HEREDITY AND ENVIRONMEN’ . EDWIN G. CONKLIN: i DIVISION OF PHYSICAL ANTHROPOLOGY U. S. NATIONAL MUSEUM THE HRDLICKA LIBRARY Dr. Ales Hrdlicka was placed in charge of the Division of Physical Anthropology when it was first established in 1903. He retired in 1942. During this time he assembled one of the largest collections of human skeletons in existence and made outstanding contributions to his science. On his death, September 5, 1943, he bequeathed his library to the Division, with the provision that —'’ ~~ ——- it be kept-exctustvely- in the said Division, where it may be consulted but not loaned out i Northwestern University THE N. W. HARRIS LECTURES FOR 1914 Che N. W. Harris Dertures were founded in 1906 through the generosity of Mr. Norman Wait Harris of Chicago, and are to be given annually. The purpose of the lecture foundation is, as expressed by the donor, “to stimulate scientific research of the highest type and to bring the results of such research before the students and friends of Northwestern Uni- versity, and through them to the world. By the term ‘scientific research’ is meant scholarly in- vestigation into any department of human thought or effort without limitation to research in the so- called natural sciences, but with a desire that such investigation should be extended to cover the whole field of human knowledge.” Dertures Already Published 1907 Personalism. Borden P. Bowne 1908 University Administration. Charles W. Eliot 1910 The Age of Mammals. Henry F. Osborn 1911 Democracy and Poetry. Francis B. Gummere 1912 The Milk Question. Milton J. Rosenau 1918 The Constitution of Matter. Joseph S. Ames REVISED SECOND EDITION NORMAN W. HARRIS LECTURES FOR 1914 AT NORTHWESTERN UNIVERSITY HEREDITY AND ENVIRONMENT DEVELOPMENT OF MEN / BY EDWIN GRANT CONKLIN PROFESSOR OF BIOLOGY IN PRINCETON UNIVERSITY PRINCETON UNIVERSITY PRESS PRINCETON LONDON: HUMPHREY MILFORD OXFORD UNIVERSITY PRESS 1918 FIRST EDITION Copyright, 1915, by PRINCETON UNIVERSITY PRESS Published February, 1915 Second Printing, June, 1915 REVISED SECOND EDITION Copyright, 1916 Published May, 1916 Second Printing, August, 1917 Third Printing, March, 1918 Printed in the United States of America PREFACE TO FIRST EDITION The origin of species was probably the great- est biological problem of the past century; the origin of individuals is the greatest biological subject of the present one. The many incon- clusive attempts to determine just how species arose led naturally to a renewed study of the processes by which individuals come into existence, for it seems probable that the prin- ciples and causes of the development of indi- viduals will be found to apply also to the evolution of races. As the doctrine of evolu- tion wrought great change in prevalent be- hefs regarding the origin and past history of man, so present studies of development are changing opinions as to the personality of man and the possibilities of improving the race. The doctrine of evolution was largely of theo- retical significance, the phenomena of develop- ment are of the greatest practical importance; indeed there is probably no other subject of such vast importance to mankind as the knowl- Vv PREFACE edge of and the control over heredity and development. Within recent years the experi- mental study of heredity and development has led to a new epoch in our knowledge of these subjects, and it does not seem unreasonable to suppose that in time it will produce a better breed of men. The lectures which compose this volume were given at Northwestern University in February, 1914, on the Norman W. Harris Foundation and were afterward repeated at Princeton Uni- versity. I gladly take this opportunity of ex- pressing to the faculties, students and friends of both institutions my deep appreciation of their interest and courtesy. In attempting to present to a general audience the results of re- cent studies on heredity and development, with special reference to their application to man, the author has had to choose between simplicity and sufficiency of statement, between apparent dogmatism and scientific caution, between a popular and a scientific presentation. These are hard alternatives, but the first duty of a lecturer is to address his audience and to make his subject plain and interesting, if he can, v1 PREFACE rather than to talk to the scientific gallery over the heads of the audience. In preparing the lectures for publication it has not been possible to avoid the technical treatment of certain sub- jects, but in the main the lectures are still addressed to the audience rather than to the scientific gallery. Unfortunately biology is still a strange subject to many intelligent people and its terminology is rather terrifying to the uninitiated; but it is hoped that the glos- sary at the end of the volume may rob these unfamiliar terms of many of their terrors. The first three chapters of this book appeared in Popular Science Monthly, June to Novem- ber 1914, by previous arrangement with the Princeton University Press, and a portion of the last chapter was first published in Science in January 1913. The illustrations are from many sources: Figures 9, 10, 19, 20, 22, 23, 26- 29, 85, 40-43, 51-54, 58, 72-75, 77, 83 are origi- nal; Figures 1-8, 11-18, 21, 24, 25, 30-34, 36- 39, 44-47, 49, 55-57, 59-62, 70 were redrawn with more or less modification from original sources which are indicated in every case; the vii PREFACE remaining figures, namely Figures 48, 50, 63- 69, 71, 76, 78-80, 84-96 were copied with little or no modification from various papers, photo- graphs and books, the original source being given in each case. ‘The writer wishes to ex- press his obligation to all authors and publish- ers upon whose works he has drawn, and he thanks especially the Columbia University Press for permission to use Figures 48 and 50, and the Open Court Publishing Company for Figures 84-87. I take this opportunity of thanking Dr. W. EK. Castle and Dr. J. H. McGregor for the use of photographs which are reproduced in Figures 81, 82 and 96; and I wish especially to thank my assistant, Marguerite Ruddiman, for her aid in preparing figures and manu- script for publication. Princeton,. December, 1914 viii PREFACE TO SECOND EDITION The interest of the public and the kindness of the publishers have made possible a revised edition of this book within a year after its first publication. This has permitted the rewriting of certain passages which were not well ex- pressed and the introduction of considerable new material which will serve, it is hoped, to further elucidate some of the questions dis- cussed, as well as to give results of more recent work. In particular Figs. 4, 5, 9, 10, 18, 19 have been added and besides minor corrections and additions to all the chapters considerable changes have been made in Chapter III, sec- tion on Maternal Inheritance, Chapter IV section on Inheritance or Non-Inheritance of Acquired Characters, and Chapter V sec- tions on Artificial Selection, Origin of Muta- tions, Past Evolution of Man, and Eugenics. January, 1916 PusBiisHER’s Nore.—In the second edition, second printing, August, 1917, the text was corrected by the author so as to em- body some of the results of the latest investigations in this subject. ix , ce CONTENTS CHAPTER I. FACTS AND FACTORS OF DEVEL- OPMENT INTRODUCTION A. PHENOMENA OF DEVELOPMENT I, DEVELOPMENT oF THE Bopy 1. The Germ Cells 2. Fertilization 3. Cleavage 4, Embryogeny 5. Organogeny 6. Oviparity and Viviparity 7. Development of Functions II. DEVELOPMENT oF THE MIND 1. Sensitivity . Tropisms, Reflexes, Instincts . Memory : . Will Q 3 4. Intellect, Reason 5 6. Consciousness B. FACTORS OF DEVELOPMENT 1. Preformation 2. Epigenesis 3. Endogenesis and Epigenesis 4. Heredity and Enviromnent xi CONTENTS CHAPTER II. CELLULAR BASIS OF HEREDITY ‘ AND DEVELOPMENT A. INTRODUCTORY 1. Definitions 2. The Transmission Hypothesis 3. Germinal Continuity and Somatic Dis- continuity 4. The Units of Living Matter 5. Heredity and Development — B. THE GERM CELLS 1. Fertilization 2. Cleavage and Differentiation 3. The Origin of the Sex Cells a. The Division Period b. The Growth Period ec. The Maturation Period 4. Sex Determination C. THE MECHANISM - OF HEREDITY I. Tue Speciricity oF GERM CELLS II. CorrELATIONS BETWEEN GERMINAL AND So- MATIC ORGANIZATION 1. Nuclear Correlations 2. Cytoplasmic Correlations a. Polarity b. Symmetry - ce. Inverse Symmetry d. Localization Pattern D. THE MECHANISM OF DEVELOPMENT 1. The Formation of Different Substances xii CONTENTS 2. Segregation and Isolation of Different Substances in Cells a. By Protoplasmic Movements b. By Differential Cell Divisions CHAPTER III. PHENOMENA OF INHERITANCE A. OBSERVATIONS ON INHERITANCE Individuals and their Characters Hereditary Resemblances and Differences I. Herepitary REsEMBLANCES 1. Racial Characters 2. Individual Characters a. Morphological Features b. Teratological and Pathological Pe- culiarities c. Physiological Peculiarities d. Psychological Characters II. Herepitary DirFrERENCES 1. New Combinations of Characters 2. New Characters or Mutations 3. Mutations and Fluctuations 4. Every Individual Unique B. STATISTICAL STUDY OF INHERITANCE 1. Galton’s “Law of Ancestral Inheri- tance” 2. Galton’s “Law of Filial Regression” C. EXPERIMENTAL, STUDY OF INHERI- TANCE I. MenpELism 1. Results of Crossing Individuals with one pair of contrasting characters xiii CONTENTS Other Mendelian Ratios 2. Results of Crossing Individuals with more than one pair of contrasting characters Dihybrids and Trihybrids 8. Inheritance Formule 4. Presence and Absence Hypothesis 5. Summary of Mendelian Principles ~a. The Principle of Unit Characters b. The Principle of Dominance ce. The Principle of Segregation II. Mopirications AND ExtTENsIoNs oF MEN- DELIAN PRINCIPLES 1. The Principle of Unit Characters and Inheritance Factors Inheritance Factors and Germinal . Units 2. Modifications of the Principle of Domi- nance Sex and Sex Limited Inheritance Sex Linked Inheritance 38. The Principle of Segregation “Blending” Inheritance Maternal Inheritance III. Menperian INHERITANCE IN Man CHAPTER IV. INFLUENCE OF ENVIRONMENT A. RELATIVE IMPORTANCE OF HEREDITY AND ENVIRONMENT 1. Former Emphasis on Environment 2. Present Emphasis on Heredity 8. Both Indispensable to Development xiv CONTENTS B. EXPERIMENTAL MODIFICATIONS OF DEVELOPMENT I. DEVELOPMENTAL STIMULI 1, Physical Stimuli 2. Chemical Stimuli II. DevELOPMENTAL RESPONSES Dependent upon (a) Nature of Organism, (b) Nature of Stimulus, (c) Stage of De- velopment 1. Modifications of Germ Cells before Fertilization 2. During Fertilization 3. After Fertilization C. FUNCTIONAL ACTIVITY AS A FACTOR OF DEVELOPMENT D. INHERITANCE OR NON-INHERITANCE OF ACQUIRED CHARACTERS E. APPLICATIONS TO HUMAN DEVELOP- MENT: EUTHENICS CHAPTER V. CONTROL OF HEREDITY: EU- GENICS A. DOMESTICATED ANIMALS AND CULTI- VATED PLANTS I. INFLUENCE OF ENVIRONMENT IN PRODUCING New Races II. ArtiriciaL SELECTION 1. The Methods of Breeders 2. How has Selection acted? III. Metruops or Mopern GENETICS 1. Mendelian Association and Dissocia- tion of Characters 2. Origin of Mutations xv CONTENTS 8B. CONTROL OF HUMAN HEREDITY I, Past Evo.tution or Man II. Can Human Evo.ution BE CONTROLLED? 1. Selective Breeding the only Method of Improving the Race. 2. No Improvement in Human Heredity within Historic Times 3. Why the Race has not Improved III. Eveenics 1. Possible and Impossible Ideals 2. Negative Eugenical Measures 3. Positive Eugenical Measures 4, Contributory Eugenical Measures 5. The Declining Birthrate CHAPTER VI. GENETICS AND ETHICS I. Tue Voutuntaristic CoNcEPTION oF Na- TURE AND OF HuMAN RESPONSIBILITY II. Tue Mecuanistic CoNcEPrTIon or NATURE AND OF PERSONALITY 1. The Determinism of Heredity 2. The Determinism of Environment III. DETERMINISM AND RESPONSIBILITY 1. Determinism not Fatalism . Control of Phenomena and of Self . Birth and Growth of Freedom . Responsibility and Will 5. Our Unused Talents IV. Tue INpivipvaL AND THE RAcE 1. The Conflict between the Freedom of the Individual and the Good of Society 2. Perpetuation and Improvement of the Race the Highest Ethical Obligation REFERENCES, GLOSSARY, INDEX xvi H O89 2% CHAPTER I FACTS AND FACTORS OF DEVELOPMENT CHAPTER I FACTS AND FACTORS OF DEVELOPMENT INTRODUCTION One of the greatest results of the doctrine of organic evolution has been the determination of man’s place in nature. For many centuries it has been known that in bodily structure man is an animal; that he is born, nourished and de- veloped, that he matures, reproduces and dies just as does the humblest animal or plant. For centuries it has been known that man belongs to that group of animals which have backbones, the vertebrates; to that class which have hair and suckle their young, the mammals, and to that order which have grasping hands, flat nails, and thoracic mamme, the primates, a group which includes also the monkeys and apes. But as long as it was supposed that every species was distinct in its origin from every other one, and that each arose by a 3 4 HEREDITY AND ENVIRONMENT special divine fiat, it was possible to maintain that man was absolutely distinct from the rest of the animal world and that he had no kinship to the beasts, though undoubtedly he was made in their bodily image. But with the establish- ment of the doctrine of organic evolution this resemblance between man and the lower ani- mals has come to have a new significance. The almost universal acceptance of this doctrine by scientific men, the many undoubted resem- blaneces between man and the lower animals, and the discovery of the remains of lower types of man, real “missing links,” have inevitably led to the conclusion that man also is a product of evolution, that he is a part of the great world of living things and not a being who stands apart in solitary grandeur in some isolated sphere. But wholly aside from the doctrine of evolu- tion, the fact that essential and fundamental resemblances exist among all kinds of organ- isms can not fail to impress thoughtful men. Life processes are everywhere the same in principle, though varying greatly in detail. All the general laws of life which apply to animals and plants apply also to man. ‘This FACTS AND FACTORS OF DEVELOPMENT 5 is no mere logical inference from the doc- trine of evolution, but a fact which has been . established by countless observations and ex- periments. ‘The essential oneness of all life gives a direct human interest to all living things. If “the proper study of mankind is man,” the proper study of man is the lower organisms in which life processes are reduced to their simplest terms, and where alone they may be subjected to conditions of rigid ex- perimentation. Upon this fundamental like- ness between the life processes of man and those of other animals are based the wonderful advances in experimental medicine, which may be counted among the greatest of all the achievements of science. The experimental study of heredity, develop- ment and evolution in forms of life below man must certainly increase our knowledge of and our control over these processes in the human race. If human heredity, development and evolution may be controlled to even a slight extent we may expect that sooner or later the human race will be changed for the better. At least no other scheme of social betterment and 6 HEREDITY AND ENVIRONMENT race improvement can compare for thorough- ness, permanency of effect, and certainty of results, with that which attempts to change - the natures of men by establishing in the blood the qualities which are desired. We hear much nowadays about man’s control over na- ture, though in no single instance has he ever changed any law or principle of nature. What he can do is to put himself into such relations to natural phenomena that he may profit by them, and all that can be done to- ward the improvement of the human race is to apply consciously to man those great princi- ples of development and evolution which have been at work, unknown to man, through all the ages. A. PHENOMENA OF DEVELOPMENT One of the greatest and most far-reaching themes which have ever occupied the minds of men is the problem of development. Whether it be the development of an animal from an ego, of a race or species from a pre-existing one, or of the body, mind and institutions of man, this problem is everywhere much the FACTS AND FACTORS OF DEVELOPMENT U same in fundamental principles, and knowl- edge gained in one of these fields must be of value in each of the others. Ontogeny and phylogeny are not wholly distmct phenomena, but are only two aspects of the one general process of organic development. ‘The evolu- tion of races and of species is sufficiently rare and unfamiliar to attract much attention and serious thought; while the development of an individual is a phenomenon of such universal occurrence that it is taken as a matter of course by most people, something so evident that it seems to require no explanation; but famili- arity with the fact of development does not remove the mystery which lies back of it, though it may make plain many of the proc- esses concerned. The development of a human being, of a personality, from a germ cell is the climax of all wonders, greater even than that involved in the evolution of a species or in the making of a world. The fact of development is everywhere ap- parent; its principal steps or stages are known for thousands of animals and plants; even the precise manner of development and its factors 8 HEREDITY AND ENVIRONMENT or causes are being successfully explored. Let us briefly review some of the principal events in the development of animals, and particu- larly of man, and then consider some of the chief factors and processes of development. Most of our knowledge in this field is based upon a study of the development of animals below man, but enough is now known of hu- man development to show that in all essential respects it resembles that of other animals, and that the problems of heredity and differ- entiation are fundamentally the same in man as in other animals. I. DEVELOPMENT OF THE Bopy The entire individual—structure and fune- tions, body and mind—develops as a single in- divisible unity, but for the sake of clarity it is desirable to deal with one aspect of the indi- vidual at a time. For this reason we shall consider first the development of the body, and then the development of the mind. 1. The Germ Cells.—In practically all ani- mals and plants individual development begins with the fertilization of a female sex cell, or FACTS AND FACTORS OF DEVELOPMENT 9 egg, by a male sex cell, or spermatozoon. The epigram of Harvey, “Omne vivum ex ovo,” has found abundant confirmation in all later studies. Both egg and spermatozoon are alive and manifest all the general properties of liv- ing things. How little this fact is appreciated by the public is shown by the repeated an- nouncements by the newspapers that “Profes- sor So-and-so has created life because he has made an egg develop without fertilization.” An egg or a spermatozoon is as much alive as is any other cell; as characteristically alive as is the adult animal into which it develops. It is difficult to define life, as it is also to de- fine matter, energy, electricity, or any other fundamental phenomenon, but it is possible to describe in general terms what living things are and what they do. Every living thing whatever, from the smallest and simplest micro-organism to the largest and most com- plex animal, from the microscopic egg or spermatozoon to the adult man, manifests the following distinctive properties: (a) It contains protoplasm, “the material basis of life,” which is composed of the most 10 HEREDITY AND ENVIRONMENT complex substances known to chemistry. Protoplasm is not a homogeneous substance, but it always exists in the form of cells, which are minute masses of protoplasm composed of many distinct parts, the most important of these bemg the nucleus and the cytoplasm (Fig. 1). Protoplasm is therefore organized, that is composed of many parts all of which are integrated into a single system, the cell. Higher animals and plants are composed of multitudes of cells, differing more or less from one another, which are bound together and integrated into a single organism. Living cells and organisms are not static structures which are fixed and stable in character, but they are systems which are undergoing continual change. They are like the river, or the whitl- pool, or the flame, which are never at two con- secutive moments composed of the same particles but which nevertheless maintain a constant general appearance; in short they are complex systems in dynamic equilibrium. The principal physiological processes by which all living things maintain this equilib- rium are: FACTS AND FACTORS OF DEVELOPMENT 11 Fic. 1. A nearly ripe human ovum in the living condition. The ovum is surrounded by a series of follicle cells (FC) in- side of which is the clear membrane (Memb.) and within this is the ovum proper containing yolk granules (Y) and a nucleus (V) embedded in a clear mass of protoplasm. Magnified 500 diameters (x 500). (From O. Hertwig.) B, two human spermatozoa drawn to about the same scale of magnification. (After G. Retzius.) 12 HEREDITY AND ENVIRONMENT (b) Metabolism, or the transformation of matter and energy within the living thing, in the course of which some substances are oxi- dized into waste products, with the liberation of energy, while other substances are built up into protoplasm, each part of every cell con- verting food substances into its own particular substance by the process of assimilation. (c) Reproduction, or the capacity of organ- isms to give rise to new organisms, of cells to give rise to other cells, and of parts of cells to give rise to similar parts by the process of division. (d) Irritability, or the capacity of receiving and responding to impinging energies, or stimuli, in a manner which is usually, but not invariably, adaptive or useful. Both the egg and the sperm are living cells with typical cell structures and functions, but with none of the parts of the mature organism into which they later develop. But although they do not contain any of the differentiated structures and functions of the developed or- ganism, they differ from other cells in that they are capable under suitable conditions of FACTS AND FACTORS OF DEVELOPMENT 18 producing these structures and functions by the process of development or differentiation, in the course of which the general structures and functions of the germ cells are converted into the specific structures and functions of the mature animal or plant. In both plants and animals the sex cells are alike in many respects, though they differ greatly in appearances. The female sex cells of animals are called ova, the male cells sperm- atozoa. ‘The corresponding cells and adjacent parts of flowering plants are known as ovules and pollen. Collectively all kinds of sex cells are called gametes, and the individual formed by the union of a male and a female gamete is known as a zygote, while the cell formed by the union of egg and sperm is frequently called the oosperm. The egg cell of animals is usually spherical in shape:and contains more or less food sub- stance in the form of yolk; it varies greatly in size, depending chiefly upon the quantity of yolk, from the great egg of a bird, in which the yolk or egg proper may be hundreds of , millimeters in diameter, to the miscroscopic 14 HEREDITY AND ENVIRONMENT eggs of oysters, worms, etc. which may be no more than a few thousandths of a millimeter in diameter. ‘The human ovum (Fig. 1) is microscopic in size (about 0.2 mm. in diameter) but it is not smaller than is found in many other animals. It has all the characteristic parts of any egg cell, and can not be distin- guished microscopically from the eggs of sev- eral other mammals, yet there is no doubt that the ova of each species differ from those of every other species, and later we shall see reasons for concluding that the ova produced by each individual are different from those produced by any other individual. The sperm, or male gamete, is among the smallest of all cells and is usually many thou- Fic. 2, Two Human Spermarozoa. A, showing the side of the flattened head; B, its edge; H, head; M, middle-piece; T, tail. (After G. Retzius.) FACTS AND FACTORS OF DEVELOPMENT 15 sands of times smaller than the egg. In most animals, and in all vertebrates, it is an elongated, thread-like cell with an enlarged head which contains the nucleus, a smaller middle-piece, and a very long and slender tail or flagellum, by the lashing of which the sper- matozoon swims forward in the jerking fashion characteristic of many monads or flagellated protozoa. In different species of animals the spermatozoa differ more or less in size and ap- pearance, and there is every reason to believe that the spermatozoa of each species are pecu- lar in certain respects even though we may not be able to distinguish any structural differ- ences under the microscope. ‘The human sper- matozoa (Fig. 2) closely resemble those of other primates but are still slightly different, and the conclusion is logically inevitable, as we shall see later, that the spermatozoa as well as the ova of each individual differ slightly from those of every other individual. 2. Fertilization—If a spermatozoon in its swimming comes into contact with a ripe but unfertilized egg, the head and middle-piece of the sperm sink into the egg while the tail is 16 - HEREDITY AND ENVIRONMENT IPB Fic. 3. ENTRANCE OF THE SPERMATOZOON INTO THE EGG OF tHE ANNELID Nereis. A, Entire SperMarozoon, showing per- foratorium (P); head (H); middle-piece (M), and tail (T). B-G, Successive Sraces IN THE MaArTurATION AND FERTILIZATION showing the progress of the entrance cone (HC) and sperm nucleus (gi) into the egg. D shows the first maturation spindle of the egg (1st Mat. Sp.); E the first polar body FACTS AND FACTORS OF DEVELOPMENT 17 usually broken off and left outside (Figs. 3, 4). The nucleus in the head of the sperm then be- gins to absorb material from the egg and to grow in size and at the same time a minute granule, the centrosome, appears, either from the middle-piece or from the head of the sperm, and radiating lines run out from the centro- some into the substance of the egg. ‘The sperm nucleus and centrosome then approach the egg nucleus and ultimately the two nuclei come to lie side by side (Figs. 3, 4). Usually when one spermatozoon has entered an egg all others are barred from entering, probably by some change in the surface layer of the egg or in the chemical substances given out by the egg. This union of a single spermatozoon with an egg is known as fertilization. Whereas egg cells are usually, but not invariably, incapable of development unless fertilized, there be- gins, immediately after fertilization, a long series of transformations and differentiations of the fertilized egg which leads to the develop- (1st PB) formed by this division; F the second maturation spindle (2d Mat. Sp.) and the sperm nucleus and spindle ($N); G the division of the male and female nuclei in the first cleavage spindle (ist Cl. Sp.). (After F. R. Lillie.) 18 HEREDITY AND ENVIRONMENT ment of a complex animal—of a person. In the fusion of egg and sperm a new indi- vidual, the oosperm, comes into being. The oosperm, formed by the union of the two sex _ cells, is really a double cell, since parts of the egg and sperm never lose their identity, and the individual which develops from this oosperm is a double being; even in the adult man this double nature, caused by the union of egg and sperm, is never lost. In by far the larger number of animal. species the oosperm, either just before or shortly after fertilization, is set free to begin its own individual existence, and in such cases it is perfectly clear that the fertilization of the egg marks the beginning of the new individual. But in practically every class of animals there are some species in which the fertilized egg is retained within the body of the mother for a varying period during which development is proceeding. In such cases it is not quite so evident that the new individual comes into be- ing with the fertilization of the egg; rather the moment of birth, or separation from the mother, is generally looked upon as the be- FACTS AND FACTORS OF DEVELOPMENT 19 ginning of the individual’s existence. And yet in all cases the new individual is always dis- tinguishable from the body of the mother since there is no protoplasmic connection between the two. In mammals generally, including also the human species, not a strand of proto- ‘ plasm, not a nerve fiber, not a blood vessel passes over from the mother to the embryo; the latter is from the moment of fertilization onward a distinct individual with particular individual characteristics, and this is just as true of viviparous animals in which the egg undergoes a part of its development within the body of the mother as it is of oviparous ones in which the eggs are laid before devel- opment begins. The fertilized egg of a star-fish, or frog, or man is not a different individual from the adult form into which it develops, rather it is a star-fish, a frog, or a human being in the one- celled stage. This fertilized egg fuses with no other cells, it takes into itself no living substance, but manufactures its own proto- plasm from food substances; it receives food and oxygen from without and it gives out Fic. 4 Fic. 4. Fervmization or THE Ecos or STARFISH AND SEA- uRCHIN. A-C, Successive stages in the entrance of a sperma- tozoon into the egg of the starfish, Asterias glacialis. Only FACTS AND FACTORS OF DEVELOPMENT 21 carbonic acid and other waste products; it is sensitive to certain alterations in the environ- ment such as thermal, chemical and electrical changes—it is, in short, a distinct living thing, an individuality. Under proper environ- mental conditions this fertilized egg cell de- velops, step by step, without the addition of anything from the outside except food, water, oxygen, and such other raw materials as are necessary to the life of any adult animal, into the immensely complex body of a star-fish, a frog, oraman. At the same time, from the relatively simple reactions and activities of the fertilized egg there develop, step by step, without the addition of anything from without except raw materials and environmental stimuli, the multifarious activities, reactions, one sperm has penetrated the jelly layer (jl) which surrounds the egg and the peripheral protoplasm (pp) of the egg pro- trudes as an entrance cone (ec) to meet it. (After Fol.) D, Mature spermatozoon of the sea-urchin, Toxopneustes, showing head (h); middle-piece (m); and tail t). H-H, Suc- cessive stages in the penetration of the sperm nucleus (gj) and centrosome (gC) into the egg of Towxopneustes. I-L, Stages in the approach of the sperm nucleus (@V) to the egg nucleus (QV), and in the division of the sperm centrosome ($C) and the formation of the first cleavage spindle. (D-L after Wilson.) rE Eco or THE SEA URCHIN, Nuclear d indle with a centrosome at each pole and with the First CLeAvAGE OF TH omes from the egg ini, a mi- ion figure, or ivis 3 A crotuberculatus mus mi Ech . ic sp tot t the equator. 1 a m nucle and sper chromos FACTS AND FACTORS OF DEVELOPMENT 23 instincts, habits, and intelligence of the ma- ture animal. Is not this miracle of development more wonderful than any possible miracle of cre- ation? And yet as one watches this marvellous process by which the fertilized egg grows into the embryo, and this into the adult, each step appears relatively simple, each perceptible change is minute; but the changes are in- numerable and unceasing and in the end they accomplish this miracle of transforming the fertilized egg cell into the fish, or frog, or man—a thing which would be incredible were it not for the fact that it has been seen by hundreds of observers and can be verified at any time by those who will take the trouble to study the process for themselves. 3. Cleavage——After its entrance into the egg the sperm nucleus moves toward the egg nucleus until the two meet when they divide B and C, Later stages showing the separation of the daughter chromosomes from one another and their movement toward the two poles of the spindle. D and #, Still later stages showing the swelling of the chromosomes and their fusion to form nuc- lear vesicles. #, Complete division of egg into two cells, each containing one daughter nucleus and centrosome. (After Boveri.) Fic. 6. Successive STaces IN THE CLEAVAGE AND GasTRULA- TION OF Amphioxus. A, one cell; B, two cells; C and D, four cells; H, eight cells; F’, sixteen cells; G, blastula stage of about ninety-six cells; H, section through the same showing the cleavage cavity; I, blastula seen from the left side showing three zones of cells, viz. an upper clear zone of ectoderm, a middle (faintly shaded) zone of mesoderm and a lower (deeply shaded) zone of endoderm cells; J, section through the same showing these three types of cells; K and L, successive stages in the infolding of the endoderm; cells indicated as in the pre- ceding figure. In all figures except D the polar body is shown at the upper pole. Figs. A-H after Hatschek. a, anterior; p, posterior; v, ventral; d, dorsal; be, blastoccel; gc, gastroceel. I and J should be reversed to correspond in position with K and L. 0 POUUED: Som WS Ye Fic. 7, 4d and B. Two Larter Sraces In THE DEVELOPMENT or Amphioxus, showing the elongation of the embryo in the antero-posterior axis (a@ p), and formation of the somites (som); neural groove (ng) and neural tube (nt); ect, ecto- derm; mes, mesoderm; ac, alimentary canal. (After Hatschek.) pee Fic. 8. Cross Section or Amphiowus Larvar IN SUCCESSIVE Straces or DreveLopmMentT. A, through a larva similar to 74; B and C, of a larva similar to 7B; D, of a still older larva, ect, ectoderm; enf, endoderm; mes, mesoderm; ch, notochord; mp, neural plate; gc, gastroccel; ac, alimentary canal; coel, ccelom. Fics. 9 anp 10. Dracrams or Froc’s Eccs SHOWING THE PROBABLE RELATIONS OF THE AxEs OF THE EGG To THE AXES AND PRINCIPAL OrGANS OF THE Empryo. All eggs viewed from right side, polar bodies above; A anterior, P posterior, D dorsal, V ventral, s spermatozoon, WV sperm nucleus, 2NV egg nucleus, m mesodermal crescent where.mesoderm will form, c and n gray crescent, where chorda (c) and nervous system (n) will form, en area of endoderm, pigmented area around polar bodies will form ectoderm of skin, se segmentation cavity, bp blasto- pore, H enteron, M region of mouth. Fic. 9. Surrace Views or Entire Ecos. A, Before entrance of spermatozoon. B, Just after en- trance of sperm. C, Union of egg and sperm nuclei. D, 2-cell stage. H, 4-cell stage. F’, 8-cell stage. Fic. 10. Srcrions 1n Mepian PLANE oF Emesryos. G, 16-32 cell stage. H, Blastula. I, Early Gastrula. J, Late gastrula. K, Early embryo. ZL, Late embryo. 28 HEREDITY AND ENVIRONMENT by a complicated process known as mitosis, or indirect nuclear division (Figs. 4 I-L, 5). The centrosome, which usually accompanies the sperm nucleus in its passage through the egg, divides and forms a spindle-shaped figure with astral radiations at its two poles (Figs. 3-5). ‘The chromatin, or stainable substance of the nucleus, takes the form of threads, the chromosomes (Fig. 5), of which there is a con- stant number for each species of animal and plant. ach chromosome then splits length- wise, its two halves moving to opposite ends of the spindle, where the daughter chromo- somes fuse together to form the daughter nuclei. In this way the chromatin of the egg and sperm nuclei is exactly halved. After the germ nuclei have divided in this manner the entire egg divides by a process of constriction into two cells (Fig. 5 F'). This is the beginning of a long series of cell divi- sions, each of them essentially like the first, by which the egg is subdivided successively into a constantly increasing number of cells. Dur- ing the earlier divisions there is little or no in- crease in the volume of the egg, consequently FACTS AND FACTORS OF DEVELOPMENT 29 Fic. 11. Four Sraces 1N THE CLEAVAGE OF THE EGG OF THE SHEEP; pb, polar bodies. (After Assheton.) successive generations of cells continually grow smaller (Figs. 6, 11). This process is known as the cleavage of the egg, and by it the egg is not only split up into a con- siderable number of small cells, but a much ‘30 “HEREDITY AND ENVIRONMENT more important result is that the different kinds of protoplasm in the egg become isolated in different cleavage cells, so that these sub- stances can no longer freely commingle. The cleavage cells, in short, come to contain dif- ferent kinds of substance, and thus to differ from one another. The differentiations of the cleavage cells appear much earlier in some forms than in others, but in all cases such differentiations appear during cleavage (Figs. 9, 10). 4. E’mbryogeny.—F rom this stage onward the course of development differs in different classes of animals to such an extent that it is difficult to formulate any general description which will apply to all of them. Usually the many cleavage cells form a hollow sphere, the blastula (Figs. 6, 10, H), and this in turn be- comes a gastrula (Figs. 6, 10, K), in which at first two, and later three, groups or layers of cells may be recognized; the outer layer, which is formed from cells nearest the upper pole of the egg, is the ectoderm; the inner layer, or endoderm, is formed from cells near- est the lower pole; a middle layer, or group FACTS AND FACTORS OF DEVELOPMENT 31 of cells, the mesoderm, is formed from cleay- age cells which in vertebrates lie between the upper and lower poles (Fig. 10, m). 5. Organogeny.—By further differentiation of the cells of these layers and by dissimilar growth and folding of the layers themselves the various organs of the embryo begin to ap- pear. From the ectoderm is formed the outer layer of the skin and the nervous system; from the endoderm arise the lining of the aliment- ary canal and its outgrowths; from the meso- derm come, in whole or in part, the skeletal, muscular, vascular, excretory, and reproduc- tive systems. In vertebrates the nervous sys- tem appears as a plate of rather large ecto- derm cells (Figs. 8, 10 np) ; this plate rolls up at its sides to form a groove (Fig. 8 C) and then a tube (Fig. 8 D); and by enlarge- ment of certain portions of this tube and by foldings and thickenings of its walls the brain and spinal cord are formed (Fig. 10 K, DL, 13, C, D). The retina or sensory portion of the eye is formed as an outgrowth from the fore part of the brain (Fig. 18, D); the sensory portion of the ear comes from a cup-shaped 32 HEREDITY AND ENVIRONMENT depression of the superficial ectoderm which covers the hinder portion of the head (Fig. 13, H and F’). The back-bone begins to ap- pear as a delicate cellular rod (Fig. 8, ch, 10 c), which then in higher vertebrates becomes surrounded successively by a fibrous, a cartila- ginous, and a bony sheath. And so one might go on with a description of all the organs of the body, each of which begins as a relatively simple group or layer of cells, which gradu- ally become more complicated by a process of growth and differentiation, until these embry- onic organs assume more and more the ma- ture form. 6. Oviparity and Viviparity—This very brief and general statement of the manner of embryonic development applies to all verte- brates, man included. ‘There are many special features of human development which are treated at length in works on embryology, but which need not detain us here since they do not affect the general principles of develop- ment already outlined. In one regard the de- velopment of the human being or of any mam- mal is apparently very different from that of FACTS AND FACTORS OF DEVELOPMENT — 33 a bird or frog or fish, viz., in the fact that in the former the embryonic development takes place within the body of the mother whereas in the latter the eggs are laid before or soon after fertilization. In man, after the cleavage of the egg, a hollow vesicle is’ formed, which becomes attached to the uterine walls by means of processes or villi which grow out from it (Fig. 12, D, E, F') while only a small portion of the vesicle becomes transformed | into the embryo. There is thus established a connection between the embryo and the uterine walls through which nutriment is absorbed by the embryo. And yet this difference is not a fundamental one for in different animals there are all stages of transition between these two modes of development. While in most fishes, amphibians and reptiles the eggs are laid at the beginning of development and are free and independent during the whole course of ontogeny, there are certain species in each of these classes in which the development takes place within the body of the mother. Even in birds a portion of the development takes place within the body of the female before the 34 HEREDITY AND ENVIRONMENT Fic. 12. Diacrams SHowr1nc THE Earty DEVELOPMENT OF THE Human Oosperm. 4, cleavage stage which has just come into the uterus; B and C, blastodermic vesicles embedded in the mucous membrane of the uterus; D, # and F, longitudinal sections of later stages, the anterior and posterior poles being marked by the axis a p. In C cavities have appeared in the ectoderm, entoderm and mesoderm. D, villi forming from the trophoblast (nutritive layer, tr); black indicates ectoderm (ect) ; oblique lines, entoderm; few stipples, mesoderm; V, villi; am, amnion; ys, yolk sac; n, neurenteric canal; x 25. (After Keibel.) FACTS AND FACTORS OF DEVELOPMENT 35 egos are laid, and there are mammals (mono- tremes) which lay eggs, while in others (mar- supials) the young are born in a very imperfect condition. These facts indicate that there is no fundamental difference between oviparity and viviparity. In the latter the union between the embryo and the mother is a nutritive but not a protoplasmic one. Blood plasma passes from one to the other by a process of soakage, and the only maternal influences which can affect the developing embryo are such as may be conveyed through the blood plasma and are chiefly nutritive in character. Careful studies have shown that supposed “maternal impres- sions” of the physical, mental, or emotional conditions of the mother upon the unborn child have no existence in fact, except in so far as the quality of the mother’s blood may be changed and may affect the child. At no time, whether before or after birth, is the mother more than nurse to the child. Hereditary in- fluences are transmitted only through the egg cell and the sperm cell and these influences are not affected by intra-uterine development. The principles of heredity and development 36 HEREDITY AND ENVIRONMENT Fic. 13. A-H, successive stages in the early development of the human embryo. 4, blastodermic vesicle showing primitive axis in embryonic area; age unknown. B, blastodermic vesicle attached to uterine wall at the posterior pole, showing neural groove; age unknown, (C, later stage in which the neural] folds FACTS AND FACTORS OF DEVELOPMENT 37 are the same in oviparous and in viviparous animals—ain fishes, frogs, birds and men. Summary.—This is a very brief and incom- plete statement of some of the important stages or phases of the development of the body of man or of any other vertebrate. In all cases development begins with the fertilized egg which contains none of the structures of the de- veloped animal, though it may exhibit the polarity and symmetry of the adult and may also contain specific kinds of protoplasm which will give rise to specific tissues or organs of the adult. From this egg cell arise by divi- sion many cells which differ from one another more and more as development proceeds, until finally the adult animal results. A specific type of development is due to a specific organ- ization of the germ cells with which develop- ment begins, but the earlier differentiations are closing and five pairs of somites have appeared; age, ten to fourteen days. D, stage of fourteen somites showing en- largements of the neural folds at the anterior end which will form the brain; age, fourteen to sixteen days. H and F later stages, the latter with twenty-three somites and three visceral clefts. The ear shows as a depression at the dorsal angle of the second cleft. G, embryo of thirty-five somites showing eye, - branchial arches and limb buds. H, embryo of thirty-six somites showing nasal pit, eye, branchial arches and clefts, limb buds and heart. (After Keibel.) 38 HEREDITY AND ENVIRONMENT Fic. 14. A, human embryo of forty-two somites; age about twenty-one days. B, embryo of about four weeks. C, still older embryo showing the beginnings of the formation of digits. D, embryo of about two months. C and D are drawn on a smaller scale than 4 and B. (After Keibel.) FACTS AND FACTORS OF DEVELOPMENT 39 of the egg are relatively few and simple as compared with the bewildering complexities of the adult, and the best way of understand- ing adult structures is to trace them back in development to their simpler beginnings and to study them in the process of becoming. 7. Development of Functions—The de- velopment of functions goes hand in hand with the development of structures; indeed function and structure are merely different aspects of one and the same thing, namely organization. All the general functions of living things are present in the germ cells, viz., (1) Constructive and destructive metabolism, (2) Reproduction, as shown in the division of cells and cell constituents, (3) Irritability, or the capacity of receiving and responding to stimuli. All these general functions of living things are manifested by germ cells, but as de- velopment advances each of these functions be- comes more specialized, more complicated and more perfect. A cell which at an early stage was protective, locomotor and sensory in func- tion may give rise to daughter cells in which these functions are distributed to different 40 - HEREDITY AND ENVIRONMENT cells; cells which at an early stage were sensi- tive to many kinds of stimuli give rise to daughter cells which are especially sensitive to one particular kind of stimulus, such as vi- bration, light, or chemicals. Functions develop from a generalized to a specialized condition by the process of “physi- ological division of labor’? which accompanies morphological division of substance. But as in the union of hydrogen and oxygen a new substance, water, appears which was not present before, and as in the development of structures new parts, which were not present in the germ, appear by a process of “creative synthesis,” so new functions appear in the course of development, which are not merely sorted out of the general functions present at the beginning, but which are created by the interaction and synthesis of parts and func- tions previously present. Jor example, Lane has shown that young rats are quite insensible to light until several days after birth although the eye begins to form at a very early stage. Doubtless every part of the eye is functioning in a general way during the entire develop- FACTS AND FACTORS OF DEVELOPMENT 41 ment but not until all parts are formed and connected and all their functions are synthe- sized does the new function, vision, spring into existence. Undoubtedly the same is true of many other complex functions which have no existence until all their constituents are pres- ent and integrated, when they suddenly appear. Much less attention has been paid to the de- velopment of functions than to the develop- ment of structures, and consequently it is not possible to describe the former with the same degree of detail as the latter. But in spite of the lack of detailed knowledge regarding the development of particular functions the gen- eral fact of such development is well estab- lished. To what extent structures may modify functions or functions structures, in the course of development, is a problem which has been much discussed, and upon the answer to it de- pends the fate of certain important theories, for example Lamarckism; but this problem can be solved only by thoroughgoing experi- mental and analytical work. In the meantime it seems safe to conclude that living structures 42 HEREDITY AND ENVIRONMENT and functions are inseparable and that any-. thing which modifies one of these must of necessity modify the other also; they are merely different aspects of organization, and are dealt with separately by the morphologists and physiologists only as a matter of conven- ience. At the same time there can be no doubt that minute changes of function can frequently be detected where no corresponding change of structure can be seen, but this shows only that physiological tests may be more delicate than morphological ones. In certain lines of mod- ern biological work, such as_ bacteriology, cytology, and genetics, many functional distinctions are recognizable between or- ganisms morphologically indistinguishable. But this does not signify that functional changes precede structural ones, but only that the latter are more difficult to see than the former. For every change of function it is probable that an “unlimited microscopist” could discover a corresponding change of structure. FACTS AND FACTORS OF DEVELOPMENT 43 II. DEVELOPMENT OF THE MIND The development of the mind parallels that of the body: whatever the ultimate relations of the mind and body may be, there can be no reasonable doubt that the two develop together from the germ. It is a curious fact that many people who are seriously disturbed by scientific teachings as to the evolution or gradual de- velopment of the human race accept with equanimity the universal observation as to the development of the human individual,—mind as well as body. ‘The animal ancestry of the race 1s surely no more disturbing to philosophi- cal and religious beliefs than the germinal origin of the individual, and yet the latter is a fact of universal observation which can not be relegated to the domain of hypothesis or theory, and which can not be successfully de- nied. If we admit the fact of the develop- ment of the entire individual, surely it matters little to our philosophical or religious beliefs to admit the development or evolution of the race. The origin of the mind, or rather of the 44, HEREDITY AND ENVIRONMENT soul, is a topic upon which there has been much speculation by philosophers and theologians. One of the earliest hypotheses was that which is known as transmigration or metempsychosis. This doctrine probably reached its greatest development in ancient India, where it formed an important part of Buddhistic belief; it was also a part of the religion of ancient Egypt; it was embodied in the philosophies of Pytha- goras and Plato. According to these teach- ings, the number of souls is a constant one; souls are neither made nor destroyed, but at birth a soul which had once tenanted another body enters into the new body. This doctrine was generally repudiated by the Fathers of the Christian Church. Jerome and _ others adopted the view that God creates a new soul for each body that is generated, and that every soul is thus a special divine creation. ‘This has become the prevailing view of the Christian Church and is known as creationism. On the other hand ‘Tertullian taught that souls of children are generated from the souls of par- ents as bodies are from bodies. ‘This doctrine, which is known as traducianism, has been de- FACTS AND FACTORS OF DEVELOPMENT 45 fended by certain modern theologians, but has been formally condemned by the Roman Catholic Church. Traducianism undoubtedly comes nearer the scientific teachings as to the development of the mind than does either of the other doc- trines named, but it is based upon the preva- lent but erroneous belief that the bodies of the parents generate the body of the child, and that correspondingly the souls of the parents generate the soul of the child. Now we know that the child comes from germ cells and not from the highly differentiated bodies of the parents, and furthermore that these cells are not made by the parents’ bodies but have arisen by the division of antecedent germ cells (see p- 99). Consequently it is not possible to hold that bodies generate bodies or even germ cells, nor that souls generate souls. ‘The only possible scientific position is that the mind (or soul) as well as the body develops from the germ. No fact in human experience is more certain than that the mind develops by gradual and natural processes from a simple condition 46 HEREDITY AND ENVIRONMENT which can scarcely be called mind at all; no fact in human experience is fraught with greater practical and philosophical significance than this, and yet no fact is more generally disregarded. We know that the greatest men of the race were once babies, embryos, germ cells, and that the greatest minds in human history were once the minds of babies, em- bryos and germ cells, and yet this stupendous fact has had but little influence on our beliefs as to the nature of man and of mind. We rarely think of Plato and Aristotle, of Shake- speare and Newton, of Pasteur and Darwin, except in their full epiphany, and yet we know that when each of these was a child he “thought as a child and spake as a child,” and when he was a germ cell he behaved as a germ cell. The development of the mind from the ac- tivities of the germ cells is certainly most won- derful and mysterious, but probably no more so than the development of the complicated body of the adult animal from the structures of the germ. Both belong to the same order of phenomena and there is no more reason for supposing that the mind is supernaturally FACTS ‘AND FACTORS OF DEVELOPMENT AT created than that the body is. Indeed, we know that the mind is formed by a process of development, and the stages of this develop- ment are fairly well known. There is nowhere in the entire course of mental development a sudden appearance of psychical processes, but rather a gradual development of these from simpler and simpler beginnings. No detailed study has been made of the reactions of human germ cells and embryos, but there is every reason to believe that these reactions are simpler in the embryo and germ cell than in the infant, and that they are generally similar to the reactions of the germ cells and embryos of other animals, and to the behavior of many lower organisms. A few years ago such a statement would have been branded as “materialism” and promptly rejected without examination by those who are frightened by names. But the general spread of the scientific spirit is shown not only by the growing regard for evidence but also by the decreasing power of epithets. “Materialism,” like many another ghost, fades away into thin air or at least loses many of 48 HEREDITY AND ENVIRONMENT its terrors, when closely scrutinized. But the statement that mind develops from the germ cells is not an affirmation of materialism, for while it identifies the origin of the entire indi- vidual, mind and body, with the development of the germ, it does not assert that “matter” is the cause of “mind” either in the germ or in the adult. It must not be forgotten that germ cells are living things and that we go no | further in associating the beginnings of mind with the beginnings of body in the germ than we do in associating mind and body in the adult. It is just as materialistic to hold that the mind of the mature man is associated with his body as it is to hold that the begmnings of mind in the germ are associated with the beginnings of the body, and both of these tenets are incontrovertible. It seems to me that the mind is related to the body as function is to structure; there are those who maintain that structure is the cause of function, that the real problem in evolution or development is the transformation of one structure into another, and that the functions which go with certain structures are merely FACTS AND FACTORS OF DEVELOPMENT 49 incidental results; on the other hand are those who maintain that function is the cause of structure and that the problem of evolution or development is the change which takes place in functions and habits, these changes causing corresponding transformations of structure. Among adherents of the former view may be classed many morphologists and Neo-Darwin- lans; among proponents of the latter, many physiologists and Neo-Lamarckians. It seems to me that the defenders of each of these views fail to recognize the essential unity of the en- tire organism, structure as well as function; that neither of these is the cause of the other, though each may modify or condition the other, but that they are two aspects of one common thing, viz., organization. In the same way I think that the body or brain is not the cause of mind, nor mind the cause of body or. brain, but that both are inherent in one common or- ganization or individuality. In asserting that the mind develops from the germ as the body does, no attempt is made to explain the fundamental properties of body .or mind. As the structures of the body may 50 HEREDITY AND ENVIRONMENT be traced back to certain fundamental struc- tures of the germ cell, so the characteristics of the mind may be traced back to certain funda- mental properties and activities of the germ. Many of the psychical processes may be traced back in their development to properties of sensitivity, reflex motions, and persistence of the effects of stimuli. All organisms manifest these properties and for aught we know to the contrary they may be original and neces- sary characteristics of living things. In the simplest protoplasm we find organization, that is, structure and function, and in germinal protoplasm we find the elements of the mind as well as of the body, and the problem of the ultimate relation of the two is the same whether we consider the organism in its germi- nal or in its adult stage. In some way the mind as well as the body develops out of the germ. What are the germinal bases of mind? What are the psy- chical Anlagen in embryos and how do they develop? In this case, even more than in the development of the body, we are compelled to rely upon the comparison of human develop-. FACTS AND FACTORS OF DEVELOPMENT 51 ment with that of other animals, but the great principle of the oneness of life, as respects its fundamental processes, has never yet failed to hold true and will not fail us here. In the study of the psychical processes of organisms other than ourselves we are compelled to rely upon a study of their activities, their reactions to stimuli, since we can not approach the sub- ject in any other way. The reactions and be- havior of organisms under normal and experi- mental conditions give the only insight which we can get into their psychical processes; and this applies to men no less than to protozoa. “1. Sensitivity—The most fundamental phenomenon in the behavior of organisms is irritability or sensitivity, which is the ability of receiving and responding to stimuli: this is one of the fundamental properties of all proto- plasm. But living matter is not equally sensi- tive to all stimuli, nor to all strengths of the same stimulus. Many of the simplest unicel- lular plants and animals show that they are differentially sensitive; they often move to- ward weak light and away from strong light, 52 HEREDITY AND ENVIRONMENT away from extremes of heat and cold, into certain chemical substances and away from others; in short, all organisms, even the sim- plest, may respond differently to different kinds of stimuli or to different degrees of the same stimulus. This is what is known as dif- ferential sensitivity (Figs. 15-19). On the other hand, many organisms respond in the same way to different stimuli, and this may be taken to indicate generally that they are not differentially sensitive to such stimuli; it is COBO D E b FF g Tic. 15. Disrripurion or Bacteria IN THE SpecrruM. The largest group is in the ultra-red at the left; the next largest group is in the yellow-orange close to the line D. (From Jen- nings, after Engelmann.) not to be concluded because organisms respond differently to certain stimuli that they are therefore capable of distinguishing between all kinds of stimuli, for this is certainly not true. Even in adult men the capacity of dis- FACTS AND FACTORS OF DEVELOPMENT 53 tinguishing between different kinds of stimuli is far from perfect. | Egg cells and spermatozoa show this prop- erty of sensitivity. The egg is generally in- capable of locomotion, and since the results of stimulation must usually be detected by move- ments it is not easy to determine to what ex- tent the egg is sensitive; but though the egg lacks the power of locomotion, it possesses in a marked degree the power of intra-cellular movement of the cell contents. When a spermatozoon comes into contact with the sur- face of the egg the cortical protoplasm of the egg flows toward that point and may form a cone or protoplasmic prominence into which the sperm is received (Figs. 3, 4, ec). It is an interesting fact that the same sort of re- sponse follows when a frog’s egg is pricked by a needle, thus showing that in this case the egg does not distinguish between the prick of the needle and that of the spermatozoon. The spermatozoon is usually a locomotor cell and it responds differently to certain stimuli, just as many bacteria and protozoa do; sper- matozoa are strongly stimulated by weak 54 HEREDITY AND ENVIRONMENT alkali and alcohol, they gather in certain chemical substances and not in others, they collect in great numbers around fertilizable egg cells, etc. The movements of fertilized egg cells, cleavage cells, and early embryonic cells are usually limited to flowing movements within the individual cells. ‘These movements, which are of a complicated nature, are of the greatest significance in the differentiation of the egg into the embryo; they are caused chiefly by in- ternal stimuli and by non-localized external ones. Modifications of the external stimuli often lead to modifications of these intra- cellular movements and to abnormal types of cleavage and development—in short, these movements show that the fertilized egg is dif- ferentially sensitive. In the further course of development particular portions of the embryo become es- pecially sensitive to some kinds of stimuli, while other portions become sensitive to oth- ers. In this way the different sense organs, each especially sensitive to one particular kind of stimulus, arise from the generalized FACTS AND FACTORS OF DEVELOPMENT 55 Fic. 16, a, b, c. Reputsion or Spirilla By Common Sat. a, condition immediately after adding crystals; b and ec, later stages in the reaction. “, y, z, repulsion of Spirilla by distilled water. The upper drop consists of sea-water containing Spirilla, the lower drop, of distilled water. At «w these have just been united by a narrow neck; at y and z, the bacteria have retreated before the distilled water. (From Jennings, after Massart.) 