Boston Medical Library 8 THE Fenway HEREDITY IN RELATION TO EVOLUTION AND ANIMAL BREEDING HEREDITY IN RELATION TO EVOLUTION AND ANIMAL BREEDING BY WILLIAM E. CASTLE PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY A f^ NEW YORK AND LONDON D. APPLETON AND COMPANY 1911 Copyright, 1911, bt D. APPLETON AND COMPANY J.' i/l/i^> //^ PiihlWied Sept Printed in the United States of America PEEFACE This little book is based on a course of eight lectures delivered in November and December, 1910, before the Lowell Institute, Boston, as well as on a course of five lectures delivered before the Graduate School of Agriculture held under the auspices of the Association of Agricultural Colleges and Experiment Stations at Ames, Iowa, in July, 1910. The hope is entertained that it may be of service to students and that it will also interest the general reader. The writer wishes to express his gratitude to the Carnegie Institution of Washington for per- mission, in its , preparation, to draw freely upon published and unpublished material derived from investigations aided by the Institution. Acknowledgment is also due to the following persons, or to their publishers, for permission to use figures from their publications, as indicated in the text : Prof. E. B. Wilson and The Mac- millan Co., Prof. H. S. Jennings and The Ameri- can Naturalist, Dr. W. B. Kirkham and The American Book Co. W. E. CASTLE June, 1911 CONTENTS PAGE Introduction. — Genetics, A New Science ... 1 Chapter I. — The Duality of Inheritance 6 11. — Germ-Plasm and Body, Their Mutual Independence 27 III. — Mendel's Law of Heredity 33 IV. — The Determination of Dominance; Heterozygous Characters and Their Fixation; Atavism or Revision. ... 52 V. — Evolution of New Races by Loss or Gain of Characters 72 VI. — Evolution of New Races by Variations IN the Potency of Characters .... 87 VII. — Can Mendelian Unit-Characters be Modified by Selection? 106 VIII. — Mendelian Inheritance Without Domi- nance, ''Blending" Inheritance ... 128 IX. — The Effects of Inbreeding 143 X. — Heredity and Sex 153 INDEX 183 LIST OF ILLUSTRATIONS FIG. PAGE 1. — Egg and sperm of the sea-urchin, Toxopneustes 9 2. — Fertilization of the egg of iVe?'eis 12 3. — Egg of a mouse previous to maturation Facing 14 4. — Maturation and fertilization of the eog of a mouse Facing 14 5. — Diagrams showing the essential facts of chromosome reduction in the development of the sperm-cells 17 6. — An ordinary fern „ 21 7. — The prothallus of a fern 23 8. — Diagram showing the chromosome number in the spermatogenesis of ordinary animals and of the wasp . 24 9. — Diagram showing the relation of the body to the germ-cells in heredity 29 10. — A young, black guinea-pig .... .Facing 30 11. — An albino female guinea-pig Facing 30 12. — An albino male guinea-pig Facing 30 13. — Pictures of three living guinea-pigs and of the preserved skins of three others . . . Facing 32 14. — A black, female guinea-pig, and her young Facing 34 15. — An albino male guinea-pig Facing 34 16. — Two of the grown-up young of a black and of an albino guinea-pig Facing 34 IK X LIST OF ILLUSTRATIONS FIG. PAGE 17. — A group of four young, produced by the animals shown in Fig. 16 ... .Facing 34 18. — Diagram to explain the result shown in Fig. 17 35 19. — A shortened condition of the skeleton, par- ticularly of the fingers Facing 36 20. — Radiograph of a hand similar to those shown in Fig. 19 Facing 38 21. — Diagram showing the descent, through five generations, of the condition shown in Figs. 19 and 20 . 40 22. — A smooth, dark guinea-pig Facing 40 23. — A rough, white guinea-pig Facing 40 24. — A dark, rough guinea-pig Facing 40 25. — A smooth, white guinea-pig Faciyig 42 26. — A short-haired, pigmented guinea-pig Facing 42 27. — A long-haired albino guinea-pig . . .Facing 42 28. — Offspring produced by animals of the sorts shown in Figs. 26 and 27 . . . .Facing 42 29. — A long-haired, pigmented guinea-pig, "Dutch- marked " with white Facing 42 30. — Diagram to explain the results of a cross between the sorts of guinea-pigs shown in Figs. 26 and 27 43 31. — Diagram showing the kinds and relative frequencies of the young to be expected in Fa from the crossing of animals shown in Figs. 26 and 27. . . 46 32. — Along-haired, rough albino guinea-pig Facing 46 33. — Five new combinations of unit-characters obtained in generation Fg, by crossing the animal shown in Fig. 32 with animals like that shown in Fig. 22 Facing 48 LIST OF ILLUSTRATIONS xi FIG. PAGE 34. — Diagram to show the gametic combinations and segregations involved in a cross be- tween guinea-pigs differing in three unit- characters. 49 35. — Diagram to show the gametic combination and recombinations which occur in the production and fixation of an atavistic coat- character in guinea-pigs 66 36. — An imperfectly rough guinea-pig . . .Facing 101 37. — A silvered guinea-pig Facing 101 38. — A. Front feet of an ordinary guinea-pig. B. Its hind feet D. Hind feet of a race four-toed on all the feet. G. Ordinary condition of the hind feet of young obtained by crossing 5 with Z) Faciyig 101 39. — Diagram showing variation in the color- pattern of hooded rats Facing 101 40. — Diagram showing the variations in size of eight different races of Paramecium ... 