day, for taken +broadly, the interaction between the organism and its environment was a +mechanistic conception of evolution even though the details of the theory +were inadequate to establish his contention. + +In our own time the French metaphysician Bergson in his _Evolution +Creatrice_ has proposed in mystical form a thought that has at least a +superficial resemblance to St. Hilaire's conception. The response of living +things is no longer hit in one species and miss in another; it is precise, +exact; yet not mechanical in the sense at least in which we usually employ +the word mechanical. For Bergson claims that the one chief feature of +living material is that it responds favorably to the situation in which it +finds itself; at least so far as lies within the possible physical +limitations of its organization. Evolution has followed no preordained +plan; it has had no creator; it has brought about its own creation by +responding adaptively to each situation as it arose. + +But note: the man of science believes that the organism responds today as +it does, because at present it has a chemical and physical constitution +that gives this response. We find a specific chemical composition and +generally a specific physical structure already existing. We have no reason +to suppose that such particular reactions would take place until a specific +chemical configuration had been acquired. Where did this constitution come +from? This is the question that the scientist asks himself. I suppose +Bergson would have to reply that it came into existence at the moment that +the first specific stimulus was applied. But if this is the answer we have +passed at once from the realm of observation to the realm of fancy--to a +realm that is foreign to our experience; for such a view assumes that +chemical and physical reactions are guided by the needs of the organism +when the reactions take place inside living beings. + +USE AND DISUSE + +_From Lamarck to Weismann_ + +The second of the four great historical explanations appeals to a change +not immediately connected with the outer world, but to one within the +organism itself. + +Practice makes perfect is a familiar adage. Not only in human affairs do we +find that a part through use becomes a better tool for performing its task, +and through disuse degenerates; but in the field of animal behavior we find +that many of the most essential types of behavior have been learned through +repeated associations formed by contact with the outside. + +It was not so long ago that we were taught that the instincts of animals +are the inherited experience of their ancestors--lapsed intelligence was +the current phrase. + +Lamarck's name is always associated with the application of the theory of +the inheritance of acquired characters. Darwin fully endorsed this view and +made use of it as an explanation in all of his writings about animals. +Today the theory has few followers amongst trained investigators, but it +still has a popular vogue that is widespread and vociferous. + +To Weismann more than to any other single individual should be ascribed the +disfavor into which this view has fallen. In a series of brilliant essays +he laid bare the inadequacy of the supposed evidence on which the +inheritance of acquired characters rested. Your neighbor's cat, for +instance, has a short tail, and it is said that it had its tail pinched off +by a closing door. In its litter of kittens one or more is found without a +tail. Your neighbor believes that here is a case of cause and effect. He +may even have known that the mother and grandmother of the cat had natural +tails. But it has been found that short tail is a dominant character; +therefore, until we know who was the father of the short-tailed kittens the +accident to its mother and the normal condition of her maternal ancestry is +not to the point. + +Weismann appealed to common sense. He made few experiments to disprove +Lamarck's hypothesis. True, he cut off the tails of some mice for a few +generations but got no tailless offspring and while he gives no exact +measurements with coefficients of error he did not observe that the tails +of the descendants had shortened one whit. The combs of fighting cocks and +the tails of certain breeds of sheep have been cropped for many generations +and the practice continues today, because their tails are still long. While +in Lamarck's time there was no evidence opposed to his ingenious theory, +based as it was on an appeal to the acknowledged facts of improvement that +take place in the organs of an individual through their own functioning (a +fact that is as obvious and remarkable today as in the time of Lamarck), +yet now there is evidence as to whether the effects of use and disuse are +inherited, and this evidence is not in accord with Lamarck's doctrine. + +THE UNFOLDING PRINCIPLE + +_Naegeli and Bateson_ + +I have ventured to put down as one of the four great historical +explanations, under the heading of the unfolding principle, a conception +that has taken protean forms. At one extreme it is little more than a +mystic sentiment to the effect that evolution is the result of an inner +driving force or principle which goes under many names such as +Bildungstrieb, nisus formativus, vital force, and orthogenesis. +Evolutionary thought is replete with variants of this idea, often naively +expressed, sometimes unconsciously implied. Evolution once meant, in fact, +an unfolding of what pre-existed in the egg, and the term still carries +with it something of its original significance. + +Naegeli's speculation written several years after Darwin's "Origin of +Species" may be taken as a typical case. Naegeli thought that there exists +in living material an innate power to grow and expand. He vehemently +protested that he meant only a mechanical principle but as he failed to +refer such a principle to any properties of matter known to physicists and +chemists his view seems still a mysterious affirmation, as difficult to +understand as the facts themselves which it purports to explain. + +Naegeli compared the process of evolution to the growth of a tree, whose +ultimate twigs represent the living world of species. Natural selection +plays only the role of the gardener who prunes the tree into this or that +shape but who has himself _produced_ nothing. As an imaginative figure of +speech Naegeli's comparison of the tree might even today seem to hold if we +substituted "mutations" for "growth", but although we know so little about +what causes mutations there is no reason for supposing them to be due to an +inner impulse, and hence they furnish no justification for such a +hypothesis. + +In his recent presidential address before the British Association Bateson +has inverted this idea. I suspect that his effort was intended as little +more than a _tour de force_. He claims for it no more than that it is a +possible line of speculation. Perhaps he thought the time had come to give +a shock to our too confident views concerning evolution. Be this as it may, +he has invented a striking paradox. Evolution has taken place through the +steady loss of inhibiting factors. Living matter was stopped down, so to +speak, at the beginning of the world. As the stops are lost, new things +emerge. Living matter has changed only in that it has become simpler. + +NATURAL SELECTION + +_Darwin_ + +Of the four great historical speculations about evolution, the doctrine of +Natural Selection of Darwin and Wallace has met with the most widespread +acceptance. In the last lecture I intend to examine this theory critically. +Here we are concerned only with its broadest aspects. + +Darwin appealed to _chance variations_ as supplying evolution with the +material on which natural selection works. If we accept, for the moment, +this statement as the cardinal doctrine of natural selection it may appear +that evolution is due, (1) _not_ to an _orderly_ response of the organism +to its environment, (2) _not_ in the main to the activities of the animal +through the use or disuse of its parts, (3) _not_ to any innate principle +of living material itself, and (4) above all _not_ to purpose either from +within or from without. Darwin made quite clear what he meant by chance. By +chance he did not mean that the variations were not causal. On the contrary +he taught that in Science we mean by chance only that the particular +combination of causes that bring about a variation are not known. They are +accidents, it is true, but they are causal accidents. + +In his famous book on "Animals and Plants under Domestication", Darwin +dwells at great length on the nature of the conditions that bring about +variations. If his views seem to us today at times vague, at times +problematical, and often without a secure basis, nevertheless we find in +every instance, that Darwin was searching for the _physical causes of +variation_. He brought, in consequence, conviction to many minds that there +are abundant indications, even if certain proof is lacking, that the causes +of variation are to be found in natural processes. + +Today the belief that evolution takes place by means of natural processes +is generally accepted. It does not seem probable that we shall ever again +have to renew the old contest between evolution and special creation. + +But this is not enough. We can never remain satisfied with a negative +conclusion of this kind. We must find out what natural causes bring about +variations in animals and plants; and we must also find out what kinds of +variations are inherited, and how they are inherited. If the circumstantial +evidence for organic evolution, furnished by comparative anatomy, +embryology and paleontology is cogent, we should be able to observe +evolution going on at the present time, i.e. we should be able to observe +the occurrence of variations and their transmission. This has actually been +done by the geneticist in the study of mutations and Mendelian heredity, as +the succeeding lectures will show. + + * * * * * + + +CHAPTER II + +THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITARY CHARACTERS + +Between the years 1857 and 1868 Gregor Mendel, Augustinian monk, studied +the heredity of certain characters of the common edible pea, in the garden +of the monastery at Bruenn. + +In his account of his work written in 1868, he said: + + "It requires indeed some courage to undertake a labor of such a + 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." + +He tells us also why he selected peas for his work: + + "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." + +Why do biologists throughout the world to-day agree that Mendel's discovery +is one of first rank? + +A great deal might be said in this connection. What is essential may be +said in a few words. Biology had been, and is still, largely a descriptive +and speculative science. _Mendel showed by experimental proof that heredity +could be explained by a simple mechanism. His discovery has been +exceedingly fruitful._ + +Science begins with naive, often mystic conceptions of its problems. It +reaches its goal whenever it can replace its early guessing by verifiable +hypotheses and predictable results. This is what Mendel's law did for +heredity. + +MENDEL'S FIRST DISCOVERY--SEGREGATION + +[Illustration: FIG. 13. Diagram illustrating a cross between a red (dark) +and a white variety of four o'clock (Mirabilis jalapa).] + +Let us turn to the demonstration of his first law--the law of segregation. +The first case I choose is not the one given by Mendel but one worked out +later by Correns. If the common garden plant called four o'clock (Mirabilis +jalapa) with red flowers is crossed to one having white flowers, the +offspring are pink (fig. 13). The hybrid, then, is intermediate in the +color of its flowers between the two parents. If these hybrids are inbred +the offspring are white, pink and red, in the proportion of 1:2:1. All of +these had the same ancestry, yet they are of three different kinds. If we +did not know their history it would be quite impossible to state what the +ancestry of the white or of the red had been, for they might just as well +have come from pure white and pure red ancestors respectively as to have +emerged from the pink hybrids. Moreover, when we test them we find that +they are as pure as are white or red flowering plants that have had all +white or all red flowering ancestors. + +Mendel's Law explains the results of this cross as shown in figure 14. + +The egg cell from the white parent carries the factor for white, the pollen +cell from the red parent carries the factor for red. The hybrid formed by +their union carries both factors. The result of their combined action is to +produce flowers intermediate in color. + +When the hybrids mature and their germ cells (eggs or pollen) ripen, each +carries only one of these factors, either the red or the white, but not +both. In other words, the two factors that have been brought together in +the hybrid separate in its germ cells. Half of the egg cells are white +bearing, half red bearing. Half of the pollen cells are white bearing, half +red bearing. Chance combinations at fertilization give the three classes of +individuals of the second generation. + +[Illustration: FIG. 14. Diagram illustrating the history of the factors in +the germ cells of the cross shown in Fig. 13.] + +The white flowering plants should forever breed true, as in fact they do. +The red flowering plants also breed true. The pink flowering plants, having +the same composition as the hybrids of the first generation, should give +the same kind of result. They do, indeed, give this result i.e. one white +to two pink to one red flowered offspring. + +[Illustration: FIG. 15. Diagram illustrating a cross between special races +of white and black fowls, producing the blue (here gray) Andalusian.] + +Another case of the same kind is known to breeders of poultry. One of the +most beautiful of the domesticated breeds is known as the Andalusian. It is +a slate blue bird shading into blue-black on the neck and back. Breeders +know that these blue birds do not breed true but produce white, black, and +blue offspring. + +[Illustration: FIG. 16. Diagram showing history of germ cells of cross of +Fig. 15. The larger circles indicate the color of the birds; their enclosed +small circles the nature of the factors in the germ cells of such birds.] + +The explanation of the failure to produce a pure race of Andalusians is +that they are like the pink flowers of the four o'clock, i.e., they are a +hybrid type formed by the meeting of the white and the black germ cells. If +the whites produced by the Andalusians are bred to the blacks (both being +pure strains), all the offspring will be blue (fig. 15); if these blues are +inbred they will give 1 white, to 2 blues, to 1 black. In other words, the +factor for white and the factor for black separate in the germ cells of the +hybrid Andalusian birds (fig. 16). + +[Illustration: FIG. 17. Diagram of Mendel's cross between yellow (dominant) +and green (recessive) peas.] + +The third case is Mendel's classical case of yellow and green peas (fig. +17). He crossed a plant belonging to a race having yellow peas with one +having green peas. The hybrid plants had yellow seeds. These hybrids inbred +gave three yellows to one green. The explanation (fig. 18) is the same in +principle as in the preceding cases. The only difference between them is +that the hybrid which contains both the yellow and the green factors is in +appearance not intermediate, but like the yellow parent stock. Yellow is +said therefore to be dominant and green to be recessive. + +[Illustration: FIG. 18. Diagram illustrating the history of the factors in +the cross shown in Fig. 17.] + +Another example where one of the contrasted characters is dominant is shown +by the cross of Drosophila with vestigial wings to the wild type with long +wings (fig. 19). The F_1 flies have long wings not differing from those of +the wild fly, so far as can be observed. When two such flies are inbred +there result three long to one vestigial. + +[Illustration: FIG. 19. Diagram illustrating a cross between a fly +(Drosophila ampelophila) with long wings and a mutant fly with vestigial +wings.] + +The question as to whether a given character is dominant or recessive is a +matter of no theoretical importance for the principle of segregation, +although from the notoriety given to it one might easily be misled into the +erroneous supposition that it was the discovery of this relation that is +Mendel's crowning achievement. + +Let me illustrate by an example in which the hybrid standing between two +types overlaps them both. There are two mutant races in our