56 HEREDITY AND ENVIRONMENT sensitivity of the oosperm, and thus general sensitivity, which is a property of all proto- plasm, becomes differential sensitivity and special senses in the process of embryonic dif- ferentiation. Such sensitivity is the basis of all psychic processes; sensations are the ele- ments of the mind. 2. Tropisms, Reflexes, Instincts—AI| the responses of germ cells, and of the simplest organisms, to stimuli are in the nature of trop- isms or reflexes, that is, relatively simple, machine-like responses. Such tropisms or re- flexes are seen in the movements of bacteria, protozoa and many higher animals and plants (Figs. 15-19) as well as in movements of ‘spermatozoa, the movements of the proto- plasm in egg cells and embryonic cells, the movements of cells and cell masses in the for- mation of the gastrula, alimentary canal, ner- vous system and other organs. Indeed the entire process of development, whether accom- panied by visible movements or not, may be re- garded as a series of automatic responses to stimuli. When the embryo becomes differentiated FACTS AND FACTORS OF DEVELOPMENT 57 HO Paes 2a Fic. 17. Reactions or Paramecium to Hear ann Comp, At a the infusoria are uniformly distributed in a trough, both ends of which have a temperature of 19°; at b the infusoria are shown collected at the cooler end of the trough; at c¢ they have collected at the warmer end of the trough. (From Jen- nings, after Mendelssohn.) to such an extent as to have specialized organs for producing movement its capacity for mak- ing responsive movements to stimuli becomes much increased. If the responses of animals and plants to stimuli are of such a sort that 58 HEREDITY AND ENVIRONMENT Fic. 18. PHororropism or SEEDLING oF Wuite Musrarp sup- ported by a sheet of cork (K, K) floating on water. The di- rection from which light comes is shown by arrows; the stem and leaves are turned toward the light (positive phototrop- ism), the root away from the light (negative phototropism). (After Strasburger. ) FACTS AND FACTORS OF DEVELOPMENT 59 the organism turns or moves toward or away from a source of stimulus they are termed tropisms; if the responses are very compli- cated, one response calling forth another and involving many reflexes, as is frequently the case in animals, they are known as instincts. In the embryo the rhythmic contractions of heart, amnion and intestine are early manifes- tations of reflex motions. These appear chiefly in the involuntary muscles before nervous con- Fic. 19. Grorropism or Seeptinc Oar. After starting to grow with the axis A-d in vertical position the seedling was gradually turned through 90° until the axis B-B was vertical; during this change of position the stem continues to grow upward (negative geotropism), the root downward (positiv geotropism). 60 HEREDITY AND ENVIRONMENT nections are formed, the protoplasm of the muscle cells probably responding directly to the chemical stimulus of certain salts in the body fluids, as Loeb has shown in other cases. Reflexes which appear later are the random movements of the voluntary muscles of limbs and body, which are called forth by nerve im- pulses. Tropisms are manifested only by or- ganisms capable of considerable free movement and hence are absent in the foetus though present in many free living larve. Some in- stincts are present immediately after birth, such as the instinct of sucking or crying, though these are so simple when compared with some instincts which develop later that they might be classed as reflexes; it is doubtful whether any of the activities before birth could properly be designated as instincts. Reflexes, tropisms and instincts have had a phylogenetic as well as an ontogenetic origin, and conse- quently we might expect that they would in general make for the preservation of the species, and as a matter of fact we usually find that they are remarkably adapted to this end. For instance the instincts of the human infant FACTS AND FACTORS OF DEVELOPMENT 61 to grasp objects, to suck things which it can get into its mouth, to cry when in pain, are complicated reflexes which have survived in the course of evolution probably because they serve a useful purpose. Very much has been written on the nature and origin of instincts, but the best available evidence strongly favors the view that instincts are complex reflexes, which, like the structures of an organism, have been built up, both onto- genetically and phylogenetically, under the stress of the elimination of the unfit, so that they are usually adaptive. 3. Memory.—Another general character- istic of protoplasm is the capacity of storing up or registering the effects of previous stimuli. A single stimulus may produce changes in an organism which persist for a longer or shorter time, and if a second stimulus occurs while the effect of a previous one still persists, the response to the second stimulus may be very different from that to the first. Macfarlane found that if the sensitive hairs on the leaf of Dionaea, the Venus fly-trap (Fig. 20, SH), be stroked once no visible response 62 HEREDITY AND ENVIRONMENT TUR CUAAAL \" ny 3 x, \\ \\ a \\ c “MA yA Fic. 20. Dionaea muscipula (Venus Fry-rrar). Three leaves showing marginal teeth and sensitive hairs (SH). The teaf at the left is fully expanded, the one at the right is closed. is called forth, but if they be stroked a second time within three minutes the leaf instantly closes. If a longer period than three minutes elapses after the first stimulus and before the second no visible response follows, 7.e., two successive stimuli are necessary to cause the leaves to close, and the two must not be more FACTS AND FACTORS OF DEVELOPMENT 63 than three minutes apart; the effects of the first stimulus are in some way stored or regis- tered in the leaf for this brief time. This kind of phenomenon is widespread among living things and is known as “summation of stim- uli.” In all such cases the effects of a former stimulus are in some way stored up for a longer or shorter time in the protoplasm. It is possible that this is the result of the forma- tion of some chemical substance which remains in the protoplasm for a certain time, during which time the effects of the stimulus are said to persist, or it may be due to some physical change in the protoplasm analogous to the “set” in metals which have been subjected to mechanical strain. Probably of a similar character is the per- sistence of the effects of repeated stimuli and responses on any organ of a higher animal. A muscle which has contracted many times in a definite way ultimately becomes “trained”’ so that it responds more rapidly and more ac- curately than an untrained muscle; and the nervous mechanism through which the stimu- lus is transmitted also becomes trained in the 64 HEREDITY AND ENVIRONMENT same way. Indeed such training is probably | chiefly a training of the nervous mechanism. The skill of the pianist, of the tennis player, of the person who has learned the difficult art of standing and walking, or the still more diffi- cult art of talking, is probably due to the per- _ sistence in muscles and nerves of the effects of many previous activities. All such phe- nomena were called by Hering “organic memory,’ to indicate that this persistence of the effects of previous activities in muscles and other organs is akin to that persistence of the effects of previous experiences in the nervous mechanism which we commonly call memory. It seems probable that this ability of proto- plasm in general to preserve for a time the effects of former stimuli is fundamentally of the same nature as the much greater power of nerve cells to preserve such effects for much longer periods and in complex associations, a faculty which is known as associative memory. The embryos, and indeed even the germ cells of higher animals, may safely be assumed to be endowed with protoplasmic and organic memory, out of which, in all probability, de- FACTS AND FACTORS OF DEVELOPMENT 65 velop associative and conscious memory in the mature organism. 4. Intellect, Reason—KEven the intellect and reason which so strongly characterize man have had a development from relatively simple beginnings. All children come gradually to an age of intelligence and reason. In its simpler forms at least reason may be defined as the power of predicting future events and of reaching conclusions regarding unexperi- enced phenomena under the influence of past experience. In the absence of individual ex- perience young children have none of this power, but it comes gradually as a result of remembering past experiences and of fitting such experiences into new conditions. Young infants and many lower animals lack the power of reason, though their behavior is fre- quently of such a sort as to suggest that they are reasoning. Even the lowest animals avoid injurious substances and conditions and find beneficial ones; more complex animals learn to move objects, solve problems, and find their way through labyrinths in the shortest and most economical way; but this apparently in- 66 HEREDITY AND ENVIRONMENT telligent and purposive behavior has been shown to be due to the gradual elimination of all sorts of useless activities, and to the per- sistence of the useful ones. The ciliated infusorian, Paramecium, moves by the beating of cilia which are arranged in such a way that they drive the animal forward in a spiral course. However, when it is strongly irritated, the normal forward move- ment is reversed; the cilia beat forward instead of backward and the animal is driven back- ward for some distance (Fig. 21, 1, 2, 3); it then stands nearly still, merely rolling over and swerving toward the aboral side, and finally it goes ahead again, usually on a new course’ (Hig: (21,8, 4; 5,6). Bhese tmove- ments seem to be conditioned rather rigidly by the organization of the animal: they are more or less fixed and mechanical in character, though to a certain extent they may be modi- fied by experience or physiological states. Paramecium behaves as it does in virtue of its constitution, just as an egg develops in a particular way because of its particular organization. FACTS AND FACTORS OF DEVELOPMENT 67 But although limited in its behavior to these relatively simple motor reactions, Paramecium does many things which seem to show intelli- gence and purpose. It avoids many injurious substances, such as strong salts or acids, and it collects in non-injurious or beneficial sub- stances, such as weak acids, masses of bacteria upon which it feeds, etc. It avoids extremes of heat and cold and if one end of a dish con- tainng Paramecia is heated and the other end is cooled by ice, the Paramecia collect in the region somewhere between these two extremes (Fig. 17). Jennings, by studying carefully Fic. 21. Diacram or THE Avorpinc Reaction or Parame- cuum. A is a solid object or other source of stimulation. 1-6, successive positions occupied by the animal. The rotation on the long axis is not shown. (After Jennings.) 68 HEREDITY AND ENVIRONMENT the behavior of single individuals, established the fact that this apparently intelligent action is due to differential sensitivity and to the single motor reaction of the animal. If in the course of its swimming a Paramecium comes into contact with an irritating substance or condition, it backs a short distance, swerves toward its aboral side, and goes ahead in a new path; if it again comes in contact with the irritating conditions this reaction is repeated, and so on indefinitely until finally a path is found in which the source of irritation is avoided altogether. In short, Paramecium continually tries its environment, and backs away from irritating substances or conditions. Its apparently intelligent reactions are thus explained as due to a process of “trial and Errore The behavior of worms, star-fishes, crusta- ceans, mollusks, as well as of fishes, frogs, reptiles, birds and mammals, has been studied *In Paramecium, there is certainly no consciousness of trial and error, and probably no unconscious attempt on the part of the animal to attain certain ends. Its responses are reflexes or tropisms, which are determined by the nature of the animal and the character of the stimulus, The fact that these re- FACTS AND FACTORS OF DEVELOPMENT 69 and in all cases it is found that their method of responding to stimuli is not at first really purposive and intelligent but by the gradual elimination of useless responses and_ the preservation (or remembering) of useful ones the behavior may come to be purposive and intelligent. Thorndike found that when dogs, cats and monkeys were confined in cages which could be opened from the inside by turning a button, or pressing upon a lever, or pulling a cord, they at first clawed around all sides of the cage until by chance they happened to operate the mechanism which opened the door. ‘There- after they gradually learned by experience, that is, by trial and error, and finally by trial and success, just where and how to claw in order to get out at once. When a dog has learned to turn a button at once and open a door we say he is intelligent, and if he can sponses are in the main self-preservative is due to the teleo- logical organization of Paramecium which has been evolved, according to current opinion, as the result of long ages of the elimination of the unfit. If, in the opinion of any one, the expression “trial and error” necessarily involves a striving after ends, it would be advisable to replace it in this case by some such term as “useful or adaptive reactions.” 70 HEREDITY AND ENVIRONMENT learn to apply his knowledge of any particular cage to other and different cages, a thing which Thorndike denies, we should be justified in saying that he reasons, though in this case intelligence and reason are founded upon memory of many past experiences, of many trials and errors and of a few trials and successes. There is every evidence that human beings arrive at intelligence and reason by the same process, a process of many trials and errors and a few trials and successes, a remembering of these past experiences and an application of them to new conditions. A baby grasps for things which are out of its reach, until it has learned by experience to appreciate distances; it tests all sorts of pleasant and unpleasant things until it has learned to avoid the latter and seek the former; it experiments with its own body until it has learned what it can do and what it can not do. Is not this learning by experience akin to the same process in the dog and more remotely to the trial and error of the earthworm or the adaptive reflexes of Paramecium? Is not intelligence and reason FACTS AND FACTORS OF DEVELOPMENT 71 in all of us, and upon all subjects, based upon the same processes of trial and error, memory of past experiences and application of this to new conditions? Surely this is true in all ex- perimental and scientific work. Indeed the scientific method is the method of trial and error, and finally trial and success—the method recommended by St. Paul to prove all things and hold fast that which is good. In Paramecium the reflex type of behavior is relatively complete; there is no associative memory and no ability to learn by experience. In the earthworm associative memory is but slightly developed and the animal learns but little by experience and can make no applica- tion of past experiences to new conditions. In the dog associative memory is well developed; the animal learns by experience and can, to a limited extent, apply such memory of past ex- periences to new conditions. In adult man all of these processes are fully developed and par- ticularly the last, viz., the ability to reason. But in his development the human individual passes through the more primitive stages of intelligence, represented by the lower animals 72 HEREDITY AND ENVIRONMENT named; the germ cells and embryo represent only the stages of reflex behavior, to these trial and error and associative memory are added in the infant and young child, and to these the application of past experience to new condi- tions, or reason, is added in later years. 5. Will—Another characteristic, which many persons regard as the supreme psychical faculty, is the will. This faculty also under- goes development and from relatively simple beginnings. ‘The will of the child has devel- oped out of something which is far less perfect in the infant and embryo than in the child. Observations and experiments on lower ani- mals and on human beings, as well as intro- spective study of our own activities, appear to justify the following conclusions: (1.) Every activity of an organism is a response to one or more stimuli, external or internal in origin. ‘These stimuli are in the main, if not entirely, energy changes outside or inside the organism. In lower organisms as well as in the germ cells and embryos of higher animals the possible number of re- sponses are few and prescribed owing to their FACTS AND FACTORS OF DEVELOPMENT 13 relative simplicity, and the response follows the stimulus directly. In more complex or- ganisms the number of possible responses to a stimulus is greatly increased, and the visible response may be the end of a long series of internal changes which are started by the original stimulus. (2.) The response to a stimulus may be modified or inhibited in the following ways: (a) Through conflicting stimuli and changed physiological states, due to fatigue, hunger, etc. Many stimuli may reach the organism at the same time and if they conflict they may nullify one another or the organism may respond to the strongest stimulus and disregard the weaker ones. When an organ- ism has begun to respond to one stimulus it is not easily diverted to another. Jennings found that the attached infusorian, Stentor, which usually responds to strong stimuli by closing up, may, when repeatedly stimulated, loosen its attachment and swim away, thus respond- ing in a wholly new manner when its physi- ological state has been changed by repeated stimuli and responses. Whitman found that 74 HEREDITY AND ENVIRONMENT leeches of the genus Clepsine prefer shade to bright light, and other things being equal they always seek the under sides of stones and shaded places; but if a turtle from which they normally suck blood is put into an aquarium with the leeches, they at once leave the shade and attach themselves to the turtle. They prefer shade to bright light but they prefer their food to the shade. The tendency to re- main concealed is inhibited by the stronger stimulus of hunger. On the other hand he found that the salamander, Necturus, is so timid that it will not take food, even though starving, until by gradual stages and gentle treatment its timidity can be overcome to a certain extent. Here fear is at first a stronger stimulus than hunger and unless the stimulus of fear can be reduced the animal will starve to death in the presence of the most tempting food. (b) Responses may also be modified through compulsory limitation of many pos- sible responses to a particular one, and the con- sequent formation of a habit. This is the method of education employed in training all FACTS AND FACTORS OF DEVELOPMENT 75 sorts of animals. Thus Jennings found that a star-fish could be trained to turn itself over, when placed on its back, by means of one pair of arms simply by persistently preventing the use of the other arms. Many responses of organisms are modified in a similar way, not only by artificial limitations but also by natural ones. (c) Responses which have become fixed and constant through natural selection or other means of limitation may become more varied and general when the compulsory limitation is relaxed. Behavior in the former case is fixed and instinctive, in the latter more varied and plastic. ‘Thus Whitman found that the be- havior of domesticated pigeons is more vari- able and their instincts less rigidly fixed than in wild species. If the eggs are removed to a little distance from the nest the wild passenger pigeon returns to the nest and sits down as if nothing had happened. She soon finds out, not by sight but by feeling, that something is missing, and she leaves the nest after a few minutes without heeding the eggs. The ring- neck pigeon also misses the eggs and some- 76 HEREDITY AND ENVIRONMENT times rolls one of them back into the nest, but never attempts to recover more than one. The dove-cote pigeon generally tries to recover both eggs. According to Whitman: In these three grades the advance is from extreme blind uniformity of action, with little or no choice, to a stage of less rigid uniformity. . . . Under con- ditions of domestication the action of natural selec- tion has been relaxed, with the result that the rigor of instinctive co-ordination, which bars alternative action, is more or less reduced. Not only is the door to choice thus unlocked, but more varied opportuni- ties and provocations arise, and thus the internal mechanism and the external conditions and stimuli work both in the same direction to favor greater freedom of action. When choice thus enters no new factor is introduced. ‘There is greater plasticity within and more provocation without, and hence the same bird, without the addition or loss of a single nerve cell, becomes capable of higher action and is encouraged and even constrained by circumstances to learn to use its privileges of choice. Choice, as I conceive it, is not introduced as a little deity en- capsuled in the brain. . . . But increased plasticity invites greater interaction of stimuli and gives more even chances for conflicting impulses. (d) Finally in all animals behavior is modi- fied through previous experience, just as structure is also. Where several responses to a FACTS AND FACTORS OF DEVELOPMENT 17 stimulus are possible and where experience has taught that one response is more satisfactory than another, action may be limited to this particular response, not by external compul- sion but by the internal impulse of experience and intelligence. This is what we know as conscious choice or will. Whitman says: Choice runs on blindly at first and ceases to be blind only in proportion as the animal learns through nature’s system of compulsory education. The tele- ological alternatives are organically provided; one is taken and fails to give satisfaction, another is tried and gives contentment. This little freedom is the dawning grace of a new dispensation, in which educa- tion by experience comes in as an amelioration of the law of elimination. . . . Intelligence implies varying degrees of freedom of choice, but never complete emancipation from automatism. Freedom of action does not mean action without stimuli, but rather the introduction of the results of experience and intelligence as additional stimuli. ‘The activities which in lower animals are “cabined, cribbed, confined,” reach in man their fullest and freest expres- sion; but the enormous difference between the relatively fixed behavior of a protozoan or a 78 HEREDITY AND ENVIRONMENT germ cell and the relatively free activities of a mature man is bridged not only in the proc- ess of evolution, but also in the course of indi- vidual development. 6. Consciousness.—The most complex of all psychic phenomena, indeed the one which includes many if not all of the others, is con- sciousness. Like every other psychic process this has undergone development in each of us; we not only came out of a state of unconscious- ness, but through several years we were grad- ually acquiring consciousness by a process of development. Whether consciousness is the sum of all the psychic faculties, or is a new product dependent upon the interaction of the other faculties, it must pass through many stages in the course of its development, stages which would commonly be counted as uncon- scious or subconscious states, and complete consciousness must depend upon the complete development and activity of the other facul- ties, particularly associative memory and in- telligence. ‘The question is sometimes asked whether germ cells, and indeed all living things, may not be conscious in some vague FACTS AND FACTORS OF DEVELOPMENT 79 manner. One might as well ask whether water is present in hydrogen and oxygen. Doubtless the elements out of which consciousness de- velops are present in the germ cells, in the same sense that the elements of the other psychic processes or of the organs of the body are there present; not as a miniature of the adult condition, but rather in the form of ele- ments or factors, which by a long series of combinations and transformations, due to in- teractions with one another and with the en- vironment, give rise to the fully developed condition. Finally there seems good reason for believ- ing that the continuity of consciousness, the continuing sense of identity, is associated with the continuity of organization, for in spite of frequent changes of the materials of which we are composed our sense of identity remains undisturbed. However, the contin- uity of protoplasmic and cellular organization generally remains undisturbed throughout life, and the continuity of consciousness is asso- ciated with this continuity of organization, especially in certain parts of the brain. It is 80 HEREDITY AND ENVIRONMENT an interesting fact that in man, and in several other animals which may be assumed to have a sense of identity, the nerve cells, especially those of the brain, cease dividing at an early age, and these identical cells persist through- out the remainder of life. If nerve cells con- tinued to divide throughout life, as epithelial cells do, there would be no such persistence of identical cells, and one is free to speculate that i such cases there would be no persis- tence of the sense of identity. Organization includes both structure and function, and continuity of organization im- plies not only persistence of protoplasmic and cellular structures but also persistence of the functions of sensitivity, reflexes, memory, in- stincts, intelligence, and will; the continuity of consciousness is associated with the continuity of these activities, as well as with the struc- tures of the body in general and of the brain in particular. It is well known that things which interrupt or destroy these functions or struc- tures interrupt or destroy consciousness. Lack of oxygen, anesthetics, normal sleep cause in some way a temporary interruption FACTS AND FACTORS OF DEVELOPMENT 81 of these functions and consequently tempo- rary loss of consciousness; while certain in- juries or diseases of the brain which bring about the destruction of certain centers or as- sociation tracts may cause permanent loss of consciousness. The development of all of these psychical faculties runs parallel with the development of bodily structures and apparently the method of development in the two cases is similar, viz., progressive differentiation of complex and specialized structures and func- tions from relatively simple and generalized beginnings. Indeed the entire organism, structure and function, body and mind, is a unity, and the only justification for dealing with these constituents of the organism as if they were separate entities, whether they be regarded in their adult condition or in the course of their development, is to be found in the increased convenience and effectiveness of such separate treatment. Development, like many other vital phe- nomena, may be considered from several dif- ferent points of view, such as (1) physico- 82 HEREDITY AND ENVIRONMENT chemical events involved, (2) physiological processes, (3) morphological features, (4) ecological correlations and adaptations, (5) psychological phenomena, (6) social and moral characteristics. All of these phases of development are correlated, indeed they are parts of one general process, and a complete account of this process must include them all. General considerations may lead us to the be- lief that each of the succeeding aspects of de- velopment named above may be causally ex- plained in terms of the preceding ones, and hence all be reducible to physics and chemistry. But this is not now demonstrable and may not be true. Function and structure may be related causally, or they may be two aspects of one substance. ‘The same is true of body and mind or of matter and energy. But even if each of these different phases in the develop- ment of personality may not be causally ex- plained by the preceding ones, at least the principle of explanation employed for any aspect of development ought to be consistent — and harmonious with that employed for any other aspect. FACTS AND FACTORS OF DEVELOPMENT 83 The phenomena of mental development in man and other animals may be summarized as follows: DEVELOPMENT OF PSYCHICAL PROCESSES IN ONTO- GENY AND PHYLOGENY Att Livine Tunes, Iy- CLUDING GERM CELLS AND Empryos, SHow: 1. Differential Sensitivity — Different Responses to Stimuli differing in Kind or Quantity. 2. Tropisms, Refler Mo- tions = Relatively Simple, Me- chanical Responses. _ 3. Organic Memory= Results of Previous Ex- perience registered in General Protoplasm. 4. Adaptive Responses = Results of Elimination of Useless Responses through Trial and Error. 5. Varied Responses Dependent upon Conflict- ing Stimuli and Physi- ological States. 6. Identity = Continuity of Individual Organization. 2. Instincts Marure Forms or HicGHEer ANIMALS SHOW: 1. Special Senses and Sen- sations = Sensations are the Ele- ments of Mind. (Inherited), Habits (Acquired) = Complex Reflexes, involv- ing Nerve Centers. 3. Associative Memory = Results of Experience registered in Nerve Cen- ters and _ Association Tracts. 4, Intelligence, Reason — Results of Trial and Er- ror plus’ Associative Memory, i. e. Experi- ence. 5. Inhibition, Choice, Will Dependent upon Associa- tive Memory, Intelli- gence, Reason. 6. Consciousness — Continuity of Memory, Intelligence, Reason, Will. 84, HEREDITY AND ENVIRONMENT B. FACTORS OF DEVELOPMENT These are some of the facts of development, —a very incomplete résumé of some of the stages through which a human being passes in the course of his development from the germ. What are the factors of development? By what processes is it possible to derive from a relatively simple germ cell the complexities of an adult animal? How can mind and con- sciousness develop out of the relatively simple psychical elements of the germ? ‘These are some of the great problems of development— the greatest and most far-reaching theme which has ever occupied the minds of men. 1. Preformation.—When the mind is once lost in the mystery of this ever-recurring mira- cle it is not surprising to find that there have been those who have refused to believe it possible and who have practically denied de- velopment altogether. The old doctrine of “evolution,” as it was called by the scientists of the eighteenth century, or of preformation as we know it to-day, held that all the organs or parts of the adult were present in the germ in FACTS AND FACTORS OF DEVELOPMENT 85 a minute and transparent condition as the leaves and stem are present in a bud, or as the shoot and root of the little plant are present in the seed.* In the case of animals it was generally impossible to see the parts of the future animal in the germ, but this was sup- posed to be due to the smaller size of the parts and to their greater transparency, and with poor microscopes and good imagination some observers thought they could see the little ani- mals in the egg or sperm, and even the little man, or “homunculus,” was described and figured as folded up in one or the other of the sex cells. This doctrine of preformation was not only an attempt to solve the mystery of develop- ment, but it was also an attempt to avoid the theological difficulties supposed to be involved in the view that individuals are produced by a process of gradual development rather than by supernatural creation. If every individual * The little plant in the seed is itself the product of the de- velopment of a single cell, the ovum, in which no trace of a plant is present, but of course this fact was not known until after careful microscopical studies had been made of the earliest stages of development. 86 HEREDITY AND ENVIRONMENT of the race existed within the germ cells of the first parents, then in the creation of the first parents the entire race with its millions of in- dividuals was created at once. ‘Thus arose the theory of “emboitement,”’ or infinite encase- ment, the absurdities of which contributed to the downfall of the entire doctrine of prefor- mation, which, in the form given it by many naturalists of the eighteenth century, 1s now only a curiosity of biological literature. 2. E'pigenesis—As opposed to this doctrine of preformation, which was founded largely on speculation, arose the theory of epigenesis, which was in its main features founded upon the direct observation of development, and which maintained that the germ contains none of the adult parts, but that it is absolutely simple and undifferentiated, and that from these simple beginnings the individual grad- ually becomes complex by a process of differ- entiation. We owe the theory of epigenesis, at least so far as its main features are con- cerned, to William Harvey, the discoverer of the circulation of the blood, and to Caspar Friedrich Wolff, whose doctoral thesis, pub- FACTS AND FACTORS OF DEVELOPMENT 87 lished in 1759 and entitled “T'heoria Genera- tionis,’ marked the beginning of a great epoch in the study of development. Wolff demon- strated that adult parts are not present in the germ, either in animals or in plants, but that these parts gradually appear in the process of development. He held, erroneously, that the germ is absolutely simple, homogeneous and undifferentiated, and that differentiation and organization gradually appear in this undiffer- entiated substance. How to get differentia- tions out of non-differentiated material, heterogeneity out of homogeneity, was the great problem which confronted Wolff and his followers, and they were compelled to assume some extrinsic or environmental force, some vis formativa or spiritus rector, which could set in motion and direct the process of development. The doctrine of preformation, by locating - in the germ all the parts which would ever arise from it, practically denied development altogether; epigenesis recognized the fact of development, but attributed it to mysterious and purely hypothetical external forces; the 88 HEREDITY AND ENVIRONMENT one placed all emphasis upon the germ and its structures, the other upon outside forces and conditions. 3. E’ndogenesis and Epigenesis—Modern students of development recognize that neither of these extreme views is true—adult parts are not present in the germ, nor is the latter homogeneous—but there are in germ cells many different structures and functions which are, however, very unlike those of the adult, and by the transformation and differentiation of this germinal organization the complicated organization of the adult arises. Development is not the unfolding of an infolded organism, nor the mere sorting of materials already pres- ent in the germ cells, though this does take place, but rather it consists in the formation of new materials and qualities, of new struc- tures and functions—by the combination and interaction of the germinal elements present in the oosperm. In similar manner the combina- tion and interaction of chemical elements yield new substances and qualities which are not to be observed in the elements themselves. Such new substances and qualities, whether in the FACTS AND FACTORS OF DEVELOPMENT — 89 organic or in the inorganic world, do not arise by the gradual unfolding of what was present from the beginning, but they are produced by a process of “creative synthesis.” Modern studies of germ cells have shown that they are much more complex than was formerly believed to be the case; they may even contain different ‘“organ-forming sub- stances” which in the course of development give rise to particular organs; these substances may be so placed in the egg as to foreshadow the polarity, symmetry and pattern of the embryo, but even the most highly organized egg is relatively simple as compared with the animal into which it ultimately develops. In- creasing complexity, which is the essence of development, is caused by the combination and interaction of germinal substances under the influence of the environment. The organiza- tion of the oosperm may be compared to the arrangement of tubes and flasks in a compli- cated chemical operation; they stand in a defi- nite relation to one another and each contains specific substances. The final result of the operation depends not merely upon the sub- 90 HEREDITY AND ENVIRONMENT stances used, nor merely upon the way in which the apparatus is set up, but upon both of these things, as well as upon the environmental con- ditions represented by temperature, pressure, moisture or other extrinsic factors. 4. Heredity and Environment.—Unques- tionably the factors or causes of development are to be found not merely in the germ but also in the environment, not only in intrinsic but also in extrinsic forces; but it is equally certain that the directing and guiding factors of development are in the main intrinsic, and are present in the organization of the germ cells, while the environmental factors exercise chiefly a stimulating, inhibiting or modifying influence on development. In the same dish and under similar environmental conditions, one egg will develop into a worm, another into a sea urchin, another into a fish, and it is cer- tain that the different fate of each egg is de- termined by conditions intrinsic in the egg itself, rather than by environmental conditions. We should look upon the germ as a living thing, and upon development as one of its functions. Just as the character of any func- FACTS AND FACTORS OF DEVELOPMENT 91 tion is determined by the organism, though it may be modified by environment, so the char- acter of development is determined by hered- ity, 7. e., by the organization of the germ cells, though the course and results of develop- ment may be modified by environmental conditions. SUMMARY In conclusion, we have briefly reviewed in this lecture the well known fact that every liv- ing thing in the world has come into existence by a process of development; that the entire human personality, mind as well as body, has thus arisen; and that the factors of develop- ment may be classified as intrinsic in the or- ganization of the germ cells, and extrinsic as represented in environmental forces and con- ditions. ‘The intrinsic factors are those which are commonly called heredity, and they direct and guide development in the main; the ex- trinsic or environmental factors furnish the conditions in which development takes place and modify, more or less, its course. CHAPTER II THE CELLULAR BASIS OF HERED- ITY AND DEVELOPMENT CHAPTER II THE CELLULAR BASIS OF HEREDITY AND DEVELOPMENT A. INTRODUCTORY Heredity is to-day the central problem of biology. This problem may be approached from many sides, that of the observer, the statistician, the practical breeder, the experi- menter, the embryologist, the cytologist; but these different aspects of the subject may be reduced to three general methods of study, (1) the observational and statistical, (2) the experimental, (3) the cytological and embry- ological. Before taking up these different as- pects of heredity it is important that we should have clear definitions of the terms employed and a fairly accurate conception of the pro- cesses involved. 96 HEREDITY AND ENVIRONMENT 1. Definitions —Heredity originally meant the transmission of property from parents to children, and in the field of biology it has been defined erroneously as “the transmission of qualities or characteristics, mental or physical, from parents to offspring”. ‘The colloquial meaning of the word has led to much confusion in biology, for it carries with it the idea of the transmission from one generation to the next of ownership in property. A son may inherit a house from his father and a farm from his mother, the house and farm remaining the same though the ownership has passed from par- ents to son. And when it is said that a son in- herits his stature from his father and _ his complexion from his mother, the stature and complexion are usually thought of only in their developed condition, while the great fact of de- velopment is temporarily forgotten. Of course there are no “qualities” or “characteristics” which are “transmitted” as such from one gen- eration to the next. Such terms are not with- out fault when used merely as figures of speech, but when interpreted literally, as they frequently are, they are altogether misleading; THE CELLULAR BASIS 97 they are the result of reasoning about names rather than facts, of getting far from phen- omena and philosophizing about them. The comparison of heredity to the transmission of property from parents to children has pro- duced confusion in the scientific as well as in the popular mind. It is only necessary to re- call the most elementary facts about develop- ment to recognize that in a literal sense developed characteristics of parents are never transmitted to children. 2. The Transmission Hypothesis—And yet the idea that the characteristics of adult persons are transmitted from one generation to the next is a very ancient one and was uni- versally held until the most recent times. Be- fore the details of development were known it was natural to suppose, as Hippocrates did, that white-flowered plants gave rise to white- flowered seeds and that blue-eyed parents pro- duced blue-eyed germs, without attempting to define what was meant by white-flowered seeds or blue-eyed germs. And even after the facts of development were fairly well known it was generally held that the germ cells were made 98 HEREDITY AND ENVIRONMENT by the adult animal or plant and that the characteristics of the adult were m some way carried over to the germ cells; but the manner in which this supposed transmission took place remained undefined until Darwin attempted to explain it by his “provisional hypothesis of pangenesis.” Darwin assumed that minute particles or “gemmules’” were given off by every cell of the body, at every stage of development, and that these gem- mules then collected in the germ cells which thus became storehouses of little germs from all parts of the body. Afterward, in the devel- opment of the embryo, the gemmules, or little germs, developed into cells and organs similar to those from which they originally came. 3. Germinal Continuity and Somatic Dis- continuity—Many ingenious hypotheses have been devised to explain things which are not real, and this is one of them. ‘The doctrine that adult organisms manufacture germ cells and transmit their characters to them is now known to be erroneous. Neither germ cells nor any other kind of cells are formed by the body as a whole, but every cell in the body THE CELLULAR BASIS 99 comes from a preceding cell by a process of di- vision, and germ cells are formed, not by con- tributions from all parts of the body, but by division of preceding cells which are derived ultimately from the fertilized egg (Fig. 22). The hen does not produce the egg, but the egg produces the hen and also other eggs. Indi- vidual traits are not transmitted from the hen to the egg, but they develop out of germinal factors which are carried along from cell to cell, and from generation to generation. There is a continuity of germinal substance, and usually of germinal cells, from one gener- ation to the next. In some animals the germ cells are set apart at a very early stage of de- velopment, sometimes in the early cleavage stages of the egg. In other cases the germ cells are first recognizable at later stages, but in practically every case they arise from germinal or embryonic cells which have not differentiated into somatic tissues. In gen- eral then germ cells do not come from dif- ferentiated body cells, but only from un- differentiated germinal cells, and if in a few doubtful cases differentiated cells may reverse 100 HEREDITY AND ENVIRONMENT the process of development and become embry- onic cells and even germ cells it does not destroy this general principle of germinal con- Gametes N@ 2 ND Zygote Ge Fic. 22. Dtacram SHow- ING THE “CeLL LINEAGE” OF THE Bopy CrLts aND GERM Cetts In A Worm ork Mot- LusK. The lineage of the germ cells (“germ track”) is shown in black, of ecto- derm in white, and of endo- derm and mesoderm in shaded circles. The whole course of spermatogenesis and oogenesis is shown in the lower right of the figure beginning with the primitive sex cells (Prim. Sex Cells) and ending with the game- tes, the genesis of the sper- matozoa being shown on the rents 3 left and that of the ova on the right. THE CELLULAR BASIS 101 tinuity and somatic discontinuity of successive generations. Thus the problem which faces the student of heredity and development has been cut in two; he no longer inquires how the body pro- duces the germ cells, for this does not happen, but merely how the latter produce the body and other germ cells. The germ is the unde- veloped organism which forms the bond be- tween successive generations; the body is the developed organism which arises from the germ (under) the influence of environmental conditions. ‘The body develops and dies in each generation; the germ-plasm is the contin- uous stream of living substance which con- nects all generations. ‘The body nourishes and protects the germ; it is the carrier of the germ-plasm, the mortal trustee of an immortal] substance. This contrast between the germ and the body, between the undeveloped and the devel- oped organism, is fundamental in all modern studies of heredity. It was especially empha- sized by Weismann in his germ-plasm theory and recently it has been made prominent by 102 HEREDITY AND ENVIRONMENT Johannsen under the terms “genotype” and “phenotype”; the genotype is the fundamental hereditary constitution of an organism, it is the germinal type; the phenotype is the de- veloped organism with all of its visible char- acters, it is the somatic type. But important as this distinction is between germ and soma it has sometimes been over- emphasized. This is one of the chief faults of Weismann’s theory. The germ and the soma are generically alike, but specifically different. Both germ cells and somatic cells have come from the same oosperm, but have differentiated in different ways; the tissue cells have lost certain things which the germ cells retain and have developed other things which remain un- developed in the germ cells. But the germ cells do not remain undifferentiated; both egg and sperm are differentiated, the former for receiving the sperm and for the nourishment of the embryo, the latter for locomotion and for penetration into the egg. But while the differentiations of tissue cells are usually irre- versible, so that they do not again become germinal cells, the differentiations of the sex THE CELLULAR BASIS 103 cells are reversible, so that these cells, after their union, again become germinal cells.