112 41. — Chart showing effects of selection in eight successive generations upon the color- pattern of hooded rats 122 42. — Skulls of three rabbits Facing 128 43. — A long-haired^ albino rabbit, having erect ears Facing 132 44. — A short-haired, sooty yellow rabbit, having lop ears Facing 132 45. — A short-haired, black rabbit, son of the rabbits shown in Figs. 43 and 44. .Facing 132 46.- — An F2 descendant of the rabbits shown in Figs. 44 and 45 ..,.,», » .Facing 132 xii^^^^ ^' LIST OF ILLUSTRATIONS FIG. PAGE . ^7. : — ^ Diagrams to show the number and size of the classes of individuals to be expected from a cross involving Mendelian segregation without dominance 135 48. — Photographs to show variation in ear length of two varieties of maize, of their Fi off- spring, and of their Fa offspring . .Facing 138 49. — Diagram of sex-determination in partheno- genesis 162 50. — Diagram of sex determination when the female is homozygous, the male heterozygous . . 167 51. — Diagram of sex-determination when the female is heterozygous, the male homozygous . . 170 52. — Diagram of sex-limited inheritance when the female is a heterozygote 173 53. — Dia2:ram of sex-limited inheritance when the female is a homozygote, as in the red-eyed Drosophila 175 HEREDITY INTEODUCTION GENETICS;, A NEW SCIENCE THE theory of organic evolution has prob- ably influenced more fields of human activity and influenced them more pro- foundly than has any other philosophic deduc- tion of ancient or modern times. By this theory philosophy, religion, and science have been rev- olutionized, while in the practical arts of educa- tion and agriculture, twin foundation stones of the state, man has been forced to adopt new methods of procedure or to justify the old ones in the light of a new principle. The evolutionary idea has forced man to con- sider the probable future of his own race on earth and to take measures to control that fu- ture, a matter he had previously left largely to fate. With a realization of the fact that or- 1 HEREDITY ganisms change from age to age and that he himself is one of these changing organisms man has attained not only a new ground for humility of spirit but also a new ground for optimism and for belief in his own supreme importance, since the forces which control his destiny have been placed largely in his own hands. The existence of civilized man rests ultimately on his ability to produce from the earth in suf- ficient abundance cultivated plants and domes- ticated animals. City populations are apt to forget this fundamental fact and to regard with indifference bordering at times on scorn agri- cultural districts and their workers. But let the steady stream of supplies coming from the land to any large city be interrupted for only a few days by war, floods, a railroad strike, or any similar occurrence, and this sentiment vanishes instantly. Man to live must have food, and food comes chiefly from the land. A knowledge of how to produce useful animals and plants is therefore of prime importance. Civilization had its beginning in the attainment of such knowledge and is limited by it at the present day. If, therefore, this knowledge can 2 GENETICS, A NEW SCIENCE be increased, civilization may be advanced in a very direct and practical way. Before Darwin tbe practices of animal and plant breeders were largely empirical, based on unreasoned past ex- perience, just as was in antiquity the practice of metallurgy. Good plows and good swords were made long before a scientific knowledge of the metals was attained, but without that sci- entific knowledge the wonderful industrial de- velopment of this present age of steel would have been quite impossible. In a similar way, if not in like measure, we may reasonably hope for an advance in the productiveness of animal and plant breeding when the scientific principles which underlie these basic arts are better under- stood. Two practical problems present them- selves to the breeder: (1) how to make best use of existing breeds, and (2) how to create new and improved breeds better adapted to the con- ditions of present-day agriculture. We shall concern ourselves with the second of these only. The production of new and improved breeds of animals and plants is historically a matter about which we know scarcely more than about the production of new species in nature. Selec- 3 HEREDITY tion has been undoubtedly the efficient cause of change in both cases, but how and why applied and to what sort of material is as uncertain in one case as in the other. The few great men who have succeeded in producing by their individual efforts a new and more useful type of animal or plant have worked largely by empirical methods. They have produced a desired result but by methods which neither they nor any one else fully understood or could adequately explain. So there exists as yet no true science of breeding but only a highly developed art which was practiced as successfully by the ancient Egyptians, the Saracens, and the Romans as by us. The present, however, is an age of science; we are not satisfied with rule-of-thumb methods, we want to know the why as well as the how of our practical operations. Only such knowl- edge of the reasons for methods empirically successful can enable us to drop out of our practice all superfluous steps and roundabout methods and to proceed straight to the mark in the most direct way. The industrial his- tory