cultures of the +fruit fly Drosophila that have dark body color, one called sooty, another +which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives +offspring that are intermediate in color. Some of them are so much like +sooty that they cannot be distinguished from sooty. At the other extreme +some of the hybrids are as dark as the lightest of the ebony flies. If +these hybrids are inbred there is a continuous series of individuals, +sooties, intermediates and ebonies. Which color here shall we call the +dominant? If the ebony, then in the second generation we count three +ebonies to one sooty, putting the hybrids with the ebonies. If the dominant +is the sooty then we count three sooties to one ebony, putting the hybrids +with the sooties. The important fact to find out is whether there actually +exist three classes in the second generation. This can be ascertained even +when, as in this case, there is a perfectly graded series from one end to +the other, by testing out individually enough of the flies to show that +one-fourth of them never produce any descendants but ebonies, one-fourth +never any but sooties, and one-half of them give rise to both ebony and +sooty. + +[Illustration: FIG. 20. Cross between two allelomorphic races of +Drosophila, sooty and ebony, that give a completely graded series in F_2.] + +MENDEL'S SECOND DISCOVERY--INDEPENDENT ASSORTMENT + +Besides his discovery that there are pairs of characters that disjoin, as +it were, in the germ cells of the hybrid (law of segregation) Mendel made a +second discovery which also has far-reaching consequences. The following +case illustrates Mendel's second law. + +If a pea that is yellow and round is crossed to one that is green and +wrinkled (fig. 21), all of the offspring are yellow and round. Inbred, +these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green +wrinkled. All the yellows taken together are to the green as 3:1. All the +round taken together are to the wrinkled as three to one; but some of the +yellows are now wrinkled and some of the green are now round. There has +been a recombination of characters, while at the same time the results, for +each pair of characters taken separately, are in accord with Mendel's Law +of Segregation, (fig. 22). The second law of Mendel may be called the law +of independent assortment of different character pairs. + +[Illustration: FIG. 21. Cross between yellow-round and green-wrinkled peas, +giving the 9: 3: 3: 1 ratio in F_2.] + +We can, as it were, take the characters of one organism and recombine them +with those of a different organism. We can explain this result as due to +the assortment of factors for these characters in the germ cells according +to a definite law. + +[Illustration: FIG. 22. Diagram to show the history of the factor pairs +yellow-green and round-wrinkled of the cross in Fig. 21.] + +As a second illustration let me take the classic case of the combs of +fowls. If a bird with a rose comb is bred to one with a pea comb (fig. 23), +the offspring have a comb different from either. It is called a walnut +comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1 +single. This proportion shows that the grandparental types differed in +respect to two pairs of characters. + +[Illustration: FIG. 23. Cross between pea and rose combed fowls. (Charts of +Baur and Goldschmidt.)] + +A fourth case is shown in the fruit fly, where an ebony fly with long wings +is mated to a grey fly with vestigial wings (fig. 24). The offspring are +gray with long wings. If these are inbred they give 9 gray long, 3 gray +vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25). + +[Illustration: FIG. 24. Cross between long ebony and gray vestigial flies.] + +The possibility of interchanging characters might be illustrated over and +over again. It is true not only when two pairs of characters are involved, +but when three, four, or more enter the cross. + +[Illustration: FIG. 25. Diagram to show the history of the factors in the +cross shown in Fig. 24.] + +It is as though we took individuals apart and put together parts of two, +three or more individuals by substituting one part for another. + +Not only has this power to make whatever combinations we choose great +practical importance, it has even greater theoretical significance; for, it +follows that the individual is not in itself the unit in heredity, but that +within the germ-cells there exist smaller units concerned with the +transmission of characters. + +The older mystical statement of the individual as a unit in heredity has no +longer any interest in the light of these discoveries, except as a past +phase of biological history. We see, too, more clearly that the sorting out +of factors in the germ plasm is a very different process from the influence +of these factors on the development of the organism. There is today no +excuse for confusing these two problems. + +If mechanistic principles apply also to embryonic development then the +course of development is capable of being stated as a series of +chemico-physical reactions and the "_individual_" is merely a term to +express the sum total of such reactions and should not be interpreted as +something different from or more than these reactions. So long as so little +is known of the actual processes involved in development the use of the +term "individuality", while giving the appearance of profundity, in reality +often serves merely to cover ignorance and to make a mystery out of a +mechanism. + +THE CHARACTERS OF WILD ANIMALS AND PLANTS FOLLOW THE SAME LAWS OF +INHERITANCE AS DO THE CHARACTERS OF DOMESTICATED ANIMALS AND PLANTS. + +Darwin based many of his conclusions concerning variation and heredity on +the evidence derived from the garden and from the stock farm. Here he was +handicapped to some extent, for he had at times to rely on information much +of which was uncritical, and some of which was worthless. + +Today we are at least better informed on _two_ important points; one +concerning the _kinds_ of variations that furnish to the cultivator the +materials for his selection; the other concerning the modes of inheritance +of these variations. We know now that new characters are continually +appearing in domesticated as well as in wild animals and plants, that these +characters are often sharply marked off from the original characters, and +whether the differences are great or whether they are small they are +transmitted alike according to Mendel's law. + +Many of the characteristics of our domesticated animals and cultivated +plants originated long ago, and only here and there have the records of +their first appearance been preserved. In only a few instances are these +records clear and definite, while the complete history of any large group +of our domesticated products is unknown to us. + +Within the last five or six years, however, from a common wild species of +fly, the fruit fly, Drosophila ampelophila, which we have brought into the +laboratory, have arisen over a hundred and twenty-five new types whose +origin is completely known. Let me call attention to a few of the more +interesting of these types and their modes of inheritance, comparing them +with wild types in order to show that the kinds of inheritance found in +domesticated races occur also in wild types. The results will show beyond +dispute that the characters of wild types are inherited in precisely the +same way as are the characters of the mutant types--a fact that is not +generally appreciated except by students of genetics, although it is of the +most far-reaching significance for the theory of evolution. + +A mutant appeared in which the eye color of the female was different from +that of the male. The eye color of the mutant female is a dark eosin color, +that of the male yellowish eosin. From the beginning this difference was as +marked as it is to-day. Breeding experiments show that eosin eye color +differs from the red color of the eye of the wild fly by a single mutant +factor. Here then at a single step a type appeared that was sexually +dimorphic. + +Zoologists know that sexual dimorphism is not uncommon in wild species of +animals, and Darwin proposed the theory of sexual selection to account for +the difference between the sexes. He assumed that the male preferred +certain kinds of females differing from himself in a particular character, +and thus in time through sexual selection, the sexes came to differ from +each other. + +[Illustration: FIG. 26. Clover butterfly (Colias philodice) with two types +of females, above; and one type of male, below.] + +In the case of eosin eye color no such process as that postulated by Darwin +to account for the differences between the sexes was involved; for the +single mutation that brought about the change also brought in the +dimorphism with it. + +In recent years zoologists have carefully studied several cases in which +two types of female are found in the same species. In the common clover +butterfly, there is a yellow and a white type of female, while the male is +yellow (fig. 26). It has been shown that a single factor difference +determines whether the female is yellow or white. The inheritance is, +according to Gerould, strictly Mendelian. + +[Illustration: FIG. 27. Papilio turnus with two types of females above and +one type of male below.] + +In Papilio turnus there exist, in the southern states, two kinds of +females, one yellow like the male, one black (fig. 27). The evidence here +is not so certain, but it seems probable that a single factor difference +determines whether the female shall be yellow or black. + +Finally in Papilio polytes of Ceylon and India three different types of +females appear, (fig. 28 to right) only one of which is like the male. Here +the analysis of the breeding data shows the possibility of explaining this +case as due to two pairs Mendelian factors which give in combination the +three types of female. + +[Illustration: FIG. 28. Papilio polytes, with three types of female to +right and one type of male above to left.] + +Taking these cases together, they furnish a much simpler explanation than +the one proposed by Darwin. They show also that characters like these shown +by wild species may follow Mendel's law. + +[Illustration: FIG. 29. Mutant race of fruit fly with intercalated +duplicate mesothorax on dorsal side.] + +There has appeared in our cultures a fly in which the third division of the +thorax with its appendages has changed into a segment like the second (fig. +29). It is smaller than the normal mesothorax and its wings are imperfectly +developed, but the bristles on the upper surface may have the typical +arrangement of the normal mesothorax. The mutant shows how great a change +may result from a single factor difference. + +A factor that causes duplication in the legs has also been found. Here the +interesting fact was discovered (Hoge) that duplication takes place only in +the cold. At ordinary temperatures the legs are normal. + +[Illustration: FIG. 30. Mutant race of fruit fly, called eyeless; a, a' +normal eye.] + +In contrast to the last case, where a character is doubled, is the next one +in which the eyes are lost (fig. 30). This change also took place at a +single step. All the flies of this stock however, cannot be said to be +eyeless, since many of them show pieces of the eye--indeed the variation is +so wide that the eye may even appear like a normal eye unless carefully +examined. Formerly we were taught that eyeless animals arose in caves. This +case shows that they may also arise suddenly in glass milk bottles, by a +change in a single factor. + +I may recall in this connection that wingless flies (fig. 5 f) also arose +in our cultures by a single mutation. We used to be told that wingless +insects occurred on desert islands because those insects that had the best +developed wings had been blown out to sea. Whether this is true or not, I +will not pretend to say, but at any rate wingless insects may also arise, +not through a slow process of elimination, but at a single step. + +The preceding examples have all related to recessive characters. The next +one is dominant. + +[Illustration: FIG. 31. Mutant race of fruit fly called bar to the right +(normal to the left). The eye is a narrow vertical bar, the outline of the +original eye is indicated.] + +A single male appeared with a narrow vertical red bar (fig. 31) instead of +the broad red oval eye. Bred to wild females the new character was found to +dominate, at least to the extent that the eyes of all its offspring were +narrower than the normal eye, although not so narrow as the eye of the pure +stock. Around the bar there is a wide border that corresponds to the region +occupied by the rest of the eye of the wild fly. It lacks however the +elements of the eye. It is therefore to be looked upon as a rudimentary +organ, which is, so to speak, a by-product of the dominant mutation. + +The preceding cases have all involved rather great changes in some one +organ of the body. The following three cases involve slight changes, and +yet follow the same laws of inheritance as do the larger changes. + +[Illustration: FIG. 32. Mutant race of fruit fly, called speck. There is a +minute black speck at base of wing.] + +At the base of the wings a minute black speck appeared (fig. 32). It was +found to be a Mendelian character. In another case the spines on the thorax +became forked or kinky (fig. 52b). This stock breeds true, and the +character is inherited in strictly Mendelian fashion. + +[Illustration: FIG. 33. Mutant race of fruit fly called club. The wings +often remain unexpanded and two bristles present in wild fly (b) are absent +on side of thorax (c).] + +In a certain stock a number of flies appeared in which the wing pads did +not expand (fig. 33). It was found that this peculiarity is shown in only +about twenty per cent of the individuals supposed to inherit it. Later it +was found that this stock lacked two bristles on the sides of the thorax. +By means of this knowledge the heredity of the character was easily +determined. It appears that while the expansion of the wing pads fails to +occur once in five times--probably because it is an environmental effect +peculiar to this stock,--yet the minute difference of the presence or +absence of the two lateral bristles is a constant feature of the flies that +carry this particular factor. + +In the preceding cases I have spoken as though a factor influenced only one +part of the body. It would have been more accurate to have stated that the +_chief_ effect of the factor was observed in a particular part of the body. +Most students of genetics realize that a factor difference usually affects +more than a single character. For example, a mutant stock called +rudimentary wings has as its principle characteristic very short wings +(fig. 34). But the factor for rudimentary wings also produces other effects +as well. The females are almost completely sterile, while the males are +fertile. The viability of the stock is poor. When flies with rudimentary +wings are put into competition with wild flies relatively few of the +rudimentary flies come through, especially if the culture is crowded. The +hind legs are also shortened. All of these effects are the results of a +single factor-difference. + +[Illustration: FIG. 34. Mutant race of fruit fly, called rudimentary.] + +One may venture the guess that some of the specific and varietal +differences that are characteristic of wild types and which at the same +time appear to have no survival value, are only by-products of factors +whose most important effect is on another part of the organism where their +influence is of vital importance. + +It is well known that systematists make use of characters that are constant +for groups of species, but which do not appear in themselves to have an +adaptive significance. If we may suppose that the constancy of such +characters may be only an index of the presence of a factor whose _chief_ +influence is in some other direction or directions, some physiological +influence, for example, we can give at least a reasonable explanation of +the constancy of such characters. + +I am inclined to think that an overstatement to the effect that each factor +may affect the entire body, is less likely to do harm than to state that +each factor affects only a particular character. The reckless use of the +phrase "unit character" has done much to mislead the uninitiated as to the +effects that a single change in the germ plasm may produce on the organism. +Fortunately, the expression "unit character" is being less used by those +students of genetics who are more careful in regard to the implications of +their terminology. + +There is a class