* In many theories of heredity it is assumed that there is a specific “inheritance material,” distinct from the general protoplasm, the function of which is the “transmission” of her- editary properties from generation to genera- tion, and the chief characteristics of which are independence of the general protoplasm, con- tinuity from generation to generation and ex- treme stability in organization. This is the idioplasm of Nageli, the germ-plasm of Weis- mann. But there is no reason to suppose that germ-plasm is anything other than germinal protoplasm, which is found in all cells in the earliest stages of development but which be- comes limited in quantity or altered in quality in tissue cells. A germ-plasm which is abso- lutely distinct from and independent of the general protoplasm is a mere fiction which finds no justification in reality. 4. The Units of Living Matter—The en- tire cell, nucleus and cytoplasm, is the small- est unit of living matter which is capable of independent existence. Neither the nucleus *See pp. 183-184 for a more complete statement. 104 HEREDITY AND ENVIRONMENT nor the cytoplasm can for long live independ- ently of each other, but the entire cell can perform all the fundamental vital processes. It transforms food into its own living material, it grows and divides, it is capable of respond- ing to many kinds of stimuli. But while the parts of a cell are not capable of independent existence they may perform certain of these vital processes. Not only is the cell as a whole capable of assimilation, growth and division, but every living part of the cell has this power. The nucleus builds foreign substances into its own substance, and after it has grown to a certain size it divides into two; the cytoplasm does the same, and this process of assimilation, growth 3 and division occurs in many parts of the nu- cleus and cytoplasm, such as the chromosomes, chromomeres, centrosomes, ete. In all cases cells come from cells, nuclei from nuclei, chromosomes from chromosomes, centrosomes from centrosomes, etc. Indeed, the manner in which all living mat- ter grows indicates that every minute particle of protoplasm has this power of taking in food THE CELLULAR BASIS 105 substance and of dividing into two particles when it has grown to maximum size. Pre- sumably this power of assimilation, growth and division is possessed by particles of protoplasm which are invisible with the highest powers of our microscopes, though it is probable that these particles are much larger than the largest molecules known to chemistry. The smallest particle which can be seen with the most powerful microscope in ordinary light is about 250 wu (millionths of a millimeter) in diameter. The largest molecules are probably about 10 win diameter. Between these in- visible molecules and the just visible particles of protoplasm there may be other units of or- ganization. These hypothetical particles of protoplasm have been supposed by many authors to be the ultimate units of assimila- tion, growth and division, and in so far as these units are supposed to be the differential causes of hereditary characters, they are known as in- heritance units. It is assumed in practically all theories of heredity that the “inheritance material,” or the germinal protoplasm, is composed of 106 HEREDITY AND ENVIRONMENT ultra-microscopial inheritance units which have the power of individual growth and di- vision and which are capable of undergoing many combinations and dissociations during the course of development, by which combina- tions and dissociations they are transformed into the structures of the adult. Various names have been given to these units by differ- ent authors; they are the “physiological units” of Herbert Spencer, the ““gemmules” of Dar- win, the “plastidules” of Elsberg and Haeckel, the “pangenes” of de Vries, the “plasomes”’ of Wiesner, the “idioblasts” of Hertwig, the “bio- phores” and “determinants” of Weismann. With the publication of Weismann’s work on the germ-plasm in 1892 speculation with regard to these ultra-microscopic units of life and of heredity reached a climax and began to decline, owing to the highly speculative character of the evidence as to the existence, nature and activities of such units. But with the rediscovery of Mendel’s principles of heredity the necessity of assuming the exist- ence of inheritance units of some kind once more became evident, and, without being able | THE CELLULAR BASIS 107 to define just what such units are or just how they behave, modern students of heredity as- sume their existence. They are now called determiners or factors or genes, and they are usually thought of as units in the germ cells which condition the characters of the devel- oped organism, and which are in a measure in- dependent of one another; though of course neither they nor any other parts of a cell are really mdependent in the sense that they can exist apart from one another. They are to be thought of as analogous to chemical radicals which are never independent but exist only in combination with other chemical elements in the form of molecules, and yet preserve their identity in many different combinations. If there are inheritance units, such as de- terminers or genes, as practically all students of heredity maintain, they must be contained in the germ cells, and it becomes one of the fundamental problems of biology to find out where and what these units are. But whether we assume the existence of these units or not we know that the germ cells are exceedingly complex, that they contain many visible units 108 HEREDITY AND ENVIRONMENT such as chromosomes, chromomeres, plasto- somes and microsomes, and that with every great improvement in the microscope and in microscopical technique other structures are made visible which were invisible before, and whether the particular hypothetical units just named are invisible or not seems to be a matter of no great importance, seeing that, so far as the analysis: of the microscope is able to go, there are in all protoplasm differentiated units which are combined into a system; in short, there is organization. 5. Heredity and Development.—The germ cells are individual organisms and after the fertilization of the egg the new individual thus formed remains distinct from every other one. Furthermore, from its earliest to its latest stage of development it is one and the same organism; the egg is not one being and the embryo another and the adult a third, but the egg of a human being is a human being in the one-celled stage of development, and the char- acteristics of the adult develop out of the egg and are not in some mysterious way grafted upon it or transmitted to it. _ THE CELLULAR BASIS 109 Parents do not transmit their characters to their offspring, but their germ cells in the course of long development give rise to adult _ characters similar to those of the parents. The thing which persists more or less completely from generation to generation is the organiza- tion of the germ cells which differentiate in similar ways in successive generations if the ex- trinsic factors of development remain similar. In short, heredity may be defined as the particular germinal organization which 1s transmitted from parents to offspring. Herit- age is the sum of all those qualities which are determined or caused by this germinal or- gamzation. Development is progressive and coordinated differentiation of this germinal or- gamzation, by which it is transformed into the adult organization. Differentiation is the for- mation and localization of many different kinds of substances out of the germinal substance, of many different structures and functions out of the relatively simple structures and functions of the oosperm. This germinal organization influences not merely adult characters but also the character 110 HEREDITY AND ENVIRONMENT of every stage from the egg to the adult con- dition. For every inherited character, whether embryonic or adult, there is some germinal basis. We receive from our parents germ cells. of a particular kind and constitution, and under given conditions of environment these cells undergo regular transformations and differentiations in the course of development, which differentiations lead to particular adult characteristics. In the last analysis the causes of heredity and development are problems of cell structures and functions, problems of the formation of particular kinds of germ cells, of the fusion of these cells in fertilization, and of the subsequent formation of the various types of somatic cells from the fertilized egg cell. B. THE GERM CELLS Observations and experiments on developed animals and plants have furnished us with a knowledge of the finished products of inheri- tance, but the actual stages and causes of in- heritance, the real mechanisms of heredity, are to be found only in a study of the germ cells and their development. Although many THE CELLULAR BASIS 111 phenomena of inheritance have been dis- covered in the absence of any definite knowl- edge of the mechanism of heredity, a scientific explanation of these phenomena must wait upon the knowledge of their causes. In the absence of such knowledge it has been neces- sary to formulate theories of heredity to ac- count for the facts, but these theories are only temporary scaffolding to bridge the gaps in our knowledge, and if we knew all that could be known about the germ cells and their de- velopment we should have little need for theories. In the first lecture we looked at the germ cells and their development from the outside, as it were; let us now look inside these cells and study their minuter structures and functions. : Only a beginning has been made in this minute study of the germ cells and of their transformation into the developed animal, and it seems probable that it may engage the attention of many future generations of bi- ologists, but nevertheless we have come far since that day, only about thirty-five years ago, when Oscar Hertwig first saw the ap- _18t Mat. Sp.“ 7 ~.y “a All fe) Fic. 23—For Description see page 113 THE CELLULAR BASIS 113 proach and union of the egg and sperm nuclei within the fertilized egg. Indeed so rapid has been the advance of knowledge in this field that many of the pioneers in this work are still active in research. 1. Fertilization—The development of the individual may be said to begin with the ferti- lization of the egg, though it is evident that both egg and sperm must have had a more re- mote beginning, and that they also have un- dergone a process of development by which their peculiar characteristics of structure and function have arisen,—a subject to which we shall return later. But the developmental processes which lead to the formation of fully developed ova and spermatozoa come to a full stop before fertilization and they do not usu- Fic. 23. DiackaAMs ofr THE MATuRATION AND FERTILIZATION OF THE Ecc or a MottuusxK (Crepidula). A, B. First matura- tion division (1st Mat. Sp.) C. Second maturation division (2d Mat. Sp.) and first polar body (1st PB) resulting from first division. QIN, sperm nucleus, fC, sperm centrosome. D. Approach of sperm nucleus (gN) and sphere (dS) to egg nucleus (?91V) and sphere (9S); the second polar body (2d PB) has been formed and the first has divided (Js¢ PB). E. Meeting of egg and sperm nuclei and origin of cleavage centrosomes. HF. First cleavage of egg showing direction of currents in the cell. 114 HEREDITY AND ENVIRONMENT | ally begin again until a spermatozoon has entered an ovum, or until the latter has been stimulated by some other outside means. In some animals and plants, eggs may develop regularly without fertilization, the stimulus to development being supplied by certain ex- ternal or internal conditions; in other cases, as Loeb discovered, eggs which would never develop if left to themselves may be experi- mentally stimulated by physical or chemical changes in the environment, so that they un- dergo regular development. ‘The development of an egg without previous fertilization is known as parthenogenesis or virgin reproduc- tion; if it occurs in nature it is natural par- thenogenesis, if in experiments it is artificial ' parthenogenesis. Natural parthenogenesis is relatively rare and in the vast majority of animals and plants the egg does not begin to develop until a spermatozoon has entered it. But the spermatozoon not only stimulates the egg to develop, as environmental condi- tions may also do, but it carries into the egg living substances which are of great signifi- THE CELLULAR BASIS 115 cance in heredity. Usually only the head of the spermatozoon enters the egg (Figs. 3, 4) and this consists almost entirely of nuclear material which has a strong chemical affinity for certain dyes, and hence is called chromatin (Fig. 4 D-H, 23 A-B) ; when the egg has ma- tured and is ready to be fertilized its nucleus also consists of a small mass of chromatin (Fig. 23 C). Both of these condensed chro- matic nuclei then grow in size and become less chromatic by absorbing from the egg a sub- stance which is not easily stained by dyes and hence is called achromatin (Figs. 4 I-L, 23 D-E). The chromatin then becomes scattered through each nucleus in the form of granules or threads which are embedded in the achro- matin; this is the condition of a typical “rest- ing” nucleus. The spermatozoon also brings into the egg a centrosome or division center, around which an aster appears consisting of radiating lines in the protoplasm of the egg (Fig. 4 F-D). The moment that the spermatozoon touches the surface of the egg the latter throws out at 116 HEREDITY AND ENVIRONMENT the pomt touched a prominence, or reception cone (Fig. 4 A-E’), and as soon as the head of the sperm has entered this cone some of the superficial protoplasm of the egg flows to this point and then turns into the interior of the egg in a kind of vortex current. Probably as" a result of this current the sperm nucleus and centrosome are carried deeper into the egg and finally are brought near to the egg nucleus (Fig. 23 D and FE). In the movements of egg and sperm nuclei toward each other it is probable that they are passively carried about by currents in the cytoplasm; the entrance of the sperm serves as a stimulus to the egg cyto- plasm which moves according to its preestab- lished organization. 2. Cleavage and Diff erentiation—When the sperm nucleus has come close to the egg nucleus the sperm centrosome usually divides into two minute granules, the daughter centro- somes, which move apart forming a spindle with the centrosomes at its poles and with astral radiations running out from these into the cytoplasm (Figs. 3, 4,23 F'). At the same time the chromatin granules and threads in the _THE CELLULAR BASIS 117 ego and sperm nuclei run together into a smooth thick thread, the spireme, which is coiled within the nucleus. At this stage it is sometimes possible to see that the spireme is composed of a series of granules, like beads on a string; these granules are the chromomeres (Fig. 4). The spireme then breaks up into a number of pieces in the form of short threads or rods (Fig. 24 C and D); these are the chromosomes. ‘The number of these chromo- somes is constant for every species and race, though the number may vary in different species. In the thread worm, Ascaris mega- locephala, there are usually two chromosomes in the egg nucleus and two in the sperm nu- cleus (Fig. 24 D). In the gastropod, Crepid- ula, there are about thirty chromosomes in each germ nucleus and sixty in the two. Then the spindle and asters grow larger and the nuclear membrane grows thinner and finally disappears altogether, leaving the chro- mosomes in the equator of the spindle (Figs. 5A, 23 F',24 HK and F'). Each of the chromo- somes then splits lengthwise into two equal parts, and in the splitting of the chromosomes E Fic. 24. FeErrmizarion or THE Ecc or THE NEMATODE Worm Ascaris megalocephala; QN, egg nucleus; GN, sperm nucleus; Arch, archiplasm; C, centrosome; 4A, B, approach of germ nuclei; C, D, formation of two chromosomes in each germ nucleus; H, F, stages in the division of the chromosomes which are split in # and are separating in F; only three chromosome pairs are shown in F. (From Wilson after Boveri.) THE CELLULAR BASIS 119 it is sometimes possible to see that each bead- like chromomere divides through its middle. The daughter chromosomes then separate and move to opposite poles of the spindle, where they form the daughter nuclei, and at the same time the cell body begins to divide by a con- striction which pinches the cell in two in the plane which passes through the equator of the spindle (Figs. 5, 24 F', 26 B). Finally the daughter nuclei grow in size by the absorp- tion of achromatin from the cell body and the substance of the chromosomes is again scat- tered through the achromatin in the form of threads and granules and thus the daughter nuclei come back to a “resting” stage similar to that with which the division began, thus completing the “division cycle” of the cell. During the whole division cycle it is possi- ble in a few instances to distinguish the chro- mosomes of the egg from those of the sperm, and in every instance where this can be done it is perfectly clear that these chromosomes do not fuse together nor lose their identity, but that every chromosome splits lengthwise and its halves separate and go into the two daugh- 120 HEREDITY AND ENVIRONMENT Fic. 25. MaruratioN AND FERTILIZATION OF THE Eco oF tHE Movse. 4, first polar body and second maturation spin- dle; B, second polar body and maturation spindle; C, entrance of the spermatozoon into the egg; D-G, successive stages in the approach of egg and sperm nuclei; H, formation of chromosomes in each germ nucleus; J, first cleavage spindle showing chromosomes from egg and sperm on opposite sides of spindle. (After Sobotta.) THE CELLULAR BASIS 121 ter cells where they form the daughter nuclei. Each of these cells therefore receives half of its chromosomes from the egg and half from the sperm. Even in cases where the individual chromosomes are lost to view in the daughter nuclei those nuclei may sometimes be clearly double, one-half of each having come from the egg chromosomes and the other half from the sperm chromosomes (Fig. 26). At every subsequent cleavage of the egg the chromosomes divide in exactly the same way as has been described for the first cleav- age. Every cell of the developing animal re- ceives one-half of its chromosomes from the egg and the other half from the sperm, and if the chromosomes of the egg differ in shape or in size from those of the sperm, as is some- times the case when different races or species are crossed, these two groups of chromosomes may still be distinguished at advanced stages — of development. Where the egg and sperm chromosomes are not thus distinguishable it may still be possible to recognize the half of the nucleus which comes from the egg and the half which comes from the sperm even up to 122 HEREDITY AND ENVIRONMENT Fic. 26. Successive Sraces 1N THE CLEAVAGE OF THE Eco or A Moxtusk (Crepidula), showing the separateness of the male and female chromosomes (¢ ch, 2 ch) and of the male and female halves of each nucleus (@N, 9IV). THE CELLULAR BASIS 123 an advanced stage of the cleavage (Fig. 26). - At the same time that the maternal and paternal chromosomes are being distributed with such precise equality to all the cells of the developing organism the different sub- stances in the cell body outside of the nucleus” may be distributed very unequally to the cleav- age cells. The movements of the cytoplasm of the egg, which began with the flowing of the surface layer to the point of entrance of the sperm, lead to the segregation of different kinds of plasms in different parts of the egg and to the unequal distribution of these sub- stances to different cells (Figs. 9, 27, 28). One of the most striking cases of this is found in the ascidian, Styela, in which there . are four or five substances in the egg which dif- fer in color, so that their distribution to differ- ent regions of the egg and to different cleavage cells may be easily followed, and even photo- graphed, while in the living condition. The peripheral layer of protoplasm is yellow and it gathers at the lower pole of the egg, where the sperm enters, forming a yellow cap (Fig. 27, 1). This yellow substance then moves, 124 HEREDITY AND ENVIRONMENT following the sperm nucleus, up to the equator of the egg on the posterior side and there forms a yellow crescent extending around the posterior side of the egg just below the equator (Fig. 27, 3). On the anterior side of the egg a gray crescent is formed in a somewhat simi- lar manner and at the lower pole between these two crescents is a slate blue substance, while at the upper pole is an area of colorless proto- plasm. ‘The yellow crescent goes into cleavage cells which become muscle and mesoderm, the gray crescent into cells which become nervous system and notochord, the slate blue substance into endoderm cells and the colorless sub- stance into ectoderm cells. (See also Figs. 9 and 10). Thus within a few minutes after the ferti- lization of the egg, and before or immediately after the first cleavage, the anterior and pos- terior, dorsal and ventral, right and left poles are clearly distinguishable, and the substances which will give rise to ectoderm, endoderm, mesoderm, muscles, notochord and nervous system are plainly visible in their character- istic positions. THE CELLULAR BASIS 125 At the first cleavage of the egg each of these substances is divided into right and left halves (Fig. 27,5). The second cleavage cuts off two anterior cells containing the gray crescent from two posterior ones containing the yellow crescent (Fig. 27, 6 and Fig. 28, 1). The third cleavage separates the colorless proto- plasm in the upper hemisphere from the slate blue in the lower (Fig. 28, 2). And at every successive cleavage the cytoplasmic substances are segregated and isolated in particular cells, and in this way the cytoplasm of the differ- ent cells comes to be unlike (Figs. 28 and 29). When once partition walls have been formed between cells the substances in the different cells are permanently separated so that they can no longer commingle. What is true of Styela in this regard is - equally true of many other ascidians, as well as of Amphioxus and of the frog (Figs. 6, 9, 10), though the segregation of substances and the differentiation of cells are not so evident in the last named animals because these sub- stances are not so strikingly colored. Indeed the segregation and isolation of different S843 0.2 {9 Se A -I20,00 Oo) 02-0! 9; Is) ao 3.0:q'0 Fic. 27. Fic. 27. Secrions or THE Ecc or Styela, showing matura- tion, fertilization and early cleavage; 7 P. S., first polar spindle, p.b., polar bodies, GV, sperm nucleus, 9, egg nucleus, p.l., peripheral layer of yellow protoplasm, Cr. crescent of yellow protoplasm, 4, A,, anterior cells, B,, B,, posterior cells of the 4-cell stage. In 1 the sperm nucleus and centrosome are at the lower pole near the point of Fic. 28. entrance; in 2 and 3 they have moved up to the equator on the posterior side of the egg; in 4 the egg and sperm nuclei have come together and the sperm centrosome has divided and formed the cleavage spindle; in 5 the egg is dividing into right and left halves; in 6 it is dividing into anterior and posterior halves. Fic. 28. Crravacr or tHE Ece or Styela, showing distribu- tion of the yellow protoplasm (stippled) and of the clear and gray protoplasm to the various cells, each of which bears a definite letter and number. 128 HEREDITY AND ENVIRONMENT protoplasmic substances in different cleavage cells occurs during the cleavage of the egg in all animals, though such differentiations are much more marked in some cases than in others. This same type of cell division, with equal division of the chromosomes and more or less unequal division of the cell body, continues long after the cleavage stages, imdeed throughout the entire period of embryonic development. Sometimes the division of the cell body is equal, the daughter cells being alike; sometimes it is unequal or differential, but always the division of the chromosomes is equal and non-differential. When once the various tissues have been differentiated the further divisions in these tissue cells are usu- ally non-differential even in the case of the | cell bodies. There can be no doubt that this remarkably complicated process of cell division has some deep significance; why should a nucleus divide in this peculiarly indirect manner instead of merely pinching in two, as was once supposed to be the rule? What is the relation of cell THE CELLULAR BASIS 129 division to embryonic differentiation? In this process of mitosis, or indirect cell division, two important things take place: (1) Each chromosome, chromomere and centrosome is divided exactly into two equal parts so that each daughter structure is at the time of its formation quantitatively one-half the size and qualitatively precisely like its mother struc- ture. (2) Accompanying the formation of radiations, which go out from the centrosomes into the cell body, diffusion currents are set up in the cytoplasm which lead to the localiza- tion of different parts of the cytoplasm in definite regions of the cell, and this cytoplas- mic localization is sometimes of such a sort that one of the daughter cells may contain one kind of cell substance and the other another kind. Thus while mitosis brings about a scrupulously equal division of the elements of the nucleus, it may lead to a very unequal and dissimilar division of the cytoplasm. In this is found the significance of mitosis, and it sug- gests at once that the nucleus contains non-dif- ferentiating material, viz., the idioplasm or germ-plasm, which is characteristic of the race 130 HEREDITY AND ENVIRONMENT Fic. 29. Gasrruta anp Larva or Styela, showing the cell lineage of various organs, and the distribution of the differ- ent kinds of protoplasm to these organs. Muscle cells are shaded by vertical lines, mesenchyme by horizontal lines, ner- yous system and chorda by stipples. THE CELLULAR BASIS 131 and is carried on from cell to cell and from generation to generation; whereas the cell body contains the differentiating substance, the personal plasm or somatoplasm which gives rise to all the differentiations of cells, tissues and organs in the course of ontogeny. Weismann supposed that the mitotic divi- sion of the chromosomes during development was of a differential character, the daughter chromosomes differing from each other at every differential division in some constant and characteristic way, and that these differ- entiations of the chromosomes produced the characteristic differentiations of the cytoplasm which occur during development. But there is not a particle of evidence that the division of chromosomes is ever differential; on the contrary, there is the most complete evidence that their division is always remarkably equal both quantitatively and qualitatively. If daughter chromosomes and nuclei ever become unlike, as they sometimes do, this unlikeness occurs long after division and is probably the result of the action of different kinds of cyto- plasm upon the nuclei, as is true, for example, 132 HEREDITY AND ENVIRONMENT in the differentiation of the chromosomes in the somatic cells as contrasted with the germ cells of Ascaris (Fig. 30). But while the chromosomes invariably divide equally, other portions of the nucleus may not do so. Achromatin and oxychromatin, like the cyto- plasm, may divide unequally and differenti- ally, and this is probably a prime factor in development. | On the other hand, the differential division of the cytoplasm is a regular and character- istic feature of ontogeny; indeed, the segrega- tion and isolation of different kinds of cytoplasm in different cells is the most im- portant function of cell division during de- velopment. Thus we find in the division apparatus of the cell a mechanism for the pres- ervation in unaltered form of the species plasm or germ-plasm of the nucleus, and for the progressive differentiation of the personal plasm or somatoplasm of the cell body. 38. The Origin of the Sex Cells—The sex cells are the latest of all cells of a developing organism to reach maturity, and yet they may be among the earliest to make their appear- Fic. 30. DirFERENTIATION oF GERM CELLS AND SoMAaTIC Certs In THE Ecco or Ascaris. A and B, second cleavage division showing that the chromosomes remain entire in the lower cell, which is in the line of descent of the sex cells (“germ track”), but that they throw off their ends and break up into small granules in the upper cells, which become somatic cells. ,C, 4-cell stage, the nuclei in the upper (somatic) cells being small and the ends of the chromosomes remaining as chromatic masses in the cell body outside of the nuclei, while the nuclei in the lower cells are much larger and con- tain all the chromatin. D, third nuclear division, showing the somatic differentiation of the chromosomes in all the cells except the lower right one, which alone is in the germ track and will ultimately give rise to sex cells. (After Boveri.) 184 HEREDITY AND ENVIRONMENT ance. Every sex cell, like every other type of cell, is a lineal descendant of the fertilized egg (Fig. 22), but the period at which the sex cells become visibly different from other cells varies from the first cleavage of the egg in some species to a relatively advanced a of de- velopment in others. (a) The Division Period. Oogonia and Spermatogonia.—When the primitive sex cells are first distinguishable they differ from other cells only in the fact that they are less differentiated; they have relatively larger nuclei and smaller cell bodies, a condition which is indicative of little differentiation of the cell body since the products of differentia- tion such as fibres, secretions, ete., swell the size of the cell body but do not contribute to the growth of the nucleus. These primitive sex cells or gonia divide repeatedly, but the oogonia grow more rapidly and divide less frequently than the spermatogonia. As a re- sult of this difference in the rate of growth and division the spermatogonia become much smaller and immensely more numerous than the oogonia. ‘This period in the genesis of the THE CELLULAR BASIS 135 sex cells is known as the division period (Fig. 22). | (b) The Growth Period. Oocytes and Spermatocytes——This period of rapid cell di- vision is followed by a period of growth with- out division during which the developing sex cells are called primary oocytes or spermato- cytes. This growth period may be very long in the case of the oocytes, lasting, for example, in the human female from the time of birth to the end of the reproductive period; durmg this long time the oocytes in the ovary prob- ably never divide, there are as many of them at birth as at any later time; during this period | of growth the ovarian egg becomes relatively large, in some animals, e. g. birds, the largest of all cells. The growth period of a sper- matocyte lasts for a briefer time than does that of an oocyte so that the former remains relatively small (Fig. 22). | All of the cell divisions which take place during the division period are of the usual kind, m which every chromosome splits lengthwise into two and the two halves then separate and move to opposite poles of the 136 HEREDITY AND ENVIRONMENT spindle where they break up into threads and granules and form the daughter nuclei, as is shown in Fig. 24. But during the growth period of the oocytes and spermatocytes the Hic. 31. Dirrerent Sraces IN THE DEVELOPMENT OF THE Ecc or THE Rassir. A, at the beginning of the growth period showing slender chromatic threads in the nucleus; B, later stage in which these threads ball up and parallel threads conjugate forming the shorter, thicker thread shown in C; D and E, segmentation of the long thread into chromosomes each of which shows its double nature; F’, later stage in which the distinctness of the chromosomes is temporarily lost. (After Winiwarter.) THE CELLULAR BASIS 137 chromosomes form a closely wound coil of long chromatin threads (Fig. 31), and when these threads uncoil later it is seen that the chromosomes have united in pairs (Figs. 31 D and EH, 32 B, 33 B); this process is known as synapsis, or the conjugation of the chromo- somes, and there is evidence that one member of each synaptic pair is derived from the father, and the other from the mother. ‘The union of these chromosomes is probably not so close that they lose their identity, though there may possibly be some mterchange of substance between them. By this union of the chromosomes into pairs the number of sepa- rate chromosomes is reduced to half the normal number; if there are usually 4 chromo- somes, as in Ascaris, they are reduced to 2 pairs; if 48 chromosomes, as in man, there are 24 of these pairs.* In the conjugation of the chromosomes it is plain that, generally speaking, those chromosomes which are similar in shape and size unite; big chromosomes unite with big ones, little ones with little ones, and those of peculiar shape with others of similar shape * See footnote p. 153. Fic. 32. SPERMATOGENESIS OF A Nematope Worm (Ancyra- canthus). A, chromosomes of sperm mother cell, 11 in num- ber, before their union into pairs. B, early stage of first division; 10 of the chromosomes have united into 5 pairs and each of these has split lengthwise; 1 chromosome remains un- THE CELLULAR BASIS. 139 (Figs. 32 B, 33 B, 34). It is probable that the two members of a pair of conjugating chromo- somes are homologous not merely in shape and size but also in function, though this homology does not amount to identity. In some instances it can be proved that one member of each conjugating pair of chromo- somes comes from one parent and the other from the other parent, and it is probable that this is always true. In every cell of every individual which has developed from a ferti- lized egg there are two full sets of chromo- somes, one of which came from the sperm and the other from the egg; but when this indi- vidual in its turn produces germ cells homolo- gous chromosomes of each set unite in pairs, during the growth period. These synaptic pairs are the bivalent chrom- paired. C, first maturation division after the 5 pairs of chrom- osomes have pulled apart; the unpaired chromosome is going entire to one pole of the spindle. D, two cells resulting from this division, one containing 5 and the other 6 chromosomes. E, four cells resulting from the division of two cells like D, in which every chromosome has split into two so that. two of the cells contain 5 and two contain 6 chromosomes. fF, two of these cells changing into spermatozoa, one containing 5 and the other 6 chromosomes. (After Mulsow.) Bigs os: For description see p. 141. THE CELLULAR BASIS 141 osomes, and in addition to showing the line of junction by which they are united they fre- quently show a longitudinal split through the middle of each chromosome and at right angles to the line of junction. It thus happens that these bivalent chromosomes are frequently four-parted and such four-parted chromo- somes are known as tetrads (Figs. 32 B, 33 B). (c) The Maturation Period—Finally at the close of the growth period both oocyte and spermatocyte undergo two peculiar divisions, one following immediately after the other, which are unlike any other cell divisions. These are known as the first and second mat- Fic. 33. Oocenesis or A NematopE Worm (Ancyracanthus). A, egg mother cell containing 12 chromosomes before their union into pairs. B, early stage of first maturation division; all the chromosomes have united.into 6 pairs, and all but one of these has split in two so that the pairs are really four-parted (tetrads). C, the six tetrads in the first maturation division. D, egg containing 6 chromosomes, after both first and second maturation divisions; the eliminated chromosomes are shown as the polar bodies at the margin of the egg. H and F, eggs after fertilization; the egg nucleus is above and contains 6 chromosomes, the sperm nucleus is below and contains 5 chro- mosomes in one case and 6 in the other; in the former case the egg becomes a male with 11 chromosomes, in the latter a female with 12 chromosomes, (After Mulsow.) 142 HEREDITY AND ENVIRONMENT uration divisions and they are the last divi- sions which take place in the formation of the egg and sperm. In one or the other of these two maturation divisions the pairs of chromo- somes separate along the line of junction, one member of each pair going to one pole of the spindle and the other to the other pole, so that in each of the daughter cells thus formed only a single set of chromosomes is present (Figs. 832 C and D, 35); but since the position of the pairs of chromosomes in the spindle is a matter of chance it rarely happens that all the paternal chromosomes go to one pole and all the maternal ones to the other; thus each of the sex cells comes to contain a complete set of chromosomes, though particular indi- vidual chromosomes may have come from the father while others have come from the mother. There is reason to believe that homologous chromosomes show general re- semblances but individual differences, and con- sequently when the members of each pair separate and go into the sex cells these cells differ among themselves because the individual chromosomes in different cells are not the same in hereditary value. THE CELLULAR BASIS 143 In this way the number of chromosomes in the mature egg or sperm comes to be one-half the number present in other kinds of cells, and when the egg and sperm unite in fertili- zation the whole number is again restored. The double set of chromosomes is known as the diploid number, the single set as the hap- loid number, and the maturation division in which this reduction from the double to the single set takes place.is the reduction division. It is generally held that this reduction takes place in the first of the two maturation di- visions (Fig. 32, C, D), and that the second of these divisions is like an ordinary mitosis in that each chromosome splits ito two and the halves move apart, such a division bemg known as an equation division (Fig. 32 E), but it is possible that some chromosome pairs undergo an equation division in the first ma- turation mitosis and a reduction division in the second, while other chromosome pairs may reverse this order. It is an interesting fact that long before the reduction of chromosomes had been actu- ally seen Weismann maintained on theoret- 144 HEREDITY AND ENVIRONMENT ical grounds that such a reduction must occur, otherwise the number of chromosomes would double in every generation, and he held that this reduction must take place in one of the maturation divisions; this hypothesis of Weismann’s is now an estab- lished fact. As the result of these two maturation divi- sions four cells are formed from each cell (spermatocyte or oocyte) of the growth period. In the spermatogenesis each of these four cells is transformed into a functional spermatozoon (Figs. 22, 32 F'), by the con- densation of the nucleus into the sperm head and the outgrowth of the centrosome and cyto- plasm to form the tail. In the oogenesis only one of these four cells becomes a functional egg while the other three are small rudimentary egos which are called polar bodies and which take no further part in development (Figs. 22, 23, C-F'). The fertilization of the egg usually takes place coincidently with the formation of the polar bodies, and so we come back once more to the stage from which we started, thus completing the life cycle. THE CELLULAR BASIS 145 4. Sex Determination—In the formation of the sex cells one can frequently distinguish at an early stage differences between the larger oogonia and the smaller and more nu- merous spermatogonia; this difference is the first visible distinction in the development of the two sexes. In the case of the human embryo this distinction can be made as early as the fifth week, and it is evident that the real causes of this difference must be found at a still earlier period of development. The cause of sex has been a favorite subject of speculation for thousands of years. Hun- dreds of hypotheses have been advanced to explain this perennially interesting phenome- non. ‘The causes of sex determination have been ascribed to almost every possible external or internal influence and the world is full of people who think they have discovered by personal experience just how sex is deter- mined. Unfortunately these hypotheses and rules are generally founded upon a few obser- vations of selected cases. Since there are only two sexes the chances are that any hypothesis will be right half the time, and if only one for- 146 HEREDITY AND ENVIRONMENT gets the failures of a rule and remembers the times when it holds good it is possible to be- lieve in the influence of food or temperature or age, of war or peace or education on the relative numbers of the sexes, or on almost any other thing. By statistics it has been shown that each of these things influences the sex ratio, and by more extensive statistics it has been proved that they do not. This was the condition regarding the causes of sex determination which prevailed up to the year 1902. Immediately preceding that year it had been found that two kinds of sper- matozoa were formed in equal numbers in cer- tain insects; one of these kinds contained a peculiar “accessory” chromosome, and the other lacked it. The manner in which these two types of spermatozoa were formed had been carefully worked out by several investi- gators without any suspicion of the real sig- nificance of the facts. It was shown that an uneven number of chromosomes might be _ present in the spermatogonia of certain in- sects and that when maternal and paternal chromosomes united in pairs in synapsis one THE CELLULAR BASIS 147 “odd” chromosome was left without a mate (Fig. 82 B). Later, in the reduction division, when the synaptic pairs separated, the odd chromosome went entire into one of the daugh- ter cells, and the spermatozoa formed from this cell contained one chromosome more than those formed from the other daughter cell (Fig. 82 C and D). Chiefly because these two kinds of sperma- tozoa occur in equal numbers McClung in 1902 concluded that this accessory chromo- some was a sex-determinant. In 1905 Wilson discovered in a number of bugs that while there were two types of spermatozoa, one of which contained and the other lacked the ac- cessory chromosome, there was only one type of egg, since every egg contained the accessory chromosome, and he pointed out that if an egg were fertilized by a sperm containing an accessory, two accessories would be present in the zygote, this being the condition of the female, while if it were fertilized by a sperm without an accessory there would be present in the zygote only the accessory derived from the egg (Fig. 33 E and F, Fig. 34). 148 HEREDITY AND ENVIRONMENT In other cases Miss Stevens as well as Wil- son discovered that two accessory chromo- somes, differmg in size, might be present in the male whereas in the female they are of equal size (Fig. 35). In such cases two types of spermatozoa are produced in equal num- bers, one containing a large and the other a Protenor Type Oocyte Reduction — Ova Ferlilfzed Egg Division @) 2 OO) Spermatocyte Reduction Spermatids Division i Fic. 34. Dracrams or Sex DIFFERENTIATION IN THE Bue, Protenor. The oocyte contains 6 chromosomes and the sper- matocyte 5 chromosomes which are not yet united into synap- tic pairs; the “sex” chromosomes are shown in black, two are present in the oocyte, but only one in the spermatocyte. In the reduction division the synaptic pairs separate, giving rise to two types of spermatids, one of which has the sex chrom- osome and the other lacks it; all ova are alike in this regard. If an egg is fertilized by a sperm without the sex chrom- osome a male results; if fertilized by a sperm containing the sex chromosome a female results. (After Wilson with modifications. ) THE CELLULAR BASIS 149 small accessory chromosome, whereas every ege contains one large accessory chromosome. If such an egg is fertilized by a sperm con- taining a large accessory (the XY chromosome) it gives rise to a female, if by a sperm contain- ing a small accessory (the Y chromosome) it gives rise to a male (Fig. 35). In other animals one may not be able to distinguish separate X or Y chromosomes and Tenebrio Type Reduction Fertilized Spermatocyte Reduction Spermatids Division Fic. 35. Dtacrams or SEx DIFFERENTIATION IN THE BEETLE, Tenebrio, showing 5 synaptic pairs of chromosomes (there are actually 10 pairs) ; in the oocyte the members of each pair are equal in size; in the spermatocyte the members of one pair are unequal. ‘These pairs separate in the reduction division giving rise to two types of spermatozoa and one type of ova; eggs fertilized by one type of sperm give rise to females, those fertilized by the other type give rise to males. (After Stevens with modifications.) 150 HEREDITY AND ENVIRONMENT yet such structures may be joined to one or two ordinary chromosomes. ‘This is the case in the thread worm, Ascaris (Fig. 36), where two such accessory elements are present in the female, each being joined to the end of an ordinary chromosome, whereas in the male only one such element is present. Here also two classes of spermatozoa are found one with and the other without the accessory element, Ascaris Type Mature Egg and Sasa Reduction Polar Body Fertilized Pee (B) gg et a Ve) WW AME @) & Uae Sperms Fic. 36. Dracrams or Sex DIFFERENTIATION IN THE THREAD Worm, Ascaris. The X chromosomes (black) are here joined to ordinary chromosomes, there being two in the egg mother cell and one in the sperm mother cell. All eggs contain one of these X chromosomes, while half of the spermatozoa have it and half do not. Eggs fertilized by oné type of sperm produce females, those fertilized by the other type produce males. (From Wilson.) Al At.) ¢ es iat a THE CELLULAR BASIS 151 whereas all ova have this element, and in this case also sex is probably determined by the type of spermatozoon which enters the egg (Fig. 36). Even in man sex is determined in the same manner, according to several recent investi- gators. There are in the spermatogonia of man 47 chromosomes, according to Winiwar- ter, one of which is the XY or accessory chromo- some (Fig. 87 A). ‘These unite in synapsis into 23 pairs, leaving the X chromosome un- paired (Fig. 37, B) and in the reduction division the pairs separate, while the X chro- mosome goes entire into one of the daughter cells, which consequently contains 23 + X chromosomes, whereas the other daughter cell contains 23 chromosomes (Fig. 37 C and D). The former gives rise to spermatozoa with 24 chromosomes, the latter to spermatozoa with 23 chromosomes. In the female there are probably 48 chromosomes, according to Wini- warter, there being two X chromosomes, one from each parent, and after the reduction di- visions every egg contains 24 chromosomes. If an egg is fertilized by a sperm containing 24 152 HEREDITY AND ENVIRONMENT chromosomes an individual with 48 chromo- somes, or a female, is produced; if fertilized ae I. ra Ci = i y ye VS 238+ 2 23 23 Fic. 37. Dracrams or Sex DirrerENTIATION IN Man. 4A, spermatogonium with 47 chromosomes one of which (small circle) is the X chromosome. B, spermatocyte showing 23 syn- aptic pairs and a single unpaired X chromosome. C, reduction division in which the synaptic pairs separate while the X chromosome does not divide, consequently the second sper- matocytes D and D’ contain respectively 23 +. X and 23 chromosomes. # and EH’, second maturation division in which every chromosome divides, giving rise (Ff) to two equal classes of spermatids and spermatozoa, one of which has 24 chromo- somes and the other 23. If an egg containing 24 chromo- somes is fertilized by a sperm with 24, a female with 48 chromosomes is produced; if an egg with 24 chromosomes is fertilized by a sperm with 23, a male with 47 chromosomes results. (After Morgan.) RA en THE CELLULAR BASIS 153 by a sperm with 23 chromosomes an individual with 47 chromosomes, or a male, results (Fig. 37). It must be said that other investigators, notably Guyer and Montgomery, have not found 47 chromosomes in the spermatogonia of man, but about 22. Since both the latter investigators worked on negroes whereas Wini- warter worked on white men it has been sug- gested quite recently by Morgan and Guyer that there may be twice as many chromosomes in the white race as in the black. A similar con- dition in which one race has twice as many chromosomes as another race of the same species is found in two races of the thread worm, Ascaris megalocephala.* Similar correlations between chromosomes and sex have been observed in more than one hundred species of animals belonging to widely different phyla. In a few classes of animals, particularly echmoderms and birds, the evi- dence while not entirely convincing seems to point to the fact that two types of ova are *Still more recent work by Wieman shows that the number of chromosomes in white men and in negroes is 24; in the male there is presumably an XY pair, in the female an XX pair. 154 HEREDITY AND ENVIRONMENT produced and but one type of spermatozoa; but the general principle that sex is determined by the chance union of male-producing or fe- male-producing gametes is not changed by such cases. On the other hand there are many observa- tions which seem to indicate that the sex ratio may be changed by environmental conditions acting before or after fertilization and that therefore sex is determined by extrinsic rather than by intrinsic causes. Many of these ob- servations, as already remarked, are now known to be erroneous or misleading, since they do not prove what they were once sup- posed to demonstrate. But there remain a few cases which can not at present be ex- plained away in this manner. Perhaps the best attested of these are the observations of R. Hertwig and some of his pupils on the effects of the time of fertilization on the de- termination of sex. If frog’s eggs, which are always fertilized after they are laid, are kept for some hours before spermatozoa are mixed with them, or if the female is prevented for two or three days from laying the eggs after ee THE CELLULAR BASIS 155 they have entered the oviducts, the proportion of males to females is enormously increased. A wholly similar result has been observed by Pearl and Parshley in the case of cattle, where delayed fertilization of the egg leads to a great preponderance of males. Hertwig attempts to explain his extremely interesting and im- portant observations as due to the relative size of nucleus and cytoplasm of the egg; but in general this nucleus-plasma ratio may vary greatly irrespective of sex and there is no clear evidence that it is a cause of sex determination. Miss King also, working on toad’s eggs, has increased the proportion of females by slightly drying the eggs or by withdrawing water from then: by placing them in solutions of salts, acids, sugar, etc., but the manner in which drying increases the proportion of females is wholly unknown. Quite recently Whitney has shown that in certain species of rotifers a scanty diet produces only female offspring while a copious diet produces as high as 95 per cent of male offspring, thus confirming the conclusions of many earlier investigators that 156 HEREDITY AND ENVIRONMENT the quantity or kind of food influences sex determination. Extensive statistics show that in many ani- mals including man more males are born than females whereas according to the chromosome theory of sex determination as many female- producing spermatozoa are formed as male- producing ones. It is possible to explain such departure from the 1: 1 ratio of males and fe- males in conformity with the chromosome theory if one class of spermatozoa are more ac- tive or have greater vitality than the other class, or if after fertilization one sex is more likely to live than the other. In the human species it is known that mortality is greater in male babies before and after birth than in female babies, and if before fertilization the activity or vitality of male-producing sper- matozoa is greater than of female-producing ones it would offer a possible explanation of the greater number of males than of females at the time of birth. In certain insects it is known that only females develop from ferti- lized eggs, and in one of these cases, viz., Phylloxera, Morgan has discovered that this THE CELLULAR BASIS 157 is due to the fact that all the male-producing spermatozoa degenerate and that only female- producing spermatozoa become functional. Possibly experimental alterations of the sex ratio, such as Hertwig, King and others have brought. about may be explained in a similar way. At least the chromosomal theory of sex determination is so well supported in so many cases and has been found to apply in so many instances where at first it seemed impossible of application, that it ought not to be abandoned until unmistakable evidence can be adduced against it.* C. THE MECHANISM OF HEREDITY The mechanism of heredity, as contrasted with the mechanism of development, consists in the formation of particular kinds of germ cells and in the union of certain of these cells in fertilization. We have briefly traced the origin, maturation and union of male and fe- male sex cells in a number of animals, and in these phenomena we have the mechanism of * More recent work by Riddle, Goldschmidt, Lillie, ef al. in- dicates that chemical substances such as ‘sex hormones” or enzymes (see pp. 337-339) may modify the chromosomal de- termination of sex, and Goldschmidt suggests that even chromo- somal determination may be due to such enzymes. 