of the last century is full of instances in 4 GENETICS, A NEW SCIENCE which a knowledge of causes in relation to processes, i. e. a scientific knowledge, has shortened and improved practice in quite un- expected ways. So we may not doubt the ulti- mate value in practice of a science of breeding, if such a science can be created. A beginning has been made during the last ten years, starting with the rediscovery of MendePs law of heredity in 1900. This book will be concerned largely with the operations of that law. CHAPTER I THE DUALITY OF INHEEITANCE AT the outset we may with profit inquire / % what is meant by heredity. When a ^ -^^ child resembles a parent or grand- parent in some striking particular, we say it inherits such-and-such a characteristic from the parent or grandparent in question. By heredity, then, we mean organic resemblance based on descent. Resemblances due to heredity may exist even between individuals not related as ancestor and descendant, as for example between uncle and nephew. Here the resemblance rests on the fact that uncle and nephew are both descended from a common ancestor, and they resemble each other simply because they have both in- herited the same characteristic from that an- cestor. This form of inheritance is sometimes spoken of as collateral in distinction from direct 6 THE DUALITY OF INHEEITANCE inheritance. In all cases alike community of descent is the basis of resemblances which can be ascribed to heredity, whether direct or col- lateral. Mother and child, no less than uncle and nephew, resemble each other because they have received a common inheritance from a common ancestor. Three biological facts of fundamental im- portance to a right understanding of heredity were known imperfectly or not at all in the time of Darwin and Mendel. These are (1) the fertilization of the egg, (2) the maturation of the egg, which must precede its fertilization, and (3) the non-inheritance of ** acquired '' characters. These we may consider in order. Every new organism is derived from a pre- existing organism, so far as our present ex- perience goes. It may not have been so al- ways. Indeed, on the evolution theory, we must suppose that living matter originally arose from lifeless, inorganic matter. But if it did, this may have occurred, and probably did occur, under physical conditions quite different from those now existing. At the present time the most exhaustive researches H HEREDITY fail to reveal the occurrence of spontaneous generation, that is, the origin of living beings other than from pre-existing living beings. In asexual methods of reproduction a new individual arises out of a detached portion of the parent individual. Such methods of origin are varied and interesting, but do not concern us at present. In all the higher animals and plants a new individual arises, by what we call a sexual process, from the union of two minute bodies called the reproductive cells. They are an egg-cell furnished by the mother and a sperm-cell furnished by the father. There is a great difference in size between egg and sperm. The egg is many thousand times greater in bulk, as seen in Fig. 1, for example, yet the influence of each in heredity appears to be equal to that of the other. This fact shows unmistakably that the bulk of the reproductive cell is not significant in heredity. A large part of the relatively huge egg can have no part in heredity. It serves merely as food for the new organism, furnishing it with building material until such a time as it can begin to secure food for itself. The essential 8 THE DUALITY OF INHEKITAISrCE material, so far as heredity is concerned, is evidently found in egg and sperm alike. It is plainly small in amount and possibly con- sists merely in ferment-like bodies which ini- lv-,".v-'-A-.V>A'-:r7':l ri'^.^-^-i-'vii-'-o'v-'-.v. Fig. 1. — Egg and sperm (s) of the sea-urchin, ToxopneusteSf both shown at the same enlargement. (After Wilson.) tiate certain metabolic processes in a suitable medium represented by the bulk of the egg. The amount of a ferment used in starting a chemical change bears no relation, as is well known, to the amount of the chemical change which it can bring about in a suitable medium. 9 HEEEDITY The equal share of egg and sperm in deter- mining the character of offspring is well shown in the following experiment. An albino guinea- pig is one which lacks in large measure the ability to form black pigment. Apparently it does not possess some ingredient or agency necessary for the production of pigment. Now, if an albino male guinea-pig, such as is shown in Fig. 15, be mated with a black female guinea- pig of pure race, such as is shown in Fig. 14, young are produced all of which are black, like the mother, none being albinos, like the father. Fig. 16 shows black offspring produced in this way. Exactly the same result is obtained from the reverse cross, that is, from mating an al- bino mother with a black sire. It makes no difference, then, whether the black parent be mother or father, its blackness regularly domi- nates over the whiteness of the albino parent, so that only black offspring result. This fact, which has been repeatedly confirmed, shows that the black character is transmitted as readily through the agency of the minute sperm-cell as through the enormously greater egg-cell. Let