of cases of inheritance, due to the XY chromosomes, that +is called sex linked inheritance. It is shown both by mutant characters and +characters of wild species. + +For instance, white eye color in Drosophila shows sex linked inheritance. +If a white eyed male is mated to a wild red eyed female (fig. 35) all the +offspring have red eyes. If these are inbred, there are three red to one +white eyed offspring, but white eyes occur only in the males. The +grandfather has transmitted his peculiarity to half of his grandsons, but +to none of his granddaughters. + +[Illustration: FIG. 35. Diagram showing a cross between a white eyed male +and a red eyed female of the fruit fly. Sex linked inheritance.] + +The reciprocal cross (fig. 36) is also interesting. If a white eyed female +is bred to a red eyed male, all of the daughters have red eyes and all of +the sons have white eyes. We call this criss-cross inheritance. If these +offspring are inbred, they produce equal numbers of red eyed and white eyed +females and equal numbers of red eyed and white eyed males. The ratio is 1: +1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the usual 3:1 +Mendelian ratio. Yet, as will be shown later, the result is in entire +accord with Mendel's principle of segregation. + +[Illustration: FIG. 36. Diagram illustrating a cross between a red eyed +male and white eyed female of the fruit fly (reciprocal cross of that shown +in Fig. 35).] + +It has been shown by Sturtevant that in a wild species of Drosophila, viz., +D. repleta, two varieties of individuals exist, in one of which the thorax +has large splotches and in the other type smaller splotches (fig. 37). The +factors that differentiate these varieties are sex linked. + +[Illustration: FIG. 37. Two types of markings on thorax of Drosophila +repleta, both found "wild". They show sex linked inheritance.] + +Certain types of color blindness (fig. 38) and certain other abnormal +conditions in man such as haemophilia, are transmitted as sex linked +characters. + +[Illustration: FIG. 38, A. Diagram illustrating inheritance of color +blindness in man; the iris of the color-blind eye is here black.] + +[Illustration: FIG. 38, B. Reciprocal of cross in Fig. 38 a.] + +In domestic fowls sex linked inheritance has been found as the +characteristic method of transmission for at least as many as six +characters, but here the relation of the sexes is in a sense reversed. For +instance, if a black Langshan hen is crossed to a barred Plymouth Rock cock +(fig. 39), the offspring are all barred. If these are inbred half of the +daughters are black and half are barred; all of the sons are barred. The +grandmother has transmitted her color to half of her granddaughters but to +none of her grandsons. + +[Illustration: FIG. 39. Sex-linked inheritance in domesticated birds shown +here in a cross between barred Plymouth Rock male and black Langshan +female.] + +[Illustration: FIG. 40. Reciprocal of Fig. 39.] + +In the reciprocal cross (fig. 40) black cock by barred hen, the daughters +are black and the sons barred--criss-cross inheritance. These inbred give +black hens and black cocks, barred hens and barred cocks. + +There is a case comparable to this found in a wild species of moth, Abraxas +grossulariata. A wild variation of this type is lighter in color and is +known as A. lacticolor. When these two types are crossed they exhibit +exactly the same type of heredity as does the black-barred combination in +the domestic fowl. As shown in figure 41, lacticolor female bred to +grossulariata male gives grossulariata sons and daughters. These inbred +give grossulariata males and females and lacticolor females. Reciprocally +lacticolor male by grossulariata female, (fig. 42) gives lacticolor +daughters and grossulariata sons and these inbred give grossulariata males +and females and lacticolor males and females. + +[Illustration: FIG. 41. Sex-linked inheritance in the wild moth, Abraxas +grossulariata (darker) and A. lacticolor.] + +[Illustration: FIG. 42. Reciprocal of Fig. 41.] + +[Illustration: FIG. 43. Four wild types of Paratettix in upper line with +three hybrids below.] + +It has been found that there may be even more than two factors that show +Mendelian segregation when brought together in pairs. For example, in the +southern States there are several races of the grouse locust (Paratettix) +that differ from each other markedly in color patterns (fig. 43). When any +two individuals of these races are crossed they give, as Nabours has shown, +in F_2 a Mendelian ratio of 1: 2: 1. It is obvious, therefore, that there +are here at least nine characters, any two of which behave as a Mendelian +pair. These races have arisen in nature and differ definitely and +strikingly from each other, yet any two differ by only one factor +difference. + +[Illustration: FIG. 44. Diagram illustrating four allelomorphs in mice, +viz. gray bellied gray (wild type) (above, to left); white bellied gray +(above, to right); yellow (below, to right); and black (below, to left).] + +Similar relations have been found in a number of domesticated races. In +mice there is a quadruple system represented by the gray house mouse, the +white bellied, the yellow and the black mouse (fig. 44). In rabbits there +is probably a triple system, that includes the albino, the Himalayan, and +the black races. In the silkworm moth there have been described four types +of larvae, distinguished by different color markings, that form a system of +quadruple allelomorphs. In Drosophila there is a quintuple system of +factors in the sex chromosome represented by eye colors, a triple system of +body colors, and a triple system of factors for eye colors in the third +chromosome. + +MUTATION AND EVOLUTION + +What bearing has the appearance of these new types of Drosophila on the +theory of evolution may be asked. The objection has been raised in fact +that in the breeding work with Drosophila we are dealing with artificial +and unnatural conditions. It has been more than implied that results +obtained from the breeding pen, the seed pan, the flower pot and the milk +bottle do not apply to evolution in the "open", nature "at large" or to +"wild" types. To be consistent, this same objection should be extended to +the use of the spectroscope in the study of the evolution of the stars, to +the use of the test tube and the balance by the chemist, of the +galvanometer by the physicist. All these are unnatural instruments used to +torture Nature's secrets from her. I venture to think that the real +antithesis is not between unnatural and natural treatment of Nature, but +rather between controlled or verifiable data on the one hand, and +unrestrained generalization on the other. + +If a systematist were asked whether these new races of Drosophila are +comparable to wild species, he would not hesitate for a moment. He would +call them all one species. If he were asked why, he would say, I think, +"These races differ only in one or two striking points, while in a hundred +other respects they are identical even to the minutest details." He would +add, that as large a group of wild species of flies would show on the whole +the reverse relations, _viz._, they would differ in nearly every detail and +be identical in only a few points. In all this I entirely agree with the +systematist, for I do not think such a group of types differing by one +character each, is comparable to most wild groups of species because the +difference between wild species is due to a large number of such single +differences. The characters that have been accumulated in wild species are +of significance in the maintenance of the species, or at least we are led +to infer that even though the visible character that we attend to may not +itself be important, one at least of the other effects of the factors that +represent these characters is significant. It is, of course, hardly to be +expected that _any_ random change in as complex a mechanism as an insect +would improve the mechanism, and as a matter of fact it is doubtful whether +any of the mutant types so far discovered are better adapted to those +conditions to which a fly of this structure and habits is already adjusted. +But this is beside the mark, for modern genetics shows very positively that +adaptive characters are inherited in exactly the same way as are those that +are not adaptive; and I have already pointed out that we cannot study a +single mutant factor without at the same time studying one of the factors +responsible for normal characters, for the two together constitute the +Mendelian pair. + +And, finally, I want to urge on your attention a question that we are to +consider in more detail in the last lecture. Evolution of wild species +appears to have taken place by modifying and improving bit by bit the +structures and habits that the animal or plant already possessed. We have +seen that there are thirty mutant factors at least that have an influence +on eye color, and it is probable that there are at least as many normal +factors that are involved in the production of the red eye of the wild fly. + +Evolution from this point of view has consisted largely in introducing new +factors that influence characters already present in the animal or plant. + +Such a view gives us a somewhat different picture of the process of +evolution from the old idea of a ferocious struggle between the individuals +of a species with the survival of the fittest and the annihilation of the +less fit. Evolution assumes a more peaceful aspect. New and advantageous +characters survive by incorporating themselves into the race, improving it +and opening to it new opportunities. In other words, the emphasis may be +placed less on the competition between the individuals of a species +(because the destruction of the less fit does not _in itself_ lead to +anything that is new) than on the appearance of new characters and +modifications of old characters that become incorporated in the species, +for on these depends the evolution of the race. + + * * * * * + + +CHAPTER III + +THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM + +The discovery that Mendel made with edible peas concerning heredity has +been found to apply everywhere throughout the plant and animal kingdoms--to +flowering plants, to insects, snails, crustacea, fishes, amphibians, birds, +and mammals (including man). + +There must be something that these widely separated groups of plants and +animals have in common--some simple mechanism perhaps--to give such +definite and orderly series of results. There is, in fact, a mechanism, +possessed alike by animals and plants, that fulfills every requirement of +Mendel's principles. + +THE CELLULAR BASIS OF ORGANIC EVOLUTION AND HEREDITY + +In order to appreciate the full force of the evidence, let me first pass +rapidly in review a few familiar, historical facts, that preceded the +discovery of the mechanism in question. + +[Illustration: FIG. 45. Typical cell showing the cell wall, the protoplasm +(with its contained materials); the nucleus with its contained chromatin +and nuclear sap. (After Dahlgren.)] + +Throughout the greater part of the last century, while students of +evolution and of heredity were engaged in what I may call the more general, +or, shall I say, the _grosser_ aspects of the subject, there existed +another group of students who were engaged in working out the minute +structure of the material basis of the living organism. They found that +organs such as the brain, the heart, the liver, the lungs, the kidneys, +etc., are not themselves the units of structure, but that all these organs +can be reduced to a simpler unit that repeats itself a thousand-fold in +every organ. We call this unit a cell (fig. 45). + +The egg is a cell, and the spermatozoon is a cell. The act of fertilization +is the union of two cells (fig. 47, upper figure). Simple as the process of +fertilization appears to us today, its discovery swept aside a vast amount +of mystical speculation concerning the role of the male and of the female +in the act of procreation. + +Within the cell a new microcosm was revealed. Every cell was found to +contain a spherical body called the nucleus (fig. 46a). Within the nucleus +is a network of fibres, a sap fills the interstices of the network. The +network resolves itself into a definite number of threads at each division +of the cell (fig. 46 b-e). These threads we call chromosomes. Each species +of animals and plants possesses a characteristic number of these threads +which have a definite size and sometimes a specific shape and even +characteristic granules at different levels. Beyond this point our +strongest microscopes fail to penetrate. Observation has reached, for the +time being, its limit. + +[Illustration: FIG. 46. A series of cells in process of cell division. The +chromosomes are the black threads and rods. (After Dahlgren.)] + +The story is taken up at this point by a new set of students who have +worked in an entirely different field. Certain observations and experiments +that we have not time to consider now, led a number of biologists to +conclude that the chromosomes are the bearers of the hereditary units. If +so, there should be many such units carried by _each_ chromosome, for the +number of chromosomes is limited while the number of independently +inherited characters is large. In Drosophila it has been demonstrated not +only that there are exactly as many groups of characters that are inherited +together as there are pairs of chromosomes, but even that it is possible to +locate one of these groups in a particular chromosome and to state the +_relative position_ there of the factors for the characters. If the +validity of this evidence is accepted, the study of the cell leads us +finally in a mechanical, but not in a chemical sense, to the ultimate units +about which the whole process of the transmission of the hereditary factors +centers. + +But before plunging into this somewhat technical matter (that is difficult +only because it is unfamiliar), certain facts which are familiar for the +most part should be recalled, because on these turns the whole of the +subsequent story. + +[Illustration: FIG. 47. An egg, and the division of the egg--the so-called +process of cleavage. (After Selenka.)] + +The thousands of cells that make up the cell-state that we call an animal +or plant come from the fertilized egg. An hour or two after fertilization +the egg divides into two cells (fig. 47). Then each half divides again. +Each quarter next divides. The process continues until a large number of +cells is formed and out of these organs mould themselves. + +[Illustration: FIG. 48. Section of the egg of the beetle, Calligrapha, +showing the pigment at one end where the germ cells will later develop as +shown in the other two figures. (After Hegner.)] + +At every division of the cell the chromosomes also divide. Half of these +have come from the mother, half from the father. Every cell contains, +therefore, the sum total of all the chromosomes, and if these are the +bearers of the hereditary qualities, every cell in the body, whatever its +function, has a common inheritance. + +At an early stage in the development of the animal certain cells are set +apart to form the organs of reproduction. In some animals these cells can +be identified early in the cleavage (fig. 48). + +The reproductive cells are at first like all the other cells in the body in +that they contain a full complement of chromosomes, half paternal and half +maternal in origin (fig. 49). They divide as do the other cells of the body +for a long time (fig. 49, upper row). At each division each chromosome +splits lengthwise and its halves migrate to opposite poles of the spindle +(fig. 49 c). + +But there comes a time when a new process appears in the germ cells (fig 49 +e-h). It is essentially the same in the egg and in the sperm cells. The +discovery of this process we owe to the laborious researches of many +workers in many countries. The list of their names is long, and I shall not +even attempt to repeat it. The chromosomes come together in pairs (fig. 49 +a). Each maternal chromosome mates with a paternal chromosome of the same +kind. + +[Illustration: FIG. 49. In the upper row of the diagram a typical process +of nuclear division, such as takes place in the early germ cells or in the +body cells. In the lower row the separation of the chromosomes that have +paired. This sort of separation takes place at one of the two reduction +divisions.] + +Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one of the +divisions the double chromosomes separate so that each resulting cell comes +to contain some maternal and some paternal