158 HEREDITY AND ENVIRONMENT the hereditary continuity between successive generations. But in addition to these specific facts there are certain general considerations which need to be emphasized. I. Tue SPrEcIFIcITy OF GERM CELLS The conclusion is inevitable that the germ cells of different species and even those of dif- ferent individuals are not all alike. Every in- dividual difference between organisms must be due to one or more differentiating causes or factors. Specific results come only from specific causes. ‘These causes may be found in the organization of the germ cells or in en- vironmental stimuli, 7. e., they may be intrinsic or extrinsic, but as a matter of fact experience has shown that they are generally intrinsic in the germ. In the same environment one egg becomes a chicken and another a duck; one becomes a frog, and another a fish, and an- other a snail; one becomes a black guinea-pig and another a white one; one becomes a male and another a female; one gives rise to a tall man and another to a short man, ete. Since these differences may occur in the same en- THE CELLULAR BASIS 159 vironment they must be due to differences in the germ cells concerned. On the other hand different environmental conditions may be.associated with similar de- velopmental results. Loeb and others have found that artificial parthenogenesis may be induced by a great variety of environmental stimuli, viz., by salt solutions, by acids and alkalis, by fatty acids and fat solvents, by alkaloids and cyanides, by blood serum and sperm extract, by heat and cold, by agitation and electric current. There is certainly noth- ing specific in these different stimuli. Simi- larly Stockard has discovered that cyclopia, or one-eyed monsters, may be produced by magnesium salts, alcohol, chloretone, chloro- form, and ether, and to this list McClendon has added various other salts and anesthetics. In all such cases it is evident that the specific re- sults of such treatment are due to a specific organization of the germ rather than to specific stimuli. Why does one egg give rise to a chicken and another to a duck, or a fish, or a frog? Why does one egg give rise to a black guinea- 160 HEREDITY AND ENVIRONMENT pig and another to a white one, though both may be produced by the same parents? Why does one child differ from another in the same family? Why does one cell give rise to a gland and another to a nerve, one to an egg and an- other to a sperm? If these differences are not due to environmental causes, and the evidence shows that they are not, they must be due to differences in the structures and functions of the cells concerned. Many differences in the material substances of cells are visible, and many more are invisi- ble though still demonstrable. These differ- ences may not be detectable by chemical or physical tests, and yet they may be demon- strated physiologically and developmentally. The most delicate of all tests are physiological, as 1s shown by the Weidal test in typhoid fever, the Wassermann reaction in syphilis, the reactions of immunized animals to different toxins, ete. Lillie has recently shown that ege cells give off a substance which he calls fertilizin, which can be detected only by the way in which spermatozoa react to it. No chemical or physical test can distinguish be- THE CELLULAR BASIS 16] tween the different eggs or spermatozoa pro- duced by the same individual, but the reactions of these cells in development prove that they are different. Undoubtedly chemical and physical differences are here present but no chemical methods at present available are sufficiently delicate to detect them. The de- velopmental test indicates that there are as many kinds of oosperms as there are differ- ent kinds of individuals which’ come from oosperms. It is one of the marvellous facts of biology that practically every sexually pro- duced individual is unique, the first and last of its identical kind, and although some of these individual differences are due to vary- ing environment, others are evidently due to germinal differences, so that we must conclude that every fertilized egg cell differs in some respects from every other one. But are there molecules and atoms enough in a tiny germ cell, such as a spermatozoon, to allow for all these differences? Miescher has shown that a molecule of albumin with 40 carbon atoms may have as many as one billion stereo-isomers, and in protoplasm there are 162 HEREDITY AND ENVIRONMENT many kinds of albumin and other proteins, some with probably more than 700 carbon atoms. In such a complex substance as proto- plasm the possible variations in molecular con- stitution must be well nigh infinite, and it can not be objected on this ground that it is chemically and physically impossible to have as many varieties of germ cells as there are dif- ferent kinds of individuals in the world. Even with regard to morphological elements which may be seen with the microscope it can be shown that an enormous number of permu- tations is possible. It seems probable, as Boveri has shown, that different chromosomes of the fertilized egg differ in hereditary po- tencies, and where the number of chromosomes is fairly large the number of possible combi- nations of these chromosomes in the germ cells becomes very great. In woman, where there are probably 48 chromosomes,* and, after synapsis, 24 pairs of maternal and paternal ones, the possible number of permutations in the distribution of these chromosomes to the different egg cells would be 2, or 16,777,036, and the possible number of differ- * See footnote p. 153. THE CELLULAR BASIS 163 ent types of fertilized eggs or oosperms which could be produced by a single pair of parents would be (16,777,036)°, or approximately three hundred thousand billions. But prob- ably other things than chromosomes differ in different germ cells, and it is by no means certain that individual chromosomes are al- ways composed of the same chromomeres, or units of the next smaller order, and in view of these possibilities it may well be that every hu- man germ cell differs morphologically and physiologically from every other one, in short that every oosperm and every individual which develops from it is absolutely unique. Indeed the production of unique individuals seems to be the chief purpose and result of sexual reproduction. In asexual reproduc- tion the individual variations which occur are chiefly if not entirely due to environment, but in sexual reproduction they are also due to new combinations of hereditary elements. The particular germinal organization trans- mitted from one generation to the next de- pends upon (a) the, ancestral organization, (b) the particular character of the cell di- 164 HEREDITY AND ENVIRONMENT visions by which the germ cells are formed, (c) the particular kinds of egg and sperm cells which combine in fertilization. The an- cestral organization determines all the gen- eral characteristics of race, species, genus, order, phylum. It determines the possibili- ties and limitations of individual variations. Given a certain ancestral organization, the individual peculiarities of the germ cells are determined by the particular character of cell division by which the germ cells are formed, and the peculiarities of the individuals or per- sons which develop from these cells are de- termined in large part by the particular kinds of germ cells which unite in fertilization. The behavior of chromosomes in matura- tion and fertilization is like the shuffle and deal of cards in a game, and apparently with the same object, viz., never to deal the same hand twice. ‘To make this comparison more complete suppose that kings be discarded from the pack, leaving 48 cards of two colors, red and black, which we will compare to the 48 chromosomes of maternal and paternal origin in the human oocyte; suppose that in the shuf- THE CELLULAR BASIS 165 fling of these cards corresponding cards of the red and the black suits are temporarily stuck together so that the ace of diamonds is united with the ace of clubs, the queen of hearts with the queen of spades, etc., thus forming 24 red- black pairs of the same denominations. If these cards are then dealt into two hands, one card of each pair going to one hand and the other to the other hand, we will have two cards of each denomination in each hand, but if the cards are dealt indiscriminately some of them will be red and some black. ‘This de- scription parallels what takes place in the maturation of the human ovum, except that there is no evidence that there are more than two suits of chromosomes, one of which is ma- ternal and the other paternal. To carry out this comparison in the case of the maturation of the human sperm where there are only 47 chromosomes it is necessary to take another pack and discard an additional card, say the queen of clubs; then in the union of corresponding red and black cards into pairs the queen of hearts unites with the queen of spades, but the queen of diamonds remains 166 HEREDITY AND ENVIRONMENT alone, and when the cards are dealt into two hands as before one hand will contain 24 cards and the other 23. If now we complete this comparison by ex- tending it to what takes place in fertilization we must take one hand from each of these deals and put them together into one pack; this pack would contain cards of every de- nomination from ace to queen but there would be varying numbers of red and black cards and a mixture of cards from two distinct packs. In no game of cards do corresponding cards from different packs have slightly different values nor are half of the cards taken from one pack and half from another at every game, but this is Just what happens in the shuffle and deal of the chromosomes. Because of the mix- ture of chromosomes from distinct individuals in every generation, each of which has its own peculiar value, the game of heredity becomes vastly more complex than any game of cards. This illustration may serve to make plain the fact that in the process of maturation and fertilization there is this shuffle and deal of the chromosomes, with the result that every THE CELLULAR BASIS 167 oosperm and every individual which develops from it is different from every other one. This conception of the specificity of every germ cell, as well as of every developed indi- vidual, sets the whole problem of heredity and development in a clear light. The visible peculiarities of an adult become invisible as development is traced back to the germ, but they do not wholly cease to exist. Similarly the multitudinous complexities of an adult fade out of view as development is traced to its earliest stages, but it is probable that they are not wholly lost. In short, the specificity of the germ applies not merely to those things in which it differs from other germs, but also to characters in which it resembles others; in short, to hereditary resemblances no less than to hereditary differences. The mistake of the doctrine of preformation was in supposing that germinal parts were of the same kind as adult parts; the mistake of epigenesis was in maintaining the lack of specific parts in the germ. The development of every animal and plant consists in the trans- formation of the specific characters of the germ 168 HEREDITY AND ENVIRONMENT into those of the adult. From beginning to end development is a series of morphological! and physiological changes but not of new for- mations or creations except in so far as new structures or functions appear as a result of “creative synthesis.” It is only the incomplete- ness of our knowledge of development which allows us to say that the eye or ear or brain begins to form in this or that stage. They be- come visible at certain stages, but their real be- ginnings are indefinitely remote. II. CorRELATIONS BETWEEN GERMINAL AND SoMATIC ORGANIZATION All the world knows that the organization of the germ is not the same as that of the de- veloped animal which comes from it, and yet the specificity of the germ indicates that there must be some correlation between the germinal and the developed organization; in short, there is not identity of organization but cor- relation of organization between the germ and the adult. What correlations are known to exist between the oosperm and the developed animal? THE CELLULAR BASIS 169 1. Nuclear Correlations—Many biologists maintain that the nucleus and more particu- larly the chromosomes are the exclusive seat of the “inheritance material” and that the cyto- plasm serves merely as environment for the nucleus. Against the extreme form of this theory many general and specific objections may be urged. General objections are based upon the consideration that the entire cell, cytoplasm as well as nucleus, is concerned in differentiation and that neither is capable of embryonic development in the absence of the other. Differentiation is indeed the result of the interaction of nucleus and cytoplasm, and how then can it be said that the nucleus is the only seat of the inheritance material? If held rigidly, this theory involves the assumption that the cytoplasm and all other parts of the cell are the products of the chromosomes, and that therefore the chromosome and not the cell is the ultimate independent unit of structure and function. Furthermore, since heredity in- cludes a series of fundamental vital processes such as assimilation, growth, division and dif- ferentiation, there is something primitive and 170 HEREDITY AND ENVIRONMENT naive in the view that this most general pro- cess can be localized in one specific part of the cell, something which recalls the long-past doctrine that the life was located in the heart or in the blood, or the ancient attempts to find the seat of the soul in the pineal gland or in the ventricles of the brain. Nevertheless there are certain general rea- sons for assuming that the chromosomes are important factors in heredity and differentia- tion: (1) they come in approximately equal numbers from the father and the mother, (2) one-half of each of the maternal and paternal chromosomes is distributed to each cell of the developing organism, (3) in the formation of the egg and sperm cells the normal number of chromosomes is reduced by one-half, and (4) in fertilization the normal number is re- stored by the union of the chromosomes of the egg and sperm. It is a remarkable fact that the determiners or factors of certain in- herited characters come in equal numbers from both parents and that in spite of their ultimate association in an individual they may be separated or “segregated” in the formation THE CELLULAR BASIS 171 of that individual’s germ cells. Such inherit- ance is known as Mendelian and will be treated at length in the next chapter, but it _ may be said here that the association, distribu- tion and segregation of Mendelian factors and of maternal and paternal chromosomes are exactly parallel. This is strong evidence that these factors are associated in some way with the chromosomes. There are also certain special reasons for considering that the chromosomes are import- ant factors in heredity and development: (5) Boveri has studied the abnormal distribution of chromosomes to different cleavage cells in doubly fertilized sea urchin eggs and _ has found evidence that the hereditary value of different chromosomes is different. (6) Mc- Clung, Stevens and Wilson have discovered that the determination of sex is associated with the presence or absence of a particular chromosome, the XY chromosome, in the sper- matozoon which fertilizes the egg. If an egg is fertilized by a sperm which lacks the A chromosome a male is produced, if fertilized by the other type a female results. (7) Fin- 172 HEREDITY AND ENVIRONMENT ally Morgan has found that there is a lnkage of certain somatic characters with sex in the fruit fly, Drosophila, which can be readily ex- plained by assuming that the determiners for these characters are in some way associated with the sex chromosome. By a series of the most remarkable discoveries ever made in this field he has shown that more than one hundred somatic characters of Drosophila fall into four groups corresponding in size and distribution to the four chromosomes in the sex cells of this animal, and not only is he able to locate the factors for particular characters in particular chromosomes but also he can determine the rel- ative positions of many of the factors in each chromosome. We have in these facts a remarkable corre- lation between the distribution of the chromo- somes and the occurrence of certain characters of the adult animal. The association of ma- ternal and paternal chromosomes in fertiliza- tion and their segregation in the maturation of the germ cells is parallel to the association of Mendelian characters in the zygote and their segregation in the gametes; if the distribution THE CELLULAR BASIS 173 of chromosomes in cleavage is abnormal the larva shows abnormal characters (Boveri) ; sex determination is associated with the distri- bution of a particular chromosome to one-half of the spermatozoa, and the fertilization of the ego by one type or the other of spermatozoa (Wilson) ; the linkage of certain characters of Drosophila into four groups which corre- spond with the four chromosomes of the sex- cells of that animal indicates that the differen- tial causes of these characters are located in the chromosomes (Morgan). On the other hand it is objected by certain investigators, notably by Child, Foot and Stro- bell, that chromosomes are not the causes of anything, but that they are the “results of dynamic processes,” “the expression rather than the cause of cell activities.” This objec- tion seemis to confuse the idea of natural cause with that of final cause. Science knows noth- ing of the latter; any natural cause is only a link in the chain of cause and effect, it is itself the result of antecedent causes and the cause of subsequent results. Undoubtedly the chromosomes are the results of antecedent pro- 174 HEREDITY AND ENVIRONMENT cesses, and yet they may also be the causes of subsequent results. No thoughtful person has ever maintained that chromosomes or any other things in nature are autonomous, abso- lute, uncaused causes. 2. Cytoplasmic Correspondences.—How- ever the most direct and the earliest recog- nized correlations between the oosperm and the developed animal are found in the polarity and symmetry of the egg cytoplasm and of the animal to which it gives rise. (a) Polarity—tIn all eggs there is polar differentiation, one pole, at which the matura- tion divisions take place, bemg known as the animal pole, and the opposite one being known as the vegetative pole. The substance of the egg in the vicinity of the animal pole usually gives rise to the ectoderm, or outer cell layer of the embryo; the portion of the egg sur- rounding the vegetative pole usually becomes the endoderm or inner cell layer. The axis which connects these poles, the chief axis of the egg, becomes the gastrular axis of the em- bryo and in every great group of animals it bears a constant relationship to the chief axis THE CELLULAR BASIS 175 of the adult animal. The polarity of the devel- oped animal is thus directly connected with the fe] Paar 3 Ly ; 0° Boe Fic. 38. Fic. 39. Fic. 38. OvtLines oF THE UNFERTILIZED Ecc or a Sauin, Loligo, showing the polarity and symmetry of the egg with reference to the axes of the developed animal; d, dorsal; v, ventral; J, left; r, right; a, anterior; p, posterior. (After Watase.) Fic. 39. Mepian Section THROUGH Ecc or Aa Fry, Musca, just after fertilization, showing the relations of the polarity and symmetry of the egg to the axes of the developed animal; the long axis of the egg corresponds to the antero-posterior axis of the animal; d, dorsal; v, ventral; m, micropyle through which sperm enters the egg; g, glutinous cap over the micro- pyle; r, polar bodies; p, egg and sperm nuclei; do, yolk; k, peripheral layer of protoplasm; dh, vitelline membrane of egg; ch, chorion. (After Korschelt and Heider.) 176 HEREDITY AND ENVIRONMENT polarity of the egg from which it came (Figs. 23, 26, 27, 28, 38, 39). (b) Symmetry.—tn many cases the sym- metry of the developed animal is foreshadowed in the cytoplasm of the egg. The eggs of cephalopods (Fig. 38) and of insects (Fig. 39) are bilaterally symmetrical while they are still in the ovary; in other cases, such as ascid- ians, Amphiowus and the frog, bilateral sym- metry appears immediately after fertilization | (Figs. 9, 27, 28), though in some of these cases there is reason to believe that the eggs are bilateral even before fertilization; in still other cases bilaterality does not become visible until later in development and we do not now know whether it is present in earlier stages or not; but wherever it can be recognized in the earlier stages it is certain that the bilateral symmetry of the egg becomes the bilateral symmetry of the developed animal. (c) Inverse Symmetry.—tIn most animals bilateral symmetry is not perfect, certain or- gans being found on one side of the mid line and not on the other, or being larger or dif- ferently located on one side as compared with THE CELLULAR BASIS 177 the other; among all such animals variations occasionally occur which show a complete re- versal of these asymmetrical organs, 7. é., in man the heart and arch of the aorta may occur on the right side instead of the left, the pyloris and chief portion of the liver on the left in- stead of the right, etc. Among certain snails this inversion of symmetry may occur regu- larly in certain species and not in others, the inverse form being known as sinistral and the ordinary form as dextral (Fig. 42). In these sinistral snails, and probably in all animals showing inverse symmetry, the embryo is in- versely symmetrical and every cleavage of the egg from the first to the last is the inverse of that which occurs in dextral snails (Figs. 40- 42). 'There is good reason to believe that in such cases the unsegmented egg is also in- versely symmetrical as compared with the more usual type (Fig. 40). In all of these cases there is a direct correspondence between the polarity and symmetry of the oosperm and the polarity and symmetry of the devel- oped animal (Figs. 38-42). (d) Localization Pattern.—In many ani- Fics. 40, 41, 42. Tue Cause or INVERSE SYMMETRY IN Swatrs. In each case the right-hand column represents dextral forms, the left-hand column sinistral ones. Fic. 40. Norman AND INveRSE SYMMETRY IN THE UN- SEGMENTED EGG AND IN THE First AND SECOND CLEAVAGES. Fic. 42. For description see p. 181. THE CELLULAR BASIS 181 mals the ectoderm, endoderm and mesoderm may be traced back to areas of peculiar proto- plasm in the oosperm, but in addition to this one can recognize in the ascidian egg areas of peculiar protoplasm which will give rise to mesenchyme, muscles, nervous system and notochord, and these substances are present in the oosperm in the approximate positions and proportions which they will have in the embryo and larva (Figs. 9, 10, 27-29). Indeed there are types of localization of these cytoplasmic materials in the egg which are characteristic of certain phyla; thus there are thé ctenophore, the flat-worm, the echino- derm, the annelid-mollusk and the chordate types of cytoplasmic localization (Fig. 43). The polarity, symmetry and pattern of a jellyfish, starfish, worm, mollusk, insect or vertebrate are foreshadowed by the character- Fic. 42. Norman aAnp INVERSE Symmetry In Late Em- BRYOs AND ApuLtt Sraces. In 1, cross-hatched area is blasto- pore; cells shaded by lines, mesoderm, other cells, endoderm; the spiral twist of the snail begins in opposite directions in the two embryos. In 2, the adult organization is shown with all organs inversely symmetrical; os, olfactory organ; a, anus; ZL, lung; V, ventricle; K, kidney. In 8, sinistral and dextral shells of adult snails are shown. CTENOPHORE TURBELLARIAN ect end. = : = end. ASCIDIAN II ect. ms. Fic. 43. ‘Types or Eco OrGaNnizarion IN DIFFERENT PHYLA; cross-hatched area, mesoderm or mesenchyme (mes); _hori- zontal lines, endoderm (end); clear area, ectoderm (ect). In the first four figures the pattern of localization is that which is found at the close of the first cleavage; in Ascidian II the pattern is that which is found at the close of the second cleavage; in the annelid egg the localization of later stages is projected upon the egg; m.p., neural plate; ch., chorda; c.g., cerebral ganglion; v.g., ventral ganglion; proto., prototroch. THE CELLULAR BASIS 183 istic polarity, symmetry and pattern of the cytoplasm of the egg either before or immedi- ately after fertilization. In all of these phyla eggs may develop without fertilization, either by natural or by artificial parthenogenesis, and in such cases the characteristic polarity, symmetry and pattern of the adult are found in the cytoplasm of the egg just as if the lat- ter had been fertilized. The conclusion seems to be justified that these earliest and most fundamental differentiations which distin- guished the eggs of various phyla are not de- pendent upon the entering spermatozoon. All of these correspondences between the polarity, symmetry and pattern of the egg and of the developed animal are found in the cytoplasm. No doubt the differentiations of cytoplasm of the egg as well as the peculiar form and structure of the spermatozoon have arisen during the genesis of these cells under the influence of paternal and maternal chro- mosomes contained within their nuclei, just as in the differentiation of any tissue cell; but in the case of the spermatozoon these cytoplasmic differentiations are lost when it enters the egg, 184 HEREDITY AND ENVIRONMENT whereas those of the egg persist. In short ontogeny begins in the ege before fertiliza- tion whereas the sperm can influence ontogeny only after, and usually a considerable time after, it enters the egg. The fact remains that at the time of fertiliza- tion the hereditary potencies of the two germ cells are not equal, the polarity, symmetry, type of cleavage, and the pattern, or relative positions and proportions of future organs, being foreshadowed in the cytoplasm of the egg cell, while only the differentiations of later development are influenced by the sperm. In short the egg cytoplasm determines the early development and the sperm and egg nuclei control only later diff erentiations. We are vertebrates because our mothers were vertebrates and produced eggs of the vertebrate pattern; but the color of our skin and hair and eyes, our sex, stature, and mental peculiarities were determined by the sperm as well as by the egg from which we came. ‘There is evidence that the chromosomes of the egg and sperm are the seat of the differential fac- tors or determiners for Mendelian characters, THE CELLULAR BASIS 185 but the general polarity, symmetry and pat- tern of the embryo are egg characters which were determined before fertilization. It will be observed that the correlation be- tween chromosomes and adult characters is different in kind from that between the cyto- plasm of the egg and adult characters; in the latter case polarity, symmetry and pat- _tern are of the same kind in the egg and in the adult, and the correspondence is comparatively close; in the former there is no correspondence in kind between the chromosomal peculiarities and the peculiarities of the adult. This fact suggests that the chromosomal organiza- tion may be more fundamental than that of the cytoplasm. There are reasons for believ- ing that many substances of the cell are formed by the interaction of nucleus and cytoplasm, and undoubtedly the chromosomes are an im- portant factor in this process. Butin no case is the cytoplasm a neglible factor, in no case does it serve merely as food or environment for the chromosomes. ‘The entire cell, nucleus and cytoplasm, is concerned in heredity and differentiation. 186 HEREDITY AND ENVIRONMENT D. THE MECHANISM OF DEVELOPMENT Development consists in the transformation of the oosperm into the adult. What is the mechanism by which this transformation is effected? There is progressive differentiation of the germ into the developed organism but by what process is this differentiation ac- complished ? Many different processes are concerned in embryonic differentiation. From the stand- point of the cell the most important of these are (1) the formation of different kinds of substances in cells, (2) the localization and isolation of these substances, (3) the trans- formation of these substances into the various structures which are characteristic of the dif- ferent kinds of tissue cells. We shall here de- scribe only the first and second of these pro- cesses which are of more general interest than the last. 1. The Formation of Different Substances in Cells—LEmbryonic differentiation consists primarily in the formation of different kinds of THE CELLULAR BASIS 187 protoplasm out of the protoplasm of the germ cells. It is plain that different kinds of proto- plasm are present in the two germ cells before they unite in fertilization, but in the course of development the number of substances present and the degree of difference among them greatly increase. Actual observation shows that by the inter- action with one another of substances or parts originally present and by their reactions to external stimuli new substances and parts ap- pear which had no previous existence just as new substances result from chemical reactions. This is “creative synthesis” in philosophy, epigenesis in development. MDifferentiations appear chiefly in the cytoplasm but only as the result of interaction between cytoplasm and nucleus. Similarly, it may be argued, smaller units of organization such as chromosomes or chromomeres do not in themselves give rise to any adult part, but only as they interact upon other units are new parts formed. In many cases the first formation of such new substances appears in the immediate vicinity of the nucleus and, like assimilation 188 HEREDITY AND ENVIRONMENT itself, is evidently brought about by the inter- action of nucleus and cytoplasm. In certain cases it can be seen that the achromatin and oxychromatin which escape from the nucleus during division take part in the formation of new substances in the cell body, and since the oxychromatin is derived from the chromo- somes of the previous cell division, it is prob- able that the chromosomes are a factor in this process. Weismann maintained that the chromo- somes and the inheritance units contained in them undergo differentiation by a process of disintegration and that these disintegrated units escape into the cell body and there pro- duce different kinds of cytoplasm in different cells. A,somewhat similar view was advanced by deVries in his theory of intra-cellular pan- genesis. However, as we have seen already, there is good evidence that the chromosomes do not undergo progressive differentiation in the course of development; they always divide with exact equality, and even in highly differ- entiated tissue cells their number and form usually remain as in embryonic cells. THE CELLULAR BASIS fe1s9 On the other hand the cytoplasm undergoes progressive differentiation, and when by pres- sure or centrifugal force it is brought into rela- tions with other nuclei the differentiations of the cytoplasm are not altered thereby, thus showing that the different nuclei are essentially alike and that differentiations are mainly lim- ited to the cytoplasm. Thus the differentiations of cells are not due to the differentiations of their nuclei, but rather the reverse is true, such differentiations of nuclei as occur are due to differentiations of the cytoplasm in which they lie. Nevertheless differentiations do not take place in the absence of nuclear material, and it seems probable that the interaction of nucleus and cytoplasm is necessary to the formation of the new cytoplasmic substances which appear in the course of development. 2. Segregation and Isolation of Different _ Substances in Cells——But differentiation con- sists not only in the formation of different kinds of substances in cells but also in the sep- aration of these substances from one another. This separation is brought about to a great extent by flowing movements within cells 190 HEREDITY AND ENVIRONMENT which are associated especially with cell division. In all these processes of heredity and de- velopment cell division plays a particularly important part. If cell divisions were always exactly alike there could be no initial differ- ence between the daughter cells, and unless acted upon by different stimuli all cells would remain exactly: alike. But there is much evi- dence that daughter cells are often unlike from the time of their formation, and that different stimuli act upon them to increase still further this initial difference. (a) Differential and Non-differential Cell Division—When each half of any dividing unit is like the other half the division is non- differential. So far as we know the di- visions of all the smallest elements of the cell are of this sort; there is no good evidence that the plastosomes, the chromomeres, or the chro- mosomes ever divide into unlike halves, though in the maturation divisions the separation of whole chromosomes leads to the appearance of a differential division of the chromosomes. But while all of the cell elements may be sup- THE CELLULAR BASIS 191 posed to grow and divide into equivalent halves there may be an unequal distribution of these elements in cell division, so that the two daughter cells are unlike. This is what is known as differential cell division and it plays a most important part in differentiation. While the chromatin is equally distributed to the daughter cells, except in the case of the maturation divisions, the achromatin and the oxychromatin of the nucleus are not always distributed equally and this is probably an important factor in development. The divi- sions of the cytoplasm of the egg are fre- quently differential and such divisions are known to play a great part in embryonic dif- ferentiation. In the differential divisions of the cytoplasm unlike substances become localized in certain parts of the cell body, chiefly by means of definite flowing movements of the cytoplasm, and when cell division occurs these substances become permanently separated by partition walls. In this way irreversible differentiations are formed. If the formation of partition walls is prevented the different substances 192 HEREDITY AND ENVIRONMENT within the cell body may freely commingle, es- pecially during nuclear division when the cyto- plasmic movements are especially active; in such cases differentiation may be arrested even though nuclear division continues. In the de- veloping eggs of most animals partition walls between daughter cells are necessary to pre- vent the commingling of different kinds of substances, which are sorted by the movements within the cell and are isolated by the partition walls. In some cases, as for example in cer- tain protozoa, the commingling of different kinds of protoplasm within a cell may be pre- vented by the viscosity of portions of the pro- toplasm, or by the formation of intracellular membranes, or by a reduction to a minimum of the mitotic movements within the cell by the persistence of the nuclear membrane during division. In general the degree of differentia- tion may be measured by the degree of un- likeness between different cells, and by the completeness with which the protoplasm of different cells is kept from intermingling. THE CELLULAR BASIS 193 SUMMARY All the phenomena of life, including hered- ity and development, are cellular phenomena in that they include only the activities of cells or of cell aggregates. The cell is the ultimate independent unit of organic structure and function. The only living bond between one generation and the next is found in the sex cells and all inheritance must take place through these cells. Inherited traits are not transmitted from parents to offspring but the germinal factors or causes are transmitted, and under proper conditions of environment these give rise to developed characters. Every oosperm as well as every developed organism differs more or less from every other one and this remarkable condition is brought about by extremely numerous permutations in the dis- tribution of certain parts of the sex cells in maturation and fertilization. Sex is an in- herited character dependent, primarily, upon an alternative distribution of certain chromo- somes of the nucleus. There is much evidence that the factors for all sorts of alternative 194 HEREDITY AND ENVIRONMENT characters are associated with the chrom- osomes. ‘The differentiation of the oosperm into the developed organism is accomplished in part by the associations and dissociations of germinal units which lead to the formation of new materials, and in part by the segregation and localization of these in definite cells. Germ cells and probably all other kinds of cells are almost incredibly complex. We know that former students of the cell greatly under- estimated this complexity and there is no reason to suppose that we have fully compre- hended it. What Darwin said of the entire organism may now be said of every cell: “An organic being is a microcosm—a little universe, formed of a host of self-propagating organ- isms, inconceivably minute and numerous as the stars in heaven.” CHAPTER III PHENOMENA OF INHERITANCE CHAPTER III PHENOMENA OF INHERITANCE A. OBSERVATIONS ON INHERITANCE The observations of men in all ages have established the fact that in general “like pro- duces like,” and that, in spite of many excep- tions, children are in their main characteristics like their parents. And yet offspring are never exactly like their parents, and this has led to the saying that “like does not produce like but only somewhat like.”” What is meant is that there are general resemblances but particular differences between parents and offspring. INDIVIDUALS AND THEIR CHARACTERS In considering organic individuals one may think of them as wholes or as composed of parts, as indivisible unities or as constituent 197 198 HEREDITY AND ENVIRONMENT characters; either aspect is a true one and yet neither is complete in itself. Formerly in discussions on heredity the individual was re- garded in its entirety and when all hereditary resemblances and differences were averaged it was said that one child resembled the father, another child the mother. ‘This method of lumping together and averaging resemblances and differences led to endless confusion. In heredity no less than in anatomy it is neces- sary to deal with the constituents of organ- isms; in short, the organism must be analyzed and each part studied by itself. Francis Gal- ton was one of the first to brmg order out of chaos by dealing with traits or characters singly instead of treating all together. He made careful studies on the mheritance of weight and size in the seeds of sweet peas, and on the inheritance of stature, eye-color, intel- lectual capacity, artistic ability and certain diseases in man. At the same time that Gal- ton was thus laying the foundations for a scientific study of heredity by dealing with characters separately, another and_ even greater student of heredity, Gregor Mendel, PHENOMENA OF INHERITANCE 199 was doing the same thing in his experiments with garden peas, but inasmuch as Mendel’s work remained practically unknown for many years, Galton has been rightly recognized as the founder of the scientific study of heredity. Of course, neither Galton nor anyone else who has followed his method of dealing with the characters of organisms singly, ever sup- posed that such characters could exist inde- pendently of other characters and apart from the entire organism. This is such a self-evi- dent fact that it may seem needless to mention it, and yet there have been critics who have believed, or have assumed to believe, that modern students of heredity attempt to an- alyze organisms into independently existing characters, whereas in most cases they have done only what the anatomist does in treating separately the various organs of the body. HEREDITARY RESEMBLANCES AND DIFFERENCES The various characters into which an organ- ism may be analyzed show a greater or smaller degree of resemblance to the corresponding characters of its parents. Whenever the dif- 20U HEREDITY AND ENVIRONMENT ferential cause of a character is a germinal one the character is, by definition, mnherited; on the other hand, whenever this differential cause is environmental the character is not inherited. While it is true that inheritance is most clearly recognized in those characters in which offspring resemble their parents, even characters in which they differ from their parents may be inherited, as is plamly seen when, in any character, a child resembles a grandparent or a more distant ancestor more than either parent. Sometimes actually new characters arise in descendants which were not present in ascendants, but which are there- after inherited. Accordingly inherited char- acters may be classified as resemblances and differences, though both are determined by germinal organization, or heredity. ‘There is therefore no fundamental difference between inherited similarities and dissimilarities. Her- edity and variation are not opposing nor con- trasting tendencies which make offspring like their parents in one case and unlike them in another; really inherited characters may be like or unlike those of the parents. PHENOMENA OF INHERITANCE 201 On the other hand many resemblances and differences between parents and offspring are due not to heredity at all, but to environ- mental conditions. By means of experiment it is possible to distinguish between hereditary and environmental resemblances and differ- ences, but among men where experiments are generally out of the question it is often diffi- cult or impossible to make this distinction. I. Herepitary RESEMBLANCES 1. Racial Characters——All peculiarities which are characteristic of a race, species, genus, order, class and phylum are of course inherited, otherwise there would be no con- stant characteristics of these groups and no possibility of classifying organisms. ‘The chief characters of every living thing are unalter- ably fixed by heredity. Men do not gather grapes of thorns nor figs of thistles. Every living thing produces offspring after its own kind. Men, horses, cattle; birds, reptiles, fishes; insects, mollusks, worms; polyps, sponges, micro-organisms,—all of the million 202 HEREDITY AND ENVIRONMENT known species of animals and plants differ from one another because of inherited peculi- arities, because they have come from differ- ent kinds of germ cells. 2. Individual Characters—Many charac- ters which are peculiar to certain individuals are known to be inherited, and in general use the word “inheritance” refers to the repetition in successive generations of such individual peculiarities. Among such individual charac- ters are the following: (a) Morphological Features—Hereditary resemblances are especially recognizable in the gross and minute anatomy of every organism, in the form, structure, location, size, color, ete., of each and every part. The number of such individual peculiarities which are inherited is innumerable and only a few of the more strik- ing of these can be mentioned. It is a matter of common knowledge that unusually great or small stature runs in cer- tain families, and Galton developed a formula for determining the approximate stature of children from the known stature of the parents and from the mean stature of the race. How- PHENOMENA OF INHERITANCE 203 ever, his statistical and mathematical formule give only general or average results, from which there are many individual departures and exceptions. In the same way the color of the skin, the color and form of hair and the color of eyes are in general like those of one or more of the parents or grandparents. We all know that certain facial features such as the shape and size of eyes, nose, mouth and chim are generally characteristic of certain families. But the inheritance of anatomical features extends to much more minute characters than those just mentioned. In certain families a few hairs in the eyebrows are longer than the others, or there may be patches of parti- colored hair over the scalp, or dimples in the cheek, chin, or other parts of the skin may oc- cur, and these triflmg peculiarities are in- herited with all the tenacity shown in the transmission of more important characters. Johannsen has found races of beans in which the average weight of individual seeds differed only by .02 to .03 gram, and yet these minute differences in weight were characteristic of 204 HEREDITY AND ENVIRONMENT each race and were of course inherited. Jen- nings has found races of Parameciwm which show hereditary differences of .005 mm. in average length (Fig. 44). Nettleship says nvygcgeage tenant Ayaaiedaeeedecaaae Aaaaaqecesecaeace iN | abt ntageracaeadas = GOO RIIKIOIUOEG B68 FAAMHIGGIGGG8 006.08 o (14000440006 008 | (AGOGO 090 seus ee oe th i" ; ae tl “a ae aa ; in a millimeter), x 43. (After Jennings.) PHENOMENA OF INHERITANCE 205 that the lens of the human eye weighs only 175 milligrams, or about one three-millionth part of the body weight, and in hereditary cataract only about one twentieth part of the lens be- comes opaque, and yet this minute fraction of the body weight shows the influence of hered- ity. Even the size, shape and number of the cells in certain organs, and in given embry- onic stages, may be repeated generation after generation; and if our analysis were suffi- ciently complete we should doubtless find that even the minute parts of cells, such as nuclei, chromosomes and centrosomes, show individ- ual peculiarities which are inherited. (b) Teratological and Pathological Peculi- arities are really only unusual or abnormal an- atomical characters, but of such interest and importance as to deserve special mention. Many such abnormalities are undoubtedly in- herited, among which are the following: poly- dactylism, in which more than the normal number of digits are present; syndactylism, or a condition of webbed fingers and toes; brachydactylism, in which the fingers are short and stumpy and usually contain less than the 206 HEREDITY AND ENVIRONMENT normal number of joints (Fig. 67) ; acondro- plasy, or short and crooked limbs, such as occur in certain breeds of dogs and sheep and in cer- tain human dwarfs (Fig. 68); myopia, in which the eyeball is elongated; glaucoma, or pressure within the eyeball; coloboma, or open suture of the iris; otosclerosis, or rigidity of tympanum and ossicles, causing “hardness of hearing’; some forms of deaf-mutism, due to certain defects of the inner ear; and many other characters too numerous to mention here. On the other hand many abnormal or mon- strous conditions are due to abnormal environ- ment and are nct inherited. The question of the inheritance of diseases may be briefly considered here. If a disease is due to some defect in the hereditary constitu- tion, it is inherited; otherwise, according to our definition of heredity, it is not. Of course no disease develops without extrinsic causes but when one individual takes a disease while another under the same conditions does not, the differential cause may be an inherited one, or it may be due to differences in the previous conditions of life. ‘There is no doubt that PHENOMENA OF INHERITANCE 207 certain diseases run in families and have the appearance of being inherited, but in this case as in many others it is extremely difficult in the absence of experiments to distinguish be- tween effects due to intrinsic causes and those due to extrinsic ones. Where the specific cause of a disease is some micro-organism the individual must have been infected at some time or other, almost invariably after birth. In few instances is the oosperm itself infected, and even when it is, this is not, strictly speak- ing, a case of inheritance, but rather one of early infection. Pearson has found that there is a marked correlation (represented by the number .55 when complete correlation is 1.) between tuberculous parents and_ tubercu- lous children, but there is very little evidence that the child is ever infected before birth. What is inherited in this case is probably slight resistance to the tubercle bacillus. There is evidence that almost all adult persons have been infected at one time or another by this bacillus, but it has not developed far in all of them because some have superior powers of resistance. Such greater or smaller resistance, 208 HEREDITY AND ENVIRONMENT stronger or weaker build, is inherited, and while diminished resistance is not the direct cause of tuberculosis it is a predisposing cause. The same is probably true of many other diseases, the immediate causes of which are extrinsic, while only the more remote, or pre- disposing causes, are hereditary. (c) Physiological peculiarities are inherited as well as morphological ones; indeed function and structure are only two aspects of one and the same thing, namely organization. For all morphological characters there are functional correlatives, for functional characters morpho- logical expressions, and if the one is inherited so is the other. But there are certain char- acters im which the physiological aspect is more striking than the morphological one. For example, longevity is a physiological character which is undoubtedly dependent upon many causes, but in the case of species which differ greatly in length of life there can be little doubt that we are dealing with an in- herited character. The great differences in the length of life of an elephant and a mouse, of a parrot and a pigeon, of a cicada and a PHENOMENA OF INHERITANCE 209 squash bug, are as surely the result of in- herited causes as are the structural differences between those animals. Within the same species different races or lines show character- istic differences in length of life; in the case of man the average length of life is much greater in some families than in others, and life-insurance companies take account of this fact. Even within the same organism certain organs or cells are short-lived, whereas others are long-lived; some cells and organs live only through the early embryonic period, while others live as long as the general organism. Obesity is another physiological character- istic which may be inherited; the members of certain families grow fat in spite of themselves, while members of other families remain thin however well fed they may be. Here also many factors enter into the result, but it seems probable that the differentiating factor is an hereditary one. Baldness affects the male members of certain families when they have reached a given age, while in others neither care, dissipation nor age can rob a man of his bushy top. Hemophilia, or excessive bleed- 210 HEREDITY AND ENVIRONMENT ing after an injury, which is due to a defi- ciency in the clotting power of the blood, is strongly inherited in the male line in certain families. Fecundity and a tendency to bear twins or triplets, left-handedness, a peculiar lack of resistance to certain diseases, and many other physiological peculiarities are probably inherited. ‘(d) Psychological characters appear to be inherited in the same way that anatomical and physiological traits are; indeed all that has been said regarding the correlation of morpho- logical and physiological characters applies also to psychological ones. No one doubts that particular instincts, aptitudes and ca- pacities are inherited among both animals and men, nor that different races and species differ hereditarily in psychological characteristics. Certain breeds of dogs such as the mastiff, the bull dog, the terrier, the collie and many others are characterized by peculiarities of temperament, affection, intelligence and dis- position. No one who has much studied the subject can doubt that different human races and families show characteristic differences in PHENOMENA OF INHERITANCE 211 these same respects. It is quite futile to argue that exceptional individuals may be found in one race with the mental characteristics of an- other race; the same could be said of different breeds of dogs, or of the sizes of different races of beans or of Paramecia (Fig. 44). The fact is that racial characteristics are not determined by exceptional and extreme individuals but by the average or mean qualities of the race; and measured in this way there is no doubt that certain types of mind and disposition are char- acteristic of certain families. There is no longer any question that some kinds of feeble-mindedness, epilepsy and in- _ sanity are inherited, and that there is often an hereditary basis for nervous and phlegmatic temperaments, for emotional, judicial and calculating dispositions. Nor can it be de- nied that strength or weakness of will, a ten- dency to moral obliquity or rectitude, capacity or incapacity for the highest intellectual pur- suits, occur frequently in certain families and appear to be inherited. In spite of certain noteworthy exceptions, which may perhaps be due to remarkable variations, statistics col- 212 HEREDITY AND ENVIRONMENT lected by Galton show that genius runs in cer- tain families; while the work of certain recent investigators, particularly Goddard, Daven- port and Weeks, proves that feeble-mindedness and epilepsy are also inherited; and the careful work of Mott and of Rosanoff leaves no room for doubt that certain types of insanity are hereditary. It frequently happens that fami- lies in which hereditary insanity occurs also have other members afflicted with epilepsy, hysteria, alcoholism, etc., which would indicate that the thing inherited is an unstable condi- tion of the nervous system which may take various forms under slightly different condi- tions. Indeed there is a good deal of evidence that extraordinary ability, or genius, is fre- quently associated with an unstable nervous organization which may sometimes take the form of insanity or epilepsy or alcoholism. There is perhaps more truth than poetry in Dryden’s lines: ‘Great wits are sure to madness near allied, And thin partitions do their bounds divide.” Woods has collected data concerning ‘“Hered- ity in Royalty” which seem to show that PHENOMENA OF INHERITANCE 213 very high or low grades of intellect and virtues may be traced through the royal fami- lies of Europe for several generations. The general tendency of recent work on heredity is unmistakable, whether it concerns man or lower animals. ‘The entire organism, consisting of structures and functions, body and mind, develops out of the germ, and the organization of the germ determines all the possibilities of development of the mind no less than of the body, though the actual reali- zation of any possibility is dependent also upon environmental stimuli. ’ II. Herepirary DIFrrereNnces There are many limitations or exceptions to the general rule that children resemble their parents. Sometimes these differences are due to new combinations of ancestral characters, sometimes they are actually new characters not present so far as known in any of the an- cestors, though even such new characters must arise from new combinations of the elements of old characters, as we shall see later. 1. New Combinations of Characters.—In all cases of sexually produced organisms new 214 HEREDITY AND ENVIRONMENT combinations of ancestral characters are evi- dent. Usually a child inherits some traits from one parent and other traits from the other parent, so that it is a kind of mosaic of ancestral traits. Such inheritance, bit by bit, of this character from one progenitor and that from another was described by Galton as “par- ticulate” (Fig. 45). On the other hand Galton supposed that in some instances a child might inherit all or nearly all of his traits from one parent, a thing which is most improbable; such inheritance he called “alternative” (Fig. 45). BLENDING ALTERNATIVE PARTICULATE og : Oe OM Fic. 45. Diagram to illustrate three kinds of inheritance described by Galton. (After Walter.) PHENOMENA OF INHERITANCE 215 In other cases the traits of the parents ap- pear to blend in the offspring, as for example, in the skin color of mulattoes; such cases were called by Galton “blending” inheritance (Fig. 45). Sometimes characters appear in off- spring which were “latent” in the parents but were “patent” in one or more of the grand- parents; such skipping of a generation, during which a character remains “latent,” has long been known as “atavism.” At other times characters which were present in distant ances- tors, but which have since dropped out of sight or have remained “latent,” reappear in descendants; such cases are known as “reversions.” ; In still other cases certain characters ap- pear only in the male sex, others only in the female, this being called “sex-limited” inheri- tance; while in some instances characters are transmitted from fathers through daughters to grandsons or from mothers to sons, all such cases being known as “‘sex-linked” inheritance. 