us now consider what happens when egg 10 THE DUALITY OF INHEEITANCE and sperm unite, in what we call the fertiliza- tion of the egg. The egg is a rounded body incapable of motion, but the sperm is a minute thread-like body which moves like a tadpole by vibrations of its tail. In the case of most animals which live in the water, egg and sperm- cells are discharged into the water and there unite and develop into a new individual, but in the case of most land animals this union takes place within the body of the mother. We may consider an illustration of either sort. The fertilization of the egg of a marine worm. Nereis, is shown in Fig. 2. The thread- like sperm penetrates into the egg. Its en- larged head-end forms there a small nuclear body, which increases in size nntil it equals that of the egg-nucleus, with which it then fnses. The egg next begins to divide up to form the different parts of a new worm-embryo. To each of these parts the nuclear material of egg and sperm is distributed equally. Since this development takes place wholly outside the body of either parent it is necessary that the egg contain enough food to last until the young worm can feed itself. This food material is 11 HEREDITY Fig. 2, — Fertilization of the egg of Nereis. A . The sperm has entered the egg and is forming a minute nucleus at o^. The egg-nucleus is breaking up preparatory to the first maturation division. B. The egg-nucleus is undergoing the first maturation division. Notice the con- spicuous rod-like chromosomes separating into two groups. The sperm-nucleus ( (^ ) is now larger and lies deeper in the egg. C. A small polar-cell has been formed above by the first maturation division of the egg. A second division is in progress at the same point. The sperm-nucleus is now deep in the egg and is preceded by a double radiation (am- phiaster). D. Two polar-cells are fully formed. The ma- tured egg-nucleus is now fusing with the sperm-nucleus. An amphiaster indicates that division of the egg will soon take place. (After Wilson.) 1% THE DUALITY OF INHEEITANCE represented in part by the conspicuous oil- drops seen in the egg (the heavy circles in Fig. 2). The egg of a mouse needs no such store of nourishment, since in common with the young of other mammals the mouse-embryo nourishes itself by osmosis from the body fluids of the mother. The mouse-egg is accordingly smaller. Stages in its fertilization are shown in Fig. 4. In A the sperm has already entered the egg. Eemnants of its thread-like tail may still be seen there. Nearby is seen a nuclear body derived from the sperm-head. Opposite is seen the nuclear body furnished by the egg itself. The two nuclear bodies fuse and their united substance is then distributed to all parts of the embryo-mouse, just as happens in the development of the worm, Nereis. There are reasons for thinking that the nuclear material is especially important in re- lation to heredity and that the equal share of the two parents in contributing it to the em- bryo is not without significance, for inheritance, as we have seen, is from both parents in equal measure. In cases where the inheritance from 13 HEEEDITY each parent is different it can be shown that the offspring possess two inherited possi- bilities, though they may show but one. Thus in the case of a black guinea-pig, one of whose parents was white, the other black, it can be shown that the animal transmits both qualities (black and white) which it received from its respective parents, and transmits them in equal measure. For, if the cross-bred black animal be mated with a white one, half the offspring are black and half of them white. The cross- bred black animal inherited black from one parent, white from the other. It showed only the former, but on forming its reproductive cells it transmitted black to half of these, white to the other half. Hence the cross-bred black individual was a duality, containing two possi- bilities, black and white, but its reproductive cells were again single, containing either black or white, but not both. Now it has been shown in recent years that the nuclear material in the reproductive cells behaves exactly as do black and white in the cross just described. This nuclear material becomes doubled in amount at fertilization, 14 Fig. 3. — Egg _ of a mouse previous to maturation. (After Kirkham.) Fig. 4. — Maturation and fertilization of the egg of a mouse. A. The first maturation division in progress. B. The first polar-cell fully formed ; the second maturation division in progress. C. The second maturation division com- pleted ; the second polar-cell is the smaller one ; near it, in the egg, is the egg-nucleus, and at the left is the sperm- nucleus. D. A view similar to the last, but showing only one polar-cell, the second; note its twelve distinct chromosomes ; near the sperm-nucleus in the egg, at the left, is seen the thread-like remains of the sperm-tail. (After Kirkham.) THE DUALITY OF INHEEITANCE equal contributions being made by egg and sperm. This double condition persists through- out the life of the new individual in all its parts and tissues. But if the individual forms eggs or sperm, these, before they can function in the production of a new individual, must undergo reduction to the single condition. This reduction process is called maturation; it is well illustrated