chromosomes, i.e. one or the +other member of each pair. At the other division each chromosome simply +splits as in ordinary cell division. + +[Illustration: FIG. 50. The two maturation divisions of the sperm cell. +Four sperms result, each with half (haploid) the full number (diploid) of +chromosomes.] + +The upshot of the process is that the ripe eggs (fig. 51) and the ripe +spermatozoa (fig. 50) come to contain only half the total number of +chromosomes. + +[Illustration: FIG. 51. The two maturation divisions of the egg. The +divisions are unequal, so that two small polar bodies are formed one of +these subsequently divides. The three polar bodies and the egg are +comparable to the four sperms.] + +When the eggs are fertilized the whole number of chromosomes is restored +again. + +THE MECHANISM OF MENDELIAN HEREDITY DISCOVERED IN THE BEHAVIOR OF THE +CHROMOSOMES + +If the factors in heredity are carried in the chromosomes and if the +chromosomes are definite structures, we should anticipate that there should +be as many _groups_ of characters as there are kinds of chromosomes. In +only one case has a sufficient number of characters been studied to show +whether there is any correspondence between the number of hereditary groups +of characters and the number of chromosomes. In the fruit fly, Drosophila +ampelophila, we have found about 125 characters that are inherited in a +perfectly definite way. On the opposite page is a list of some of them. + +It will be observed in this list that the characters are arranged in four +groups, Groups I, II, III and IV. Three of these groups are equally large +or nearly so; Group IV contains only two characters. The characters are put +into these groups because in heredity the members of each group tend to be +inherited together, i.e., if two or more enter the cross together they tend +to remain together through subsequent generations. On the other hand, any +member of one group is inherited entirely independently of any member of +the other groups; in the same way as Mendel's yellow-green pair of +characters is inherited independently of the round-wrinkled pair. + + _Group I_ _Group II_ _Group III_ _Group IV_ + Abnormal Antlered Band Bent + Bar Apterous Beaded Eyeless + Bifid Arc Cream III + Bow Balloon Deformed + Cherry Black Dwarf + Chrome Blistered Ebony + Cleft Comma Giant + Club Confluent Kidney + Depressed Cream II Low crossing over + Dot Curved Maroon + Eosin Dachs Peach + Facet Extra vein Pink + Forked Fringed Rough + Furrowed Jaunty Safranin + Fused Limited Sepia + Green Little crossover Sooty + Jaunty Morula Spineless + Lemon Olive Spread + Lethals, 13 Plexus Trident + Miniature Purple Truncate intensifier + Notch Speck Whitehead + Reduplicated Strap White ocelli + Ruby Streak + Rudimentary Trefoil + Sable Truncate + Shifted Vestigial + Short + Skee + Spoon + Spot + Tan + Truncate intensifier + Vermilion + White + Yellow + +If the factors for these characters are carried by the chromosomes, then we +should expect that those factors that are carried by the same chromosome +would be inherited together, provided the chromosomes are definite +structures in the cell. + +[Illustration: FIG. 52. Chromosomes (diploid) of D. ampelophila. The sex +chromosomes are XX in the female and XY in the male. There are three other +pairs of chromosomes.] + +In the chromosome group of Drosophila, (fig. 52) there are _four_ pairs of +chromosomes, three of nearly the same size and one much smaller. Not only +is there agreement between the number of hereditary groups and the number +of the chromosomes, but even the size relations are the same, for there are +three great groups of characters and three pairs of large chromosomes, and +one small group of characters and one pair of small chromosomes. + +THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA AMPELOPHILA + +The following description of the characters of the wild fly may be useful +in connection with the account of the modifications of these characters +that appear in the mutants. + +The head and thorax of the wild fly are grayish-yellow, the abdomen is +banded with alternate stripes of yellow and black. In the male, (fig. 4 to +right), there are three narrow bands and a black tip. In the female there +are five black bands (fig. 4 to left). The wings are gray with a surface +texture of such a kind that at certain angles they are iridescent. The eyes +are a deep, solid, brick-red. The minute hairs that cover the body have a +very definite arrangement that is most obvious on the head and thorax. +There is a definite number of larger hairs called bristles or chaetae which +have a characteristic position and are used for diagnostic purposes in +classifying the species. On the foreleg of the male there is a comb-like +organ formed by a row of bristles; it is absent in the female. The comb is +a secondary sexual character, and it is, so far as known, functionless. + +Some of the characters of the mutant types are shown in figures 53, 54, 55, +56. The drawing of a single fly is often used here to illustrate more than +one character. This is done to economize space, but of course there would +be no difficulty in actually bringing together in the same individual any +two or more characters belonging to the same group (or to different +groups). Without colored figures it is not possible to show many of the +most striking differences of these mutant races; at most dark and light +coloring can be indicated by the shading of the body, wings, or eyes. + +_Group I_ + +In the six flies drawn in figure 53 there are shown five different wing +characters. The first of these types (a) is called cut, because the ends of +the wings look as though they had been cut to a point. The antennae are +displaced downward and appressed and their bristle-like aristae are +crumpled. + +[Illustration: FIG. 53. Group I. (See text)] + +The second figure (b) represents a fly with a notch in the ends of the +wings. This character is dominant, but the same factor that produces the +notch in the wings is also a recessive lethal factor; because of this +latter effect of the character no males of this race exist, and the females +of the race are never pure but hybrid. Every female with notch wings bred +to a wild male, will produce in equal numbers notch winged daughters and +daughters with normal wings. There will be half as many sons as daughters. +The explanation of this peculiar result is quite simple. Every notch winged +female has one X chromosome that carries the factor for notch and one X +chromosome that is "normal". Daughters receiving the former chromosomes are +notched because the factor for notch is dominant, but they are not killed +since the lethal effect of the notch factor is recessive to the normal +allelomorph carried by the other chromosome that the daughters get from +their father. This normal factor is recessive for notch but dominant for +life. This same figure (b) is used here to show three other sex linked +characters. The spines on the thorax are twisted or kinky, which is due to +a factor called "forked". The effect is best seen on the thorax, but all +spines on the body are similarly modified; even the minute hairs are also +affected. Ruby eye color might be here represented--if the eyes in the +figure were colored. The lighter color of the body and antennae is intended +to indicate that the character tan is also present. The light color of the +antennae is the most certain way of identifying tan. The tan flies are +interesting because they have lost the positive heliotropism that is so +marked a feature in the behavior of D. ampelophila. As this peculiarity of +the tan flies is inherited like all the other sex linked characters, it +follows that when a tan female is bred to a wild male all the sons inherit +the recessive tan color and indifference to light, while the daughters show +the dominant sex linked character of their father, i.e., they are "gray", +and go to the light. Hence when such a brood is disturbed the females fly +to the light, but the males remain behind. + +One of the first mutants that appeared in D. ampelophila was called +rudimentary on account of the condition of the wings (c). The same mutation +has appeared independently several times. In the drawing (c) the dark body +color is intended to indicate "sable" and the lighter color of the eyes is +intended to indicate eosin. This eye color, which is an allelomorph of +white, is also interesting because in the female the color is deeper than +in the male. In other cases of sex linked factors the character is the same +in the two sexes. + +In the fourth figure (d) the third and fourth longitudinal veins of the +wing are _fused_ into one vein from the base of the wing to the level of +the first cross-vein and in addition converge and meet near their outer +ends. The shape of the eye is represented in the figure as different from +the normal, due to another factor called "bar". This is a dominant +character, the hybrid condition being also narrow, but not so narrow as the +pure type. Vermilion eye color might also be here represented--due to a +factor that has appeared independently on several occasions. + +In the fifth figure (e) the wings are shorter and more pointed than in the +wild fly. This character is called miniature. The light color of the +drawing may be taken to represent yellow body color, and the light color of +the eye white eye color. + +In the last figure (f) the wings are represented as pads, essentially in +the same condition that they are in when the fly emerges from the pupa +case. Not all the flies of this stock have the wings in this condition; +some have fully expanded wings that appear normal in all respects. +Nevertheless, about the same percentage of offspring show the pads +irrespective of whether the parents had pads or expanded wings. + +The flies of this stock show, however, another character, which is a +product of the same factor, and which is constant, i.e., repeated in all +individuals. The two bristles on the sides of the thorax are constantly +absent in this race. The lighter color of the eye in the figure may be +taken to indicate buff--a faint yellowish color. The factor for this eye +color is another allelomorph of white. + +There are many other interesting characters that belong to the first group, +such as abnormal abdomen, short legs, duplication of the legs, etc. In +fact, any part of the body may be affected by a sex-linked factor. + +_Group II_ + +In the first figure (a) of figure 54 that contains members of Group II the +wings are almost entirely absent or "vestigial". This condition arose at a +single step and breeds true, although it appears to be influenced to some +extent by temperature, also by modifiers that sometimes appear in the +stock. Purple eye color belongs in Group II; it resembles the color of the +eye of the wild fly but is darker and more translucent. + +[Illustration: FIG. 54. Group II. (See text.)] + +In the second figure (b) the wing is again long and narrow and sometimes +bent back on itself, as shown here. In several respects the wing resembles +strap (d) but seems to be due to another factor, called antler, +insufficiently studied as yet. + +In the third figure (c) the wings turn up at the end. This is brought about +by the presence of the factor called jaunty. + +In the fourth figure the wings are long and narrow and several of the veins +are unrepresented. This character, "strap", is very variable and has not +yet been thoroughly studied. On the thorax there is a deep black mark +called trefoil. Even in the wild fly there is a three pronged mark on the +thorax present in many individuals. Trefoil is a further development and +modification of this mark and is due to a special factor. + +In the fifth figure (e) the wings are arched. The factor is called arc. The +dark color of the body, and especially of the wings, indicates the factor +for black. + +The sixth figure (f) shows the wings "curved" downwards. In addition there +is present a minute black speck at the base of each wing, due to another +factor called speck. + +In the seventh figure (g) the wing is truncate. Its end is obliquely +squared instead of rounded; it may be longer than the body, or shorter when +other modifying factors are present. The mutation that produces this type +of wing is of not infrequent occurrence. It has been shown by Muller and +Altenburg that there are at least two factors that modify this +character--the chief factor is present in the second chromosome; alone it +produces the truncate wing in only a certain percentage of cases, but when +the modifiers are also present about ninety percent of the individuals may +show the truncate condition of the wing. But the presence of these factors +makes the stock very infertile, so that it is difficult to maintain. + +In the eighth figure (h) the legs are shortened owing to the absence of a +segment of the tarsus. The stock is called dachs--a nickname given to it +because the short legs suggested the dachshund. + +_Group III_ + +In figure 55, (a), a mutant type called bithorax is shown. The old +metathorax is replaced by another mesothorax thrust in between the normal +mesothorax and the abdomen. It carries a pair of wings that do not +completely unfold. On this new mesothorax the characteristic arrangement of +the bristles is shown. Thus at a single step a typical region of the body +has doubled. The character is recessive. + +[Illustration: FIG. 55. Group III. (See text.)] + +The size of the adult fly of D. ampelophila varies greatly according to the +amount of nourishment obtained by the larva. After the fly emerges its size +remains nearly constant, as in many insects. Two races have, however, been +separated by Bridges that are different in size as a result of a genetic +factor. The first of these, called dwarf, is represented by figure 55, (b). + +The race is minute, although of course its size is variable, depending on +food and other conditions. The same figure shows the presence of another +factor, "sooty", that makes the fly very dark. Maroon eye color might be +here represented, due to still another factor. + +In the third figure (c) the other mutation in size is shown. It is called +"giant". The flies are twice the size of wild flies. An eye color, called +peach, might here be represented. It is an allelomorph of pink. + +In the fourth figure (d) the mutant called dichaete is shown. It is +characterized by the absence of two of the bristles on the thorax. Other +bristles may also be absent, but not so constantly as the two just +mentioned. Another effect of the same factor is the spread-out condition of +the wings. The very dark eye color in this figure may be taken to indicate +the presence of another factor, "sepia", which causes the eyes to assume a +brown color that becomes black with age. Most of the other mutations in eye +color that have occurred tend to give a lighter color: this one, which is +also recessive, makes the eye darker. + +In the fifth figure (e) the color of the darkest fly is due to a factor +called ebony, which is an allelomorph of sooty. + +In the sixth figure (f) the wings are beaded, i.e., the margin is defective +at intervals, giving a beaded-like outline to the wings. This condition is +very variable and much affected by other factors that influence the shape +of the wings. The lighter eye color of the drawing may be taken to +represent pink. + +In the seventh figure (g) the wings are curled up over the back. This is a +recessive character. + +_Group IV_ + +Only two mutants have been obtained that do not belong to any of the +preceding groups; these are put together in Group IV. It has been shown +that they are linked to each other and the linkage is so close that it has +thus far been impossible to obtain the dominant recessive. One of these +mutants, called "eyeless" (fig. 56, a, a^1), is variable--the eyes are +often entirely absent or represented by one or more groups of ommatidia. +The outline of the original eye, so to speak, is strongly marked out and +its area might be called a rudimentary organ, if such a statement has any +meaning here. + +[Illustration: FIG. 56. Group IV. (See text.)] + +The other figure (b) represents "bent", so called from the shape of the +wings. This mutant is likewise very variable, often indistinguishable from +the wild type, yet when well developed strikingly different from any other +mutant. + +This brief account of a few of the mutant races that can be most easily +represented by uncolored figures will serve to show how all parts of the +body may change, some of the changes being so slight that they would be +overlooked except by an expert, others so great that in the character +affected the flies depart far