2. New Characters or Mutations—But in addition to these permutations in the distri- bution and combination of ancestral characters 216 HEREDITY AND ENVIRONMENT new and unexpected characters sometimes de- velop in the offspring, which were not present, so far as known, in any of the ascendants, but which, after they have once appeared, are passed on by heredity to descendants. Such inherited variations are usually of two kinds, continuous or slight, and discontinuous or sudden variations. The latter are especially noticeable when variations occur in the normal number of parts, as in four-leaved clover, or six-fingered men, and such numerical varia- tions have been called by Bateson “meristic.” However, sudden variations may include any marked departure from the normal type, in color, shape, size, chemical compositions, etc. Such sudden variations have long been known to breeders as “sports,” and both Darwin and Galton pointed out the fact that such sports have sometimes given rise to new races or breeds, though Darwin was not inclined to assign much importance to them in the gen- eral process of evolution. Galton, on the other hand, maintained that variations, or what would now be called “‘continuous variations,” cannot be of much significance in the process PHENOMENA OF INHERITANCE Q17 of evolution, but that the case is quite different with “sports” (“Hereditary Genius,” prefa- tory chapter). | More recently the entire biological world has been greatly influenced by the “Mutation Theory” of deVries, which has placed a new emphasis upon the importance of sudden vari- ations in the process of evolution. At first deVries was inclined to emphasize the degree of difference, that is the discontinuity, in these variations, but in later works this distinction is given a minor place as compared with the question whether variations are inherited or not. Inherited variations, whether large or small, are called by deVries “mutations,” whereas non-inherited variations are known as “fluctuations.” The former are caused by changes in germinal constitution, the latter by alterations in environmental conditions; the former represent changes in heredity, the latter changes in development. 3. Mutations and Fluctuations —This clear cut distinction between mutations and fluctu- ations marks one of the most important ad- vances ever made in the study of development 218 HEREDITY AND ENVIRONMENT and evolution. ‘Thousands of fluctuations oc- cur which are purely somatic in character and which do not affect the germ cells, for every single mutation or change in the hereditary constitution; and yet only the latter are of significance in heredity and evolution. This distinction between variations due to environ- ment (fluctuations) and those due to hered- — itary causes (mutations) was recognized by Weismann and many of his followers, but the actual demonstration on a large scale of the importance of this distinction is due mainly to deVries. All hereditary variations, whether due to new combinations of old characters or to the appearance of actually new characters, whether small and continuous or large and discontinuous, have their causes in the organi- zation of the germ cells, just as do inherited resemblances. Heredity is not to be con- trasted with variation, nor are hereditary like- ness and unlikeness due to _ conflicting principles; both are the results of germinal organization and both are phenomena of heredity. 4. Every Individual Unique.—As a result PHENOMENA OF INHERITANCE 219 of the permutations of ancestral characters, the appearance of mutations, and the fluctua- tions of organisms due to environmental changes, it happens that in all cases offspring differ more or less from their parents and from one another. No two children of the same family are ever exactly alike (except in the case of identical twins which have come from the same oosperm). Every living being ap- pears on careful examination to be the first and last of its identical kind. | This is one of the most remarkable peculiarities of living things. The elements of chemistry are con- stant, and even the compounds fall into definite categories which have constant charac- teristics. But the individuals of biology are apparently never twice the same. This may be due to the immense complexity of living units as contrasted with chemical ones,—in- deed lack of constancy is evidence in itself of lack of analysis into real elements or of lack of uniform conditions,—but whatever its cause the extraordinary fact remains that every liv- ing being appears to be unique. “Reproduc- tion is the generation of unique beings that 220 HEREDITY AND ENVIRONMENT are, on the average, more like their kind than like anything else” (Brooks). There seems to be no reason to doubt that all the extraordinary differences which organ- isms show, as well. as all of their resemblances, are due to differences or resemblances in the hereditary and environmental factors which have been operative in their development. But in view of this universal variability of organ- isms it is not surprising that inheritance has seemed capricious and uncertain,—‘‘a sort of maze in which science loses itself.” B. STATISTICAL STUDY OF INHERITANCE Francis Galton was one of the first who at- tempted to reduce the mass of conflicting ob- servations on heredity and variation to some system and to establish certain principles as a result of statistical study. He was the real founder of the scientific study of inheritance; he studied characters singly and he introduced quantitative measures. Galton’s researches, which were published in several volumes, con- sisted chiefly in a study of certain families PHENOMENA OF INHERITANCE 221 with regard to several selected traits, viz., genius or marked intellectual capacity, artis- — tic faculty, stature, eye color and disease. As a result of his very extensive studies two main principles appeared to be established: 1. The Law of Ancestral Inheritance which he stated as follows: The two parents contribute between them on the average one-half of each inherited faculty, each of them contributing one-quarter of it. The four grandparents contribute between them one-quarter, or each of them one-sixteenth; and so on, the sum of the series 1/2 + 1/4 + 1/8 + 1/16... being equal to 1, as it should be. It is a property of this infinite series that each term is equal to the sum of all those that follow: thus 1/2 = 1/4 + 1/8 + 1/16 eee er Aa / Sh /AiGeal- beeen aNd SOON: The prepotencies of particular ancestors in any given pedigree are eliminated by a law which deals only with average contributions, and the various prepotencies of sex with respect to different quali- ties are also presumably eliminated. The average contribution of each ancestor was thus stated definitely, the contribution di- minishing with the remoteness of the ancestor. This Law of Ancestral Inheritance ‘is repre- sented graphically in the accompanying dia- 222 HEREDITY AND ENVIRONMENT gram (Fig. 46). Pearson has somewhat modified the figures given by Galton, holding that in horses and dogs the parents contribute 1/2, the grandparents 1/8, the great grand- parents 2/9, etc. Theoretically the number of ancestors Parents Grand Pis Saae slle[e [ele] a[e bam 2{s].4{o}s|2[a[e] [2] s[e[e[ela[s]ot ot oi rte Fic. 46. Diagram of Galton’s “Law of Ancestral Inheri- tance.” The whole heritage is represented by the entire rect- angle; that derived from each progenitor by the smaller squares; the number of the latter doubles in each ascending generation while its area is halved. (After Thompson.) doubles in each ascending generation; there are two parents, four grandparents, eight ereat-grandparents, ete. If this continued to be true indefinitely the number of ancestors in any ascending generation would be (2)*, in PHENOMENA OF INHERITANCE 223 which n represents the number of generations. There have been about 57 generations since the beginning of the Christian Era, and if this rule held true indefinitely each of us would have had at the time of the birth of Christ a number of ancestors represented by (2)°" or about 120 quadrillions,—a number far greater than the entire human population of the globe since that time. As a matter of fact, owing to the intermarriage of cousins of various de- grees the actual number of ancestors is much smaller than the theoretical number. For ex- ample, Plate says that the present Emperor of Germany had only 162 ancestors in the 10th ascending generation, instead of 512, the theoretical number. Nevertheless this calcu- lation will serve to show how widespread our ancestral lines are, and how nearly related are all people of the same race. Davenport concludes that no people of ‘English descent are more distantly related than 30th cousins, while most people are much more closely related than that. If we allow three generations to a century, and calculate that the degree of cousinship is determined by 224 HEREDITY AND ENVIRONMENT the number of generations less two, since first cousins appear only in the third generation, the first being that of the parents and the sec- ond that of the sons and daughters, we find that 80th cousins at the present time would have had a common ancestor about one thou- sand years ago or approximately at the time of William the Conqueror. As a matter of fact most persons of the same race are much more closely related than this, and certainly we need not go back to Adam nor even to Shem, Ham, or Japheth to find our common ancestor. 2. The second principle which Galton de- duced from his statistical studies is known as the Law of Filial Regression, or what might be called the tendency to mediocrity. He found that, on the average, extreme peculiari- ties of parents were less extreme in children. Thus, “the stature of adult offspring must on the whole be more mediocre than the stature of their parents, that is to say more near to the mean or mid of the general population”; and again, “the more bountifully a parent is gifted by nature, the more rare will be his PHENOMENA OF INHERITANCE 225 good fortune if he begets a son who is as richly endowed as himself.” ‘This so-called law of filial regression is represented graphi- cally in Fig. 47 in which the actual stature of individual parents is shown by the oblique line, the stature of children by the dotted curve, and the mean stature of the race in the horizontal dotted line. One of the chief aims and results of statis- tical studies is to eliminate individual peculiari- ties and to obtain general and average results. Such work may be of great importance in the study of heredity, especially where questions of the occurrence or distribution of particular phenomena are concerned; but the causes of heredity are individual and physiological, and averages are of less value in finding the causes of such phenomena than is the intensive study of individual cases. By observation alone it is usually impossi- ble to distinguish between inherited and en- vironmental resemblances and differences, and yet this distinction is essential to any study of inheritance. If all sorts of likenesses and unlikenesses are lumped together, whether in- 226 HEREDITY AND ENVIRONMENT Inches ies) 69 68 4 67 A nor (/) 66 res 65 YY Gti 63 Fic. 47. Scheme to illustrate Galton’s “Law of Filial Re- gression” as shown in the stature of parents and children. The mean height of all parents is shown by the dotted line between 68 and 69 inches. The circles through which the diagonal line runs represent the heights of graded groups of parents and the arrow heads indicate the average heights of their children. The offspring of undersized parents are taller and of oversized parents are shorter than their respective parents. (After Walter.) PHENOMENA OF INHERITANCE 227 herited or not, our study of inheritance can only end in confusion. ‘The value of statistics depends upon a proper classification of the things measured and enumerated, and if things which are not commensurable are grouped together the results may be quite misleading and worthless. Unfortunately Galton and Pearson, as well as some of their followers, have not always carefully distin- guished between hereditary and environmental characters. Furthermore much of their ma- terial was drawn from a general population in which were many different families and lines not closely related genetically. Consequently their statistical studies are of little value in dis- covering the physiological principles or laws of heredity. Jennings (1910) well says, “Galton’s laws of regression and of ancestral! inheritance are the product mainly of a lack of distinction between two absolutely diverse things, between non-inheritable fluctuations on the one hand, and permanent genotypic differentiations on the other.” In the case of man we have few certain tests to determine whether the differential cause of any character 228 HEREDITY AND ENVIRONMENT is hereditary or environmental, but in the case of animals and plants, where experiments may be performed on a large scale, it is possible to make such tests by (1) experiments in which the environment is kept as uniform as possible while the hereditary factors differ, and (2) ex- periments in which, in a series of cases, the hereditary factors are fairly constant while the environment differs. In this way the dif- ferential cause or causes of any character may be located in heredity, in environment or in both. The observational and statistical study of inheritance helped to outline the problem but did little to solve it. Certain phenomena of hereditary resemblance between ascendants and descendants were made intelligible, but there were many peculiar and apparently ir- regular or lawless phenomena which could not be predicted before they occurred nor ex- plained afterward. For example when Dar- win crossed different breeds of domestic pigeons, no one of which had a trace of blue in its plumage, he sometimes obtained off- spring with more or less of the blue color and PHENOMENA OF INHERITANCE 229 markings of the wild rock pigeon from which domestic pigeons are presumably descended. He described many cases of dogs, cattle and swine, as well as many cultivated plants, in which offspring resembled distant ancestors and differed from nearer ones; such cases had long been known and were spoken of as “re- versions.” He observed many cases in which certain characters of one parent prevailed over corresponding characters of the other parent in the offspring, this bemg known as “pre- potency”; but there was no satisfactory ex- planation of these curious phenomena. They did not come under either of Galton’s laws, and their occurrence was apparently so ir- regular that every such case seemed to be a law unto itself. 230 HEREDITY AND ENVIRONMENT C. EXPERIMENTAL STUDY OF INHERI- TANCE Il. MENDELISM The year 1900 marks the beginning of a new era in the study of inheritance. In the spring of that year three botanists, deVries, Correns, and ‘Tschermak, discovered inde- pendently an important principle of heredity and at the same time brought to light a long neglected and forgotten work on “Experi- ments in Plant Hybridization” by Gregor Mendel, in which this same principle was set forth in detail. This principle is now gener- ally known as “Mendel’s Law.” Mendel, who was a monk, and later abbot, of the Kénigs- kloster, an Augustinian monastery in Brinn, Austria, published the results of his experi- ments on hybridization in the Proceedings of the Natural History Society of Brinn in 1866. The paper attracted but little attention at the time although it contained some of the most important discoveries regarding inheri- tance which had ever been made, and it re- PHENOMENA OF INHERITANCL 931 mained buried and practically unknown for thirty-five years. Plant hybridization had been studied extensively before Mendel began his work, but he carried on his observations of the hybrids and of their progeny for a longer time and with greater analytical ability than any previous investigator had done. ‘The methods and results of his work are so well known through the writings of Bateson, Pun- nett, and many others that it is unnecessary to dwell at length upon them here. In brief Mendel’s method consisted in crossing two forms having distinct characters, and then in counting the number of offspring in successive generations showing one or the other of these characters. During the eight years preceding the publi- cation of his paper in 1866 Mendel hybridized some twenty-two varieties of garden peas. This group of plants was chosen because the different varieties could be cross-fertilized or self-fertilized and were easily protected from the influence of foreign pollen; because the hybrids and their offspring remained fertile through successive generations; and because 232 HEREDITY AND ENVIRONMENT the different varieties are distinguished by constant differentiating characters. Mendel devoted his attention to seven of these charac-_ ters, which he followed through several gener- ations of hybrids, viz., (1) Differences in the form of the ripe seeds, whether round or wrinkled. (2) Differences in the color of the food material within the seeds, whether pale yel- low, orange or green. (3) Differences in the color of the seed coats (and in some cases of the flowers also), whether white, gray, gray brown, leather brown, with or without violet spots. (4) Differences in the form of the ripe pods, whether simply inflated or constricted between the seeds. (5) Differences in the color of the unripe pods, whether light to dark green, or vividly yellow. (6) Differences in the positions of the flowers, whether axial, that is distributed along the stem, or terminal, that is bunched at the top of the stem. PHENOMENA OF INHERITANCE 233 (7) Differences in the length of the stem, whether tall or short. 1. Results of Crossing Individuals with one Pair of Contrasting Characters—Having de- termined that these characters were constant for certain varieties (or species) Mendel then proceeded to cross one variety with another, by carefully removing the unripe stamens, with their pollen, from the flowers of one variety and dusting upon the stigmas of such flowers the pollen of a different variety. In this way he crossed varieties of peas which differed from each other in some one of the characters men- tioned above, and then studied the offspring of several successive generations with respect to this character. In every case he discovered that the plants that developed from such a cross showed only one of the two contrasting characters of the parent plants, 7. e. all were round-seeded, yel- low-seeded, tall, etc., although one of the parents had wrinkled seeds, green seeds, or short stem, ete. ‘Those characters which are transmitted entire or almost unchanged in the hybridization are termed dominant, and those 234 HEREDITY AND ENVIRONMENT which become latent in the process, recessive.” These hybrids* when self-fertilized gave rise to a second filial generation of individuals some of which showed the dominant character and others the recessive, the relative numbers of the two being approximately three to one. Thus the hybrids produced by crossing yellow- seeded and green-seeded peas yielded when self-fertilized 6,022 yellow seeds and 2,001 green seeds, or very nearly three yellow to one green (Fig. 48). The hybrids produced by crossing round and wrinkled seeded varie- ties yielded in the second filial generation 5,474 round and 1,850 wrinkled seeds, or approxi- mately three round to one wrinkled (Fig. 52). The hybrids from tall-stemmed and short- stemmed parents produced in the second filial generation 787 long-stemmed and 277 short- stemmed plants, or again approximately three ‘Bateson introduced the term “homozygote” for pure-bred individuals resulting from the union of gametes which are hereditarily similar, and “heterozygote” for hybrids resulting from the union of hereditarily dissimilar gametes. The gametes formed from a homozygote are all of the same hereditary type, those formed from a heterozygote are of two different types for every unit difference of the parents. PHENOMENA OF INHERITANCE 233 PARENTS Fic. 48. Diagram showing the results of crossing yellow- seeded (lighter colored) and green-seeded (darker colored) peas. (From Morgan after Thompson.) 236 HEREDITY AND ENVIRONMENT tall to one short. And in every other case Mendel found that the ratio of dominants to recessives in the second filial generation was approximately three to one. ‘These recessives derived from hybrid parents are pure and are known as “extracted” recessives; when self- fertilized they produce recessives indefinitely. One-third of the dominants are also pure hom- ozygotes, or “extracted” dominants, and when self-fertilized produce pure dominants indefi- nitely. On the other hand two-thirds of the dominants are heterozygotes and when self- fertilized give rise in the next generation to pure dominants, mixed dominant-recessives and pure recessives in the proportion of 1: 2: 1. These general results are sum- marized in the accompanying diagram (Fig. 49) in which dominant characters are indicated by the letter D, recessive characters by R, and mixed dominant-recessives, with the recessive character unexpressed, by D (R&R); while DD or RR indicate extracted dominants or reces- sives, that is, pure dominants or recessives which have separated out from mixed domi- nant-recessives, D (Rf). The parental gener- PHENOMENA GF INHERITANCE 237 ation is indicated by the letter P, and the suc- cessive filial generations by F',, F2, F3, etc. In the case of the peas studied by Mendel the hybrids of the F, generation show only the dominant character, the contrasted recessive character being present but not expressed. However in certain cases it has been found that the hybrids differ from either parent and in successive generations split up into both parental types and into the hybrid type; thus Correns found that when a white-flowered va- riety of Mirabilis, the four o’clock, was crossed with a red-flowered variety all of the hybrids DR D R Parent Generation @X O XO Homozygotes Fi a XY D(R) D(R)® Heterozygqotes Fe Op DD(R D DUR OR Fs D D Or D D OO Fé i OM 1 Dy mele Dm O80 PM OOO F5 DOOO / DOO O 8 OOOO DOOO OO Fic. 49. Diagram showing results of Mendelian splitting where the parents are pure dominants and pure recessives (homozygotes). All pure dominants are represented by black circles, all pure recessives by white ones, while mixed domi- nant-recessives (heterozygotes) are represented by circles half white and half black. Successive generations are marked F.,, Hae uete: 938 HEREDITY AND ENVIRONMENT in the F; generation had pink flowers and from these in the F, generation there came white- flowered, pink-flowered and red-flowered forms in the proportion of 1 white : 2 pink : 1 red, as shown in Fig. 50. This is a better illustra- tion of Mendel’s principle of splitting than is PARENTS Fic. 50. Results of crossing white-flowered and red-flow- ered races of Mirabilis Jalapa (four o’clocks) giving a pink hybrid in F,, which when inbred gives in F, 1 white, 2 pink, 1 red. (From Morgan, after Correns.) PHENOMENA OF INHERITANCE 239 offered by the peas, since in this case the mixed dominant-recessives D (#) are always distin- guishable from the pure dominants DD. In the F, generation and in all subsequent ones the pure dominants and the pure reces- sives always breed true when self-fertilized, whereas the mixed dominant-recessives con- tinue to split up in each successive generation into pure dominants, mixed dominant-reces- sives and pure recessives in the proportion 1:2:1. The result of this is that the relative number of dominants and recessives increases in successive generations, whereas the relative number of mixed dominant-recessives de- creases, and in a few generations a hybrid race will revert in large part to its parental types if contmued hybridization is prevented. On. the other hand there is no tendency for the relative number of dominants to increase and of recessives to decrease in successive genera- tions; an equal number of pure dominants and pure recessives is produced in each generation. With remarkable insight Mendel recog- nized that the real explanation of the splitting of pure recessives and pure dominants from 240 HEREDITY AND ENVIRONMENT hybrid parents must be found in the composi- tion of the male and female sex cells. Since such extracted dominants and recessives breed true, just as pure species do, it must be that their germ cells are pure. In the cross be- tween pure races of white-flowered and red- flowered Mirabilis the germ cells which unite in fertilization must be pure with respect to white and red, though the individual which develops from this cross is a pink hybrid. But the fact that one-quarter of the progeny of this hybrid are pure white, and another quarter pure red, and that these thereafter breed true, proves that the hybrid produces germ cells which are pure with respect to red and white. Further- more the fact that one-half the progeny of this _ hybrid are themselves hybrid may be explained by assuming that they were produced by the union of germ cells carrying pure white and pure red, as in the parental generation. Mendel therefore concluded that individual germ cells are always pure with respect to any pair of contrasting characters, even though those germ cells have come from hybrids in which the contrasting characters are mixed. PHENOMENA OF INHERITANCE 241 A single germ cell can carry the factor for red flowers or white flowers, for green seeds or yellow seeds, for tall stem or short stem, etc., but not for both pairs of these contrasting characters. ‘The hybrids formed by crossing white and red four o’clocks carry the factors for both white and red, but the in- dividual germ cells formed by such a hybrid carry the factors for white or red, but not for both; these factors segregate or separate in the formation of the germ cells so that one-half of all the germ cells formed carry the factor for white and the other half that for red. This is the most important part of Mendel’s Law,—the central doctrine from which all other conclusions of his radiate. It explains not only the segregation of dominant and re- cessive characters from a hybrid in which both are present, but also the relative numbers of pure dominants, pure recessives and mixed dominant-recessives in each generation. For if all germ cells are pure with respect to any particular character the hybrid offspring of any two parents with contrasting characters will produce in equal numbers two classes of 24.2 HEREDITY AND ENVIRONMENT germ cells, one bearing the dominant and ihe other the recessive factor, and the chance com- bination of these two classes of male and fe- male gametes will yield on the average one union of dominant with dominant, two unions of dominant with recessive and one union of recessive with recessive, thus producing the typical Mendelian ratio, 1DD : 2D(R) : 1 RR, as shown in the accompanying diagram, (Hier 5i)asib)’ Fic. 51. Diagram of Mendelian inheritance, in which the individual is represented by the large circle, the germ cells by the small ones, dominants being shaded and _ recessives white. a, Pure dominant pure recessive = all dominant- recessives, b, Dominant-recessive dominant-recessive = 1 pure dominant : 2 dominant-recessives : 1 pure recessive, c, Dominant-recessive pure dominant — 2 pure dominant : 2 dominant-recessive, d, Dominant-recessive \< pure reces- sive — 2 dominant-recessive : 2 pure recessive. PHENOMENA OF INHERITANCE 243 Other Mendelian Ratios When a pure dominant is crossed with a mixed dominant-recessive (Fig. 51 c) all the offspring show the dominant character, though one-half are pure dominants and the other half dominant-recessives. Thus if a pure round- seeded variety of pea is crossed with a hybrid between a round-seeded and a wrinkled-seeded one, all the progeny are round-seeded, though one half of them carry the factor for wrinkled seed; this may be graphically represented as follows, # representing the factor for round seed and W that for wrinkled seed: é germ cells it ane | 2 germ cells Possible combinations 2 RR:2 R (W). In subsequent generations the progeny of the pure round (RR) breed true and produce only round-seeded peas, whereas the progeny of the hybrid round-wrinkled (RW) split up into pure round, hybrid round-wrinkled, and pure wrinkled in the regular Mendelian ratioof 1RR:2R(W) :1WW (Fig. 52). 244, HEREDITY AND ENVIRONMENT When a pure recessive is crossed with a mixed dominant-recessive (Fig. 51, d) another typical ratio results. Thus if a wrinkled- seeded variety of pea is crossed with a hybrid between a round- and wrinkled-seeded one, round-seeded and wrinkled-seeded peas are produced in the proportion of 1:1. ‘This is due to the fact that the hybrid produces two kinds of germ cells, the pure-bred but one, and the possible combinations of these are as follows: 2 germ cells W Y W é germ cells E ares be Possible combinations 2 RW:2 WW. This ratio of 1 : 1 is approximately the ratio of the two sexes in many animals and plants, and there is good reason to believe that sex is a Mendelian character of this sort, in which one parent is heterozygous for sex and the other homozygous (Fig. 51, d). 2. Results of Crossings where there 1s more than one Pair of Contrasting Characters.—It rarely happens that two individuals differ in a single character only; more frequently they PHENOMENA OF INHERITANCE Q45 differ in many characters, and this leads to a great increase in the number of types of off- spring in the F, generation. But however many pairs of contrasting characters the par- ents may show each pair may be considered by itself as if it were the only contrasting pair, and when this is done all the offspring may be classified according to the regular Mendelian formula given above. But when two or more contrasting charac- ters of the parents are followed to the F: gen- eration many permutations of these characters occur, thus giving rise to a larger number of types of individuals than when a single pair of characters is concerned. When there is only one pair of contrasting characters there are usually but two types of offspring apparent in the IF’, generation, viz., dominants and reces- sives in the ratio of 3:1 (Fig. 52); where there are two pairs of contrasting characters in the parents there are four types of offspring in the F generation in the ratio of (3 : 1)? = 9 : 3:3: 1; when there are three pairs of con- trasting characters in the parents there are eight types of offspring apparent in the F, 246 HEREDITY AND ENVIRONMENT generation in the proportions of (3 : 1)? = 27 :9:9:9:3:3:3:1, ete. Thus when Mendel crossed a variety of peas bearing round and yellow seeds with another variety having wrinkled and green seeds all the offspring of the F', generation bore round and yellow seeds, round being dominant to wrinkled, and yellow to green. But the plants raised from these seeds, when self-fertilized, yielded seeds of R W Sacer R(W) R(W) R(W) R(W) Ce} R Ww : aaa QO] @ 1O}@ Fic. 52. Monohybrid Diagram showing results of crossing round (R) seeded with wrinkled (W) seeded peas. Large cir- cles represent zygotes, small ones, or single letters, gametes. In F, all individuals are round but contain round and wrinkled gametes. In F, the ¢ gametes are placed above the square, the 2 ones to the left, and the possible combinations of g and @ gametes are shown in the small squares, the relative numbers of different types being 1 RR:2 R(W):1 WW. Fa PHENOMENA OF INHERITANCE Q47 four types, round and yellow (RY), wrinkled and yellow (WY), round and green (#G), and wrinkled and green (WG) in the propor- tion of 9 : 3 : 3: 1 as shown in Fig. 53. In this case also this ratio may be explained P YR Fi Y R(GW) g YR YW GR GW = SYR \VYR \/YR V/ YR YR J, YW GR \ GW IN Wi wy i : Fp Cae GR \//GR \~ GR ae coh : sw “GW MG Wh e YW }6a Fic. 53. Dihybrid Diagram showing results of crossing peas having yellow-round (YR) seeds with others having green- wrinkled (GR) ones. Four types of germ cells are formed by such a hybrid, viz. YR, YW, GR, GW, and the 16 possible combinations (genotypes) of these ¢ and 2 gametes are shown in the small squares. Since recessive characters do not ap- pear when mated with dominant ones these 16 genotypes pro- duce 4 phenotypes in the following relative numbers: 9YR: 3YW:3GR:1GW. There is 1 pure dominant (upper left cor- ner), 1 pure recessive (lower right corner), 4 homozygotes in diagonal line between these corners, and 12 heterozygotes. 248 HEREDITY AND ENVIRONMENT by assuming that the germ cells (ovules and pollen) are pure with respect to each of the contrasting characters, round-wrinkled, yel- low-green, and therefore any combination of these may occur in a germ cell except the com- binations RW and YG. Accordingly there are four possible kinds of germ cells as follows: bg G | Ne | ie VR,YW, GR, GW. RC Ae, Each of these four kinds of pollen may fertilize any of the four kinds of ovules, thus giving rise to sixteen combinations, no two of which are alike, as shown in Fig. 58. ‘The dominant characters are in this case round and yellow, and only when one of these is absent can its contrasting character, wrinkled or green, develop. Accordingly the sixteen possible combinations yield seeds of four different ap- pearances and in the following proportions: 9RY :3RG:3WY:AWG. Only one individual in each of these four classes is pure (homozy- gous) and continues to breed true in successive generations; in Fig. 53 these are found in the diagonal from the upper left to the lower PHENOMENA OF INHERITANCE 249 right corner. All other individuals are heter- ozygous and show Mendelian splitting in the next generation. When parents differ in three contrasting characters a much larger number of combina- tions is possible in the F, generation. ‘Thus if a pea with round (Ff) and yellow (Y) seeds, and with tall (7') stem is crossed with one having wrinkled (W) and green (G) seeds, and dwarf (D) stem all the progeny of the F’, generation have round and yellow seeds and tall stem, R, Y, and T' being dominant over W, G, and D. In the F, generation there are sixty-four possible combinations (genotypes) of these six characters; but since a recessive character does not develop if its contrasting dominant character is present there are only eight types which come to expression (phenotypes) and in the following numbers: 27RYT :9RYD : 9RGT :3RGD :9WYT :3WYD :3WGT :1WGD. Of these sixty- four genotypes only eight are homozygous and breed true (those lying in the diagonal be- tween upper left and lower right corners in Fig. 54), while only one is pure dominant 250 HEREDITY AND ENVIRONMENT RYT sag WGD Fi S RyT RYD RGT ROD WYT WYD wet wen Fic. 54. Trihybrid Diagram showing results of crossing peas having round-yellow seeds and tall stem (RYT) with peas having wrinkled-green seeds and dwarf stem (WGD). Eight types of germ cells result from such a hybrid, as shown in the g gametes above the square and the 9 ones to the left of it, and the possible combinations (genotypes) of these ¢ and 2 gametes are shown in the 64 small squares of which only 1 is pure dominant (upper left corner), 1 pure recessive (lower right corner) and 8 homozygotes (in di- agonal line between these corners). The relative numbers of the different phenotypes are 27RYT: 9RYD: 9RGT: 9WYT: 3RGD: 3WYD: 3WGT: 1WGD. PHENOMENA OF INHERITANCE 251 and one pure recessive (the ones in the upper left and lower right corners of Fig. 54). When the parents differ in one character only, the offspring formed by their crossing are called monohybrids, when there are two contrasting characters in the parents the off- spring are dihybrids, when three, trihybrids, and when the parents differ in more than three characters the offspring are called polyhybrids. There are certainly few cases in which parents actually differ in only a single character, but since each contrasting character may be dealt with separately, as if it were the only one, and since the number of types of offspring in- creases greatly when more than one or two characters are considered at the same time, it is customary to deal simultaneously with only one or two characters of hybrids, even though the parents may have differed in many characters. 3. Inheritance Formulae—Mendel repre- sented the hereditary constitution of the plants used in his experiments by letters employed as symbols, dominant characters being repre- sented by capitals and recessives by small let- 252 HEREDITY AND ENVIRONMENT ters. The seven contrasting characters of his peas could be represented as follows: Seeds, round (4), or wrinkled (a); yellow (B), or green (b); with gray seed coats (C’), or white seed coats (c). Pods, green (D), or yellow (d); inflated (EH), or constricted (e). Habit, tall (#"), or dwarf (f). Flowers, axial (G), or terminal (g). It is possible for one plant to have all of these dominant characters or all of the reces- sive ones, or part of one kind and part of the other. The inheritance formula of a plant having all seven of the domimant characters is ABCDEFG;; of one having all of the recessive characters abcdefg. When two such plants are crossed the inheritance formula of the hy- brid is AaBbCcDdEeFfGg, and since the domimant and recessive characters (or rather determiners of characters) represented by these seven pairs of letters separate in the for- mation of the gametes, and since each separate determiner may be associated with either mem- ber of the six other pairs, the number of possi- PHENOMENA OF INHERITANCE 253 ble combinations of these determiners in the gametes is (2) or 128. ‘That is, in this case 128 kinds of germ cells may be produced, each having a different inheritance formula; and since each of these 128 kinds of male germ cells may unite with any one of the 128 kinds of female germ cells, the number of possible combinations is (128)* or 16,384, which repre- sents the number of combinations of these char- acters which are possible in the F, generation. Every one of these more than sixteen thousand genotypes may be represented by various com- binations of the letters ABCDEFG and abcdefg. When many characters are concerned it is difficult to remember what each letter stands for, and consequently it is customary in such © cases to designate characters by the initial let- ter in the name of that character. By this form of short hand one can show in a graphic way the possible segregations and combina- tions of hereditary units in gametes and zy- gotes through successive generations, and as a result many modern works on Mendelian inheritance look like pages of algebraic formule. ; 254 HEREDITY AND ENVIRONMENT Some progress has been made, as was pointed out in the last lecture, in identifying certain structures of the germ cells with cer- tain hereditary units, but quite irrespective of what these units may be and where they may be located it is possible, by means of the Men- delian theory of segregation of units in the germ cells and of chance combinations of these in fertilization to predict the number of geno- types and phenotypes which may be expected as the result of a given cross. 4. Presence and Absence Hypothesis.— Mendel ‘spoke of the presence of contrasting or differentiating characters in the plants which he crossed, such as round or wrinkled seeds, tall or short stems, etc. Many other writers regard these contrasting characters as positive and negative expression of a single character, and consequently they speak of the presence or absence of single characters: thus round seeds are due to the presence of a factor for roundness (4) while wrinkled seeds are characterized by the absence of that factor (a). Round seeds are wrinkled seeds plus the factor for roundness. Most of the phenomena PHENOMENA OF INHERITANCE 255 of Mendelian inheritance are more simply stated in terms of presence or absence of single characters than in terms of contrasting characters. But it is by no means certain that recessive characters are due to the absence of factors for dominant characters; there are some genetical and many philosophical objec- tions to such a view, which leads logically to some strange conclusions, such as Bateson’s speculations on evolution (p. 404). When both gametes carry similar positive factors the zygote has a “double dose” of such factors and is said to be duplex; when only one of the gametes carries such a factor the zygote has a “single dose” and is simplex, when neither gamete carries a positive factor or fac- tors, the zygote receives only negative factors and is said to be nullaplex. Thus the union of gametes AB (*) and AB (¢) yields zygote AABB, which is duplex in constitution: gametes Ab (*) and aB (¢) yield zygote AaBb, which is simplex; gametes ab (?) and ab (2) yield zygote aabb, which is nulliplex. In some instances a character comes to full expression only when it is derived from both 256 HEREDITY AND ENVIRONMENT parents, that is, when it is duplex; if derived from one parent only, that 1s, if simplex, it is diluted in appearance and is intermediate be- tween the two parents. For example, when white-flowered four o’clocks which are nulli- plex are crossed with red-flowered ones which are duplex the progeny, which are simplex, bear pink flowers; in this case red flowers are produced only when the factor for red is de- rived from both parents, pink flowers when it is derived from one parent, white flowers when it is derived from neither parent (Fig. 50). 5. Summary of Mendelian Principles.— Since the rediscovery in 1900 of Mendel’s work many investigators have carried out similar ex- periments on many species of animals and plants and have greatly extended our knowl- edge of the principles of inheritance discovered by Mendel, but in the main Mendel’s conclu- sions have been confirmed again and again, so that there is no doubt that they constitute an important rule of inheritance among all organisms. In brief the “Mendelian Law of Alternative Inheritance” or of hereditary “splitting” con- sists of the following principles: PHENOMENA OF INHERITANCE 257 (a) The principle of wnit characters.—The heritage of an organism may be analyzed into a number of characters which are inherited as a whole and are not further divisible; these are the so-called “unit characters” (deVries). (b) The principle of dominance——When contrasting unit characters are present in the parents they do not as a rule blend in the offspring, but one is dominant and usually ap- pears fully developed, while the other is reces- sive and temporarily drops out of sight. (c) The principle of segregation—KEvery individual germ cell is “pure” with respect to any given unit character, even though it come from an “impure” or hybrid parent. In the germ cells of hybrids there is a separation of the determiners of contrasting characters so that different kinds of germ cells are pro- duced, each of which is pure with regard to any given unit character. This is the principle of segregation of unit characters, or of the “purity” of the germ cells. Every sexually produced individual is a double being, double in every cell, one-half having been derived from the male and the other half from the fe- 258 HEREDITY AND ENVIRONMENT male sex cell. ‘This double being, or zygote, again becomes single in the formation of the germ cells only once more to become double when the germ cells unite in fertilization. II. MopIFricaTIONs AND EXTENSIONS oF MEN- DELIAN PRINCIPLES It is a common experience that natural phenomena are found to be more complex the more thoroughly they are investigated. Na- ture is always greater than our theories, and with few exceptions hypotheses which were satisfactory at one stage of knowledge have to be extended, modified or abandoned as knowl- edge increases. ‘This observation is well illus- trated in the case of the Mendelian theory. The principles proposed by Mendel were rela- tively simple, but in attempting to apply them to the many phenomena of inheritance now known it has become necessary to modify or extend them in many ways. And yet the gen- eral and fundamental truth of these principles has been established in a surprisingly large number of cases, and they have been extended PHENOMENA OF INHERITANCE 259 to forms of inheritance where at first it was supposed that they could not apply. 1. The Principle of Umt Characters and of Inheritance Factors—There has been much criticism on the part of some biologists of the principle of unit characters. It is said that unit characters cannot be independent and dis- crete things; the organism itself is a unity and every one of its parts, every one of its charac- ters, must influence more or less every other part and every other character. Certainly unit characters cannot be absolutely independent of one another; the various parts and organs of the body, and even the organism as a whole, are not absolutely independent, and yet there are varying degrees of independence in organ- isms, organs, cells, parts of cells, hereditary units and characters which make it possible for purposes of analysis to deal with these things as if they were really independent though we know they are not. Of course characters of adult individuals do not exist as such in germ cells, but there is no escape from the conclusion that in the case of inherited differences between mature organ- 260 HEREDITY AND ENVIRONMENT isms there must have been differences in the constitution of the germ cells from which they developed. For every inherited character there must have been a germinal cause in the fertilized egg. ‘This germinal cause, whatever it may be, is often spoken of as a determiner of a character. But the character in question is not to be thought of as the result of a single cause nor as the product of the development of a single determiner; undoubtedly many causes are involved in the development of every character, but the differential cause or combination of causes is that which is peculiar to the development of each _ particular character. Again it is not necessary to suppose that every developed character is represented in the germ by a distinct determiner, or inheritance unit, just as it is not necessary to suppose that every chemical compound contains a peculiar chemical element; but it is necessary to suppose that each hereditary character is caused by some particular combination of inheritance units and that each compound is produced by some particular combination of chemical ele- PHENOMENA OF INHERITANCE 261 ments. An enormous number of chemical compounds exists as the result of various com- binations of some eighty different elements, and an almost endless number of words and combinations of words—indeed whole litera- tures—may be made with the twenty-six let- ters of the alphabet. It is quite probable that the kinds of mheritance units are few in num- ber as compared with the multitudes of adult characters, and that different combinations of the units give rise to different adult characters ; but it is certain that every inherited difference in adult organization must have had some differential cause or factor in germinal organization. Mendel did not speculate about the nature of hereditary units though he evidently con- ceived that there was something in the germ which corresponded to each character of the plant. Weismann postulated a determinant in the germ for every character which is inde- pendently heritable, and many recent students of heredity hold a similar view. But it is evident that there is not an exact one to one correspondence of inheritance units 262 HEREDITY AND ENVIRONMENT and adult characters. Many different characters may be determined by a single unit or factor; for example, all the numerous secondary sex- ual characters which distinguish males from fe- males may be determined by the original factor which determines whether the germ cells shall be ova or spermatozoa. On the other hand two or more factors may be concerned in the production of a single character. In many cases among both plants and animals the development of color appears to depend upon the presence in the germ cells and the cooperation in development of at least two factors, viz. (1) a pigment factor for black B, for brown Br, for yellow Y, for red Ff, etc., and (2) a color developer C. When both of these factors are present color develops, when either one is absent no color appears. Such cases have been described for mice, guinea-pigs, and rabbits as well as for several species of plants. Bateson and Punnett found two varieties of white sweet peas which were apparently alike in every respect except the shapes of their pollen grains, one of them hav- ing long and the other round pollen. But PHENOMENA OF INHERITANCE 263 when these were crossed a remarkable thing occurred for the progeny “instead of being white were purple like the wild Sicilian plant from which our cultivated sweet peas are de- scended.” This is apparently a typical case of reversion and its cause was found in the fact that at least two factors are necessary in this case for the production of color, a pig- ment factor R and a color developer C. One of these was lacking in each of the white parents, their gametic formule being Cr and cR, but when these two factors came together in the offspring a purple-flowered type was produced with the somatic formula CcfRr. These I’, plants produced colored and white F, plants in the proportion of 9 colored to 7 white and the colored forms were of six dif- ferent kinds (Fig. 55). For the production of these six colored forms five different factors must be present in the gametes, according to Punnett, viz.: (1) a color base R, (2) a color developer C, (3) a purple factor P, (4) a light wing factor L, (5) a factor for intense color I. When all of these factors are present the result is the purple wild form with blue VERY PALE PURPLE PINK PALE PURPLE (#7 RED PURPLE BLUE DEEP PURPLE Fic. 55. Results of crossing two different races (A and B) of white sweet peas; all the F, hybrids (C) are purple with blue wings like the wild ancestral stock; in F, six colored varieties are formed ranging from purple with blue wings (D) to tinged white (I) and several kinds (genotypes) of white varieties (K). (After Punnett.) PHENOMENA OF INHERITANCE 265 wings, while the omission of one or more of these factors leads to the production of six forms of colored and various types of white- flowered plants of the F. generation. Castle found that eight different factors may be involved in producing the coat colors of rabbits; these are: C acommon color factor necessary to produce any color. B a factor acting on C to produce black. Br a factor acting on C to produce brown. Y a factor acting on C to produce yellow. I a factor which determines intensity of color. U a factor which determines uniformity of color. A_ a factor for agouti, or wild gray pattern, in which the tip of every hair is black, below which is a band of yellow, while the basal part of the hair is gray. FE’ a factor for the extension of black or brown but not of yellow. Plate found that all of these factors except the last, H’, are also involved in the production of the coat colors of mice. Baur has recog- 266 HEREDITY AND ENVIRONMENT nized more than twenty different factors for the color and form of flowers in the snap- dragon, Antirrhinum. These factors are probably complex chemi- cal substances which preserve their individu- ality in various combinations, just as groups of atoms or radicals do in chemical reactions; they may be dropped out or added, substituted or transposed, just as chemical radicals may be in chemical compounds. ‘To this extent they maintain continuity and independence, but they are not absolutely independent for they react upon one another as well as to en- vironmental changes, so that the characters of the developed organism are the resultants of all these reactions and interactions. Inheritance Factors and Germinal Units If it is asked whether there are particular structures in germ cells which correspond to particular inheritance factors it must be ad- mitted that we have no certain knowledge on this subject and that opinions differ greatly with respect to it. On the one hand it is maintained that the entire germ is concerned PHENOMENA OF INHERITANCE 267 in the development of every character, and on the other hand that the differential cause of any character may be located in some differ- entiated structure or function of the germ. These views are not mutually exclusive and it may well be that both are true. We know that germ cells are composed of many parts which differ from one another in both structure and function, and it is highly probable that there are enough of these parts to provide a locus for every inheritance factor. There was a time when the cell was the Ultima Thule of biological analysis and when the contents of cells were supposed to be “per- fectly homogeneous, diaphanous, structureless slime.” Then the nucleus was discovered within the cell, then the chromosomes within the nucleus, then the chromomeres within the chromosomes, and there is no reason to sup- pose that organization ceases with the powers of our present microscopes. With every im- provement of the microscope and of micro- scopical technique, structures have been found in cells which were undreamed of before, and 268 HEREDITY AND ENVIRONMENT it is not probable that the end has been reached in this regard. We know that cells contain nuclei and chromosomes and chromomeres, centrosomes and plastosomes and microsomes, and we know that some of these parts differ in function as well as in structure. And there is no reason to doubt that if we had sufficiently powerful microscopes we should find still smaller and smaller units until we came at last to molecules and atoms. Fic. 56. Diagram showing union of factors in fertilization and their segregation in the formation of germ cells. With 4 pairs of factors (Aa, Bb, Cc, Dd) 16 types of gnmetes are possible as shown in the two series of small circles at the right. (After Wilson.) PHENOMENA OF INHERITANCE 269 The fact that inheritance units from the two parents unite in fertilization and later segregate in the formation of gametes, so that the latter are pure with respect to any char- acter, is a familiar part of Mendelian inheri- tance (Fig. 56). Even if these units be re- garded as physiological processes they must be associated with particular structures, since function and structure are inseparable in life Fertilization 2 Union of Cleavage implex Groups Duplex Groups O ABCD 2 ABCD abed aS p N A) Aa. Bb. Ce. Dd. AbeD Synapsis Reduction Germ Cells Division Simplex Groups Somatic Divisions Duplex Groups Fic. 57. Diagram of germ cells corresponding to Fig. 56, showing the union of maternal chromosomes (ABCD) and pa- ternal ones (abcd) in fertilization, their distribution in cleav- age, their union into 4 pairs (Aa, Bb, Cc, Dd,) in synapsis and the separation of the pairs in the reduction division. Only 2 of the 16 possible types of germ cells are shown at the lower right. (After Wilson.) 