in the case of the mouse- egg, whose fertilization has already been de- scribed. The large nucleus of the egg-cell, as it leaves the ovary, is either broken up or about to break up preparatory to a cell-division. The most conspicuous of the nuclear constituents are some dense, heavily staining bodies called chromosomes, about twenty-four in number. In Fig. 3 each of these is split in two, prepara- tory to the first maturation division. The egg now divides twice, both times very unequally (Fig. 4), forming thus two smaller cells called polar cells, or polar bodies. They take no part in the formation of the embryo. The chromo- somes left in the egg after these two divisions are only about half as numerous as before, or about twelve in number. These form the chro- 15 HEEEDITY matin contribution of the egg to the production of a new individual. It is possible that other cell constituents undergo a similar reduction by half during maturation, but of this we have no present knowledge. The known fact of chromosome reduction, of course, favors the current interpretation that the chromosomes are bearers of heredity, though it by no means proves the correctness of that interpretation. In the egg of Nereis, as well as in that of the mouse, two matura- tion divisions precede the fertilization of the egg. See Fig. 2. In B the first maturation division is in progress; in C the second is in progress; and in D both polar cells are fully formed, while egg and sperm nuclei are unit- ing. Similar processes occur in eggs gener- ally, prior to their fertilization. Like changes occur also in the development of the sperm-cells. In Fig. 5 the original or unreduced condition of the chromosomes in a cell of the male sexual gland is shown (at ^) as one of four chromosomes to a cell. After a series of changes involving as in the maturation of the egg two cell-divisions, we find (at fl") that the 16 THE DUALITY OF INHERITANCE products of tlie original cell contain in each case two chromosomes, half the original number. Fig. 5. — Diagrams showing the essential facts of chromosome reduction in the development of the sperm-cells. (After Wilson.) These chromosomes make up the bulk of the head of the sperm which forms from each of 17 HEEEDITY these cells, its tail being derived from other portions of the cell. It follows that not only eggs but also sperms, prior to their union in fertilization have passed into a reduced or single state as regards their chromatin constituents, whereas the fertilized egg, and the organism which develops from it, is in a double condition. It will be convenient to refer to the single condition as the N condi- tion, the double as the 2 N condition. From a wholly different source we have evidence strongly confirmatory of the conclu- sion that the fertilized egg contains a double dose of the essential nuclear material. By arti- ficial means it has been found possible to cause the development of an unfertilized egg. The means employed may be of several different sorts, such as stimulation with acids, alkalies, or solutions of altered density. In such ways the development has been brought about of the eggs of sea-urchins, star-fishes, worms, and mol- lusks, which normally require fertilization to make them develop. The sea-urchin egg has been made to develop more successfully than any other. This has 18 THE DUALITY OF INHERITANCE occurred even after the egg had undergone maturation, being reduced to the N condition. From the development of such reduced but unfertilized eggs fully normal sea-urchins have been obtained which even contain developed sexual glands. On the other hand it has been found possible to break the egg into fragments by shaking it, or cutting it into bits with fine knives or scissors. It has also been found possible to bring about the development of an egg fragment so obtained, — a fragment which contained no egg nucleus. This result has been attained by allowing a sperm to enter it and form there a nuclear body. No adult organism has yet been reared from such a fertilized egg-fragment, but so far as the de- velopment has been followed it progressed normally. There can accordingly be no doubt that the nuclear material of a sperm-cell has all the capabilities of that of an egg-cell and can in- deed replace it in development. Accordingly, when, as in normal fertilization, both an egg nucleus and a sperm nucleus are present in the cell, a double dose of the necessary nuclear 19 HEREDITY material is supplied. The second or extra dose is, however, not superfluous. It probably adds to the vigor of the organism produced, and in some cases at least, materially affects its form. For many animals and plants exist in two different conditions, in one of which the nu- clear components are simple, N, while in the other they are double, 2 N. Thus in bees, rotifers, and small Crustacea the egg may under certain conditions develop without being fertilized. If the egg develops before matura- tion is complete, that is in the 2 N condition, the animal produced is a female, like the mother which produced the egg. But if the egg undergoes reduction to the N condition before beginning its development, then it pro- duces a male individual, an organism, so far as reproduction is concerned, of lower meta- bolic activity. In many plants, too, individuals of N and of 2 N constitution occur, which differ markedly in appearance. Thus the ordinary fern-plant is a 2 N individual, but it never produces 2 N offspring. Fig. 6 shows an ordinary fern- plant, which produces spores on the under 20 THE DUALITY OF INHERITANCE Fig. 6. — An ordinary fem, which reproduces by asexual spores. The fern is shown reduced in size at 382; a portion of a frond seen from below and slightly enlarged, at 383; a cross-section of the same more highly magnified, at 384. Notice in 384 the sporangia, and in 385 one of these dis- charging spores. (After Wossidlo, from Coulter Barnes and Cowle's Textbook of Botany.) 