from the original species. + +_It is important to note that mutations in the first chromosome are not +limited to any part of the body nor do they affect more frequently a +particular part. The same statement holds equally for all of the other +chromosomes. In fact, since each factor may affect visibly several parts of +the body at the same time there are no grounds for expecting any special +relation between a given chromosome and special regions of the body. It can +not too insistently be urged that when we say a character is the product of +a particular factor we mean no more than that it is the most conspicuous +effect of the factor._ + +If, then, as these and other results to be described point to the +chromosomes as the bearers of the Mendelian factors, and if, as will be +shown presently, these factors have a definite location in the chromosomes +it is clear that the location of the factors in the chromosomes bears no +spatial relation to the location of the parts of the body to each other. + +LOCALIZATION OF FACTORS IN THE CHROMOSOMES + +_The Evidence from Sex Linked Inheritance_ + +When we follow the history of pairs of chromosomes we find that their +distribution in successive generations is paralleled by the inheritance of +Mendelian characters. This is best shown in the sex chromosomes (fig. 57). +In the female there are two of these chromosomes that we call the X +chromosomes; in the male there are also two but one differs from those of +the female in its shape, and in the fact that it carries none of the normal +allelomorphs of the mutant factors. It is called the Y chromosome. + +The course followed by the sex chromosomes and that by the characters in +the case of sex linked inheritance are shown in the next diagram of +Drosophila illustrating a cross between a white eyed male and a red eyed +female. + +[Illustration: FIG. 57. Scheme of sex determination in Drosophila type. +Each _mature_ egg contains one X, each mature sperm contains one X, or a Y +chromosome. Chance union of any egg with any sperm will give either XX +(female) or XY (male).] + +[Illustration: FIG. 58. Cross between white eyed male of D. ampelophila and +red eyed female. The sex chromosomes are indicated by the rods. A black rod +indicates that the chromosome carries the factor for red; the open +chromosome the factor for white eye color.] + +The first of these represents a cross between a white eyed male and a red +eyed female (fig. 58, top row). The X chromosome in the male is represented +by an open bar, the Y chromosome is bent. In the female the two X +chromosomes are black. Each egg of such a female will contain one "black" X +after the polar bodies have been thrown off. In the male there will be two +classes of sperm--the female-producing, carrying the (open) X, and the +male-producing, carrying the Y chromosome. Any egg fertilized by an X +bearing sperm will produce a female that will have red eyes because the X +(black) chromosome it gets from the mother carries the dominant factor for +red. Any egg fertilized by a Y-bearing sperm will produce a male that will +also have red eyes because he gets his (black) X chromosome from his +mother. + +When, then, these two F_1 flies (second row) are inbred the following +combinations are expected. Each egg will contain a black X (red eye +producing) or a white X (white eye producing) after the polar bodies have +been extruded. The male will produce two kinds of sperms, of which the +female producing will contain a black X (red eye producing). Since any egg +may by chance be fertilized by any sperm there will result the four classes +of individuals shown on the bottom row of the diagram. All the females will +have red eyes, because irrespective of the two kinds of eggs involved all +the female-producing sperm carry a black X. Half of the males have red eyes +because half of the eggs have had each a red-producing X chromosome. The +other half of the males have white eyes, because the other half of the eggs +had each a white-producing X chromosome. Other evidence has shown that the +Y chromosome of the male is indifferent, so far as these Mendelian factors +are concerned. + +[Illustration: FIG. 59. Cross between red eyed male and white eyed female; +reciprocal cross of Fig. 58.] + +The reciprocal experiment is illustrated in figure 59. A white eyed female +is mated to a red eyed male (top row). All the mature eggs of such a female +contain one white-producing X chromosome represented by the open bar in the +diagram. The red eyed male contains female-producing X-bearing sperm that +carry the factor for red eye color, and male-producing Y chromosomes. Any +egg fertilized by an X-bearing sperm will become a red eyed female because +the X chromosome that comes from the father carries the dominant factor for +red eye color. Any egg fertilized by a Y-bearing sperm will become a male +with white eyes because the only X chromosome that the male contains comes +from his mother and is white producing. + +When these two F_1 flies are inbred (middle row) the following combinations +are expected. Half the eggs will contain each a white producing X +chromosome and half red producing. The female-producing sperms will each +contain a white X and the male-producing sperms will each contain an +indifferent Y chromosome. Chance meetings of egg and sperm will give the +four F_2 classes (bottom row). These consist of white eyed and red eyed +females and white eyed and red eyed males. The ratio here is 1:1 and not +three to one (3:1) as in other Mendelian cases. But Mendel's law of +segregation is not transgressed, as the preceding analysis has shown; for, +the chromosomes have followed strictly the course laid down on Mendel's +principle for the distribution of factors. The peculiar result in this case +is due to the fact that the F_1 male gets his single factor for eye color +from his mother only and it is linked to or contained in a body (the X +chromosome) that is involved in producing the females, while the mate of +this body--the Y chromosome--is indifferent with regard to these factors, +yet active as a mate to X in synapsis. + +[Illustration: FIG. 60. Diagram of sex determination in type with XX female +and XO male (after Wilson).] + +In man there are several characters that show exactly this same kind of +inheritance. Color blindness, or at least certain kinds of color blindness, +appear to follow the same scheme. A color blind father transmits through +his daughters his peculiarity to half of his grandsons, but to none of his +grand-daughters (fig. 38A). The result is the same as in the case of the +white eyed male of Drosophila. Color blind women are rather unusual, which +is expected from the method of inheritance of this character, but in the +few known cases where such color blind women have married normal husbands +the sons have inherited the peculiarity from the mother (fig. 38B). Here +again the result is the same as for the similar combination in Drosophila. + +[Illustration: FIG. 61. Spermatogenesis in man. There are 47 chromosomes +(diploid) in the male. After reduction half of the sperm carry 24 +chromosomes (one of which is X) and half carry 23 chromosomes (no X).] + +In man the sex formula appears to be XX for the female and XO for the male +(fig. 60), and since the relation is essentially the same as that in +Drosophila the chromosome explanation is the same. According to von +Winiwarter there are 48 chromosomes in the female and 47 in the male (fig. +61). After the extrusion of the polar bodies there are 24 chromosomes in +the egg. In the male at one of the two maturation divisions the X +chromosome passes to one pole undivided (fig. 61, C). In consequence there +are two classes of sperms in man; female producing containing 24 +chromosomes, and male producing containing 23 chromosomes. If the factor +for color blindness is carried by the X chromosome its inheritance in man +works out on the same chromosome scheme and in the same way as does white +eye color (or any other sex linked character) in the fly, for the O sperm +in man is equivalent to the Y sperm in the fly. + +In these cases we have been dealing with a single pair of characters. Let +us now take a case where two pairs of sex linked characters enter the cross +at the same time, and preferably a case where the two recessives enter the +cross from the same parent. + +If a female with white eyes and yellow wings is crossed to a wild male with +red eyes and gray wings (fig. 62), the sons are yellow and have white eyes +and the daughters are gray and have red eyes. If two F_1 flies are mated +they will produce the following classes. + +[Illustration: FIG. 62. Cross between a white eyed, yellow winged female of +D. ampelophila and a red eyed, gray winged male. Two pairs of sex linked +characters, viz., white-red and yellow-gray are involved. (See text.)] + + Yellow Gray Yellow Gray + White Red Red White + ------------ ------------- + | | + 99.% 1.% + +Not only have the two grandparental combinations reappeared, but in +addition two new combinations, viz., grey white and yellow red. The two +original combinations far exceed in numbers the new or exchange +combinations. If we follow the history of the X chromosomes we discover +that the _larger classes_ of grandchildren appear in accord with the way in +which the X chromosomes are transmitted from one generation to the next. + +The _smaller classes_ of grandchildren, the exchange combinations or +cross-overs, as we call them, can be explained by the assumption that at +some stage in their history an interchange of parts has taken place between +the chromosomes. This is indicated in the diagrams. + +The most important fact brought out by the experiment is that the factors +that went in together tend to stick together. It makes no difference in +what combination the members of the two pairs of characters enter, they +tend to remain in that combination. + +If one admits that the sex chromosomes carry these factors for the +sex-linked characters--and the evidence is certainly very strong in favor +of this view--it follows necessarily from these facts that at some time in +their history there has been an interchange between the two sex chromosomes +in the female. + +There are several stages in the conjugation of the chromosomes at which +such an interchange between the members of a pair might occur. There is +further a small amount of direct evidence, unfortunately very meagre at +present, showing that an interchange does actually occur. + +At the ripening period of the germ cell the members of each pair of +chromosomes come together (fig. 49, e). In several forms they have been +described as meeting at one end and then progressively coming to lie side +by side as shown in fig. 63, e, f, g, h, i. At the end of the process they +appear to have completely united along their length (fig. 63, j, k, l). It +is always a maternal and a paternal chromosome that meet in this way and +always two of the same kind. It has been observed that as the members of a +pair come together they occasionally twist around each other (fig. 63, g, +l, and 64, and 65). In consequence a part of one chromosome comes to be now +on one side and now on the other side of its mate. + +[Illustration: FIG. 63. Conjugation of chromosomes (side to side union) in +the spermatogenesis of Batracoseps. (After Janssens.)] + +When the chromosomes separate at the next division of the germ cell the +part on one side passes to one pole, the part on the other to the opposite +pole, (figs. 64 and 65). Whenever the chromosomes do not untwist at this +time there must result an interchange of pieces where they were crossed +over each other. + +[Illustration: FIG. 64. Scheme to illustrate a method of crossing over of +the chromosomes.] + +Janssens has found at the time of separation evidence in favor of the view +that some such interchange probably takes place. + +We find this same process of interchange of characters taking place in each +of the other three groups of Drosophila. An example will show this for the +Group II. + +[Illustration: FIG. 65. Scheme to illustrate double crossing over.] + +If a black vestigial male is crossed to a gray long-winged female (fig. 66) +the offspring are gray long. If an F_1 female is back-crossed to a black +vestigial male the following kinds of flies are produced: + + Black Gray Black Gray + vestigial long long vestigial + ----------------- ----------------- + | | + 83% 17% + +The combinations that entered are more common in the F_2 generations than +the cross-over classes, showing that there is linkage of the factors that +entered together. + +Another curious fact is brought out if instead of back-crossing the F_1 +female we back-cross the F_1 male to a black vestigial female. Their +offspring are now of only two kinds, black vestigial and gray long. This +means that in the male there is no crossing-over or interchange of pieces. +This relation holds not only for the Group II but for all the other groups +as well. + +Why interchange takes place in the female of Drosophila and not in the male +we do not know at present. We might surmise that when in the male the +members of a pair come together they do not twist around each other, hence +no crossing-over results. + +[Illustration: FIG. 66. Cross between black vestigial and gray long flies. +Two pairs of factors involved in the second group. The F_1 female is back +crossed (to right) to black vestigial male; and the F_1 male is back +crossed to black vestigial female (to left). Crossing over takes place in +the F_1 female but not in the F_1 male.] + +Crossing-over took place between white and yellow only once in a hundred +times. Other characters show different values, but the same value under the +same conditions is obtained from the same pair of characters. + +[Illustration: FIG. 67. Map of four chromosomes of D. ampelophila locating +those factors in each group that have been most fully studied.] + +If we assume that the nearer together the factors lie in the chromosome the +less likely is a twist to occur between them, and conversely the farther +apart they lie the more likely is a twist to occur between them, we can +understand how the linkage is different for different pairs of factors. + +On this basis we have made out chromosomal maps for each chromosome (fig. +67). The diagram indicates those loci that have been most accurately +placed. + +_The Evidence from Interference_ + +There is a considerable body of information that we have obtained that +corroborates the location of the factors in the chromosome. This evidence +is too technical to take up in any detail, but there is one result that is +so important that I must attempt to explain it. If, as I assume, crossing +over is brought about by twisting of the chromosomes, and if owing to the +material of the chromosomes there is a most frequent distance of internode, +then, when crossing over between nodes takes place at same level at a-b in +figure 68, the region on each side of that point, a to A and b to B, should +be protected, so to speak, from further crossing over. This in fact we have +found to be the case. No other explanation so far proposed will account for +this extraordinary relation. + +[Illustration: FIG. 68. Scheme to indicate that when the members of a pair +of chromosomes cross (at a-b) the region on each side is protected +inversely to the distance from a-b.] + +What advantage, may be asked, is there in obtaining numerical data of this +kind? It is this:--whenever a new character appears we need only determine +in which of the four groups it lies and its distance from two members +within that group. With this information we can predict with a high degree +of probability what results it will give with any other member of any +group. Thus we can do on paper what would require many months of labor by +making the actual experiment. In a word we can predict what will happen in +a situation where prediction is impossible without this numerical +information. + +_The Evidence from Non-Disjunction_ + +In the course of the work on Drosophila exceptions appeared in one strain +where certain individuals did not conform to the scheme of sex linked +inheritance. For a moment the hypothesis seemed to fail, but a careful +examination led to the suspicion that in this strain something had happened +to the sex chromosomes. It was seen that if in some way the X chromosomes +failed to disjoin in certain eggs, the exceptions could be explained. The +analysis led to the suggestion that if the Y chromosome had got into the +female line the results would be accounted for, since its presence there +would be expected to cause this peculiar non-disjunction of the X +chromosomes. + +That this