270 HEREDITY AND ENVIRONMENT processes. What are these units in terms of cell structures and where are they located in the cell? : We have in the chromosomes, as Wilson especially has emphasized, an apparatus which fulfils all the requirements of carriers of Mendelian factors (Fig. 57). Both factors and chromosomes come in equal numbers from both parents; both maternal and paternal factors and chromosomes pair in the zygote and separate in the gamete as shown in Fig. 56 and Fig. 57; and so far as is known the chromosomes are the only portions of the germ cells which fulfil these conditions. Further- more there is much additional evidence that the chromosomes are especially concerned in heredity, as was pointed out in the last chapter, and it is not reasonable to suppose that this remarkable coincidence between the distribu- tion of Mendelian factors and of chromosomes is without significance. Of course Mendelian factors are not the only factors of development but merely the differ- ential factors which cause, for example, one guinea-pig to be white and its brother to be PHENOMENA OF INHERITANCE 271 black. Very many factors are involved in the production of white or black color but there is at least one differential factor for every unit character and this alone is the Mendelian fac- tor. Of course there is no such thing as a “sex-producing chromosome,” sex being the result of the teraction of many intrinsic and extrinsic causes.* The X chromosome is only one factor in the determination of sex, but if it is a factor which differs in the case of the two sexes it is a “sex determining factor.” There are many parts of a germ cell, all of which may be concerned in heredity and de- velopment, but the chromosomes appear to be the seat of the differential factors for Men- delian characters. 2. Modifications of the Principle of Domi- nance.—A. large number of animal and plant hybrids show one contrasting character com- pletely dominant over the other one as Mendel observed in the case of his peas. But in a considerable number of cases this dominance is incomplete or imperfect. When white- flowered strains of four o’clocks are crossed with red-flowered ones the F, plants bear =see pp- 157 and 338. 272 HEREDITY AND ENVIRONMENT neither white nor red flowers but pink ones, and the F, plants bear white, red and pink flowers. ‘The whites and reds are always homozygous, the pinks heterozygous; pure white and pure red are produced only when their factors are duplex (WW), (R&) ; when they are simplex (W#) pink is produced. In this case red is not completely dominant over white, but the hybrid is more or less interme- diate between the two parents (Fig. 50). It has long been known that the race of fowls called Blue Andalusian does not breed true, but in each generation produces a certain number of blacks and whites as well as blues. Bateson found that the blues are really hybrids between blacks and whites in which neither of the latter is completely dominant. Black and white appear only when they are pure (homo- zygous), blue only when both black and white are present (heterozygous). Again a cross of red and white cattle pro- duces roan offspring, but the latter when in- terbred give rise to reds, roans and whites in the proportion of 1:2:1, showing that the roans are heterozygotes in which red is not com- PHENOMENA OF INHERITANCE 273 pletely dominant over white, while the reds and whites are homozygotes and consequently breed true. Lang found that when snails with uniformly colored shells were crossed with snails having bands of color on the shells the hybrids were faintly banded, thus being more or less inter- mediate between the two parents; but when these hybrids were interbred they produced banded, faintly banded and uniformly colored snails in the ratio of 1:2:1, thus proving that Mendelian segregation takes place in the F, generation, and that dominance is incomplete in the heterozygotes. Many other similar cases of incomplete dominance are known. Sometimes dominance is incomplete in early stages of development but becomes complete in adult stages. Davenport found that when pure white and pure black Leghorn fowls are crossed the chicks are speckled white and black, but in the adult fowl dominance is com- plete and the plumage is black. Similar con- ditions of delayed dominance are well known in the color of the hair and eyes of children, though dominance may become complete wren they have reached adult life. QTA HEREDITY AND ENVIRONMENT In a few instances a character may be domi- nant at one time and recessive at another. Thus Davenport found that an extra toe in fowls is dominant under certain circumstances and recessive under others. ‘Tennent found that characters which are usually dominant in hybrid echinoderms may be made recessive if the chemical or physical nature of the sea water is changed. Such cases seem to show that dominance may depend sometimes upon en- vironmental conditions, sometimes upon a par- ticular combination of hereditary units. Sex and Sexv-Limited Inheritance Sex and sex-limited inheritance may be con- sidered here since they involve questions of dominance, certain characters remaining un- developed in one sex which are fully developed in the other. ‘There is good evidence, as was shown in Chapter II, that sex is a Mendel- - jan character, in which the female has a double dose of the determiner for sex, whereas the male has only a single dose. Consequently in the formation of the gametes every egg re- ceives one sex-determiner, while only one-half PHENOMENA OF INHERITANCE 275 of the spermatozoa receive such a determiner, the other half of them being without it. If then, an egg is fertilized by a sperm without one of these determiners, a male results; but if an egg is fertilized by a sperm with one of these determiners, a female is produced. This is graphically represented in Fig. 58 in which X represents the sex-determiner, which is duplex in the female and simplex in the male, and the chance unions of male and fe- male gametes yield females (XX) and males (XO) in equal numbers. In either sex many secondary sexual charac- ters of the other sex are present during de- velopment, and traces of these may persist in the adult; but one set of these characters de- velops in the male and another in the female, so that they may be called sea-limited. The de- velopment of the secondary sex characters is usually determined by the ovaries or testes, which are the primary sex characters, though in some instances they may develop in animals which have lost their ovaries or testes, but in the last analysis both primary and secondary sex characters are dependent upon the sex- 276 HEREDITY AND ENVIRONMENT determiner. Sex and sex-limited inheritance are only special cases of Mendelian mheritance in which conditions of dominance differ in the two sexes, depending upon whether the factor for sex is duplex or simplex. 2 9 (@)@) cia Gametes es ve 2 (ax) 2 CO) Fic. 58. Diagram showing sex as a Mendelian character, the female being homozygous, the male heterozygous for sex. The female forms gametes all of which contain the X chromosome; the male forms two sorts of gametes one-half of which contain the X chromosome and the other half lack it. All possible combinations of these gametes give a 2:2 or 1:1 ratio of females to males. PHENOMENA OF INHERITANCE Qi7 Sex-Linked Inheritance In this connection we may consider another class of characters, which are linked with sex but are in no wise connected with sexual repro- duction. Such characters are not necessarily limited to one sex or the other, as are many Flies Chromosomes 0) ca Parents KE ics : vas : } Gamete 2X0: 2XX F1 3 2 2 Gametes Kan? (es XX XO XO Fe v| 2 CG uC) 2 Fic. 59. Sex-linked inheritance of white and red eyes in Drosophila. Parents, white-eyed male and red-eyed female; F,, red-eyed males and females; F’,, red-eyed females and equal numbers of red-eyed and white-eyed males. The distribution of sex chromosomes is shown to right of flies; K carries the factor for red eyes, the factor for white eyes, 0) stands for absence of KX, or presence of Y chromosome which is found only in males. (After Morgan.) 278 HEREDITY AND ENVIRONMENT primary and secondary sexual characters, but they may appear in either sex though they are usually transmitted from fathers to daughters, or from mothers to sons (“criss-cross” inheri- tance) in exactly the way in which the sex chromosomes (X) are transmitted. Morgan has therefore concluded that the factors for these characters are carried by the sex chromo- somes and has named them sew-linked charac- ters. In the fruit fly, Drosophila, he has discovered more than twenty-five such charac- ters, applying to the color of the eyes and of the body, to the length of the wings, ete. A typical case is shown in Figs. 59 and 60. The eye color of this fly is normally red, but muta- tions have arisen in which the eye is white. Such a mutation always appears in males, though it may later be transferred to females, as we shall see. If now a white-eyed male and a red-eyed female are crossed all the F, s are red-eyed, but if these F, s are interbred all the females of F. have red eyes while half of the males have red eyes and the other half have white eyes (Fig. 59). On the other hand if one of the F, females of this cross is bred PHENOMENA OF INHERITANCE 279 with a white-eyed male half of the females of F, are red-eyed and half are white-eyed, and half of the males are red-eyed and half are white-eyed. If now one of these white-eyed females is bred with a red-eyed male all the females of the EF, generation are red-eyed and all the males white-eyed (“criss-cross” inheritance) Flies Chromosomes XO Ox Parents 3 2 SC eae X0 XX Parents 3 2 6) 2 i< a Gametes oO GO 2XX: 2X0 Fi DUS) 2 Co » a amie >| Gametes x0 all —= =~ a OO oO B® XX XX XO X0 Fe Aaah AO Oe. i te Fic. 61. Diagram of inheritance of color blindness through the male. A color blind male (here black) transmits his defect to his grandsons only. The corresponding distribution of the sex chromosomes is shown on the right, the one carry- ing the factor for color blindness being black. (After Morgan.) PHENOMENA OF INHERITANCE 283 to produce a color blind male, just as is true of sex. The accompanying diagrams (Figs. 61, 62) illustrate the method of inheritance of color blindness. As in the previous diagrams X represents the sex-determiner, © its absence, and X the sex-determiner which carries the factor for color blindness. It will be seen that a color blind father and a normal mother have only normal children, but the father transmits to his daughters Eyes Chromosomes < ° KO XX Parents 3 2 3 fe) i 2) G tes ane XO EX ae Cw 3-2 VX: 2X0 Fy Qayis © 3 a | Gametes Cs ere ne — Cl & oO w@m MX XX X0 X0 Fe ¢ Tbe One O a) uae Tot Tol Fic. 62. Diagram of inheritance of color blindness through the female. A color blind female transmits her defect to all her sons, to half of her granddaughters and to half of her grandsons. Corresponding distribution of sex chromosomes on right. (After Morgan.) 284 HEREDITY AND ENVIRONMENT and not to his sons the sex-determiner which carries the factor for color blindness. But since color blindness does not develop in fe- males unless it is duplex (i.e. comes from both father and mother) whereas it develops in males if it is simplex (z.e. comes from either parent) all the daughters of a color blind father and normal mother will appear normal although carrying one determiner for color blindness, while all the sons will be normal be- cause they carry no determiner for color blind- ness. But these daughters transmit to one-half of their children the single determiner for color blindness and if any of those receiving this de- terminer are males they will be color blind. Consequently we have the curious phenome- non of simplex color blindness appearing only in males and being transmitted to them only through apparently normal females. On the other hand if a female is color blind she has inherited it from both father and mother, 7.e. the character in her is duplex, and in all of her children by a normal male the character will be simplex; accordingly all of her sons will be color blind and all of her PHENOMENA OF INHERITANCE 285 daughters will be normal, though carrying the simplex determiner for color blindness. In all cases dominance means merely the development in offspring of certain characters of one parent, while contrasting characters of the other parent remain undeveloped. ‘The appearance of any developed character in an organism depends upon many complicated reactions of germinal units to one another and to the environment. Under certain conditions of the germ or of the environment some char- acters may develop in hybrids to the exclusion of their opposites whereas under other condi- tions these results may be reversed or the char- acters may be intermediate. The principle of dominance is not a fundamental part of Men- delian inheritance. Eiven when the characters of hybrids are intermediate between those of their parents, if the parental types reappear in the F’, generation we may be certain that we are dealing with cases of Mendelian inheritance. 3. The Principle of Segregation—The in- 286. HEREDITY AND ENVIRONMENT dividuality of mheritance units, and their seg- regation or separation in the sex cells and recombination in the zygote are fundamental principles of the Mendelian doctrine. Indeed the evidence for the individuality and contin- uity of inheritance units is based entirely upon such segregation and recombination, so that the entire Mendelian theory may be said to rest upon the principle of segregation. If there are cases in which such segregation does not take place they belong to other forms of inheritance than the Mendelian; if segregation occurs in every instance there is no other type of inheritance than that discovered by Mendel. Are there cases which do not segregate ac- cording to Mendelian expectation? When the Mendelian theory was new it was generally supposed that there were forms of inheritance which differed materially from the Mendelian type; indeed it was supposed that the latter was one of the less common forms of heredity and that blending of parental traits and not segregation was the rule. All cases in which the characters of the parents appeared to blend in the offspring or in which PHENOMENA OF INHERITANCE 287 there was not a clear segregation of the par- ental types in the F, generation or in which the ratio for a monohybrid differed from the well known 8 to 1 ratio were supposed to be non-Mendelian. However further work has shown that some of these are really Mendelian. Sometimes offspring are intermediate between their parents owing to incompleteness of dominance, rather than to incompleteness of segregation; in such cases the parental types reappear in the F, generation as in the cross between red and white four o’clocks. Sometimes depar- tures from the 3 to 1 ratio are caused by the fact that two or more factors of the same sort _are involved in the production of a single char- acter. Nilsson-Ehle found that when oats with black glumes were crossed with varieties having white glumes the ratio of 3 white to 1 black was usually found in the second gen- eration; but one variety of black oats when crossed with white gave in the second gener- ation approximately 15 blacks to 1 white which is the dihybrid ratio. From this and other evidence he concludes that in this va- 288 HEREDITY AND ENVIRONMENT riety of oats two hereditarily separable fac- tors are involved in the production of black. In crosses between red-grained and white- grained wheat he usually got in the second generation the monohybrid ratio of 3 red to 1 white, but three strains gave the dihybrid ratio of 15 to 1 and two gave the trihybrid ratio of 63 to 1. Consequently he concludes that while the red color of wheat grains is usually due to one factor for red, it may in some cases be due to two or even three factors; notable de- partures from expected ratios may thus be explained. Blending Inheritance But the most serious objections which can be presented against the universality of the Mendelian doctrine are found in phenomena of “blending” inheritance. In some instances contrasting characters of parents appear to blend in offspring and even in the F, and in subsequent generations the descendants remain more or less intermediate between the par- ents. One of the best known illustrations of this is found in the skin color of the mulatto PHENOMENA OF INHERITANCE 289 which is intermediate between the white parent and the black one, and even in the F, and in subsequent generations mulattoes do not usu- ally produce pure white or pure black chil- dren, though the children of mulattoes show considerable variation in color. Hence there seems to be a failure of the Mendelian princi- ple of segregation. But white skin is not really white nor is black skin ever perfectly black. Davenport has shown that there is a mixture of black, yellow and red pigment in both white and black skins, though the amount of each of these pigments varies greatly in negroes and whites. The relative amounts of these pig- ments in any given case may be determined by means of a rotating color disk. A white person may have a skin color composed of black (b) 8 per cent., yellow (y) 9 per cent., red (r) 50 per cent., and absence of pigment or white (w) 33 per cent. On the other hand a very black negro may have b 68 per cent., y 2 per cent., r 26 per cent., w 4 per cent. The nine children of two mulattoes, the father having 13 per cent. of black and the mother 45 per cent., ranged all 290 HEREDITY AND ENVIRONMENT the way from 46 per cent. to 6 per cent. of black, the latter so far as skin color is con- cerned being virtually white. On the other hand where both parents have about the same degree of pigmentation the children are more nearly uniform in color; thus seven children of two mulattoes, the father having 36 per cent. and the mother 30 per cent. of black, ranged only from 27 per cent. to 39 per cent. of black.* Such variations in color in the F; and in subsequent generations are exactly what one would expect in a Mendelian character in which more than one factor is involved, as for example in the case of the color of the sweet peas shown in Fig. 55. Davenport, who has made an extensive study of this case, con- cludes that “there are two double factors (4A, BB) for black pigmentation in the full blooded negro of the west coast of Africa, and these are separably inheritable.” These fac- tors are lacking in white persons (this beng indicated by the formula aa, bb). Since the germ cells carry only single factors and not *In another family shown in Fig. 64 the father has 18 per cent black pigment, the mother 38 per cent and the children range from 17 per cent to 54 per ¢ent. PHENOMENA OF INHERITANCE 291 double ones the cross between negro and white would have only one set of these factors for black color, as shown by the-formula 4B x ab — ABab; hence the color of the F’, generation is intermediate between that of the two par- ents. In the F, generation there should be a variety of colors ranging all the way from white to black, though pure white or pure black would be expected in only a small proportion of the offsprng. As a matter of fact it is known that the children of mulattoes vary considerably in color, and in some cases a child may be darker or lighter than either parent, which would indicate that segregation does actually occur. It is very probable that this classical case of “blending” inheritance is really Mendelian inheritance in which two or more factors for skin color are involved. Similar “blending” inheritance is found in certain other cases where the parents differ in form or size. Thus Castle found that when long-eared rabbits were crossed with short- eared ones the offspring have ears of inter- mediate length, and in all subsequent gener- ations the ear length remained intermediate 292 HEREDITY AND ENVIRONMENT between that of the parents. He found the same thing true of length and breadth of the skull (Fig. 63) and of the size of other portions of the skeleton, and he concluded that such quantitative characters are not inherited in Mendelian fashion. Quite recently MacDowell, working on the inheritance of size in rabbits, concludes that this character, as well as other quantitative dif- ferences between parents which appear to blend in the offspring, such as Castle’s case of Fic. 63. Blending inheritance of size in rabbits; the skulls of two parents are shown in 1 and 3, of their intermediate offspring in 2. (From Castle.) PHENOMENA OF INHERITANCE 293 ear length in rabbits, is not due to a single fac- tor, as in the case of Mendel’s tall and dwarf peas, but to several factors. Consequently in the formation of the germ cells there is not a clean segregation of all the factors for tallness or large size or long ears in half the germ cells and their total absence in the other half of those cells, but some of these factors go into certain cells and others into others, as in the case of dihybrids, trihybrids or polyhybrids. As a result offspring appear more or less in- termediate in size between their parents. Thus it is possible to explain even “blend- ing”’ inheritance as due not to the real fusion or blending of inheritance factors but to vary- ing combinations of numerous or multiple fac- tors, according to the Mendelian rules. The Mendelian principle of segregation has been found to be of such general occurrence that there is a strong inclination among Mendel- lans of the stricter sort to make it universal, and to explain all cases of “blending” inheri- tance as due to incomplete dominance and to multiple factors. Whether or not such at- tempts may prove completely successful it is still too soon to say. 294 HEREDITY AND ENVIRONMENT Maternal Inheritance On the other hand there are some characters which are not inherited in the typical Men- delian manner. Among these are the polarity, symmetry and pattern of the ege and of the adult animal which,is derived from it (see p. 174). These characters are of such a general sort that they may not be recognized as phe- nomena of inheritance at all, and yet they form the background and framework for all the other characters. They do not come equally from the egg and sperm, and they do not undergo segregation in the formation of the gametes, but are apparently derived from the ege cytoplasm. Among characters of this sort are the normal and inverse symmetry of snails, and of many other animals, including man, which were referred to on pages 176-185. Such characters are undoubtedly inherited, though they differ from other characters not only m the fact that they are transmitted through the egg only, but also because they are of the same kind in the egg and in the developed organism; they are in a measure preformed in the egg; they are differentiated characters carried over PHENOMENA OF INHERITANCE 295 from a previous generation rather than inheri- tance factors. ‘These egg characters probably appeared in the course of oogenesis under the influence of paternal as well as of maternal chromosomes; if so this is a case of Mendelian inheritance in the previous generation or what may be called “Pre-inheritance.” Similar phenomena have been described by McCracken and by Toyama in silk-worms where several egg characters seem to be non-Mendelian, but ‘Toyama has shown that they are in reality Mendelian in the previous generation, this also _ being a case of pre-inheritance. , Somewhat similarly it has been found by Correns, Baur, and Shull that the leaf colors of certain plants are not inherited in Mendel- ian fashion, but the chromoplasts, which pro- duce the chromatophores (chloroplasts), are transmitted from one generation to the next in the cytoplasm of the egg cell and only rarely through the male sex cell. If chromoplasts are integral parts of a plant and undergo dif- ferentiation or development this may be a case of pre-inheritance; if they are symbiotic organ- isms it is an instance of the inclusion of foreign bodies in the cytoplasm and not inheritance. 296 HEREDITY AND ENVIRONMENT Plate calls all such cases ““Pseudo-heredity,” because they have nothing to do with inheri- tance units, or with special qualities of the germ plasm, but are dependent upon the cyto- plasm. We may agree with Plate that no character is inherited unless its differential cause is found in the organization of the germ cells, but it is certainly an indefensible defini- tion of heredity which limits it to inheritance units transmitted only through the nucleus. Other forms of transmission are known in which substances are carried over from one generation to the next through the egg, but they are probably not cases of true inheritance. Among these are the occasional transmission of immunity through the mother but never through the father, the carrying over of par- ticular chemical substances such as fat dyes through the egg but not through the sperm, and the transport of symbiotic or parasitic or- ganisms, such as algae, bacteria, etc., through the female sex cell but not through the male cell. These substances or micro-organisms are to be regarded as inclusions in the egg rather than as any permanent part of the germinal organization; consequently they are not in- herited in the strict sense of that term. PHENOMENA OF INHERITANCE 297 III. MENDELIAN INHERITANCE IN MAN The study of inheritance in man must al- ways be less satisfactory and the results less secure than in the case of lower animals and for the following reasons: Im the first place there are no “pure lines” but the most comph- cated intermixture of different lines. In the second place experiments are out of the ques- tion and one must rely upon observation and le Fic. 64. Mulatto husband and wife and their seven children ranging in color from the one on left who “passes for white” to thé youngest who is typically black. (From Davenport.) CAytpatayzy JO [BuINoLr ut yLOduaaeG wot) “1a}ORALYO daISsavoa B st ‘sjeutueUL Joy}0 ur se qsnf ‘ueEM UI UUSTUTGTe yey} UMOYS aAvY s}toduaARG oyy, ‘UISTUTGTe JO souvydoyut sjotduoo Surmoys ‘(ppryo ATuo) uos pue ssyjour ‘1ayyey oTUIQTY “99 “DT PHENOMENA OF INHERITANCE 299 statistics. In the third place man is a slow breeding animal; there have been less than sixty generations of men since the beginning of the Christian era, whereas Jennings gets as many generations of Paramecium within two months and Morgan almost as many genera- tions of Drosophila within two years. Finally the number of offspring are so few in human families that it is difficult to determine what all the hereditary possibilities of a family may be. Bearing in mind these serious handicaps to an . exact study of inheritance it is not surprising that the method of inheritance of many human characters is still uncertain. Davenport and Plate have catalogued more than sixty human traits which seem to be in- herited in Mendelian fashion. About fifty of these represent pathological or teratological conditions while only a relatively small num- ber are normal characters. ‘This does not sig- nify that the method of inheritance differs in the case of normal and abnormal characters, but rather that abnormal characters are more striking, more easily followed from generation to generation, and consequently statistics are 300 HEREDITY AND ENVIRONMENT Fic. 66. X-ray picture of right and left hands each with six fingers (polydactyly) caused by splitting of the little fin- gers at an early stage. (From Journal of Heredity.) . Fic. 67. X-ray picture, A of a normal, B of a short-fingered (brachydactyl) hand. (From Bateson.) PHENOMENA OF INHERITANCE 301 more complete with regard to them than in the case of normal characters. In many cases sta- tistics are not sufficiently complete to deter- mine with certainty whether the character in question is dominant or recessive, and it must be understood that in some instances the classi- fication in this respect is tentative. oe ‘ ‘ ‘ ¢ . - ‘ ry \ ’ y Se ‘ \ A z xT ‘ ‘ ‘ " 7 x on AK sy ‘ ’ - - ~ x \ c \ “0 YF, - . N ‘. if ‘ 4 ¢ we aA S s ’ ¥ Mee Be yes Ne \, A A ‘ ’ - ~ : ‘ , ? , SS SS SN . U \ A ’ eo “ oh . ‘ . ’ \ ae - oe ~ . Nas 5 \ 7h tee Aa SN hy SK \ et : me PA . SON 4 , Aina a CENCE ‘ \ MLE Nose wr ~~ NE AES ' ’ ce ~ \ \ “7 sie SSQR ‘A / fase SSN, \ Beem +l oa Mi Heredity Fic. 83. Diagram to show the influence of heredity, en- vironment and training in the development of an individual. Various types of individuals (represented by the triangles) may be produced from the same germ cells (heredity) if the environment and training are variable. INFLUENCE OF ENVIRONMENT 369 a given heredity the characteristics of the developed organism may vary enormously depending upon the extrinsic factors. He- reditary possibilities are not changed by acci- dents of environment but development is so changed. After the fertilization of the egg the hereditary potencies of every organism are un- alterably fixed but the extrinsic factors re- main variable and may be controlled. All of our social and ethical institutions such as government, education and religion deal only with extrinsic factors of development and of life. Nevertheless there is no evidence that such extrinsic influences ever modify he- redity, no evidence that the effects of good environment or good training ever change the germinal constitution. The influences of environment and education affect only the de- velopment of the individual and not the con- stitution of the race, and consequently such in- fluences are temporary in effect and must be repeated generation after generation. But though the effects of environment and training are not inherited, the environment and training and experience of former gener- 370 HEREDITY AND ENVIRONMENT ations are handed down to later generations through custom, tradition, history. We do not inherit through the germ cells the effects on our ancestors of their training and environ- ment, but we do inherit, in the property sense of that word, their environment, customs, in- stitutions. In short the experiences and ac- complishments of past generations are not inherited through the germ cells but are inher- ited through society. In this sense “We are the heirs of all the ages.” Because of this social inheritance the ex- trinsic conditions of life continue to grow more complex age after age, while our inherited na- tures remain unchanged. All moralists, all religions, have recognized the very general ex- perience among men of a sense of imperfec- tion and of disharmony with social and ethical standards. Huxley held that the spirit of ethics was opposed to the spirit of evolution. Metchnikoff finds these disharmonies due to the survival of bestial instincts in man. Galton finds the sense of sin to be due to the fact that the development of our inherited nature has not kept pace with the development of our INFLUENCE OF ENVIRONMENT 371 moral civilization. ‘Our psychical, social and moral environment has come to us from the past with ever-increasing increments, every age standing on the shoulders of the preceding one. The aspirations, impulses, responsibili- ties of modern life have become enormous and our inherited natures and abilities have not essentially improved. Social heredity has out- run germinal heredity and the intellectual, so- cial and moral responsibilities of our times are too great for many men. Civilization is a strenuous affair, with impulses and compul- sions which are difficult for the primitive man to fulfil, and many of us are hereditarily prim- itive men. The frequent result is disharmony, poor adjustment, a struggle between primitive instincts and high ideals, with a resulting sense of discouragement and defeat which often ends in abnormal states of mind. The pre- valence of crime, alcoholism, depravity and in- sanity is an ever-increasing protest and menace of weak men against high civilization. We are approaching the time when one or the other must give way, either the responsibilities of life must be reduced and the march of civiliza- 372 HEREDITY AND ENVIRONMENT tion stayed, or a better race of men, with greater hereditary abilities, must be bred. ’' The present world-war, with its appeal to the primitive instincts of men and its destruc- tion of the fairest and best fruits of civilization, points one way out of this disharmony between germinal and social inheritance; but it is a way from which all sane and sober minds recoil. Wars and revolutions shake off the burdens of social inheritance but they destroy the good along with the bad and afford only local and temporary relief. Mankind cannot return permanently to barbarism or savagery; civili- zation must be and will be preserved; but if society is really to advance from age to age the natures of men must improve as well as their environment. | CHAPTER V CONTROL OF HEREDITY: EUGENICS CHAPTER V CONTROL OF HEREDITY: EUGENICS It is the aim of science to interpret phe- nomena and as far as possible to control them. To what extent is it possible to control heredity and thus to improve the race, as well as the individual? A. DOMESTIC ANIMALS AND CULTIVATED : PLANTS The history of domesticated animals and cultivated plants shows that it is possible to control or rather guide phenomena of he- redity and evolution. Very many species of wild animals have been tamed by man but only about 40 species may be classed as domes- ticated. DeCandolle recognized 247 species of cultivated food plants, 193 of which still exist in the wild state.* In a number of in- stances the wild stocks from which these do- * Bailey says that more than 20,000 kinds of plants in cultivation are described in the “Standard Cyclopedia of Horticulture,” although not nearly all of these species are domesticated. 375 376 HEREDITY AND ENVIRONMENT mestic forms came are known and it is possible to compare them with their modified descend- ants and thus to determine the degree of change which has been brought about under human guidance. In other cases where the original wild species are unknown it is possible to determine the amount of modification which has taken place within recent times. The degree of change which has taken place under human guidance is very remarkable. In some cases dozens and even hundreds of races have been formed, showing the most re- markable differences in size, structure and proportion of parts, as well as in functions, instincts and behavior. The extent to which heredity may be guided by man is forcibly il- lustrated by our present races of domestic pigeons which Darwin said would be classed by any naturalist who did not know their ori- gin in not less than twenty different species and three different genera, though all of them have descended from the wild rock pigeon (Figs. 84, 85); or by the numerous races of dogs varying in size from a toy dog to a Great Dane or St. Bernard and showing almost un- CONTROL OF HEREDITY: EUGENICS 377 sof PCOBIN: Y 4 ———s "FRIZZLED Fic. 84. Races of Domestic Pigeons, the wild rock pigeon being shown in the center. (From Romanes, “Darwin and After Darwin.”) 378 HEREDITY AND ENVIRONMENT ws) x 2 Ta VY A, ANAT ov ee iil ive ( IVR Dy hi | Fic. 85. Races of Domestic Pigeons, continued. (Irom Romanes. ) 379 EUGENICS CONTROL OF HEREDITY: | 7 ¢ - Wy Eh, Hi ( amy Un) w 4 1=) Zs o rs) 4 (From Romanes.) Races of Fowls. lic. 86. 380 HEREDITY AND ENVIRONMENT 1 SIN® PENce HAMBUR Bs asiby’spa® HAMBURG Fic. 87. Races of Fowls, continued. (From Romanes.) CONTROL OF HEREDITY: EUGENICS 381 believable differences in structure and dispo- sition; or by the great variety of domestic fowls, all of which have probably descended from the wild jungle-fowl; or by modern races of horses, cattle, sheep, or swine. ‘These are only a few of the many illustrations which could be cited of the practical control of he- redity and evolution for human purposes. How have the present races of domesticated animals and cultivated plants been produced? Fic. 88. Wild Boar contrasted with a modern domestic Pig. (From Romanes.) 382 HEREDITY AND ENVIRONMENT Puchtan Secor. Fic. 89. Different Breeds of British Cattle. (From Romanes.) CONTROL OF HEREDITY: EUGENICS 383 Il. [THE INFLUENCE OF ENVIRONMENT IN PRO- pucING NEw RaAcEs There is a popular belief that the value of cultivated races is due to good environment, good food, good soil, protection from enemies, ete., and that if turned out to shift for them- selves they revert at once to the original wild stock. There are two ways in which it is con- ceivable that new races might be produced by environmental influences: ; 1. By the direct inheritance of somatic or personal characters acquired under the stimu- lus of the environment. In spite of popular opinion in favor of this view there is no evi- dence that this ever occurs. There is no doubt that environment has much to do with indi- vidual development, but it does not usually modify the hereditary constitution of the race. 2. It is possible that environmental changes acting upon germ cells at a sensitive period of their development may produce germinal vari- ations or mutations and thus give rise to new races. There is considerable evidence that en- vironment does sometimes act in this way. 384 HEREDITY AND ENVIRONMENT but there is no evidence that such changes in hereditary constitution are reversible and that the race reverts to its former state when the old environment is restored. Such reversible changes undoubtedly occur in somatic char- acters but they are not inherited; they are modifications of development, not of heredity; they are personal fluctuations, not racial mutations. II. ARTIFICIAL SELECTION Since the beginning of historic times, and probably through long prehistoric ages, breed- ers have followed the method of selecting de- sirable individuals for propagating their stock. There can be no doubt that almost all that man has done in the production of domestic ani- mals and cultivated plants has been accom- plished by this process of selection. How has selection acted? Until very recently it was generally believed that continued selection of individuals which showed desirable characters gradually led to the improvement of those characters and thus to the production of new races; it was CONTROL OF HEREDITY: EUGENICS 385 supposed that the character in question was “built up” by continued selection in one direc- tion, and that the average development of the character in all the offsprmg was thus in- creased in successive generations to an indefi- nite extent. It was this view as to the sup- posed action of artificial selection which formed the basis of Darwin’s theory of natural selection. On the other hand it has been known for a long time that the limits of the possible im- provement of any character by artificial selec- tion are soon reached and that thereafter selec- tion serves only to maintain the character at its high level but not to advance it. The probable explanation of this fact has been found only in recent years. The researches of deVries, Johannsen, Jennings, Tower, Pearl, and oth- ers have shown that in some cases at least selection merely isolates mutants or distinct hereditary lines which are already present in a mixed population but that it does not “build up” characters nor produce new mutations; in short it does not create the variations on which it acts. Johannsen found that from a single species 386 HEREDITY AND ENVIRONMENT of beans he was able, by keeping the progeny of each individual bean separate from the oth- ers, to isolate 19 different “pure lines,” each differing in certain respects from every other line. ‘These lines were not created by selec- tion but were merely sorted out of the general species where they existed already. He fur- ther found that when extremely large or small individuals from any pure line were selected and propagated, none of the progeny showed that character in a still more extreme degree but all merely fluctuated within the original extremes of that line. He concludes there- fore that selection within a pure line is abso- lutely without effect in modifying any char- acter in the offspring of that line. Jennings found that different races or pure lines of Paramecium differ in size, structure and rate of division, and that these differences are “as rigid as iron.” With respect to aver- age length of body he was able to isolate eight pure lines which constantly differed more or less from one another. Within each of these lines there was considerable fluctuation in size, but he was unable by selecting extremes to increase these fluctuations, the progeny of any CONTROL OF HEREDITY: EUGENICS 387 pure line always fluctuating about the mean of that line (Fig. 44). Similarly Tower found in his studies on the potato beetle that he was unable to shift the mean or the extremes of any character by se- lection of extreme forms of an inbred line. Pearl also made an extensive study of the records of breeding experiments extending over many years in which the attempt was made to increase the egg-laying capacity of hens by selecting in each generation for breed- ing only those which had a high record for egg production. It was found that certain “blood lines” produced a larger number of eggs than other lines, but by no amount of selection was it possible to increase the egg production within any line. On the other hand Castle strenuously de- fends the importance of the selection of fluctu- ations in “building up” a character. From among the descendants of piebald rats and rabbits he has selected through many genera- tions those individuals showing the largest and also those showing the smallest extent of color in the coat and he has thus produced one line 388 HEREDITY AND ENVIRONMENT which is nearly all-black and another nearly all-white. He maintains that not only inheri- ted characters, but also their factors are vari- able and that by means of selection of plus or minus variations the mode of a character may be shifted in one direction or the other. This is the old view which flourished before a distinc- tion was made between fluctuations and muta- tions. Breeders have long been acquainted with similar results of selection from a mixed population containing different hereditary lines; however Castle has been careful to em- ploy as pure a race of rats and of rabbits as he could obtain, but it is not possible to get as pure a race of these animals or of any organisms in which cross fertilization occurs as in the case of self fertilizing plants such as beans. Johann- sen defines a “pure line” as “all individuals which are derived from a_ single, abso- lutely self-fertilizing, homozygous individual.” Within such a pure line he maintains that se- lection is unable to change any character. Adherents of the “pure line” hypothesis ex- plain Castle’s results in one of two ways: either his material may not have been genetically CONTROL OF HEREDITY: EUGENICS 389 pure or mutations may have occurred during the course of his experiments and his selection has served merely to isolate distinct hereditary lines; or, more probably, extent of color in his animals may depend upon multiple factors, which are more numerous in some individuals than in others, as is the case for example in “blending” inheritance (pp. 288-293), and se- lection has merely sorted out some individuals with a larger or smaller number of factors, and consequently with a larger or smaller develop- ment of the character in question, but it has not in the least modified any individual factor. This view finds support in the recent work of McDowell and of Zeleny and Mattoon on the effects of selection on certain characters of Drosophila. The crux of this whole controversy lies in the question as to whether inheritance factors fluc- tuate or not; Castle maintains that they do, Johannsen that they do not. If inheritance factors fluctuate they may be changed grad- ually in one direction or another by selection; if they do not fluctuate, but mutate only, changing only rarely and then in one direction, 390 HEREDITY AND ENVIRONMENT and not in all directions, selection can act only by sorting out mutations, but can do nothing to produce them. In general fluctuations are due to environment and are not inherited, therefore they concern development rather than heredity, developed characters rather than inheritance factors. ‘There is much evidence that mheritance factors are relatively stable and that when they change, they undergo a complete change or mutation comparable to what occurs in a chemical reaction. As we have seen it is theoretically possible to explain almost all phenomena of inheritance on this basis even including Castle’s results. Quite recently Jennings has shown that con- tinued selection in one direction does, appar- ently, shift the mode of certain characters in a pure line of asexually reproducing Difflugia, and Middleton has found the same to be true of Stylonychia. 'These results differ totally from Jennings’ earlier work on Paramecium, which has recently been repeated and con- firmed by Ackert. It is a hard thing to believe that different organisms differ irreconcilably in so fundamental a matter and it seems much CONTROL OF HEREDITY: EUGENICS 391 more probable that these discrepancies are due to an incomplete analysis of the phenomena in question. It is possible that the divisions of the cell body in these protozoa is not always into exactly equivalent halves in which case variations might take place in the descendants. which might then be heaped up by selection. The evidence as to the ability of selection to produce new characters is conflicting and the question is still an open one but for the present, the evidence which is most clear-cut and abund- ant indicates that selection by itself is unable to change inheritance factors or unit charac- ters. Nevertheless selection is of great service in separating good lines or races from poor ones, and this is the chief significance of the artificial selection practiced by breeders. The elimination of certain races by natural selection is an important factor in evolu- tion though it may have nothing to do with the formation of new characters or new races but serve merely as a sieve, as deVries has ex- pressed it, to sort the individuals which are supplied to it. Even if selection has no power to make or change characters, it preserves certain lines and eliminates others and thus 392 HEREDITY AND ENVIRONMENT fixes the type of a species. Finally the elimi- nation of the unfit by natural selection is still the only natural explanation of fitness, or adaptation, in organisms. IlIl. Mreruops or Moprrn GENETICS 1. Mendelian Association and Dissociation of Characters——Breeders have long known that it is possible to get certain desirable char- acters of an organism from one race and others from another race. But since the discovery of the Mendelian principles of heredity such new combinations of old characters have been made repeatedly, and with almost the same certainty of results as when the chemist makes combina- tions of elements or compounds. In Fig. 90, A and B, are shown two guinea-pigs, one having long (LL), rough and tumbled (7'), white (W) hair, and the other having short (8), smooth (Sm), red (#) hair. When such animals are crossed one should get in the F. generation 64 genotypes and 8 phenotypes, one of each of the latter being homozygous and breeding true, as is shown in Fig. 54 for trihybrid peas. These 8 pheno- CONTROL OF HEREDITY: EUGENICS 393 types of this cross are STR, STW, SSmk, SSuW, LTR, LTW, LSmRk, LSmW. In Figs. 90, C and D, and 91, A, B, C, are shown 5 of these 8 phenotypes which were ob- tained by Castle from this cross. ‘These fig- ures well illustrate the new combinations of Mendelian characters which may be obtained by cross breeding. This is the chief method employed by Bur- bank in producing his really wonderful “new creations in plant life.” By extensive hybridi- zation he brings about many new combinations of old characters, a few of which may be com- mercially valuable, and sometimes actually new characters or mutations appear, possibly as a result of the interaction of old characters, or rather of their factors. Lotsy, for example, maintains that the sole source of variation is crossing, and Bateson says that the new breeds of domestic animals made in recent times are the carefully selected products of recombina- tions of pre-existing breeds, and that most of the new varieties of cultivated plants are the outcome of deliberate crossing. (aqysegQ Wor) ‘gq pue y Sutsso1s Aq poonpoad sadAz ay} Ju OM} ‘q pue DQ ‘Arey pat ‘yJOOWS 4YaoYs YAM 4103}e] 94} faArey oqIYA “YSnor “SuoT YA L9UILOF ayy fsed4y [eyuored ‘gq pue y ‘s8id-eoums ul saojovavyo yeood Jo suoTeUIquiODIY “06 “OI (CapyseQ worl) 06 ‘SI tapun pauorjueu sso.d oy} Aq poonpoad sadAjz 1943190 “1G “OT 396 HEREDITY AND ENVIRONMENT One of the striking results of modern work in plant-breeding has been the discovery of the greatly increased vigor of certain hybrids as compared with either pure-bred parent. In general it is not possible to tell without pre- vious experience what the character of the hybrid of two races or “lines” will be; some- times it is more and sometimes less vigorous than either parent, but not infrequently it is more vigorous. East and Shull have shown that hybrids between two races of corn may be very much larger and more fertile than either parent. In some instances the yield of corn per acre has been increased from 20-30 bushels to 80-90 bushels, and in one case to more than 250 bushels per acre (Figs. 92, 93). Unfortunately such hybrid races of corn do not continue to breed true and the crossing must be made anew in each generation if maximum results are to be had. Nevertheless this method of hybridization or “heterozygo- sis,” as it has been called, offers an extremely important means of quickly producing very vigorous and fruitful individuals, but not lines or races which breed true. CONTROL OF HEREDITY: EUGENICS 397 2. Origin of Mutations—Mendelian asso- ciation and dissociation of characters produces new forms of adult animals and plants but not new hereditary characters. Permutations of Mendelian characters we may have almost without number, of new combinations of these there may be no end, but no new unit charac- ters are formed by such temporary combina- tions, no new inheritance factors are created or evolved. New combinations of factors may be compared to new combinations of chemical elements; you can always get out of the combination what went into it, whereas new factors are comparable to the changes which take place in certain atoms, for example ra- dium, by which the element itself is changed in an irreversible manner. ‘The discoveries of Mendel show us how to follow old characters through many combinations and through many generations, but they do not show us how new characters arise. ‘These discoveries have given us an invaluable method of sorting and combin- ing hereditary qualities, but Mendelian inheri- tance as such does not furnish the materials for evolution. (jseq Wo1g) ‘suTetys asaq} UsaMjoq sso1d B JO SUOT}B1oUed “|W pue "iT oy} YW poseduioo (ZI pue 6 ‘SON) ULOD JUS Sulureey jo sureajs patquy ‘G6 “OT =) saaviiad vq LU 1H (‘yseq Wot) ‘yYSIL uo asay} UsaA49q SaSSO1D “JOT UO SUIVI}S PIIqUI OM} $GG “BIy UL UMOYS VSOY} 0} ALTIWIS suTeI]s JO UIOD BuIMOID “EG ‘OLY 400 HEREDITY AND ENVIRONMENT The actual origin of new hereditary charac- ters or mutations is obscure. Practically all of the earlier workers and writers on evolution found the principal causes of transmutation in the action of extrinsic or environmental forces on the organism. As the result of years of labor on this subject Darwin concluded that “variability of every sort is due to changed conditions of life”; but in the light of modern genetics such a statement is too sweeping. It is well known that environmental changes produce many kinds of modifications in organ- isms, and in general these modifications are the more profound the earlier they occur in onto- geny; it is known that slight alterations of the germ cells may produce great modifications of adult structure, and yet one of the most strik- ing results of recent work is to show the small effect of environmental changes on hereditary characters. Marked individual modifications may be produced which do not become racial. tsually not one of thousands of variations which occur has any evolutionary value. These fluctuations come with changing en- vironment and with changing environment ~ CONTROL OF HEREDITY: EUGENICS 401 they disappear. Very rarely a sudden variation or mutation arises which is per- petuated by heredity and which forms the Fic. 94. Common Colorado Potato Beetle, Leptinotarsa, a, normal undecemlineata, b, the mutant augusto-vittata, c, the mutant melanothorax, d, normal decemlineata, e, the mutant tortuosa, f, the mutant defecto-punctata. (From Plate after Tower.) 402 HEREDITY AND ENVIRONMENT basis of a new race (Figs. 94, 95). In most cases such mutations consist in the dropping out of some old character rather than in the addition of a new one, but at least they repre- sent modifications of hereditary constitution and as such they furnish material for evolu- tion. Whence and how they appear we do not know, for like the kingdom of heaven they come without observation. ‘Their infrequency amidst the multitude of fluctuations indicates the wonderful stability of racial types and teaches respect for Weismann’s doctrine of a germ plasm relatively stable, independent and continuous. This distinction between somatic and germi- nal variations, between those which concern only the individual and those which are in- herited and furnish material for evolution, marks the greatest advance in the study of evolution since the work of Darwin. And just as these germinal variations are the only ones of importance in the process of evolution so the question of their origin is the greatest evo- lutionary problem of the present day. How are such germinal variations produced? CONTROL OF HEREDITY: EUGENICS 403 There is some evidence, as was pointed out in the last chapter, that environmental changes of the right sort acting upon germ plasm at the right stage may lead to permanent modifica- tion of the hereditary organization. Extrinsic influences acting upon germ cells at the time of their maturation divisions may lead to new distributions of chromosomes or even to changes in the composition of individual chro- mosomes, thus producing new hereditary types. Certain mutants of Oenothera (Fig. 95) seem to be of this sort; for example O. lamarckiana has 14 chromosomes, O. lata 15, O. semi-gigas 21, O. gigas 28, and these vari- ations in the number of chromosomes are prob- ably due to abnormalities in the maturation divisions. It is significant that the mutants lata and semi-gigas have occurred several times, whereas gigas appeared but once; this may be explained by the fact that the chances of the doubling of chromosomes in both germ cells (gigas) are very few compared with the chances of their doubling in one germ cell (semi-gigas) or of their increase by one in one germ cell (lata). 