3 21 HEREDITY surface of its fronds. Each of those spores is a reproductive cell which, like the mature eggs and sperm of animals, is in a reduced nuclear condition (N). These spores germi- nate, however, without uniting in pairs and form a plant different from the parent, just as the mature egg of a bee, if unfertilized, develops into an individual different from the parent, in that case a male. The plant which develops from the spore of a fern is small and inconspicuous and is known as a prothallus. See Fig. 7. It produces sexual cells (eggs and sperm) which, uniting in pairs, form fern-plants, 2 N individuals. Thus there is a constant alter- nation of generations, fern-plants (2 N), which produce prothalli (N), and then these produce again fern-plants (2 N). The fact is worthy of note that in an animal or plant which is in the single or N condition, there occurs no chromatin reduction at the formation of reproductive cells. Its cells are already in the single condition, and they probably cannot be further reduced without destroying the organism. The 2 N fern-plant forms reproductive cells, its spores, which are 23 THE DUALITY OF INHEEITANCE in the reduced condition, N, and these germi- nate into the prothalhis, which accordingly is Fig. 7. — The prothallus of a fern, which reproduces by sexual cells, eggs and sperm. The eggs are borne in the sac-like "archegonia," just below the notch in the figure. They, like the sperm-forming "antheridia," lie on the under sur- face of the flattened prothallus which is here viewed from below. Notice the root-hairs or rhizoids by which the plant feeds. Highly magnified. (After Coulter, Barnes, and Cowles.) N throughout. But when the prothallus forms reproductive^ cells, no reduction occurs. Its egg-cells and its sperm-cells in common with 23 HEREDITY all other cells of the prothallus are already in the reduced condition without any matura- tion divisions. The result of their union in pairs, at fertilization, is the formation of 2 N combinations that germinate into fern-plants. Similarly in the case of a male animal which o o Fig. 8. — Diagram showing the chromosome number in the spermatogenesis of ordinary animals (upper line) and of the wasp (lower line). has developed from a reduced but unfertilized eggj no reduction occurs at the formation of its sperm-cells. In an ordinary male animal, one which is in the double or 2 N state, the development of the sperms is attended by re- duction to the N condition. In this process there occur two cell-divisions producing from each initial cell four sperms. See Fig. 5, and 24: THE DUALITY OF INHEEITANCE Fig. 8, upper line. But in the male wasp, whose cells are in the N condition at the be- ginning, one of these divisions is so far sup- pressed that the resulting cell products are of very unequal size, and the smaller one contains no nuclear material. The other then gives rise to two sperm-cells, each possessing the origi- nal N nuclear condition, while the small non- nucleated cell degenerates. See Fig. 8, lower line. In conclusion, I wish to introduce two tech- nical terms, which it will be convenient for us to use in subsequent discussions. These are gamete and zygote. A reproductive cell (either e^g or sperm) which is in the reduced condi- tion (N) ready for union in fertilization is called a gamete. The result of fertilization is a zygote, a joining together of two cells each in the N condition. The result is a new or- ganism, at first a single cell, in the 2 N condition. 25 HEREDITY BIBLIOGRAPHY Castle, W. E. 1903. ''The Heredity of Sex." Bull. Mus. Comp. Zool- ogy, 40, pp. 189-218. Delage, Y. 1898. "Embryons sans noyau maternel." Compte rendu, Academie des sciences, Paris, 127, pp. 528-531. 1909. ''Le sexe chez les Oursins issus de parthe- nogenese experimentale." Compte rendus, Academie des sciences, Paris, 148, pp. 453-455. KiRKHAM, W. B. 1907. Maturation of the Egg of the White Mouse.'' Trans. Conn. Acad, of Arts and Sciences, 13, pp. 65-87. LOEB, J. 1899. ''On the Nature of the Process of FertiHzation and the Artificial Production of Normal Larvae (Plutei) from the Unfertilized Eggs of the Sea-urchin." Amer. Journ. of Physiol, 3, pp. 135-138. LOTSY, J. P. 1905. "Die X-Generation und die 2 X-Generation." Biologisches Centralblatt, 25, pp. 97-117. Meves, F., und Duesberg, J. 1908. "Die Spermatozytenteilungen bei der Hornisse (Vespa crabo L.)