was the explanation was shown when the material was examined. The +females that gave these results were found by Bridges to have two X's and a +Y chromosome. + +The normal chromosome group of the female is shown in figure 52 and the +chromosome group of one of the exceptional females is shown in figure 69. +In a female of this kind there are three sex chromosomes X X Y which are +homologous in the sense that in normal individuals the two present are +mates and separate at the reduction division. If in the X X Y individual X +and X conjugate and separate at reduction and the unmated Y is free to move +to either pole of the spindle, two kinds of mature eggs will result, viz., +X and XY. If, on the other hand, X and Y conjugate and separate at +reduction and the remaining X is free to go to either pole, four kinds of +eggs will result--XY--X--XX--Y. As a total result four kinds of eggs are +expected: viz. many XY and X eggs and a few XX and Y eggs. + +[Illustration: FIG. 69. Figure of the chromosome group of an XXY female, +that gives non-disjunction.] + +These four kinds of eggs may be fertilized either by female-producing +sperms or male-producing sperms, as indicated in the diagram (fig. 70). + +[Illustration: FIG. 70. Scheme showing the results of fertilizing white +bearing eggs (4 kinds) resulting from non-disjunction. The upper half of +the diagram gives the results when these eggs are fertilized by normal red +bearing, female producing sperm, the lower half by normal, male producing +sperm.] + +If such an XXY female carried white bearing Xs (open X in the figures), and +the male carried a red bearing X (black X in the figures) it will be seen +that there should result an exceptional class of sons that are red, and an +exceptional class of daughters that are white. Tests of these exceptions +show that they behave subsequently in heredity as their composition +requires. Other tests may also be made of the other classes of offspring. +Bridges has shown that they fulfill all the requirements predicted. Thus a +result that seemed in contradiction with the chromosome hypothesis has +turned out to give a brilliant confirmation of that theory both genetically +and cytologically. + +HOW MANY GENETIC FACTORS ARE THERE IN THE GERM-PLASM OF A SINGLE INDIVIDUAL + +In passing I invite your attention to a speculation based on our maps of +the chromosomes--a speculation which I must insist does not pretend to be +more than a guess but has at least the interest of being the first guess +that we have ever been in position to make as to how many factors go +towards the makeup of the germ plasm. + +We have found practically no factors less than .04 of a unit apart. If our +map includes the entire length of the chromosomes and if we assume factors +are uniformly distributed along the chromosome at distances equal to the +shortest distance yet observed, viz. .04, then we can calculate roughly how +many hereditary factors there are in Drosophila. The calculation gives +about 7500 factors. The reader should be cautioned against accepting the +above assumptions as strictly true, for crossing-over values are known to +differ according to different environmental conditions (as shown by Bridges +for age), and to differ even in different parts of the chromosome as a +result of the presence of specific genetic factors (as shown by +Sturtevant). Since all the chromosomes except the X chromosomes are double +we must double our estimate to give the _total_ number of factors, but the +half number is the number of the different kinds of factors of Drosophila. + +CONCLUSIONS + +I have passed in review a long series of researches as to the nature of the +hereditary material. We have in consequence of this work arrived within +sight of a result that seemed a few years ago far beyond our reach. The +mechanism of heredity has, I think, been discovered--discovered not by a +flash of intuition but as the result of patient and careful study of the +evidence itself. + +With the discovery of this mechanism I venture the opinion that the problem +of heredity has been solved. We know how the factors carried by the parents +are sorted out to the germ cells. The explanation does not pretend to state +how factors arise or how they influence the development of the embryo. But +these have never been an integral part of the doctrine of heredity. The +problems which they present must be worked out in their own field. So, I +repeat, the mechanism of the chromosomes offers a satisfactory solution of +the traditional problem of heredity. + + * * * * * + + +CHAPTER IV + +SELECTION AND EVOLUTION + +Darwin's Theory of Natural Selection still holds today first place in every +discussion of evolution, and for this very reason the theory calls for +careful scrutiny; for it is not difficult to show that the expression +"natural selection" is to many men a metaphor that carries many meanings, +and sometimes different meanings to different men. While I heartily agree +with my fellow biologists in ascribing to Darwin himself, and to his work, +the first place in biological philosophy, yet recognition of this claim +should not deter us from a careful analysis of the situation in the light +of work that has been done since Darwin's time. + +THE THEORY OF NATURAL SELECTION + +In his great book on the _Origin of Species_, Darwin tried to do two +things: first, to show that the evidence bearing on evolution makes that +explanation probable. No such great body of evidence had ever been brought +together before, and it wrought, as we all know, a revolution in our modes +of thinking. + +Darwin also set himself the task of showing _how_ evolution might have +taken place. He pointed to the influence of the environment, to the effects +of use and disuse, and to natural selection. It is to the last theory that +his name is especially attached. He appealed to a fact familiar to +everyone, that no two individuals are identical and that some of the +differences that they show are inherited. He argued that those individuals +that are best suited to their environment are the most probable ones to +survive and to leave most offspring. In consequence their descendants +should in time replace through competition the less well-adapted +individuals of the species. This is the process Darwin called natural +selection, and Spencer the survival of the fittest. + +Stated in these general terms there is nothing in the theory to which +anyone is likely to take exception. But let us examine the argument more +critically. + +[Illustration: FIG. 71. Series of leaves of a tree arranged according to +size. (After de Vries.)] + +If we measure, or weigh, or classify any character shown by the individuals +of a population, we find differences. We recognize that some of the +differences are due to the varied experiences that the individuals have +encountered in the course of their lives, i.e. to their environment, but we +also recognize that some of the differences may be due to individuals +having different inheritances--different germ plasms. Some familiar +examples will help to bring home this relation. + +If the leaves of a tree are arranged according to size (fig. 71), we find a +continuous series, but there are more leaves of medium size than extremes. +If a lot of beans be sorted out according to their weights, and those +between certain weights put into cylinders, the cylinders, when arranged +according to the size of the beans, will appear as shown in figure 72. An +imaginary line running over the tops of the piles will give a curve (fig. +73) that corresponds to the curve of probability (fig. 74). + +[Illustration: FIG. 72. Beans put into cylindrical jars according to the +sizes of the beans. The jars arranged according to size of contained beans. +(After de Vries.)] + +[Illustration: FIG. 73. A curve resulting from arrangement of beans +according to size. (After de Vries.)] + +If we stand men in lines according to their height (fig. 75) we get a +similar arrangement. + +[Illustration: FIG. 74. Curve of probability.] + +[Illustration: FIG. 75. Students arranged according to size. (After +Blakeslee.)] + +The differences in size shown by the individual beans or by the individual +men are due in part to heredity, in part to the environment in which they +have developed. This is a familiar fact of almost every-day observation. It +is well shown in the following example. In figure 76 the two boys and the +two varieties of corn, which they are holding, differ in height. The +pedigrees of the boys (fig. 77) make it probable that their height is +largely inherited and the two races of corn are known to belong to a tall +and a short race respectively. Here, then, the chief effect or difference +is due to heredity. On the other hand, if individuals of the same race +develop in a favorable environment the result is different from the +development in an unfavorable environment, as shown in figure 78. Here to +the right the corn is crowded and in consequence dwarfed, while to the left +the same kind of corn has had more room to develop and is taller. + +[Illustration: FIG. 76. A short and a tall boy each holding a stalk of +corn--one stalk of a race of short corn, the other of tall corn. (After +Blakeslee.)] + +[Illustration: FIG. 77. Pedigree of boys shown in Fig. 76. (After +Blakeslee.)] + +Darwin knew that if selection of particular kinds of individuals of a +population takes place the next generation is affected. If the taller men +of a community are selected _the average_ of their offspring will be taller +than the average of the former population. If selection for tallness again +takes place, still taller men will _on the average_ arise. If, amongst +these, selection again makes a choice the process would, he thought, +continue (fig. 79). + +[Illustration: FIG. 78. A race of corn reared under different conditions.] + +We now recognize that this statement contains an important truth, but we +have found that it contains only a part of the truth. Any one who repeats +for himself this kind of selection experiment will find that while his +average class will often change in the direction of his selection, the +process slows down as a rule rather suddenly (fig. 80). He finds, moreover, +that the limits of variability are not necessarily transcended as the +process continues even although the average may for a while be increased. +More tall men may be produced by selection of this kind, but the tallest +men are not necessarily any taller than the tallest in the original +population. + +[Illustration: FIG. 79. Curves showing how (hypothetically) selection might +be supposed to bring about progress in direction of selection. (After +Goldschmidt.)] + +Selection, then, has not produced anything new, but only more of certain +kinds of individuals. Evolution, however, means producing more new things, +not more of what already exists. + +Darwin seems to have thought that the range of variation shown by the +offspring of a given individual about that type of individual would be as +wide as the range shown by the original population (fig. 79), but Galton's +work has made it clear that this is not the case in a general or mixed +population. If the offspring of individuals continued to show, as Darwin +seems to have thought, as wide a range on each side of their parents' size, +so to speak, as did the original population, then it would follow that +selection could slide successive generations along in the direction of +selection. + +[Illustration: FIG. 80. Diagram illustrating the results of selection for +extra bristles in D. ampelophila. Selection at first produces decided +effects which soon slow down and then cease. (MacDowell.)] + +Darwin himself was extraordinarily careful, however, in the statements he +made in this connection and it is rather by implication than by actual +reference that one can ascribe this meaning to his views. His +contemporaries and many of his followers, however, appear to have accepted +this _sliding scale_ interpretation as the cardinal doctrine of evolution. +If this is doubted or my statement is challenged then one must explain why +de Vries' mutation theory met with so little enthusiasm amongst the older +group of zoologists and botanists; and one must explain why Johannsen's +splendid work met with such bitter opposition from the English school--the +biometricians--who amongst the post-Darwinian school are assumed to be the +lineal descendants of Darwin. + +And in this connection we should not forget that just this sort of process +was supposed to take place in the inheritance of use and disuse. What is +gained in one generation forms the basis for further gains in the next +generation. Now, Darwin not only believed that acquired characters are +inherited but turned more and more to this explanation in his later +writings. Let us, however, not make too much of the matter; for it is much +less important to find out whether Darwin's ideas were vague, than it is to +make sure that our own ideas are clear. + +If I have made several statements here that appear dogmatic let me now +attempt to justify them, or at least give the evidence which seems to me to +make them probable. + +The work of the Danish botanist, Johannsen, has given us the most carefully +analyzed case of selection that has ever been obtained. There are, +moreover, special reasons why the material that he used is better suited to +give definite information than any other so far studied. Johannsen worked +with the common bean, weighing the seeds or else measuring them. These +beans if taken from many plants at random give the typical curve of +probability (fig. 74). The plant multiplies by self-fertilization. Taking +advantage of this fact Johannsen kept the seeds of each plant separate from +the others, and raised from them a new generation. When curves were made +from these new groups it was found that some of them had different modes +from that of the original general population (fig. 81 A-E, bottom group). +They are shown in the upper groups (A, B, C, D, E). But do not understand +me to say that the offspring of each bean gave a different mode. + +[Illustration: FIG. 81. Pure lines of beans. The lower figure gives the +general population, the other figures give the pure lines within the +population. (After Johannsen.)] + +On the contrary, some of the lines would be the same. + +The result means that the general population is made up of definite kinds +of individuals that may have been sorted out. + +That his conclusion is correct is shown by rearing a new generation from +any plant or indeed from several plants of any one of these lines. Each +line repeats the same modal class. There is no further breaking up into +groups. Within the line it does not matter at all whether one chooses a big +bean or a little one--they will give the same result. In a word, the germ +plasm in each of these lines is pure, or homozygous, as we say. The +differences that we find between the weights (or sizes) of the individual +beans are due to external conditions to which they have been subjected. + +In a word, Johannsen's work shows that the frequency distribution of a pure +line is due to factors that are extrinsic to the germ plasm. It does not +matter then which individuals in a pure line are used to breed from, for +they all carry the same germ plasm. + +We can now understand more clearly how selection acting on a general +population brings about results in the direction of selection. + +An individual is picked out from the population in order to get a +particular kind of germ plasm. Although the different classes of +individuals may overlap, so that one can not always judge an individual +from its appearance, nevertheless on the whole chance favors the picking +out of the kind of germ plasm sought. + +In species with separate sexes there is the further difficulty that two +individuals must be chosen for each mating, and superficial examination of +them does not insure that they belong to the same group--their germ plasm +cannot be inspected. Hence selection of biparental forms is a precarious +process, now going forward, now backwards, now standing still. In time, +however, the process forward is almost certain to take place if the +selection is from a heterogeneous population. Johannsen's work was +simplified because he started with pure lines. In fact, had he not done so +his work would not have been essentially different from that of any +selection experiment of a pure race of animals or plants. Whether Johannsen +realized the importance of the condition or not is uncertain--curiously he +laid no emphasis on it in the first edition of his "Elemente der exakten +Erblichkeitslehre". + +It has since been pointed out by Jennings and by