." Arch. f. mik. Anat. u. Entwick., 71, pp. 571-587. Wilson, E. B. 1896. "The Cell in Development and Inheritance," 370 pp., illustrated. The Macmillan Co., New York. CHAPTEE II GERM-PLASM AND BODY, THEIR MUTUAL INDEPENDENCE IN the last chapter we discussed two bio- logical principles which, if clearly grasped, greatly simplify an understanding of the process of heredity. These are as follows: (1) A sexually produced individual arises from the union of two reproductive cells (or gametes), each of which contains, so far as heredity is concerned, a full material equip- ment for the production of a new individual. Accordingly, the newly produced individual is two-fold or duplex as concerns the material basis of heredity. (2) If the new individual becomes adult and forms gametes, the production of these will be attended by a reduction to the simplex or single condition as regards the material basis of heredity. ^7 HEEEDITY To these two principles we may now add a third, viz.: — (3) The individual consists of two distinct parts: first, its body destined to die and disintegrate after a certain length of time; and, secondly, the germ-cells con- tained within that body, capable of indefinite existence in a suitable medium. The fertilized egg or zygote begins its in- dependent existence by dividing into a number of cells. These become specialized to form the various parts and tissues of the body, muscle, bone, nerve, etc., and by becoming thus specialized they lose the power to produce any- thing but their own particular kind of special- ized tissue; they cannot reproduce the whole. This function is retained only by certain un- differentiated cells found in the reproductive glands and known as germ-cells. They are direct lineal descendants of the fertilized egg itself. If they are destroyed the individual loses the power of reproduction altogether. External influences which act upon the body may of course modify it profoundly, but such modifications are not transmitted through the gametes, because the gametes are not derived 28 GEEM-PLASM x\ND BODY from body-cells, but from germ-cells. This relationship first pointed out by Weismann may be expressed in a diagram, as in Fig. 9. Only such environmental influences as directly alter the character of the germ-cells will in any way influence the character of subsequent generations of individuals derived from those S Line of succession. © Line of inheritance. G Fig. 9. — Diagram showing the relation of the body (S) to the germ-cells (G) in heredity. (After Wilson.) germ-cells. Body (or somatic) influences are not inherited. This knowledge we owe largely to Weismann, who showed experimentally that mutilations are not inherited. The tails of mice were cut ofP for twenty generations in succession, but without effect upon the char- acter of the race. Weismann also pointed out the total lack of evidence for the then current belief that characters acquired by the body are inherited. The correctness of his view that body and germ-cells are physiologically distinct 29 HEREDITY is indicated by the results obtained when germ- cells are transplanted from one individual to another. Heape showed some twenty years ago that if the fertilized egg of a rabbit of one variety (for example an angora, i. e. a long-haired, white animal) be removed from the oviduct of its mother previous to its attachment to the uterine wall, and be then transferred to the oviduct of a rabbit of a different variety (for example a Belgian hare, which is short-haired and gray), the egg will develop normally in the strange body and will produce an individual with all the characteristics of the real (an- gora) mother unmodified by those of the foster mother (the Belgian hare). Young thus ob- tained by Heape were both long-haired and albinos, like the angora mother. To this ex- periment the objection might be offered that the transplanted egg was already full-grown and fertilized when the transfer was made, and that therefore no modification need be expected, but if the egg were transferred at an earlier stage the result might have been different. In answer to such a possible objection the foUow- 30 Fig. 10. — A young, black guinea-pig, about three weeks old. Ovaries taken from an animal like this were transplanted into the albino shown below. Fig. 11. — An albino female guinea-pig. Its ovaries were removed, and in their place were introduced ovaries from a young, black guinea- pig, like that one shown in Fig. 10. Fig. 12. — An albino male guinea-pig, with which was mated the albino shown in Fig. 11. GEEM-PLASM AND BODY ing experiment performed by Dr. John C. Phil- lips and myself may be cited. A female albino guinea-pig (Fig. 11) just at- taining sexual maturity was by an operation deprived of its ovaries, and instead of the re- moved ovaries there were introduced into her body the ovaries of a young black female guinea-pig (Fig. 10), not yet sexually mature, aged about three weeks. The grafted animal was now mated with a male albino guinea-pig (Fig. 12). From numerous experiments with albino guinea-pigs it may be stated emphati- cally that normal albinos mated together, with- out exception, produce only albino young, and the presumption is strong, therefore, that had this female not been operated upon she would have done the same. She produced, however, by the albino male three litters of young, which together consisted of six individuals, all black. (See Fig. 13.) The first litter of young was produced about six months after the oper- ation, the last one about a year. The trans- planted ovarian tissue must have remained in its new environment therefore from four to ten months before the eggs attained full growth 31 HEKEDITY and were discharged, ample time, it would seem, for the influence of a foreign body upon the inheritance to show itself were such influence possible. In the light of the three principles now stated, viz. (1) the duplex condition of the zygote, (2) the simplex condition of the gametes, and (3) the distinctness of body and germ-cells, we may proceed to discuss the greatest single discovery ever made in the field of heredity, — MendePs law. BIBLIOGEAPHY Castle, W. E., and Phillips, John C. 1911. ''On Germinal Transplantation in Vertebrates." Carnegie Institution of Washington, Publication No. lU, 26 pp., 2 pi. Heape, W. 1890. "Preliminary Note on the Transplantation and Growth of Mammalian Ova within a Uterine Foster- mother." Proc. Roy. Soc, 48, pp. 457-458. , 1897. ''Further Note," etc. Id. 62, pp. 178-183. Weismann, a. 1893. "The Germ-Plasm." Translation by Parker and Romfeldt. Chas. Scribner's Sons, New York. Fig. 13. — Pictures of three living guinea-pigs (A, B, C), and of the preserved skins of three others {D, E, F); all of which were produced by the pair of albinos shown in Figs. 11 and 12. 1 CHAPTEE III MENDEL 'S LAW OF HEKEDITY GEEGOR JOHANN MENDEL was a teacher of the physical and natural f sciences in a monastic school at Briinn, Austria, in the second half of the last cen- tury. He was, therefore, a contemporary of Darwin, but unknown to him as to nearly all the great naturalists of the period. Al- though not famous in his lifetime, it is clear to us that he possessed an analytical mind of the first order, which enabled him to plan and carry through successfully the most origi- nal and instructive series of studies in hered- ity ever executed. The material which he used was simple. It consisted of garden-peas, which he raised in the garden of the monastery. The conclusions which he reached were like- wise simple. He summed them up, the results of eight years of arduous work, in a brief paper published in the proceedings of the local 33 HEREDITY scientific society. There they remained un- heeded for thirty-four years, until their author had long been dead. Meantime biological sci- ence had made steady progress. It reached the position Mendel had attained in advance of his time, and MendePs law was rediscov- ered simultaneously in 1900 by De Vries in Holland, by Correns in Germany, and by Tschermark in Austria. It gratifies our sense of poetic justice that to-day the rediscovered law bears the name, not of any one or of all of its brilliant rediscoverers, but of the all-but- forgotten Mendel. The essential features of this law can best be explained in connection with some illustra- tions, which I choose for convenience from my own experiments. If a black guinea-pig of pure race (Fig. 14) be mated with a white one (Fig. 15), the offspring will, as explained on page 10, all be black ; none will be white. To use MendePs terminology, the black char- acter dominates in the cross, while white recedes from view. The black character is, therefore, called the dominant character; white, the recessive character. 34 rt< 00 bC S a» M 03 a. a TjH 0 T-H 0 bb t>: s bh r-4 fl '?^ 6 -1 .r-( d o c3 CD M d r-l ^ bC bJC m § s O lo .s -t-3 '^ d o ^ d o3 0 rM bC d «+-( 0 03 d 73 '^ o i a 3 c3 "d >> J2 3 0) O 03 >• r^S ;h t*-i •r-t Ti 0) 0 0 03 bi3 d 0 d bS bC d 0 0 "a "9^ >i a d d d bC d d CO ■| *d bC % 0 & 1— 1 0) ^ (h d a a; 1—1 bC d 0 1— i s «+H 0 -t-3 0 1q <-l-l 0 d c3 0 '^ 0 t^ !3 d «1 ^ bC < H < 1 1 1 "^ 1 l_ 1^ 1 -H 1 '^" 10 CO'^ t>^ 1— 1 I— 1 i-H d c3 T-H 6 6 6 6 M w 1— 1 ^^ fe CiH fM MENDEL'S LAW OF HEREDITY But, if now two of the cross-bred black in- dividuals (Fig. 16) be mated with each other, the recessive white character reappears on the average in one in four of the offspring (Fig. Zysjot© ( W j Gametes I W W i Zygotes Fig. 18. — Diagram to explain the result shown in Fig. 17. 17). Its reappearance in that particular pro- portion of the offspring may be explained as follows (see Fig. 18) : The gametes which united in the original cross were, one black, the other white in character. Both characters 35 HEREDITY were then asociated together in the offspring; but black from its nature dominated, because white in this case is due merely to the lack of some constituent supplied by the black gamete. But when the cross-bred black individuals on becoming adult form gametes, the black and the white characters separate from each other and pass into different cells, since, as we have seen, gametes are simplex. Accordingly, the eggs formed by a female cross-bred black are half of them black, half of them white in char- acter, and the same is true of the sperms formed by a male cross-bred black. The com- binations of egg and sperm which would natu- rally be produced in fertilization are accord- ingly 1BB:2BW:1W W, or three combina- tions containing black to one containing only white, which is the ratio of black to white off- spring observed in the experiment. Now the white individual may be expected to transmit only the white character, never the black, because it does not contain that char- acter. Experiment shows this to be true. White guinea-pigs mated with each other pro- duce only white offspring. But the black in- 36 o o3 faD d o .-H -u h 03 ^^ ^ & 6 ^ o fl^ O c3 -u ^ rS c3 % fe S +i O «-!-( ^ ti-. O >> c +J .2 ^ "'S 0) ^ (-1 fl Si o o S Ti u