Pearl that a race that +reproduces by self-fertilization as does this bean, automatically becomes +pure in all of the factors that make up its germ plasm. Since +self-fertilization is the normal process in this bean the purity of the +germ plasm already existed when Johannsen began to experiment. + +HOW HAS SELECTION IN DOMESTICATED ANIMALS AND PLANTS BROUGHT ABOUT ITS +RESULTS? + +If then selection does not bring about transgressive variation in a general +population, how can selection produce anything new? If it can not produce +anything new, is there any other way in which selection becomes an agent in +evolution? + +We can get some light on this question if we turn to what man has done with +his domesticated animals and plants. Through selection, i.e., artificial +selection, man has undoubtedly brought about changes as remarkable as any +shown by wild animals and plants. We know, moreover, a good deal about how +these changes have been wrought. + +(1) By crossing different wild species or by crossing wild with races +already domesticated new combinations have been made. Parts of one +individual have been combined with parts of others, creating new +combinations. It is possible even that characters that are entirely new may +be produced by the interaction of factors brought into recombination. + +(2) New characters appear from time to time in domesticated and in wild +species. These, like the mutants in Drosophila, are fully equipped at the +start. Since they breed true and follow Mendel's laws it is possible to +combine them with characters of the wild type or with those of other mutant +races. + +Amongst the new mutant factors there may be some whose chief effect is on +the character that the breeder is already selecting. Such a modification +will be likely to attract attention. Superficially it may appear that the +factor for the original character has varied, while the truth may be that +another factor has appeared that has modified a character already present. +In fact, many or all Mendelian factors that affect the same organ may be +said to be modifiers of each other's effects. Thus the factor for vermilion +causes the eye to be one color, and the factor for eosin another color, +while eosin vermilion is different from both. Eosin may be said to be a +modifier of vermilion or vermilion of eosin. In general, however, it is +convenient to use the term "modifier" for cases in which the factor causes +a detectable change in a character already present or conspicuous. + +[Illustration: FIG. 82. Scheme to indicate influence of the modifying +factors, cream and whiting. Neither produces any effect alone but they +modify other eye colors such as eosin.] + +One of the most interesting, and at the same time most treacherous, kinds +of modifying factors is that which produces an effect _only_ when some +other factor is present. Thus Bridges has shown that there is a factor +called "cream" that does not affect the red color of the eye of the wild +fly, yet makes "eosin" much paler (fig. 82). Another factor "whiting" which +produces no effect on red makes eosin entirely white. Since cream or +whiting may be carried by red eyed flies without their presence being seen +until eosin is used, the experimenter must be continually on the lookout +for such factors which may lead to erroneous conclusions unless detected. +As yet breeders have not realized the important role that modifiers have +played in their results, but there are indications at least that the +heaping up of modifying factors has been one of the ways in which highly +specialized domesticated animals have been produced. Selection has +accomplished this result not by changing factors, but by picking up +modifying factors. The demonstration of the presence of these factors has +already been made in some cases. Their study promises to be one of the most +instructive fields for further work bearing on the selection hypothesis. + +In addition to these well recognized methods by which artificial selection +has produced new things we come now to a question that is the very crux of +the selection theory today. Our whole conception of selection turns on the +answer that we give to this matter and if I appear insistent and go into +some detail it is because I think that the matter is worth very careful +consideration. + +ARE FACTORS CHANGED THROUGH SELECTION? + +As we have seen, the variation that we find from individual to individual +is due in part to the environment; this can generally be demonstrated. +Other differences in an ordinary population are recognized as due to +different genetic (hereditary) combinations. No one will dispute this +statement. But is all the variability accounted for in these two ways? May +not a factor itself fluctuate? Is it not _a priori_ probable that factors +do fluctuate? Why, in a word, should we regard factors as inviolate when we +see that everything else in organisms is more or less in amount? I do not +know of any _a priori_ reason why a factor may not fluctuate, unless it is, +as I like to think, a chemical molecule. We are, however, dealing here not +with generalities but with evidence, and there are three known methods by +means of which it has been shown that variability, other than environmental +or recombinational, is not due to variability in a factor, nor to various +"potencies" possessed by the same factors. + +(1) By making the stock uniform for all of its factors--chief factors and +modifiers alike. Any change in such a stock produced by selection would +then be due to a change in one or more of the factors themselves. +Johannsen's experiment is an example of this sort. + +[Illustration: FIG. 83 a. Drosophila ampelophila with truncate wings.] + +(2) The second method is one that is capable of _demonstrating_ that the +effects of selection are actually due to modifiers. It has been worked out +in our laboratory, chiefly by Muller, and used in a particular case to +demonstrate that selection produced its effect by isolating modifying +factors. For example, a mutant type called truncate appeared, characterized +by shorter wings, usually square at the end, (fig. 83a). The wings varied +from those of normal length to wings much shorter (fig. 83b). For three +years the mutant stock was bred from individuals having the shorter wings +until at last a stock was obtained in which some of the individuals had +wings much shorter than the body. By means of linkage experiments it was +shown that at least three factors were present that modified the wings. +These were isolated by means of their linkage relations, and their mutual +influence on the production of truncate wings was shown. + +[Illustration: FIG. 83 b. Series of wings of different length shown by +truncate stock of D. ampelophila.] + +An experiment of this kind can only be carried out in a case where the +groups of linked gens are known. At present Drosophila is the only animal +(or plant) sufficiently well known to make this test possible, but this +does not prove that the method is of no value. On the contrary it shows +that any claim that factors can themselves be changed can have no finality +until the claim can be tested out by means of the linkage test. For +instance, bar eye (fig. 31) arose as a mutation. All our stock has +descended from a single original mutant. But Zeleny has shown that +selection within our stock will make the bar eye narrower or broader +according to the direction of selection. It remains to be shown in this +case how selection has produced its effects, and this can be done by +utilizing the same process that was used in the case of truncate. + +Another mutant stock called beaded (fig. 84), has been bred for five years +and selected for wings showing more beading. In extreme cases the wings +have been reduced to mere stumps (see stumpy, fig. 5), but the stock shows +great variability. It is probable here as Dexter has shown, that a number +of mutant factors that act as modifiers have been picked up in the course +of the selection, and when it is recalled that during those five years over +125 new characters have appeared elsewhere it does not seem improbable that +factors also have appeared that modify the wings of this stock. + +[Illustration: FIG. 84. Two flies showing beaded wings.] + +(3) The third method is one that has been developed principally by East for +plants; also by MacDowell for rabbits and flies. The method does not claim +to prove that modifiers are present, but it shows why certain results are +in harmony with that expectation and can not be accounted for on the basis +that a factor has changed. Let me give an example. When a Belgian hare with +large body was crossed to a common rabbit with a small body the hybrid was +intermediate in size. When the hybrid was crossed back to the smaller type +it produced rabbits of various sizes in apparently a continuous series. +MacDowell made measurements of the range of variability in the first and in +the second generations. + + _Classification in relation to parents based on skull lengths and ulna + lengths, to show the relative variability of two measurements and of + the first generation (F_1) and the back cross (B. C.)_ + + CHARACTER|GENERATION|-13|-12|-11|-10| -9| -8| -7| -6| -5| -4| -3| -2| -1| + ---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+ + Length of{ F_1 | | | | | | | | | | | | | | + skull { B.C. | | | | | | | | | | | | | 3| + Length of{ F_1 | | | | | | | | | | | | | | + ulna { B.C. | 1| | | | | 1| | 2| 3| 1| 2| 4| 4| + + _same table continued_ + + CHARACTER|GENERATION| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9| 10| 11| 12| + ---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+ + Length of{ F_1 | | | | | | | | 2| 2| 8| 5| 10| 7| + skull { B.C. | 6| 4| 13| 18| 42| 32| 38| 34| 16| 16| 8| 4| 3| + Length of{ F_1 | | 1| | | 2| | 1| 1| 1| 2| 2| 5| 3| + ulna { B.C. | 12| 11| 20| 26| 17| 19| 18| 15| 12| 13| 15| 11| 5| + + _same table continued_ + + CHARACTER|GENERATION| 13| 14| 15| 16| 17| 18| 19| 20| 21| 22| 23| 24| 25| + ---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+ + Length of{ F_1 | 3| 2| 2| | | | | | | | | | | + skull { B.C. | 1| | | | | | | | | | | | | + Length of{ F_1 | 1| 7| 3| 2| 1| | | | 2| | 1| | 1| + ulna { B.C. | 2| 4| 2| 2| | | 1| 1| | | | | | + +He found that the variability was smaller in the first generation than in +the second generation (back cross). This is what is expected if several +factor-differences were involved, because the hybrids of the first +generation are expected to be more uniform in factorial composition than +are those in the second generation which are produced by recombination of +the factors introduced through their grandparents. Excellent illustrations +of the same kinds of results have been found in Indian corn. As shown in +figure 85 the length of the cob in F_1 is intermediate between the parent +types while in F_2 the range is wider and both of the original types are +recovered. East states that similar relations have been found for 18 +characters in corn. Emerson has recently furnished further illustrations of +the same relations in the length of stalks in beans. + +[Illustration: FIG. 85. Cross between two races of Indian corn, one with +short cobs and one with long cobs. The range of variability in F_1 is less +than that in F_2. (After East.)] + +A similar case is shown by a cross between fantail and common pigeons (fig. +86). The latter have twelve feathers in the tail, while the selected race +from which the fantails came had between 28 and 38 feathers in the tail. +The F_1 offspring (forty-one individuals) showed (fig. 87) between 12 and +20 tail feathers, while in F_2 the numbers varied between 12 and 25. Here +one of the grand-parental types reappears in large numbers, while the +extreme of the other grand-parental type did not reappear (in the counts +obtained), although the F_2 number would probably overlap the lower limits +of the race of fantail grandparents had not a selected (surviving) lot been +taken for the figures given in the table. + +[Illustration: FIG. 86. Cross of pigeon with normal tail P_1 and fantail +P_1; F_1, bird below.] + +[Illustration: FIG. 87. Cross of normal and fantail pigeons. (See Fig. 86.) +The F_2 range is wider than that of F_1. The normal grand-parental type of +12 feathers was recovered in F_2 but the higher numbers characteristic of +fantails were not recovered.] + +The preceding account attempts to point out how I should prefer to +interpret the problem of selection in the light of the most recent work on +breeding. But I would give a very incomplete account of the whole situation +if I neglected to include some important work which has led some of my +fellow-workers to a very different conclusion. + +[Illustration: FIG. 88. Scheme to show classes of hooded rats used by +Castle. (After Castle.)] + +Castle in particular is the champion of a view based on his results with +hooded rats. Starting with individuals which have a narrow black stripe +down the back he selected for a narrower stripe in one direction and for a +broader stripe in the other. As the diagram shows (fig. 88) Castle has +succeeded in producing in one direction a race in which the dorsal stripe +has disappeared and in the other direction a race in which the black has +extended over the back and sides, leaving only a white mark on the belly. +Neither of these extremes occurs, he believes, in the ordinary hooded race +of domesticated rats. In other words no matter how many of them came under +observation the extreme types of his experiment would not be found. + +Castle claims that the factor for hoodedness must be a single Mendelian +unit, because if hooded rats are crossed to wild gray rats with uniform +coat and their offspring are inbred there are produced in F_2 three uniform +rats to one hooded rat. Castle advances the hypothesis that factors--by +which he means Mendelian factors--may themselves vary in much the same way +as do the characters that they stand for. He argues, in so many words, that +since we judge a factor by the kind of character it produces, when the +character varies the factor that stands for it may have changed. + +As early as 1903 Cuenot had carried out experiments with spotted mice +similar to those of Castle with rats. Cuenot found that spotted crossed to +uniform coat color gave in F_2 a ratio of three uniform to one spotted, yet +selection of those spotted mice with more white in their coat produced mice +in successive generations that had more and more white. Conversely Cuenot +showed that selection of those spotted mice that had more color in their +coat produced mice with more and more color and less white. Cuenot does not +however bring up in this connection the question as to how selection in +these spotted mice brings about its results. + +Without attempting to discuss these results at the length that they deserve +let me briefly state why I think Castle's evidence fails to establish his +conclusion. + +In the first place one of the premises may be wrong. The three to one ratio +in F_2 by no means proves that all conditions of hoodedness are due to one +factor. The result shows at most that one factor that gives the hooded +types is a simple Mendelian factor. The changes in this type may be caused +by modifying factors that can show an effect only when hoodedness is itself +present. That this is not an imaginary objection but a real one is shown by +an experiment that Castle himself made which furnishes the ground for the +second objection. + +Second. If the factor has really changed its potency, then if a very dark +individual from one end of the series is crossed to a wild rat and the +second generation raised we should expect that the hooded F_2 rats would +all be dark like their dark grandparent. When Castle made this test he +found that there were many grades of hooded rats in the F_2 progeny. They +were darker, it is true, as a group than were the original hooded group at +the beginning of the selection experiment, but they gave many intermediate +grades. Castle attempts to explain this by the assumption that the factor +made pure by selection became contaminated by its normal allelomorph in the +F_1 parent, but not only does this assumption appear to beg the whole +question, but it is in flat contradiction with what we have observed in +hundreds of Mendelian cases where no evidence for such a contamination +exists. + +Later Castle crossed some of the extracted rats of average grade (3.01) +from the plus series to the same wild race and got F_2 hooded rats from +this cross. These F_2 hooded rats did not further approach the ordinary +range but were nearer the extreme selected plus hooded rats (3.33) than +were the F_2's extracted from the first cross (2.59). Castle concludes from +this that multiple factors can not account for the result. As a matter of +fact, Castle's evidence _as published_ does not establish his conclusion +because the wild rats used in the second experiment may have carried plus +modifiers. This could only be determined by suitable tests which Castle +does not furnish. This is the crucial point, without which the evidence +carries no conviction. + +Furthermore, from Castle's point of view, these latest results would seem +to increase the difficulty of interpretation of his first F_2 extracted +cross, and it is now the first result that calls for explanation if one +accepts his later conclusion. + +These and other objections that might be taken up show, I think, that +Castle's experiment with hooded rats fails entirely to establish his +contention of change in potency of the germ or of contamination of factors, +while on the contrary they are in entire accord with the view that he is +dealing with a case of modifying factors. + +[Illustration: FIG. 89. Races of Paramecium. (After Jennings.)] + +Equally important are the results that Jennings has obtained with certain +protozoa. Paramecium multiplies by dividing across in the middle, each half +replacing its lacking part. Both the small nucleus (micronucleus) and the +large nucleus (macronucleus) divide at each division of the body. Jennings +found that while individuals descended from a single paramecium vary in +size (fig. 89), yet the population from a large individual is the same as +the population derived from a small individual. In other words, selection +produces no result and the probable explanation is, of course, that the +different sizes of individuals are due to the environment, while the +constancy of the type is genetic. Jennings found a number of races of +paramecium of different sizes living under natural conditions. The largest +individual of a small race might overlap the smallest individual of other +larger races (fig. 89); nevertheless each kind reproduced its particular +race. The results are like those of Johannsen in a general way, but differ +in that reproduction takes place in paramecium by direct division instead +of through self-fertilization as in beans, and also in that the paramecia +were probably not homozygous. Since, however, so far as known no +"reduction" takes place in paramecium at each division, the genetic +composition of parent and offspring should be the same. Whether +pseudo-parthenogenesis that Woodruff and Erdmann have found occurring in +paramecium at intervals involves a redistribution of the hereditary factors +is not clear. Jennings's evidence seems incompatible with such a view. + +[Illustration: FIG. 90. Stylonychia showing division into two. (After +Stein.)] + +More recently one of Jennings's students, Middleton, has made a careful +series of selection experiments with Stylonychia (fig. 90) in which he +selected for lines showing more rapid or slower rates of division. His +observations seem to show that his selection separated two such lines that +came from the same original stock. The rapidity of the effects of selection +seems to preclude the explanation that pseudo-parthenogenesis has +complicated the results. Nevertheless, the results are of such a kind as to +suggest that they were due to selection of vegetative (somatic) differences +and that no genetic change of factors was involved, for his conclusion that +the rapidity with which the effects gained by long selection might be +suddenly reversed when selection was reversed is hardly consistent with an +interpretation of the results based on changes in the "potencies" of the +factors present. + +Equally striking are the interesting experiments that Jennings has recently +carried out with Difflugia (fig. 91). This protozoon secretes a shell about +itself which has a characteristic shape, and often carries spines. The +opening at one end of the shell through which the protoplasm protrudes to +make the pseudopodia is surrounded by a rim having a characteristic +pattern. The protoplasm contains several nuclei and in addition there is +scattered material or particles called chromidia that are supposed to be +chromatic in nature and related to the material of the nuclei, possibly by +direct interchange. + +[Illustration: FIG. 91. Difflugia Corona. (After Cash.)] + +When Difflugia divides, part of the protoplasm protrudes from the opening +and a new shell is secreted about this mass which becomes a daughter +individual. The behavior of the nucleus and of the chromidia at this time +is obscure, but there is some evidence that their materials may be +irregularly distributed between parent and offspring. If this is correct, +and if in the protozoa the chromatin has the same influence that it seems +to have in higher animals, the mode of reproduction in Difflugia would be +expected to give little more than random sampling of the germ plasm. + +[Illustration: FIG. 92. Races of Difflugia. (After Leidy.)] + +Jennings was able by means of selection to get from the descendants of one +original individual a number of different types that themselves bred true, +except in so far as selection could affect another change in them. In this +connection it is interesting to note that Leidy has published figures of +Difflugia (fig. 92) that show that a great many "types" exist. If through +sexual union (a process that occurs in Difflugia) the germ plasm +(chromatin) of these wild types has in times past been recombined, then +selection would be expected to separate certain types again, if, at +division, irregular sampling of the germ plasm takes place. Until these +points are settled the bearing of these important experiments of Jennings +on the general problem of selection is uncertain. + +HOW DOES NATURAL SELECTION INFLUENCE THE COURSE OF EVOLUTION? + +The question still remains: Does selection play any role in evolution, and, +if so, in what sense? Does the elimination of the unfit influence the +course of evolution, except in the negative sense of leaving more room for +the fit? There is something further to be said in this connection, although +opinions may differ as to whether the following interpretation of the term +"natural selection" is the only possible one. + +[Illustration: FIG. 93. Evolution of elephant's skulls. (After Dendy.)] + +If through a mutation a character appears that is neither advantageous nor +disadvantageous, but indifferent, the chance that it may become established +in the race is extremely small, although by good luck such a thing may +occur rarely. It makes no difference whether the character in question is a +dominant or a recessive one, the chance of its becoming established is +exactly the same. If through a mutation a character appears that has an +_injurious_ effect, however slight this may be, it has practically no +chance of becoming established. + +[Illustration: FIG. 94. Evolution of elephant's trunk. (After Lull.)] + +If through a mutation a character appears that has a _beneficial_ influence +on the individual, the chance that the individual will survive is +increased, not only for itself, but for all of its descendants that come to +inherit this character. It is this increase in the number of individuals +possessing a particular character, that might have an influence on the +course of evolution. This gives a better chance for improvement by several +successive steps; but not because the species is more likely to mutate +again in the same direction. An imaginary example will illustrate how this +happens: When elephants had trunks less than a foot long, the chance of +getting trunks more than one foot long was in proportion to the length of +trunks already present and to the number of individuals; but increment in +trunk length is no more likely to occur from an animal having a trunk more +than one foot long than from an animal with a shorter trunk. + +The case is analogous to tossing pennies. At any stage in the game the +chance of accumulating a hundred heads is in proportion to the number of +heads already obtained, and to the number of throws still to be made. But +the number of heads obtained has no influence on the number of heads that +will appear in the next throw. + +[Illustration: FIG. 95. Evolution of elephant's trunk: above Maeritherium, +in the middle Tetrabelodon (After Lancaster); below African elephants +(After Gambier Bolton).] + +Owing then to this property of the germ plasm to duplicate itself in a +large number of samples not only is an opportunity furnished to an +advantageous variation to become extensively multiplied, but the presence +of a large number of individuals of a given sort prejudices the probable +future result. + +The question may be raised as to whether it is desirable to call selection +a _creative_ process. There are so many supernatural and mystical +implications that hang around the term creative that one can not be too +careful in stating in what sense the term is to be used. If by creative is +meant that something is made out of nothing, then of course there is no +need for the scientist to try to answer such a question. But if by a +creative process is meant that something is made out of something else, +then there are two alternatives to be reckoned with. + +First, if it were true that selection of an individual of a certain kind +determines that new variations in the same direction occur as a consequence +of the selection, then selection would certainly be creative. How this +could occur might be quite unintelligible, but of course it might be +claimed that the point is not whether we can explain how creation takes +place, but whether we can get verifiable evidence that such a kind of thing +happens. This possibility is disposed of by the fact that there is no +evidence that selection determines the direction in which variation occurs. + +Second, if you mean by a creative process that by picking out a certain +kind of individual and multiplying its numbers a better chance is furnished +that a certain end result will be obtained, such a process may be said to +be creative. This is, I think, the proper use of the term creative in a +mechanistic sense. + +CONCLUSIONS + +In reviewing the evidence relating to selection I have tried to handle the +problem as objectively as I could. + +The evidence shows clearly that the characters of wild animals and plants, +as well as those of domesticated races, are inherited both in the wild and +in the domesticated forms according to Mendel's Law. + +The causes of the mutations that give rise to new characters we do not +know, although we have no reason for supposing that they are due to other +than natural processes. + +Evolution has taken place by the incorporation into the race of those +mutations that are beneficial to the life and reproduction of the organism. +Natural selection as here defined means both the increase in the number of +individuals that results after a beneficial mutation has occurred (owing to +the ability of living matter to propagate) and also that this preponderance +of certain kinds of individuals in a population makes some further results +more probable than others. More than this, natural selection can not mean, +if factors are fixed and are not changed by selection. + + * * * * * + + + INDEX + + Abnormal abdomen 109 + Abraxas 78-81 + Allantois 17 + Allelomorphs 83-84 + Altenburg 112 + Amnion 16-17 + Andalusian fowl 45, 46 + Annelids 22 + Antlered wing 111 + Apterous wing 11 + Arc wing 111 + Aristae 104 + + Bar eye 67, 108, 169 + Bateson 18, 34, 36 + Beaded wing 11, 115 + Beans 147-149, 157 + Belgian hare 171 + Bent wing 116 + Bergson 30, 31 + Bildungstrieb 34 + Biogenetic law 15, 18, 19, 21 + Biometricians 156 + Bird 21, 23 + Bithorax 65, 112, 113 + Black body color 111, 133 + Blakeslee 152 + Bridges 114, 143, 163 + British Association 36 + Bruenn 40 + Buff eye color 109 + Bufon 27 + + Castle 176-180 + Cat 33 + Cell 90, 91 + Chance variations 37 + Chick 16, 17, 20 + Chromatin 184 + Chromosome group of Drosophila 102 + Chromosomes 91, 95, 96, 98, 130, 131, 132 + Cleavage 21, 22, 94 + Clover butterfly 62 + Club wing 69, 70, 108 + Colias philodice 62 + Color blindness 77, 125 + Comb of Drosophila 103 + Combs of fowls 33, 54 + Comparative anatomy 7, 8, 9, 14 + Corn 150, 153, 172 + Correns 41 + Cosmogonies 27 + Cream eye color 163, 164 + Crepidula 22 + Criss-cross inheritance 78 + Crossing over 131-133 + Cuenot 178 + Curled wing 115 + Curved wing 111 + Curve of probability 149 + Cut wing 11, 104 + + Dachs legs 112 + Dahlgren 62 + Darwin 15, 24, 28, 32, 35-37, 64, 145, 146, 152, 154-156 + Dendy 188 + De Vries 18, 147, 156 + Dexter 170 + Dichaete 114 + Difflugia 184-187 + Discontinuous variation 13 + Disuse 31 + Drosophila ampelophila 10, 12, 13, 48-50, 60, 75, 84, 85, 93, 100, 103, + 119, 155, 162, 169 + Drosophila repleta 76 + Duplication of legs 109 + Dwarf 114 + + East 170, 172 + Ebony 50, 55, 56, 115 + Egg 91, 94 + Elephant 191 + Elephants' skulls 188 + Elephants' trunks 190 + Embryology 13-23 + Emerson 172 + Environment 27 + Eosin eye color 61, 107, 163 + Erdmann 183 + Evolution Creatrice 30 + Evolution--three kinds of 1, 2, 4 + Eye color 13 + Eyeless 66, 115 + + Factorial theory 89 + Factors of Drosophila 143 + Fantails 172, 175 + Fertilization 91 + Fish 16, 20, 21 + Flatworms 22 + Fluctuations 12 + Forked bristles 106 + Fowl 77 + Fused veins 107, 108 + + Galton 154 + Geneticist 26 + Germ-plasm 142 + Geoffroy St. Hilaire 27 + Giant 114 + Gill-slits 20, 21, 23 + Groups I, II, III, IV 100-118 + + Haeckel 15 + Haemophilia 77 + Heliotropism 106, 107 + Himalyan rabbits 83 + History 1, 6 + Hoge 66 + Horse, evolution of 6 + + Indian corn 172, 173 + Interference 137, 138 + + Janssens 132 + Jaunty wing 111 + Jennings 161, 181-184, 186 + Johannsen 156, 157, 159-161, 166, 182 + + Lamarck 31-34 + Langshan 77 + Leaves 147 + Leidy 186 + Lethal 105 + Linkage groups 103 + Lizard 23 + Localization of factors 118 + + MacDowell 155, 170, 171 + Macritherium 191 + Mammal 16, 21, 23 + Man 20, 77, 125, 126 + Map of Chromosomes 136 + Maroon eye color 114 + Mendel 40, 41, 52, 89 + Mendelian heredity 39 + Mendel's law 41-59, 64, 124 + Mendel's second law 52 + Mesenchyme cells 22 + Mesoderm cells 22 + Metaphysician 30 + Mice 33, 178 + Middleton 183 + Miniature wing 108 + Mirabilis 42 + Modifiers 163, 164, 170, 171 + Molluscs 22 + Mouse 83 + Muller 112, 167 + Mutations 35, 39, 84 + + Naegeli 34, 35 + Natural Selection 36, 145, 146, 187-194 + Nisus formativus 34 + Non-disjunction 139-142 + Notch wing 104-106 + Nucleus 91 + + Origin of Species 35, 145 + Orthogenesis 34 + + Paleontology 24-27 + Papilio polytes 63 + Papilio turnus 63 + Paramecium 181, 182 + Paratettix 81 + Peach eye color 114 + Pea comb 54 + Pearl 161 + Peas 47 + Pigeons 172, 174, 175 + Pink eye color 114, 115 + Planarian 22 + Plymouth Rock 77 + Podarke 22 + Polar bodies 126 + Pole arms 5 + Protozoa 181 + Pseudo-parthenogenesis 183 + Purple eye color 109 + Purpose 4 + + Rabbits 83, 170 + Rats 176-180 + Reduction division 182 + Reproductive cells 96 + Ruby eye color 106 + Rudimentary organ 116 + Rudimentary wing 70, 71, 107 + + Sable body color 107 + Science definition of 6 + Segregation 41 + Selenka 94 + Sepia eye color 13, 114 + Sex chromosomes 118 + Sex linked inheritance 75, 118-130 + Sexual dimorphism 62 + Sheep 33 + Single comb 54 + Sooty body color 50, 114, 115 + Speck 68, 69, 111 + Spencer 145 + Spermatozoon 91, 98 + Stars, evolution of 6 + St. Hilaire 27-30 + Strap wing 110, 111 + Stumpy wing 11 + Sturtevant 76, 143 + Stylonychia 183 + Survival of the fittest 146 + Systematist 85 + + Tails 33 + Tan flies 106, 107 + Tetrabelodon 191 + Trefoil 111 + Truncate wing 111, 112, 167, 168 + + Unfolding principle 34 + Unio 22 + Unit character 74, 75 + Use 31 + + Variation discontinuous 13 + Vermilion eye color 108, 163 + Vestigial wing 11, 55, 56, 109, 133 + Vital force 34 + + Wallace 36 + Walnut comb 54 + Weismann 17, 31-33 + Wilson, E. B. 125 + Wingless 67 + Winiwarter 126 + White eye color 13, 75, 119-130 + Whiting eye color 163, 164 + Woodruff 183 + + Yellow body color 108, 133 + Yolk sac 16, 17 + + Zeleny 169 + + * * * * * + + +Corrections made to printed original. + +page 104, "shown in figures 53, 54, 55, 56": '52, 53, 54, 55' in original. + + + + + + +End of the Project Gutenberg EBook of A Critique of the Theory of Evolution, by +Thomas Hunt Morgan + +*** END OF THIS PROJECT GUTENBERG EBOOK CRITIQUE OF THEORY OF EVOLUTION *** + +***** This file should be named 30701.txt or 30701.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/3/0/7/0/30701/ + +Produced by Bryan Ness, Keith Edkins, and the Online +Distributed Proofreading Team at https://www.pgdp.net. 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