404 HEREDITY AND ENVIRONMENT But it is probable that mutations are not usually associated with changes in the number of chromosomes. Where the number of chro- mosomes remains constant the change may take place in the number or composition of the chromomeres or units of the next lower order, but it would be practically impossible to find such changes in bodies so small and so numerous. Whatever the cellular changes may be which accompany mutations, it is certain that changes take place in the inheritance fac- tors. Sometimes factors drop out, as in the white sweet peas shown in Fig. 57, and in many other cultivated races of plants and animals, thus producing regressive mutations. Indeed most of our domestic animals and cultivated plants have arisen by the omission of some- thing which their wild ancestors had. In most of the mutations studied by deVries, Bateson, Morgan and others some factor has dropped out and Bateson suggests that at present all new forms arise only by the loss of factors or by the fractionation of factors and that new factors are not added from without. This leads him to inquire “whether the course of evolution (‘soltA ap WOIyZ) “syUe {NUT s}t sO udAIS pUe jJoT 94} UO “oUNIyOIDWHT DviaYyjOUIO ‘SASOIUIgG SUIUIAT SQpiewey “9G “oy D)]AUDU ‘OC punr SLALIUILQNL "OQ DUDLYDUDWYT *Q. Sun)?)112U298 ‘CQ. svbhib ‘¢C 2197-024 “O DID) ‘QO. -Yyaunwn'TyT *O 406. HEREDITY AND ENVIRONMENT can at all reasonably be represented as an un- packing of an original complex which con- tained within itself the whole range of diversity which living things present.” ‘This is as ex- treme preformation in the field of inheritance factors as was the old theory of emboitement in the field of developed characters; it is devolu- tion rather than evolution since it assumes that the earliest organisms were genetically the most complex. But if, as is often said, evolution from the amoeba to man necessarily involves the addi- tion of many new inheritance factors it does not involve additions from without as Bateson assumes. New hereditary factors are to be thought of as we think of new chemical com- pounds, which are formed by new combinations of the same old elements, or as we think of new elements, such as helium and radium emana- tion, which are formed by dissociation of ra- dium. As compared with chemical elements the factors of heredity are probably very complex things and the new factors which appear in the course of evolution probably arise as new com- binations of factors or parts of factors previ- CONTROL OF HEREDITY: EUGENICS 407 ously present. Nowhere in the entire process of organic evolution is there any evidence that new factors are “extrinsic additions” or are cre- ated de novo. ‘The whole process is one of evo- lution, that is of new combinations of existing units, having new qualities which are the results of these new combinations. If these changes in the germ plasm may be induced by extrinsic conditions, then a real experimental evolution will be possible; if they cannot be so induced but are like the changes taking place in the radium atom we can only look on while the evolutionary processes pro- ceed, selecting here and there a product which nature gives us but unable to initiate or con- trol these processes. B. CONTROL OF HUMAN HEREDITY: EUGENICS I. Past Evolution oF MAn There is every evidence that man also, no less than domesticated animals, has evolved from a natural or wild state. The most primi- tive types of men are known only from a few 408 HEREDITY AND ENVIRONMENT fossil remains, which indicate that these primi- tive men belonged to different species, and some of them even to different genera, from Homo sapiens (Fig. 96). Later stages in the evolution of man are known from many re- mains, implements and handiwork, as well as from certain primitive races or tribes which have persisted to the present time. ‘The grades of culture represented by these extinct or per- sistent tribes and by modern men are usually classified as savagery, barbarism and civiliza- tion. ‘There must have been much greater evo- lution of human types during prehistoric times than since the beginnings of civilization. The physical, mental and moral changes which took place in men from the earliest stages of sav- agery down to the beginnings of civilization were very great, but they were nevertheless slight compared with the tremendous changes which must have occurred in those long ages before the ancestors of man actually became men. Within the historic period the evolution- ary changes in man have been very small. Minor changes have occurred and are still go- ing on, as Osborn has shown in his “Cartwright CONTROL OF HEREDITY: EUGENICS 409 Fic. 96. Extinct Types of Man. A Skull of Homo nean- derthalensis; from Chapelle-aux-Saints, France, 1908. (After Boule). B. Skull of Pithecanthropus erectus; found in Trinil, Java, 1895; parts below dotted line restored by J. H. McGregor. 410 HEREDITY AND ENVIRONMENT Lectures on Contemporary Evolution in Man,” but the species has remained relatively stable during the historic epoch as compared with the much longer prehistoric period. The past history of man has been a long one, no one can say how long, but probably not less than half a million years have passed since the genus Homo appeared, and not less than one hundred thousand years since the present species arose. There is every reason to believe that the future history of man will be even longer. Barring great secular changes, catas- trophes or cataclysms, which cannot be fore- seen nor provided against, man controls his own destiny on this planet. It is a curious fact that in prescientific times the instability of nature especially appealed to men. How often in the past have men looked forward to a “speedy end of the world”! It may well have seemed to our ancestors a use- less thing to take any thought for the morrow if very soon the heavens are to be rolled up as a parchment and the elements dissolved in fer- vent heat; it would be folly to plan for future ages if the time is at hand when the angel CONTROL OF HEREDITY: EUGENICS 411 shall stand with one foot on the sea and the other on land and declare that time shall be no more. But science has taught us something of the wonderful stability of nature, something of the immensity of past time and of future ages, something of the eternity of natural pro- cesses. Compared with this infinite stability and eternity of nature what are our little sys- tems and customs! Our years and centuries fall like grains of sand into this abyss of time. Our individual lives are like drops of water in this great ocean of life. What intellectual de- velopments, what social institutions, what con- trol of natural processes may come in the long ages of futurity it has not entered into the heart of man to conceive. And yet so far as we may judge by the small portion of the record of the past which we can read there has been no necessary progress. ‘There has been “eternal process moving on,” but not eternal progress. Stagnation, degeneration, elimination, as well as progression, have occurred all along the path of evolution. And yet on the whole evolution has been 412 HEREDITY AND ENVIRONMENT progressive and there is no reason to sup- pose that the elimination of the unfit and the preservation of the fit will cease to be the law of future evolution, as it has been of the past. There are four principal types of the hu- man species—white, yellow, brown, and black—and many subtypes and races. These types and races differ in many regards in physi- eal, mental and social characteristics, and their comparative value has frequently been dis- cussed. It is difficult to take an impartial view of such a matter, though I suppose there would be little question on the part of any well informed person that the white and yel- low types have contributed most to what we call civilization. Nevertheless every race probably has good qualities which could be made of service to society. ‘The various races of cattle, horses, sheep, etc., are all useful to man, but in different ways and degrees, and the same is true of various races of men with re- spect to civilization. In general the dominant races are the most capable intellectually and socially, while those which have been left he- CONTROL OF HEREDITY: EUGENICS 413 hind or have been eliminated have been the less capable ones. And yet some very good races, possibly with capacities for high social and intellectual development, have been com- pletely exterminated, as for instance the Luca- yan Indians of the West Indies, and the aborigines of ‘Tasmania. Few animals have suffered more wholesale destruction than have the more primitive races of men in different parts of the earth. Several species of man have become entirely extinct, leaving only, as is generally believed, a single existing species, Homo sapiens. Race exter- mination has been witnessed in relatively re- cent times and on a large scale in the West Indies, North and South America, Africa, Australia, New Zealand and the Islands of the Pacific. But in the disappearance of native races extermination is usually supplemented by amalgamation. After the most warlike members of a race have been destroyed the - more peaceful remnants are generally incor- porated in the conquering race. ‘Thus the Maoris of New Zealand, the finest native race with which the English have come in contact in 414 HEREDITY AND ENVIRONMENT their colonies, were estimated to number more than a quarter of a million at the end of the eighteenth century. Owing to imported dis- eases and to destructive wars among the tribes and with the English there are not fifty thou- sand of them today, and these are being grad- ually absorbed into the white race. Undoubtedly there has been a great growth of altruism in the modern world; there is a relatively new feelmg among men that nothing so becomes a strong nation as the exercise of justice toward weaker ones, and many idealists maintain that every race and every people has the right to live its life m its own way. But however philanthropic they may be in theory, the practice of all nations demonstrates that weaker and inferior peoples are not permitted to stand in the way of dominant ones. When such peoples occupy territory which is desired by more powerful neighbors, they are either exterminated, expelled, exploited or amalga- mated with the conquering race. In practice their rights are usually of small concern as compared with the desires of the invaders, and the inaccessible or undesirable parts of the CONTROL OF HEREDITY: EUGENICS 415 earth, the deserts and mountains and regions of polar ice, become the refuge of the less cap- able races, just as “the conies, who are but a feeble folk, make their houses in the rocks.” This is an illustration of the great law of evolu-. tion, the survival of the strong and capable and intelligent, and even though ideal justice be meted out to weaker peoples, dominant races will still dominate and possess the earth. The only recourse which the inferior peoples have, and it is a terrible revenge, is to amalgamate with the superior race and thus lower its heredi- tary qualities. From the way in which primitive races have gone down before more cultured ones there is reason to believe that in general the principle of the elimination of the unfit and the survival of the fit has characterized human evolution no less than that of other organisms. Un- doubtedly intelligence has played a great part in the evolution of man, as is at once apparent when we consider the infinitely varied experiments by which he has worked his way from savagery to civilization. And yet he has not consciously set before himself an 416 HEREDITY AND ENVIRONMENT evolutionary goal to be attained by intelligent attention to principles of good breeding. II. Can HumMan EvouuTIon BE CONTROLLED 2 All that man now is he has come to be with- out conscious human guidance. If evolution has progressed from the amoeba to man with- out human interference, if the great progress from ape-like men to the most highly civilized races has taken place without conscious hu- man control, the question may well be asked, Is it possible to improve on the natural method of evolution? It may not be possible to im- prove on the method of evolution and yet by intelligent action it may be possible to facilitate that method. Man cannot change a single law of nature but he can put himself into such relations to natural laws that he can profit by them. 1. Selective Breeding the only Method of Improving the Race.—It is surely not possible to improve on nature’s method of eliminating the worst lines from reproduction. ‘This has been the chief factor in the establishment of CONTROL OF HEREDITY: EUGENICS 417 races of domesticated animals and cultivated plants, though as we have seen it has probably had nothing to do with the origin of mutations. The history of such races shows that evolution may be guided to human advantage by intelli- gent elimination and selection, and probably any hereditary improvement of the human race must be accomplished by this means, though of ‘course such elimination and selection can ap- ply only to the function of reproduction. ‘The method of evolution by the elimination of per- sons, the destruction of the weak and cowardly and antisocial, which was the method practiced in ancient Sparta, is repugnant to the moral sense of enlightened men and cannot be al- lowed to act as in the past; but the worst types of mankind may be prevented from pro- pagating, and the best types may be encour- aged to increase and multiply. This is apparently the only way in which we may hope to improve permanently the human breed. . 2. No Improvement in Human Heredity within Historic Times.—The improvement of environment and opportunities for individual 418 HEREDITY AND ENVIRONMENT development enables men at the present day to get more out of their heredity than was possible in the past. Advance of civiliza- tion has meant only improvement of environ- ment. But neither environment nor training has changed the hereditary capacities of man. There has been no perceptible improvement in human heredity within historic times, nothing comparable with the changes which have oc- curred in domesticated animals. Indeed no modern race of men is the equal of certain an- cient ones. Galton has pointed out the fact that in the little country of Attica in the cen- tury between 530 and 430 B. C. there were produced fourteen illustrious men, one for every 4,300 of the free born, adult male popu- lation. In the two centuries from 500-300 B.C. this small, barren country with an area and total population about equal to that of the present State of Rhode Island but with less than one-fifth as many free persons produced at least twenty-five illustrious men. Among statesmen and commanders there were Miuilti- ades, Themistocles, Aristides, Cimon, Pericles, Phocion; among poets Auschylus, Euripides, CONTROL OF HEREDITY: EUGENICS 419 Sophocles, Aristophanes; among philosophers and men of science Socrates, Plato, Aristotle, Demetrius, Theophrastus; among architects and artists Ictinus, Phidias, Praxiteles, Poly- gnotus; among historians Thucydides and Xenophon; among orators Auschines, Demos- thenes, Isocrates, Lysias. In this small coun- try in the space of two centuries there appeared such a galaxy of illustrious men as has never been found on the whole earth in any two centuries since that time. These illustrious men came from a remark- able race composed of individuals drawn together from all the shores of the Mediter- ranean by a process of unconscious but severe selection. Athens was the intellectual and so- cial capital of the world and to it the most ambitious and most capable men were irresist- ibly drawn. It was good immigration as well as good native stock that made Athens famous. Galton concludes that the average ability of the Athenian race of that period was, on the lowest possible estimate, as much greater than that of the English race of the present day as the latter is above that of the African negro. 420 HEREDITY AND ENVIRONMENT But this marvellously gifted race declined, as all such races have in time declined: Social morality grew exceedingly lax, marriage be- came unfashionable and was avoided, many of the more ambitious and accomplished women were. avowed courtesans and consequently infertile, and the mothers of the incoming population were of a heterogeneous class. . . . It can be therefore no sur- prise to us, though it has been a severe misfortune to humanity, that the high Athenian breed decayed and disappeared, for if it had maintained its excel- lence and had multiplied and spread over large countries, displacing inferior populations (which it well might have done, for it was naturally very pro- lific), it would assuredly have accomplished results advantageous to human civilization, to a degree that transcends our powers of imagination. (Galton, “Hereditary Genius,” page 331.) Bateson suggests that the high intellectual qualities of the ancient Athenian race were due to the inbreeding of homogeneous and very su- perior phratries and gentes, but when foreign marriages were sanctioned, and aliens and manumitted slaves were admitted to citizenship by the “reforms” of Cleisthenes (507 B.C.) the population gradually became mongrelized and its intellectual superiority declined. 3. Why the Race has not Improved.—If mankind has made no. progress in hereditary ‘CONTROL OF HEREDITY: EUGENICS 421 characteristics since the time of the Greeks the cause is not far to seek. ‘There have been gifted races and families, doubtless many notable human mutations have occurred, but most of these have been diluted, squandered, lost. There has been persistent violation of all principles of good breeding among men. For example, there has been for ages a futile reliance upon good environment to improve heredity. Men do not so improve the races of animals and plants, and thousands of years of human history show that this method is of no avail in improving the human breed. But the case is far worse than _ this; such efforts though futile are at least well in- tentioned, but on the part of most men and governments there has been complete disre- gard of the entire question of the improvement of the human stock. Natural selection which has through countless ages eliminated the worst and conserved the best fitted and thus has led on the whole to the survival of the fit is so far as possible nullified by civilized man; the worst are preserved along with the best and all are given the same chance of reproduction. 422 HEREDITY AND ENVIRONMENT The mistake has been not in nullifying natural selection by preserving the weak and incompe- tent, for civilized men could not well do other- wise, but in failing to substitute intelligent artificial selection for natural selection in the propagation of the race. Instead of this there has been perpetuation of the worst lines through sentimental regard for personal rights, even when opposed to the welfare of society ; and both church and state have cheer- fully given consent and blessing to the mar- riage and propagation of idiots and of diseased, defective, insane and vicious persons. Fi- nally there has been extinction of the world’s most gifted lines by enforced celibacy in many religious orders and societies of scholars; by almost continuous wars which have taken the very best blood that was left outside of the monastic orders; by luxury and voluntary sterility; by vice, disease and consequent infertility. Is it any wonder that the inheritance of the human race has not improved within historic times? Is it not rather an evidence of the broadcast distribution of good and wholesome CONTROL OF HEREDITY: EUGENICS 423 qualities in the race that in spite of such serious violations of the principles of good breeding mankind remains as good as we find it to-day? III. KuGeEnics If a superior power should deal with man as man deals with domestic animals no doubt great improvement could be effected in the human breed. Society is in some respects such a power and can do what the individual, because of self-interest, short life or iack of ability, cannot accomplish. In matters of pub- lic health, comfort and security of life and property society is superior in power to the individual; in matters of the perpetuation of the race the individual is still supreme. In animal societies the race, the breed, is to the swift and strong and fit, and the same was probably true of primitive men. But it is im- possible to return to the conditions of primi- tive society in this respect, and the social body itself must in some way control the breeding of men. 424, HEREDITY AND ENVIRONMENT There are millions of men in civilized countries whose mental equipment places them on a plane with barbarians or savages, and they have on the average more offspring than their civilized contemporaries. There are mil- lions of others who are so seriously defective in body or mind, owing to hereditary causes, that they can never take care of themselves and must always be a charge upon the state, and yet in many civilized countries they are per- mitted to perpetuate their kind and produce an ever-increasing supply of mental and moral defectives, whose maintainance must seriously interfere with the proper education and development of the normai population and whose unrestrained existence constantly threatens to pollute purer streams of heredity. The practice of society regarding marriage and reproduction up to the present has been to allow all sorts, good, bad and indifferent, to propagate with the belief that good environ- ment and training may make up for deficien- cies of birth. But very recently the conviction has been growing that good environment is far less important than good heredity and that in CONTROL OF HEREDITY: EUGENICS 425 some way society must influence the race of men at its source. ‘This is the doctrine of eugenics, which Galton defines as follows: The science of improving stock, which is by no means confined to questions of judicious mating but which, especially in the case of man, takes cog- nizance of all influences that tend in however remote a degree to give to the more suitable races or strains of blood a better chance of prevailing speedily over the less suitable than they otherwise would have had. (“Inquiries into Human Faculty.”’) Fortunately, or unfortunately, the methods which breeders use cannot be rigidly applied in the case of man. It is possible for breeders to eliminate from reproduction all except the very best stocks, and this is really essential if evolu- tion is to be guided in a definite direction. If only the very worst are eliminated in each gen- eration, the standard of a race is merely main- tained, but the more severe the elimination is the more does it become a directing factor in evolution. In the case of man, however, even the most enthusiastic eugenicists have never proposed to cut off from the possibility of. re- production all human stocks except the very best, and if only the very worst stocks are thus 426 HEREDITY AND ENVIRONMENT eliminated, we must face the conclusion that practically all that can be accomplished will be to preserve the race at its present level. It is impossible, then, to apply rigidly to man the methods of animal and plant breeders. So- ciety cannot be expected to eliminate from re- production all but the very best lines. The great majority of mankind cannot be expected voluntarily to efface itself. ‘The most that can be hoped for in this direction is that the great mediocre majority may eliminate from repro- duction a very small mimority of the worst individuals. Furthermore, other and perhaps even more serious objections to the views of extreme eu- genicists are to be found in human ideals of morality. Even for the laudable purpose of producing a race of supermen, mankind will probably never consent to be reduced to the morality of the breeding-pen with a total disre- gard of marriage and monogamy. The geneti- cist who has dealt only with chickens or rabbits or cattle is apt to overlook the vast difference between controlling reproduction in lower ani- mals and in the case of man where restraints must be self-imposed. CONTROL OF HEREDITY: EUGENICS 427 Another fundamental difficulty in breeding a better race of men is to be found in a lack of uniform ideals. A breeder of domestic animals lives long enough to develop certain races and see them well established, but the devotee of eugenics cannot be sure that his or her ideals will be followed in succeeding generations. The father of Simon Newcomb is said to have walked through the length and breadth of Nova Scotia seeking for himself a suitable mate, but neither he nor any other eugenicist could be sure that his descendants would follow a similar course, and long continued selection along particular lines must be practiced if the race is to be permanently improved. Mankind is such a mongrel mixture, and it is so imprac- ticable to exercise a strict control over the breeding of men, that it is hopeless to expect to get pure or homozygous stocks except with respect to a very few characters and then only after long selection. But granting all these difficulties which con- front the eugenicist, there is no doubt that something may be gained by eliminating mere- ly the worst human kinds from the possibility 428 HEREDITY AND ENVIRONMENT of reproduction, even though no marvellous improvement in the human race can be ex- pected as a result of such a feeble measure. 1. Possible and Impossible Ideals —What the future evolution of the human race may lead to is an interesting speculation, but it is and can be only a speculation. ‘There is no present evidence that there will ever be a higher animal than man on the earth, and the only evidence that there may be a higher spe- cies than Homo sapiens is to be found in the fact that there have been lower species of men in the past and that evolution has been on the whole progressive. ‘The idea that by the aid of that infant industry eugenics a new race of supermen is shortly to be produced is an iri- descent dream, and the fantastic demand of some enthusiasts for changes in racial fashions has served to bring this whole subject of eu- genics into disrepute among thoughtful men. To a considerable extent the ideals regard- ing individuals and society have differed in different ages and races in the past, but with the closer communications which have been es- tablished between all parts of the earth in CONTROL OF HEREDITY: EUGENICS 429 modern times there has developed a greater uniformity of ideal. In a complex society all types of service are needed and many differ- ent types of individuals are socially useful. If the social good were the supreme end, as it is in a colony of ants or bees, the greatest dif- ferentiation of individuals for particular kinds of service would be desirable. ‘There should be a hereditary class of laborers, of business men, of scholars, of artists, ete., and for the im- provement of each class there should be in- breeding in that class. Such methods are now used by breeders of various races of domestic animals and cultivated plants with the best of results. No breeder would think of trying to improve draft horses by crossing with race horses, nor of improving milk cows by crossing with beef cattle. In other countries and ages the development of hereditary classes and castes in human society has been tried, and survivals of it persist to this day, but they are only vestigial remnants of an old order which is everywhere being replaced by a new ideal in which the good of the individual as well as that of society is the end desired. 430 HEREDITY AND ENVIRONMENT The whole development of modern society is in the direction of racial solidarity and away from hereditary classes. Government, educa- tion and religion; socialism, syndicalism, an- - archism all reflect the movement for individual liberty, fraternity and equality. The modern ideal individual is not the highly specialized unit in the social organism, as in the case of social insects, but rather the most general all- round type of individual, the man who can when conditions demand combine within him- self the functions of the laborer, business man, soldier and scholar. For such a generalized type the methods of inbreeding or close breed- ing used by the breeder of thoroughbreds are wholly inappropriate. On the other hand such a generalized type must include the best qualities of many types and races and Men- delian inheritance shows how it is possible to sort out the best qualities from the worst. Nowadays one hears a lot of high sounding talk about “human thoroughbreds,” which usually means that those who use this phrase desire to see certain narrow and exclusive so- cial classes perpetuated by close inbreeding; it CONTROL OF HEREDITY: EUGENICS 431 usually has no reference to good hereditary traits wherever found, indeed such traits would not be recognized if they appeared out- _ side of “the four hundred.” Such talk prob- ably does neither harm nor good; the “social thoroughbreds” are so few in number and so nearly sterile that the mass of the population is not affected by these exclusive classes. Galton advocated the segregation and inter- marriage of the most highly intellectual mem- bers of society, such as the prize scholars in the colleges and universities; but if the human ideal is the generalized rather than the spe- cialized type it would be better if the prize scholars married the prize athletes. A race of highly specialized scholars or athletes is not so desirable as a race in which these and other excellences are well balanced. From this point of view the person who is voted the “best all-round man in his class” is nearer the eugenical ideal than the prize scholar. No man can trace his lineage back through many generations without realizing that it in- cludes many hereditary lines differing greatly in value. The significance of sexual reproduc- 432 HEREDITY AND ENVIRONMENT tion les in this very fact that it brings about the commingling of distinct lines and thereby makes every individual different from every other one. ‘The entire history of past evolu- . tion testifies to the value of this process, al- though it causes the gardener, the breeder, the eugenicist serious trouble. But the gardener can propagate his choice fruits by budding and grafting, the breeder can for a time preserve his choice stock by close inbreeding, but the eugenicist cannot shut out the influence of for- eign blood, and it is well that he cannot for if he could do so the progress of the race would probably soon come to an end. In the human species the only absolute bar- rier to the intermingling of races is geographi- cal isolation. Every human race is fertile with every other one, and though races and nations and social groups may raise artificial barriers against interbreeding we know that these arti- ficial restraints are frequently disregarded and that in the long run amalgamation does take place; and in general the farther amalgama- tion progresses the faster it goes. In Aus- CONTROL OF HEREDITY: EUGENICS 433 tralia and New Zealand, after little more than a century’s contact with white races, there are about as many “half castes” as there are full blooded aborigines. In the United States one- quarter of all persons of African descent con- tain more or less white blood; there are about eight million full blooded negroes and two mil- lion mulattoes, and during the past twenty years the latter have increased at twice the rate of the former. In Jamaica, where there are about seven hundred thousand blacks and fifteen thousand whites, there are about fifty thousand mulattoes. A similar condition pre- vails wherever different races occupy the same country. Even the Jews, who were long sup- posed to be a peculiarly separate and distinct people, have received large admixtures of Gentile blood in every country in which they have lived. Whether we want it or not hybridization of human races is going on and will increase. Partition walls between classes and races are being broken down; complete isolation is no longer possible, and a gradual intermixture of human races is inevitable. We are in the grip YY . 434 HEREDITY AND ENVIRONMENT of a great world movement and we cannot re- verse the current, but we may to a certain ex- tent direct that current into the more desirable channels. There is a popular belief that hybrid races are always inferior to pure bred ones, but this is by no means the case. Some hybrids are undoubtedly inferior to either of the parents but on the other hand some are vastly supe- rior; only experience can determine whether a certain cross will yield inferior or superior types. Society may well attempt to prevent those crosses which produce inferior stock while encouraging those which produce su- perior types. It is race mixture which makes the problem of immigration so serious. Generally immigra- tion is regarded merely as an economic and political problem, but these aspects of it are temporary and insignificant as compared with its biological consequences. In welcoming the immigrant to our shores we not only share with him our country but we take him imto our families and give to him our children or our children’s children in marriage. Whatever the CONTROL OF HEREDITY: EUGENICS 435 present antipathies may be to such racial mix- tures we may rest assured that in a few hun-— dred years these persons of foreign race and blood will be incorporated in our race and we in theirs. From the amalgamation of good races good results may be expected; but fusion with inferior races, while it may help to raise the lower race, is very apt to pull the higher race down. How insignificant are considera- tions of cheap labor and rapid development of natural resources when compared with these biological consequences! 2. Negative Eugenical Measures.—Gaiton said nothing about sterilization or elimination from reproduction of less valuable lines in his “Inquiries into Human Faculty” which was first published in 1883. He proposed no radi-. - cal policy but rather one which he thought would be practical and might meet with general favor. He suggested a social policy which would delay the age of marriage among the weak and hasten it among the vigorous, whereas present social agencies act in the op- posite direction. He showed by statistics that, on the average, marriage at the age of 22 would 436 HEREDITY AND ENVIRONMENT _ produce at the end of one century four times as many offspring as marriage at 33 and at the end of two centuries ten times as many. He particularly emphasized the great harm which would be done by an application of the theory of Malthus among the better classes. For the prudent to put off marriage and to limit offsprmg while the imprudent continue to reproduce at the present rate would be to give the world to the imprudent within a few centuries at most. His suggestions, which were at first received with indifference or ridicule, were much less radical than the legal requirements in many of our States to-day. Public sentiment has been greatly aroused on this question; the apparent increase in the number of defectives and criminals has seemed to call for radical action and a flood of hasty but well intentioned legislation has been the result. We may con- fidently expect that in a very short time the marriage of the feeble-minded, hopelessly insane or epileptic, the congenitally blind, deaf and dumb, and those suffering from many other inherited defects which unfit them CONTROL OF HEREDITY: EUGENICS 437 for useful citizenship will be prohibited by law in all the States. Our immigration laws al- ready exclude such aliens, and the number of persons of the types named who seek legal consent to marry is not large so that it need not be expected that such laws will quickly improve the general population. If in addi- tion such persons are either segregated or sterilized the danger of their leaving illegiti- mate offspring will be removed; such pre- cautions have been taken in certain of our States and will probably become general, though at present few of the laws on this sub- ject are strictly enforced. The study of heredity shows that the nor- mal brothers and sisters, and even more distant © relatives, of affected persons may carry a re- cessive defect in their germ plasm and may transmit it to their descendants though not showing it themselves. It will be more diffi- cult, perhaps an impossible thing, to apply rigidly the principles of good breeding to such persons and to exclude them from reproduc- tion; but if in each generation those persons in whom this recessive trait appears are prevented 438 HEREDITY AND ENVIRONMENT from leaving offspring the number of persons affected will gradually grow less, other condi- tions being equal. But while such negative eugenical measures are wholly commendable when applied to such defects as those named, which are certainly in- herited and which render those affected unfit for citizenship, the wholesale sterilization of all sorts of eriminals, alcoholics and undesir- ables without determiming whether their de- fects are due to heredity or to conditions of development would be like burning down a house to get rid of the rats; and the only justification which could be offered for the general sterilization of the inmates of all pub- lic institutions, which is urged by some of our modern crusaders, would be the defense which some persons make for war, that there are too many people anyway and anything which will prevent the growth of population is to be welcomed. Advocates of war never cease to point out {ts beneficial effects on the race,—how it makes men strong, courageous, unselfish, how it makes nations great, powerful, progressive. CONTROL OF HEREDITY: EUGENICS 439 There is no doubt that war like any other great crisis discovers great men and furnishes op- portunities for the development of great qual- ities that might otherwise remain undeveloped and unknown. But there is also no doubt that it takes the very best blood of the nations. Those who go to war are the young, the strong, the capable, while the weak, incompetent and degenerate are left behind as unfit for military service. If conditions could be reversed and the bungled and botched, the feeble-minded and insane, the degenerate and debauched could be put in the fore front of battle some benefit to the race might result, but no increase of national greatness can compensate for the awful waste of the best thing which any nation possesses—its best blood. 3. Positive Eugenical Measures.—Positive eugenical measures are much more difficult to apply and are of more doubtful value. Of course compulsory measures are out of the _ question and encouragement and advice alone are feasible. Giving advice regarding matri- mony is proverbially a hazardous performance, and it is not much safer for the biologist than 440 HEREDITY AND ENVIRONMENT for others. Wuth much more complete knowl- edge regarding human inheritance than we now possess it may ultimately be possible to give such advice wisely, especially with respect to physical characteristics which are hereditar- ily simple and generally of mimor significance. But where the character is an extremely com- plex one as in intellectual ability, moral recti- tude, judgment and poise, which are the chief characteristics which distinguish the great man from his fellows, it will probably never be pos- sible to predict the result before the event. He would be a bold prophet who would un- dertake to predict the type of personality which might be expected in the children of a given union. Some very unpromising stocks have brought forth wonderful products. Could anyone have predicted Abraham Lincoln from a study of his ancestry? Observe I say “pre- dict,” and not “explain” after his appearance. Can anyone now predict from what kind of ancestral combinations the great scholars, statesmen, men of affairs of the next gener- ation will come? The time may come when it will be possible to predict what the chances are CONTROL OF HEREDITY: EUGENICS 441 that the children of given parents will inherit more or less than average intellectual capacity, but since germinal potentiality is transformed into intellectual ability only as the result of development such a prediction could not be extended to the latter unless the environment as well as the heredity were known. Mankind is such a mongrel race, good and bad qualities are so mixed in us, marriage is such a lottery, the distribution of the germinal units to the different germ cells and the union of particular germ cells in fertilization is so wholly a matter of chance, the influence of even bad hereditary units on one another is so un- predictably good or bad as is shown in many hybrids, even the minor influences of environ- ment and education which escape attention are so potent in development, that the chances were infinity to one against any one of us, with all his individual characteristics, ever coming into existence. If the Greeks or Romans had known of the real infinity of chances through which every human being is brought to the light of day not only would they have deified Chance but they would have made her the mother of gods and men. 442 HEREDITY AND ENVIRONMENT But granting the impossibility of predicting the character of children it may well be asked if good general advice may not be given re- garding the choosing of a mate. Many people have thought so, and if all that has been said or written on this subject were to be gathered together I suppose that there would not or should not be room for it in all the libraries of the world. It is generally admitted that few lines are wholly free from hereditary defects and the question has often been asked what the eugenical practice should be in such cases. Of course people with really serious hereditary defects should not have children. If the de- fects are slight Davenport has suggested that they may be either disregarded or weakness in any character may be mated with strength in that character. That people with only slight hereditary defects should not marry at all is a counsel of perfection. On the other hand it would be a dangerous _ rule to propose that persons having really serious hereditary defects should be mated with those who are strong in those characters on the ground that in general strength in a CONTROL OF HEREDITY: EUGENICS 443 character is dominant over weakness. It has been suggested that a normal man who marries a feeble-mmded woman would have only nor- mal children, since both genius and feeble- mindedness seem to be recessive when mated with mediocrity or normality. But in all such ~cases the weakness is not neutralized or re- moved but merely concealed in the offspring and is therefore the more dangerous. If a man chooses to marry a feeble-minded woman he at least does so with his eyes open and he need not be deceived. But the normal and perhaps capable children of such a union carry the taint concealed in their germ plasm and if they should be mated with other normal persons carrying a similar taint some of their children would be feeble-minded, and thus the sins of the parents in mating weakness with strength — would be visited upon the children to the nth generation. Such a policy of concealing weak- ness by mating it with strength is wholly com- parable with the custom once prevalent of concealing cases of contagious diseases, and may be properly characterized as the “ostrich policy.” 444 HEREDITY AND ENVIRONMENT After all in the choosing of mates a combi- nation of instinct and intelligence is probably the safest guide. Our instincts, built up through long ages, are generally adaptive and useful, and if they be guided by reason the re- sult is apt to be better than if either instinct or reason act alone. More need not be said on this subject, since it is treated ad infinitum in works of fiction and in ladies’ journals. 4. Contributory Eugenical Measures.—In addition to the negative and positive eugenical measures mentioned many conditions may be classed as contributary to eugenics. One of the most important of all contributary meas- ures is the general education of the people re- garding heredity. The wide-spread ignorance on this subject is profound and very many of- fenders against the principles of good breeding have sinned through ignorance. Any general reform must rest upon enlightened public opinion, and the schools, the churches and the press can do no more important work for man- kind than to educate the people, after they have been educated themselves, on this im- portant matter. CONTROL OF HEREDITY: EUGENICS 4AS Society too may cultivate a proper pride in good inheritance. Much of value would be ac- complished if the silly pride in ancestral wealth or position or environment which touched our forebears only superficially and never entered into their germ plasm, or the still sillier claims of long descent, in which we are all equal, could be replaced by a proper pride in ancestral heredity, a pride in those in- herited qualities of body, mind and character which have made some families illustrious. A proper pride in heredity would do much to insure the perpetuation of the line and to pro- tect it from admixture with baser blood. Among other contributory measures which serve to promote good breeding among men must be reckoned coeducation, as well as other means of promoting good and early marriages. The president of a large coeducational insti- tution once said that if marriages were made in heaven he was sure that the Lord had a branch office in his university. I had occasion a few years ago to investigate the eugenical record of a coeducational institution, which is not un- known in the world of scholarship, and found 446 HEREDITY AND ENVIRONMENT that about 33 per cent. of the recent graduates had married fellow students, that there had been no divorces and that there were many | children. ‘There is no doubt that coeducation promotes good and early marriages and that it is not necessarily inimical to good scholar- ship even though it violates the spirit of medieval monasticism. ‘There was a time when it was supposed that a scholar must live the monkish life of seclusion and contemplation, but the monasteries are disappearing the world over, and it is time that the monastic spirit should go out of the colleges and universities. On the other hand the colleges exclusively for women appear to have a bad influence on the marriage rate and birth rate of their grad- uates. Johnson has shown that 90 per cent. of all the women of the United States marry be- fore the age of 40, but that among college women only half that number have married at the same age. As a result of investigations at one of the leading women’s colleges he finds that the marriage and birth rate of the most brilliant students, who have been elected mem- hers of Phi Beta Kappa, is lowest of all. Cat- CONTROL OF HEREDITY: EUGENICS p aaT tell says that a Harvard graduate has on the average three-fourths of a son, a Vassar grad- uate one-half of a daughter. ) At present early and fruitful marriages _ among able and ambitious people are very un- fashionable and are becoming increasingly im- practicable. If society has any regard for its own welfare all this must be changed. As Galton has shown, the race that marries at 22 instead of 33 will possess the earth in two or three centuries. The good of society demands that we re- verse our methods of putting a premium upon celibacy among our most gifted and ambitious young men and women, and if monastic orders and institutions are to continue they should be open only to the eugenicially unfit. 5. The Declining Birth Rate and the Death of Families—Among animals and plants in a state of nature the number of individuals in each species remains fairly constant from year to year; that is, only enough young are born and survive to take the places of mature indi- viduals that die. But when a species is placed in new and favorable conditions it may for a 448 HEREDITY AND ENVIRONMENT while increase at an amazing rate until the pressure of population becomes sufficient to reestablish an equilibrium between the birth rate and the death rate. ‘Thus when the English sparrow was introduced into the United States it increased at a phenomenal rate for a number of years, but now the num- ber of individuals in any given locality remains about the same from year to year, the birth rate merely compensating for the death rate. This equilibrium is brought about in the main by increased mortality, especially among the young, though decreasing fecundity may play a minor part. Essentially the same principles apply to human populations. Up to two or three cen- turies ago the populations of the older coun- tries of the world were practically stationary. Fecundity was relatively high but the death rate was also very high, the excess of popula- tion in each generation being carried off in large numbers by war, pestilence and famine. Then owing to the developments of science and industry and to the opening up of new coun- tries a period of remarkable expansion of pap- CONTROL OF HEREDITY: EUGENICS 449 ulation began. The population of Europe, which was about 175 millions in 1800, increased to 420 millions in 1900, and this in spite of the fact that about 35 millions migrated from Europe to new countries during this period. This great increase in the population of EKu- rope was due primarily to reduction of the death rate since the birth rate also declined slightly during this period, while in the newer countries there was both an increase in the birth rate and a decrease in the death rate. It is perhaps an open question how long the advances of science in rendering available the natural resources of the earth may be able to keep pace with increasing population, but it is evidently impossible for this great increase in the population of the world to go on in- definitely; sooner or later it must come to an end and the population again become station- ary. Already the birth rate is decreasing more rapidly than the death rate in all the western countries of Europe and this movement must ultimately extend to all parts of the world and lead to a checking of the great increase in population which has characterized the last two 450 HEREDITY AND ENVIRONMENT hundred years. This approach to a stationary population is both a normal and a desirable thing, for no one could wish to see population increase more rapidly than the supply of food or other necessaries of life; and of the two pos- sible methods of checking population few would hesitate to choose a decreasing birth rate as preferable to an increasing death rate. It is not therefore the declining birth rate in the general population that should cause alarm but rather the declining birth rate in the best elements of a population, while it con- tinues to increase or at the least remains sta- tionary among the poorer elements, and there is abundant evidence that this is just what is taking place. The descendants of the Puri- tans and the Cavaliers who have raised the cry for “fewer and better children” are already disappearing and in a few centuries at most will have given place to more fertile races of mankind. Everybody knows that the old New England families are dying out and that their places are being taken by recent immi- grants. The few exceptions are merely eddies in the current that is bearing them to doom. * CONTROL OF HEREDITY: EUGENICS 451 In Massachusetts the birth rate of the foreign born is twice that of the native population while the death rate is about the same for both. The same is true of the older families in many parts of the world. Professor Cattell has recently made a sta- tistical study of the families of 917 Ameri- can men of science and he finds that the average size of family of the parents of these men was 4.66 children, whereas the average size of family of these men is 2.22 children. In one generation the fertility of these lines has been reduced by more than half. The causes of this decline are chiefly voluntary - being assigned to health, expense and other causes. But the causes of sterility are not only so- cial and voluntary ones, which could be changed by custom and public opinion; there are also involuntary and biological causes of a deep-seated nature. Fahlenbeck has made a study of 433 noble families of Sweden which have become extinct in the male line, and he shows that the last male died unmarried in 45 per cent. of these families, and before the 452 HEREDITY AND ENVIRONMENT age of 21 in 89 per cent., while the line ended in infertile marriage in 11 per cent. and in daughters only in 5 per cent. Broman points out that most noble families of Kurope die out after 100 to 250 years and generally do not live beyond the third gener- ation. ‘The same is true of the families of great scholars, artists and statesmen. Possi- bly one cause of such declining fertility may be found in too great brain activity, but there is no doubt that in many instances it is due to luxurious living. On the other hand bodily fatigue and simple livmg favor fertility in both animals and men. Wild animals brought into captivity where they have comfortable quarters and an unwonted abundance of rich food are usually infertile: and the conditions of life of the upper classes of society are al- most as unfavorable to fertility as is captivity with wild animals. It is evident that if we had fewer luxuries we could have, and could afford to have, more children. But animals in captivity may gradually be- come adapted to their new conditions so as to become fertile, and there is evidence that man CONTROL OF HEREDITY: EUGENICS 453 also may undergo a slow adaptation in this regard to conditions of high civilization. Some royal families of Europe go back six or eight hundred years, and in general if a family survives the new conditions of affluence and luxury for more than three generations it may become more or less adapted to the new conditions. No eugenical reform can fail to take account of the fact that the decreasing birth rate among intelligent people is a constant menace to the race. "We need not “fewer and better chil- dren” but more children of the better sort and fewer of the worse variety.’ There is great enthusiasm to-day on the part of many child- less reformers for negative eugenical meas- ures; the race is to be regenerated through sterilization. But unfortunately this reform begins at home among those who because of good hereditary traits should not be sterile. Sterility is too easily acquired; what is not so easily brought about is the fertility of the bet- ter lines. Galton was far wiser than most of his followers for he realized the necessity of 454 HEREDITY AND ENVIRONMENT increasing the families of the better types as well as of decreasing those of the worse. What Bernard Shaw regards as the greatest discovery of the nineteenth century, viz., ‘the means of artificially limiting the size of fami- lies, may prove to be the greatest menace to the human race.’ If it were applied only to those who should not have children or to those who should for various reasons have only a few chil- dren it would be a blessing to mankind. But applied to those who could and should have: many children it is no gift of the gods. No one denies that the chief motive for limiting the size of families is personal comfort and pleas- ure rather than the welfare of the race. The argument that people should have no more chil- dren than they can rear in comfort or luxury assumes that environment is more important than heredity, which is contrary to all the bio- logical evidence. In the breeding of horses or cattle or men heredity is more potent than environment; and it is more important for the welfare of the race that children with good in- heritance should be brought into the world than that parents should live easy lives and CONTROL OF HEREDITY: EUGENICS 455 have no more children than they can conven- iently rear amid all the comforts of a luxury- loving age. The method of evolution in the past has been the production of enormous numbers of indi- viduals and the elimination of the least fit. ‘The modern method of improving domestic races is to select for reproduction the best types from large numbers of individuals. One reason why human evolution has gone on so slowly is to be found in the slow breeding of men. Nature has provided an almost infinite wealth and variety of potential personalities in human germ cells but only an infinitesimal number ever come to development. If this number is still further reduced by artificial means the race will be made the poorer not merely in quantity but also in quality. “The optimism of those who believe that supermen may be pro- duced by artificially limiting the number of children is a foolish and fatal optimism. ‘ Finally for those who are denied the privi- lege of parenthood and upon whom sterility is forced by whatever circumstances there is a lesson of value to be drawn from the social in- 456 HEREDITY AND ENVIRONMENT sects. ‘The sterile members of a colony of ants or bees are forever denied the possibility of having offspring of their own, but they become foster mothers to the offspring of the queen. They tenderly nurse, care for and rear the young of the colony. There are many children in the world who need foster mothers and fathers; there are many men and women in the world, both married and unmarried, who need adopted children. “Go to the ant, thou slug- gard; consider her ways and be wise.” CHAPTER VI GENETICS AND ETHICS ey decree ee ee a iat. @ ae ios CHAPTER VI GENETICS AND ETHICS* Modern studies of development are preo- foundly changing the opinions of men with re- spect to human personality. Observation of the relentless laws of heredity, of the inevitable influences for good or bad of environmental conditions over which the individual has no control, undoubtedly tends to produce a sense of helplessness and hopelessness. What light is thrown upon the great problems of freedom and determinism, of responsibility and irre- sponsibility, of duty and necessity by modern studies of development? Such questions can- not be dealt with quantitatively and experi- mentally, and they lie outside the field of exact science, but they are involved in all inquiries which have to do with rational and social be- * A portion of this chapter was given as the presidential ad- dress before the American Society of Naturalists in January, 1913, and was published in Science under the title “Heredity and Responsibility.” 460 HEREDITY AND ENVIRONMENT ings; they lie at the foundation of the applica- tion of science to human welfare; they occupy a large place in the thought and conduct of all men. I. Tur VoLuntTaRistic CONCEPTION OF Na- TURE AND OF HuMAN RESPONSIBILITY Primitive men regarded their own activities and all phenomena of nature as the expression of will, and a similar view has been maintained by certain philosophers and theologians even in modern times. Nature was regarded as the immediate expression of a vast will which creates, rules, builds and destroys as it sees fit. The lightning is hurled from the hand of Jove, the sea is disturbed by angry deities, the winds are let loose or stilled, the earth trembles, the hills smoke, the sun and moon and stars travel in their appointed courses as the gods will. In this primitive view of nature even inani- mate objects were supposed to be endowed with wills of their own, and many modern men are sufficiently primitive to kick the chair over which they stumble, or to swear that the devil GENETICS AND ETHICS 461 has gotten into the automobile. Of course the actions of all animate things were held to be the result of choice; the fly that dances on your head or gets into the soup is doing it to annoy you; the cats that yowl, the dogs that howl, the maniacs that screech are possessed of devils, evil wills, and should be punished. All good is the result of good will, all evil of evil will. Some being, some volition, is responsi- ble for everything that happens. All nature is the expression of big or little wills, of good or bad wills, and the good should be rewarded and the bad punished. This conception of nature finds its counter- part and probably its origin in similar views concerning human conduct and responsibility. According to this belief every man is the archi- tect of his own character; the will is absolutely free; no taint of heredity or necessity rests on the mind or soul; character is a tabula rasa, upon which the self writes its own record as it chooses, and is responsible for the result. Conduct whether good or bad, benevolent or criminal, rational or irrational rests upon vol- untary choice, and for such choices men must 462 HEREDITY AND ENVIRONMENT be held responsible. To a great extent this view of freedom and responsibility is the basis of present systems of government, education, ethics and religion. II. THe Mecuanistic Conception or Na- TURE AND OF PERSONALITY As contrasted with this voluntaristic view of nature and of man consider the scientific conception of nature as a vast mechanism, an endless chain of causes and effects. Science deals with “the unfailing order of immortal — nature,” with the universality of cause and effect, with the eternal stability and inevita- bility of natural processes. Natural phe- nomena are not the result of volitions big or little, good or bad, but of all the events which have gone before. To the man of science na- ture is not the mere caprice of god or devil, to be lightly altered for a child’s whim; nature is, aS Bishop Butler said, that which is “stated, fixed, settled,” eternal process moving on, the same yesterday, to-day and forever. From sands to stars, from the immensity of the universe to the minuteness of the electron, in living things no less than in lifeless ones, GENETICS AND ETHICS 463 science recognizes everywhere the inevitable se- quence of cause and effect, the universality of natural processes, the reign of natural law. Man also is a part of nature, a part of the great mechanism of the universe, and all that he is and does is limited and prescribed by laws of nature. Every human being comes into ex- istence by a process of development, every step of which is determined by antecedent causes. 1. The Determinism of Heredity— There can be no doubt that the main characteristics of every living thing are unalterably fixed by heredity. Men differ from horses or turnips because of their inheritance. Our family traits were determined by the hereditary constitu- tions of our ancestors, our inherited personal traits by the hereditary constitutions of our fathers and mothers. By the shuffle and deal of the hereditary factors in the forma- tion of the germ cells and by the chance union of two of these cells in fertilization our heredi- tary natures were forever sealed. Our ana- tomical, physiological, psychological possibili- ties were predetermined in the germ cells from which we came. All the main characteristics 464, HEREDITY AND ENVIRONMENT of our personalities were born with us and can- not be changed except within relatively nar- row limits. “The leopard cannot change his spots nor the Ethiopian his skin,” and “though thou shouldst bray a fool in a mortar with a pestle yet will not his foolishness depart from him.” Race, sex, mental capacity are de- termined in the germ cells, perhaps in the chromosomes, and all the possibilities of our lives were there fixed, for who by taking thought can add one chromosome, or even one determiner to his organization? The thought of this age has been pro- foundly influenced by such considerations. We formerly heard that “all men were created free and equal”; we now learn that “all men are created bound and unequal.” We were once taught that acts, if oft repeated, become habits, and that habits determine character; hereditarians of the stricter sort now teach that acts, habits and character were foreordained from the foundation of the family. We once thought that men were free to do right or wrong, and that they were responsible for their deeds; now we learn that our reactions GENETICS AND ETHICS 465 are predetermined by heredity, and that we can no more control them than we can control our heart beats. For ages men have believed in the influence of example, in the uplift of high ideals, in the power of an absorbing pur- pose; for ages men have lived and died for what they believed to be duty and truth, and have received the homage of mankind; or they have lived malevolent and criminal lives and have been despised by men and punished by society. But if our reactions, habits, charac- ters are predetermined in the germ plasm such men have deserved neither praise nor blame. If personality is determined by heredity alone all teaching, preaching, government is useless; freedom, responsibility, duty are delusions; whether men are useful or useless members of society depends upon their inheritance, and the only hope for the race is in eugenics— always supposing that enough freedom is left to men or to society to control the important function of choosing a mate. Already a few enthusiastic persons have begun to apply these doctrines to practical af- fairs. We are told that children should never 466 HEREDITY AND ENVIRONMENT be admonished or punished, for they do only what their natures lead them to do; the nature of the child must be respected and must be allowed to manifest itself in its own way. Liy- ing and stealing will cure themselves like the mumps, or they will remain incurable, in which case the germ plasm is to blame and nothing could have been done anyway. Laziness is due to inheritance or to hookworms; the latter kind may be cured, but not the former. Thriftless- ness, alcoholism and uncleanness run in fami- lies and can be cured only by extermination. Men who prey upon society were born with wolfish instincts, and cannot help but eat the lambs. Villains, lawbreakers, murderers should be pitied but not punished; if blame attaches to their deeds it falls upon the mar- riage bureau and the parents. ‘The world needs hospitals and sanatoria and sterilization institutes for the criminal and the vicious, but not courts and prisons, and all punishments should be visited only upon the parents to the third and fourth generations. Do our studies of heredity lead us to any such radical conclusions? If they do we must ——- GENETICS AND ETHICS 467 accept them like brave men. “Truth is truth if it sears our eyeballs.” But when theories lead to such revolutionary results it behooves us to examine carefully those theories to see if there is not somewhere a fundamental flaw in them. One of the most difficult things in the world is to recognize a great truth, to feel its signi- ficance and yet not be carried away by it. Great scientific errors are frequently due not so much to faulty observations as to sweeping conclusions. In biology the search for univer- sal laws is a peculiarly dangerous pursuit. In philosophy great errors are often due not so much to false premises as to supposed logical necessities. As a test of truth logic is inferior to experience; its faults are not so much in its methods as in its premises and applications. For this reason a logical chain has led many a man into the bondage of error. Truth is not usually found in extremes, in “carrying out a process to its logical conclusions,” but rather in some middle course which is less striking but more judicious. Having observed that the main characteris- 468 HEREDITY AND ENVIRONMENT tics of our minds as well as of our bodies are inherited, it is easy and natural to go further and to conclude not only that all the possibili- ties of our lives are marked out in the germ but that all that will actually develop from the germ is there determined and cannot be al- tered. ‘There are many similarities between such an extreme view and the old doctrine of preformation, and it contains a like absurdity. It practically denies development altogether. If the germ is a closed system and receives nothing from without, and if adult character- istics are predetermined in the germ, they are as irrevocably fixed as if they were predelineated. At the opposite extreme is the old voluntar- istic view of absolute freedom and absolute responsibility. This view, like the old epigene- sis, virtually postulates a new creation for each individual. As far as the mind and soul are concerned there is no hereditary continuity with past generations and none with future ones. But while such a view may be logically complete and theologically satisfying, it is not scientific, for it also contradicts the evidence. GENETICS AND ETHICS ; 469 The truth then seems to lie somewhere be- tween these two extremes. Our personalities were not absolutely predetermined in the germ cells from which we came, and yet they have arisen from those germ cells and have been conditioned by them. When it is said that any characteristic is predetermined in the germ cell, what does this mean? What but that the development of that characteristic is made pos- sible? Adult characteristics are potential and not actual in the germ, and their actual ap- pearance depends upon many complicated re- actions of the germinal units with one another and with the environment. In short, our ac- tual personalities are not predetermined in the germ cells, but our possible personalities are. 2. The Determinsm of Environment.— This determinism of heredity is matched by a corresponding determinism of environment. Life is possible only within rather narrow limits of physical and chemical conditions and in the main these limits are fixed by the con- stitution of nature. But apart from these an- tecedent conditions of life in general there are many minor conditions of environment which 470 HEREDITY AND ENVIRONMENT exercise a profound influence upon organisms, especially in the course of their development. Very slight changes in food, temperature, moisture and atmospheric conditions may pro- duce great changes in the developing organ- ism, and these conditions are for the most part entirely beyond the control of the indi- vidual affected. In all organisms the potentialities of de- velopment are much greater than the actuali- ties. In many animals a small part of the body is capable, when separated from the re- mainder, of producing a whole body, though this potency would never have become an ac- tuality except under the stimulus of separa- tion. In lke manner a part of an egg may, when separated from the remainder, give rise to an entire animal. By modifying the con- ditions of development animals may be pro- duced which have one eye, many eyes or no eyes; animals in which the body is turned in- side out, or side for side; animals in which all sorts of dislocation of organs have taken place; and the earlier the environmental forces act the more profound are the modifications. GENETICS AND ETHICS AT1 But leaving out of account all forms which are so monstrous that they are incapable of reaching maturity we find that there are left many variations in the size and vigor of the body as a whole, as well as of its parts; many variations in the more or less perfect correla- tion of these parts with one another, which were determined by the conditions of develop- ment rather than by heredity. Inagiven germ cell there is the potency of any kind of organ- ism that could develop from that cell under any kind of conditions. The potencies of de- velopment are much greater than the actuali- ties. Anything which could possibly appear in the course of development is potential in heredity and under given conditions of en- vironment is predetermined. Since the en- vironment cannot be all things at once many hereditary possibilities must remain latent or undeveloped. Consequently the results of de- velopment are not determined by heredity alone but also by extrinsic causes. Things cannot be predetermined in heredity which are not also predetermined in environment. Of all animals I suppose that man enjoys 472 HEREDITY AND ENVIRONMENT the most extensive and the most varied en- vironment, and its effect upon his personality is correspondingly great. Of all animals man has the longest period of immaturity and it is during this period that the play of environ- mental stimuli on the organism is effective in modifying development. In addition to the material environment he lives in the midst of intellectual, social and moral stimuli which are potent factors in his development. By means of his power to look before and after, he lives in the future and past as well as in the pres- ent; through tradition and history he becomes an heir of all the ages. The modifying influ- ences of all these environmental conditions on personality are very great. ach of us may say with Ulysses: “I am a part of all that I have met.” So great is the power of environ- ment on the development of personality that it may outweigh inheritance; a relatively poor in- heritance with excellent environmental condi- tions often produces better results than a good inheritance with poor conditions. Of course no sort of environment can do more than bring out the hereditary possibilities, but on the GENETICS AND ETHICS “413 other hand those possibilities must remain latent and undeveloped unless they are stimu- lated into activity by the environment. Functional activity or use is one of the most important factors of development. F'unc- tional activity is response to stimuli, which may be external or internal in origin. The entire process of development may be regarded as an almost endless series of such responses on the part of the organism, whether germ cell, embryo or adult, to external and internal stimuli. It is a truism that use strengthens a part and disuse weakens it; it is likewise a truism that responses which are oft repeated become more rapid and more perfect, and in this way habits are formed. Practically all education, whether of man or of lower ani- mals, consists in habit formation, in establish- ing constant relations between certain external or internal stimuli and certain responses of the organism. At first these stimuli are largely of external origin; later the external stimuli may be replaced more and more by internal ones; but whatever the source of the stimulus the response of the organism to these stimuli is AT4 HEREDITY AND ENVIRONMENT one of the most important factors of develop- ment, whether of the body or of the mind. The influence of environment upon the minds and morals of men is especially great. To a large extent our habits, words, thoughts; our aspirations, ideals, satisfactions; our re- sponsibility, morality, religion are the results of the environment and education of our early years. It cannot be doubted that if we had been born in other countries or ages we should have been different from our present selves in many important respects; if we had been born and reared in the slums of great cities we should have been other than we are; indeed if the little ilnesses, accidents and contingencies of our lives had been different we should have been different in our bodies and minds, as iden- tical twins come to differ from each other under such circumstances. ‘The conditions of early life and education have a great influence in shaping personality and are almost as much beyond the control of the individual as is heredity. If personality in all of its main features is fixed by heredity and environment over which GENETICS AND ETHICS ATS the individual has little or no control, and this is certainly true, personality is as inevitably de- termined by its antecedents as is any other natural phenomenon. ‘This is, I believe, a conclusion from which there is no escape. How then is it possible to believe in freedom and responsibility? Is there not justification for the view so often expressed of late that man is never free and that responsibility and duty are mere delusions? III. DETERMINISM AND RESPONSIBILITY Many persons who have thought upon these subjects have felt, apparently, that there was no tenable middle ground between extreme voluntarism and extreme mechanism; man has been regarded as a “free agent” or a mere “automaton,” absolutely free or absolutely bound, wholly indeterminate or wholly prede- termined. But these extreme views are unreal, — unscientific and unjustifiable, for they contra- dict the facts of experience. We have the as- surance of experience that we are not abso- lutely free nor absolutely bound, but that we 476 HEREDITY AND ENVIRONMENT are partly free and partly bound; the alterna- tives are not merely freedom or determinism, but rather freedom and determinism. 1. Determinism not Fatalism.—Whatever the philosophical meaning of “determinism” may be, all that is meant by that term in science and in actual life is that every effect is the re- sultant of antecedent causes and that identical causes yield identical results. Determinism does not mean predeterminism: the one finds every effect to be due to a long chain of pre- ceding causes, the other attributes every effect to a single original cause; the one is scientific naturalism, the other is fatalism. Applying this to personality actual experi- ence teaches that constant conditions of he- redity and environment give constant results in development and that different conditions give different results. Undoubtedly the entire personality, body and mind, undergoes de- velopment, and modifications of either heredity or environment modify personality. This is scientific determinism, but it is not fatalism and it is- not incompatible with a certain amount of freedom and responsibility. GENETICS AND ETHICS ATT 2. Control of Phenomena and of Self.— Even the most extreme mechanists, who main- tain that we are mere automata and that we could never do otherwise than we do, admit the possibility of a certain amount of control over phenomena outside ourselves. They tell us that the aim of science is not merely to under- stand but also to control nature. But if man may to a limited extent control physical, chem- ical and biological processes in the world around him, if he may control to a limited ex- tent the behavior of a star-fish or dog or child, on what ground is it possible to deny a similar control of his own behavior? Does it not come to this that all such control means intelligent action, or rather the introduction of intelli- gence as a factor in the chain of cause and effect? Before the appearance of intelligence, whether in ontogeny or in phylogeny, no such control of phenomena or of self is possible, but when intelligence becomes a factor in be- havior a limited control of the world and of the self is made possible. Of course man has no control over events which have already happened. Our heredity and early development are accomplished facts 478 HEREDITY AND ENVIRONMENT which nothing can change. Development is not a reversible process; a man cannot enter a second time into his mother’s womb and be born again. Once the sex cells are formed their hereditary nature is determined; once the egg is fertilized the hereditary possibilities of the new individual are fixed; once any stage of development has passed that page in the book of life is closed and sealed. And yet at every step in this long process of development there were one or more alterna- - tives which might have been taken instead of the one which was taken. There were innu- merable possible alternatives in the matings of our ancestors, there were billions of possible alternatives in the union of the millions of types of germ cells which each of our parents produced; at every step in the development of the oosperm from which each of us came there were many possible alternative stimuli and re- sponses. But in each case one of these in- numerable alternatives was taken and the oth- ers left. In every imstance there was some cause that determined which alternative was taken, but these causes are so local and indi- GENETICS AND ETHICS 479 vidual that they cannot be generalized; one cause works in one instance, another in an- other, and so we say that chance determines which alternative is taken, meaning by chance only this that the causes involved cannot be generalized. At critical stages in this process of development the alternatives are so evenly balanced that minor considerations, which we call chance, determine which path shall be taken; but there are no backward steps in de- velopment and once a path has been taken that particular crisis or turning point does not occur again. Thus each of us has wandered through the maze of life, chance usually determining which path shall be taken of the many which heredity and environment offer, until he has come to a stage where associative memory makes it pos- sible to profit by experience and where intel- lect and will make possible intelligent choice. With the growth of intellect and will there. comes to be a limited degree of freedom and responsibility, and with increasing complexity of organization the number of alternative paths is greatly increased. The possible reac- 480 HEREDITY AND ENVIRONMENT tions of germ cells are relatively few and fixed, the possible reactions of a complex ani- mal are relatively many and behavior is more plastic; and thus this very complexity and plasticity allow adaptations to the minutest al- terations of environment. 3. Birth and Growth of Freedom.—In ani- mals below man and in the stages of human development one may trace the birth and growth of freedom. Even in some of the simplest organisms one can observe inhibitions of responses and modifications of behavior which seem to be due to conflicting stimuli or to changes in the physiological state. In higher organisms such inhibitions or modifica- tions proceed particularly from internal stimuli, which in turn are probably conditioned by hereditary constitution and past experience. The factors which determine behavior are not merely the present stimulus and the heredi- tary constitution, but also the experiences through which the organism has passed and the habits which it has formed. A moth cannot avoid the impulse to fly toward the light, and it does not learn by ex- GENETICS AND ETHICS 481 perience to avoid the flame. Its reactions are relatively fixed and machine-like. Many other animals learn by experience to inhibit respon- ses to certain stimuli; a tame fish or frog will take food from your hand, but if it is repeat- edly frightened when it attempts to take food. it will not come near you though it is starv- ing,—it inhibits the strong impulse of a hungry animal to take food by the counter impulse of unpleasant memories or of fear. Here we have the beginnings of what we call freedom, the immediate response to a stimulus is suppressed, internal stimuli are balanced against external ones and final action is de- termined largely by past experience. Owing to his vastly greater power of memory, reflec- tion and inhibition man is much freer than any other animal. Animals which learn little from experience have little freedom and the more they learn the freer they become. In both ontogeny and phylogeny there has been development of freedom. The reactions of germ cells and of the lowest organisms are relatively fixed. In more complex organisms _ reactions become modifiable through conflict- 482 HEREDITY AND ENVIRONMENT ing stimuli, intelligence, inhibitions. Freedom is the more or less limited capacity of the high- est organisms to inhibit instinctive and non- rational acts by intellectual and rational stimuli and to regulate behavior in the light of past experience. Such freedom is not un- caused activity, but freedom from the mechan- ical responses to external or instinctive stimula, through the intervention of internal stimula due to experience and intelligence. 'To the person accustomed to think of will and choice as absolutely free this may seem to be a sort of freedom so limited as to be scarcely worth the having; and yet “it is the dawning grace of a new dispensation,” the beginnings of rational life, social obligations, moral responsibility. The only control over natural phenomena which is possible is in choosing between alter- natives which are offered; and the only control which one who has reached the age of intelli- gence can have over his own development con- sists in choosing between the alternatives which are open to him. He may not choose his he- redity or early development for the alternative paths which were once offered here have long GENETICS AND ETHICS 483 since been passed; but to a limited extent he may choose his present environment and train- ing, he may choose a path which leads to dis- cipline and increased powers of self-control or the reverse, and to this extent only is he respon- sible for what he may become. 4. Responsibility and Will.—All organisms are capable of responding to chemical and physical stimuli but in addition normal men have the capacity of responding to stimuli of a higher order. By responsibility in this higher sense I understand the ability on the part of the organism to respond to rational, social and ethical stimuli sr impulses and to inhibit responses to stimuli of an opposite na- ture, and the corresponding expectation on the part of others that the individual will so respond. The psychical stimuli which influ- ence our behavior are not merely remembered experiences but the words, suggestions, ad- monitions, ideas which come to us from others, as well as the almost endless permutations of such memories and suggestions in our thoughts. The social and ethical stimuli are not merely such as arise from love of reward 484, HEREDITY AND ENVIRONMENT and fear of punishment or the desire for praise and the fear of blame but also from the deep seated social instinct to do good, which may reach the highest levels of altruism and self sacrifice. The higher the type of organization the larger is the range of stimuli to which it will respond and the larger the number and kind of responses which may be called forth; and at the same time the larger becomes the power of inhibition of responses whether through the balancing of one stimulus against another or from whatever cause. Human responsibility varies with the complexity of the stimuli in- volved as well as with the capacity of indi- viduals to respond to those stimuli. A man might be quite responsible in savage society who would be quite irresponsible in civilized communities. In an infant there is no capacity to respond to rational, social or ethical stimuli but with increasing capacity in this respect comes increasing responsibility. Mental and ethical imbeciles, insane and mentally defective persons have a low capacity for such responses and inhibitions and consequently less is ex- GENETICS AND ETHICS 485 pected of them. ‘There are in different men all degrees of responsibility, as there are all degrees of capacity. In one and the same in- dividual responsibility varies at different times and under different circumstances; it rises and falls, like the tides, in every life. Varying ca- pacity to respond to rational, social and ethical stimuli and to inhibit responses of an opposite nature depends not merely upon inheritance but also upon training, habits, physiological states. The common opinion that all normal men are equally responsible is not correct; in the eyes of the law this may be true, because legal obligations are so far below the capacities of normal men that all may be held equally re- sponsible before the law, though in reality their responsibilities are as varied as their in- heritance or their training. Conversely the responsibility of society to the individual is universally recognized. Iv- responsible persons must be cared for by older or wiser persons who become responsible for them; and in general the responsibility rests upon society to provide as favorable environ- ment as possible for all its members. Ex- 486 HEREDITY AND ENVIRONMENT perienced persons can to a certain extent choose their own environment and thus indi- rectly control their responses and habits but young children are almost if not quite as in- capable of choosing their environment as of choosing their heredity, and it becomes the duty of society to see to it that the environ- mental stimuli are such as to develop rational, social and ethical habits rather than the reverse. We need not think of the will as a deus ex machina, nor even as “a little deity encapsuled in the brain,” but rather as the sum of all those psychical processes, such as memory and rea- son, which regulate behavior. In this sense the will is as free as the mind, and no freer. In- deed the will is the mind acting as internal stimulus, inhibition, regulation; in this sense the existence and power of will is no more to be doubted than the existence of those other mental conditions which we call intellect or memory. Just as intellect or memory may be trained to accomplish results which would have been impossible to the untrained mind, so will may GENETICS AND ETHICS — 487 be trained to initiate, inhibit or regulate be- havior in a manner quite impossible to one who has not had this traming. It is one of the most serious indictments against modern systems of education that they devote so much attention to training memory and intellect and so little attention to the training of will, upon the proper development of which so much depends. 5. Our Unused Talents——Will is indeed the supreme human faculty, the whole mind in action, the internal stimulus which may call forth all the capacities and powers. And yet the will does not directly create nor even dis- cover these powers; they are produced by the factors of development, by heredity, environ- ment and traiming; and they are usually dis- covered by accident or under the stress of necessity. How often have we surprised our- selves by doing some unusual or prodigious task! What we have once done we feel that we can do again. We realize more or less clearly, depending upon our experience, that what we habitually do is far less than we could do. It is this reserve, upon which we can 488 HEREDITY AND ENVIRONMENT draw on special occasions, that gives us the sense of freedom. In his inspiring address on “The Energies of Men” William James showed that we have reservoirs of power which we rarely tap, great energies upon which we seldom draw, and that we habitually live upon a level which is far below that which we might occupy. Darwin held the opinion, as the result of a lifetime of observation, that men differ less in capacity than in zeal and determination to utilize the powers which they have. In playful comment on the variety and extent of his own life work he said in modest and homely phrase, “It’s dogged as does it.” It may be objected that the zeal and determination were inherited, but here also the hereditary possibilities become actualities only as the result of use, training, the formation of habits. It is generally admitted that no constant distinction can be recognized between the brain of a philosopher and that of many a peasant. Neither size nor weight of brain nor complexity of convolutions bears any constant relation to ignorance or intelligence, though GENETICS AND ETHICS 48S doubtless an “unlimited microscopist” could find differences between the trained and the untrained brain. The brains of Beethoven, Gauss and Cuvier, although unusually large, have been matched in size and visible complex- ity by the brains of unknown and unlearned persons—persons who were richly endowed by nature but who had never learned to use their talents. In all men the capacity for intellec- tual development is probably much greater than the actuality. The parable of the talents expresses a profound biological truth, men differ in hereditary endowments, one receives ten talents and another receives but one; but the used talent increases many fold, the un- used remains unchanged and undeveloped. Happy is he who is compelled to use his tal- ents; thrice happy he who has learned how to compel himself! We shall not live to see the day when human inheritance is greatly im- proved, though that time will doubtless come, but i the meantime we may console ourselves by the thought that we have many half-used talents, many latent capacities, and although we may not be able to add to our inheritance 490 HEREDITY AND ENVIRONMENT new territory we may greatly improve that which we have. Jennings has pointed out as one of the great tragedies of life the almost infinite slaughter of potential personalities in the form of germ cells which never develop. A more dreadful though less universal tragedy is the loss of real personalities who have all the native en- dowments of genius and leadership but who for lack of proper environmental stimuli have remained undeveloped and unknown; the “mute, inglorious Miltons” of the world; the Czsars, Napoleons, Washingtons who might have been; the Newtons, Darwins, Pasteurs who were ready formed by nature but who never discovered themselves. One shudders to think how narrowly Newton escaped being an unknown farmer, or Faraday an obscure bookbinder, or Pasteur a provincial tanner. In the history of the world there must have been many men of equal native endowments who missed the slender chance which came to these. We form the habit of thinking of great men as having appeared only at long intervals, and yet we know that great crises al- GENETICS AND ETHICS 491 ways discover great men. What does this mean but that the men are ready formed and that it requires only this extra stimulus to call them forth? To most of us heredity has been kind—kinder than we know. The possibilities within us are great but they rarely come to full epiphany. What is needed in education more than any- thing else is some means or system which will train the powers of self discovery and self control. Easy lives and so-called “good en- vironment” will not arouse the dormant powers. It usually takes the stress and strain of hard necessity to make us acquainted with our hidden selves, to rouse the sleeping giant within us. How often is it said that the worthless sons of worthy parents are mys- teries; with the best of heredity and environ- ment they amount to nothing, whereas the sons of poor and ignorant farmers, blacksmiths, tanners and backwoodsmen, with few oppor- tunities and with many hardships and disad- vantages, become world figures. Probably the inheritance in these last named cases was no better than in the former, but the environment 492 - HEREDITY AND ENVIRONMENT was better. “Good environment’ usually means easy, pleasant, refined surroundings, “all the opportunities that money can buy,” but little responsibility and none of that self discipline which reveals the hidden powers and which alone should be counted good environ- ment. Many schools and colleges are making the same mistake as the fond parents; luxury, soft living, irresponsibility are not only al- lowed, but are encouraged and endowed—and by such means it is hoped to bring out that in men which can only be born in travail. The chief educational value of athletics is found in this that it teaches self control. But in great athletic contests the self control of the spectators is usually inversely proportional to that of the players, and while excess of stimuli may lead to wholesome and beneficial reactions in the players it frequently leads to excess of stimulants and to other injurious reactions in the spectators. But college athletics has this much at least in its favor, it trains men who take part in the contests to do their best, to subordinate pleasure, appetite, the desire for a good time, to one controlling purpose; it GENETICS AND ETHICS 493 trains them to attempt what may often seem to them impossible, to crash into the line though it may seem a stone wall, to get out of their bodies every ounce of strength and endurance which they possess. Such training makes men acquainted with their powers and teaches courage, confidence and responsibility. If only we could make young persons ac- quainted in some similar way with their hidden mental and moral powers what a race of men and women might we not have without wait- ing for that uncertain day when the inheri- tance of the race will be improved! Whatever the stimulus required, whether pride or shame, fear or favor, ambition or loyalty, responsi- bility or ‘necessity, education should utilize each and all of these to teach men self knowl- edge and self control. But it will be said that self control depends upon inheritance, that strong wills and weak wills are such because of heredity. It is true that one man may be born with a potentiality for self contro] which another man lacks, but in all men this potentiality becomes actuality only through development, one of the princi- 494, HEREDITY AND ENVIRONMENT pal factors of which is use or functional ac- tivity. An amazing number of persons have but little self control. Is this always due to defective inheritance, or is it not frequently the result of bad habits, of arrested develop- ment? To charge defects at once to heredity removes them from any possible control, helps to make men irresponsible, excuses them for making the least of their endowments. To hold that everything has been predetermined, that nothing is self determined, that all our traits and acts are fixed beyond the possibility of change is an enervating philosophy and is not good science, for it does not accord with the evidence. It is amazing that men whose daily lives contradict this paralyzing philoso- phy still hold it, as it were in some water- tight compartment of the brain, while in all the other parts of their being their acts pro- claim that they believe in their powers of self control: they set themselves hard tasks, they overcome great difficulties, they work until it hurts, until they can say with Johannes Mil- ler, E's klebt Blut an der Arbeit, and yet in the philosophical compartment of their minds GENETICS AND ETHICS 495 they can say that it was all predetermined in heredity and from the foundations of the world. Whether all the phenomena of life and of mind can be explained on the basis of a purely mechanistic hypothesis or not, that hypothesis must square with the facts and not the facts with the hypothesis. It has always been true of those who “sat apart and reasoned high of fate, free will, foreknowledge absolute” that they have “found no end in wandering mazes lost.” Whatever the way out of these mazes may be,—whether it be found in the varied re- sponses of an organism to the same stimulus, to the introduction of memory, intelligence and reason as internal stimuli, or to some form of idealism which finds necessity not in nature but in the spectator, and freedom not in the spectator but in the agent,—tt is true for those who do not “sit apart and reason high,” but who deal merely with evident phenomena, that the way of escape is not to be found in denying the reality of inhibition, atten- tion and control. Because we can find no place in our philosophy and logic for self de- 496 HEREDITY AND ENVIRONMENT termination shall we cease to be scientists and close our eyes to the evidence? ‘The first duty of science is to appeal to fact and-to settle later with logic and philosophy. Is it not a fact that the possibilities of our inheritance depend for their realization upon development, one of the most important factors of which is use, functional activity in response to stimuli? Is it not a fact that in many animals behavior is modifiable and that impulses may be in- hibited and controlled? Is it not a fact that experience, intelligence, will are factors in human behavior and that by means of these men are often able to choose between alterna- tives and so to control their own activities as well as external phenomena? Is it not a fact that our capacities are very much greater than our habitual demands upon them? Is it not a fact that belief in our responsibility ener- gizes our lives and gives vigor to our mental and moral fiber? Is it not a fact that shifting all responsibility from men to their heredity or to that part of their environment which is beyond their contro] helps to make them irresponsible ? GENETICS AND ETHICS "497 This debilitating philosophy in which every- thing is predetermined, in which there is no possibility of change or control, in which there is hypertrophy of intellect and atrophy of will, is a symptom of senility whether in men or nations. We need to return to the joys of a childhood age in which men believed them- selves free to do, to think, to strive, in which life was full of high endeavor and the world was crowded with great emprise. We need to think of the possibilities of development as well as of the limitations of heredity. ‘ Chance, heredity, environment have settled many things for us; we are hedged about by bounds which we cannot pass, but those bounds are not so narrow as we are sometimes taught and within them we have a considerable degree of freedom and responsibility. | “That which we are we are, One equal temper of heroic hearts Made weak by time and fate, but strong in will To strive, to seek, to find, and not to yield.” 498 HEREDITY AND ENVIRONMENT LV. THe INDIVIDUAL AND THE RACE There is a larger freedom and a greater responsibility than that which characterizes the individual. What the individual cannot do because of weakness, ignorance, self interest, short life, society can accomplish with the strength, wisdom, and interest of all, and through long ages of time. There are many grades of organization from the bacterium to the vertebrate, from the germ cell to the man. Society is the last and highest grade of organi- zation and its freedom and responsibility are to those of the individual very much as the free- dom and responsibility of the developed man are to those of the germ cell from which he came. Out of the correlations, differentiations and integrations of persons has grown this higher type of organization which we call society. 1. The Conflict between the Freedom of the Individual and the Good of Society.—The freedom, power and responsibility of society are founded upon limitations of individual free- ‘GENETICS AND ETHICS 499 dom for the good of the race. Among social animals, such as ants and bees, there is so much instinct and so little reason and freedom that there is practically no conflict between the individual and the race, but with the in- crease of intelligence and freedom among men there has developed an increasing conflict be- tween the individual and society. So far as social limitations are artificial, selfish, for the good of a few rather than of all, this conflict of the ages, this struggle to be free has been the crowning glory of mankind. The struggle for freedom from tyranny in thought and speech, in religion, government and industry, no less than for the freedom that comes by the conquest of nature, is one of the greatest achievements of the human race. But social restrictions on individual freedom are not all artificial and selfish. Some of them are absolutely essential not only to the welfare but even to the continued existence of the race, and when demands for individual freedom go to the extent of fighting against these racial obligations they become a serious menace to mankind. 500 HEREDITY AND ENVIRONMENT 2. Perpetuation and Improvement of the Species the highest Ethical Obligation— Among all organisms the race or species is of paramount importance. Race preservation, not self preservation, is the first law of nature. Among all organisms the perpetuation and welfare of the race are cared for by the strong- est instincts. In very many species of animals reproduction means the death of the individual. The breeding instinct drives every male bee, every male and female salmon, to its certain death in order that the race may be perpetu- ated. Among the higher organisms the strongest of all the instincts are those con- nected with reproduction. But in the human species intellect and freedom come in to inter- fere with instinct. ‘The reproductive instincts are not merely controlled by reason, as they should be, but to an alarming extent they are thwarted and perverted among intelligent people. The struggle to be free is part of a great evolutionary movement, but the freedom must be a sane one which neither injures others nor eliminates posterity. The feminist movement GENETICS AND ETHICS 501 in so far as it demands greater intellectual and political freedom for women may be a benefit to the race but in so far as it demands freedom from marriage and reproduction it is suicidal. The cry of Rachel, “Give me children or I die,” has been turned by many modern women to, “I’d rather die than have children.” If the demand for individual freedom blinds men and women to their racial obligations the in- evitable decadence and extinction of their lines must follow. In every age and country where demands for personal freedom have been most _ insistent and extreme, where men and espe- cially women have demanded freedom from the burdens of bearing and rearing children as well as from other natural social obligations, the end has been degeneration and extinction. This has been the history of many talented races and families of mankind. The decay of the most gifted races of the ancient world, especially those of Greece and Rome, was not due primarily to bad heredity nor to bad ma- terial environment but rather to the growth of luxury and selfishness and unrestricted free- dom; marriage became unfashionable, immo- 502... HEREDITY AND ENVIRONMENT rality was widespread, and then came sterility and extinction or mixture with inferior stock and degeneracy. And then the barbarian, the immigrant, the natural man, unspoiled by too much freedom and true to his instincts, came in to take the place of the more gifted race. Truly “there is a power not ourselves that makes for righteousness.” In these days when we talk of our race and our civilization as if they were necessarily su- preme and immortal it is well to remember that there have been other races and other civilizations that regarded themselves in the same way. “Assyria, Greece, Rome, Carth- age, where are they?’ And what assurance have we that our race and our civilization will not run a similar course and come to a similar end? May we not surely predict that if we continue to put individual freedom and luxury and selfishness above social obligations our race and civilization will also see the writing on the wall, “Thou art weighed in the bal- ”? Jn these days when individuals are demanding more and more freedom it is well to remember that “the ances and art found wanting GENETICS AND ETHICS 503 best use that man has made of his freedom has been to place limitations upon it.” Again and again, age after age, men and families and nations have gone up to a climax of greatness and then have declined, while other unknown men have taken their places. Greatness has not for long perpetuated itself. An epitome of human history is contained in the words, “He hath put down the mighty from their seats and hath exalted them of low degree.” It may well be asked by those who are in- terested in breeding a better race of men whether such a thing is possible, whether the better race may not be lacking in vitality or fertility or morality and thus be doomed to an early end. Although this has been the fate of many gifted races of the past I do not think that it was a necessary fate. The history of domesticated animals and of cultivated plants, and especially the recent notable advances in genetics, indicate what eugenics might do for the human race. In time, under intelligent guidance, the worst qualities of the race might be weeded out and the best qualities preserved. This is the goal toward which intelligent effort 504 HEREDITY AND ENVIRONMENT should be directed. ‘This should be the su- preme duty of society and of all who love their fellow men. | But I think that notable human improve- ment can take place only upon two conditions: (1) The physical and intellectual improve- ment of the individual through environment and training must not interfere with his racial and ethical obligations. Individual freedom must be subordinated to racial welfare. (2) The promotion of human evolution must be undertaken by society as its greatest work. Not only has society greater freedom and greater power than the individual but it per- sists while men come and go. Our hereditary lines are so interwoven with those of other races and will be so entangled with other lines in the future that any selfish or narrow policy of improving our family or class can have little permanent value. We shall rise only as our race rises. Indeed when we con- sider all the influences of our fellow men upon our development, when we consider our he- reditary connections with multitudes of men and women of the past, when we think of the GENETICS AND ETHICS 505 nexus of hereditary strands which are woven into our personalities and which will be ccn- tinued through us to many future generations, we realize that after all the individual is not really a separate and independent being, but a minor unit in the great organism of hu- manity, and that his greatest duty is to trans- mit unimpaired and undefiled a noble heritage to generations yet unborn. It is possible greatly to improve environ- ment. Conditions of life are still hard and cruel for many. A vast amount of good hu- man material is wasted in modern society. As civilization becomes more complex the quan- tity of human wreckage and wastage ever grows greater. Many useful lives and some great possibilities are blotted out by unfavor- able environment. It is the duty of society as far as possible to conserve these lives and to develop these possibilities. It is possible greatly to improve education, to make it a potent factor in development in- stead of a conventional veneer. In spite of innumerable educational reforms the essential reform has not yet been reached; mere refine- 506 HEREDITY AND ENVIRONMENT ments of bad methods are not real reforms. The essence of all education is self discovery and self control. When education helps an individual to discover his own powers and limi- tations and shows him how to get out of his heredity its largest and best possibilities it will fulfil its real function; when children are taught not merely to know things but particu- larly to know themselves, not merely how to do things but especially how to compel them- selves to do things, they may be said to be really educated. For this sort of education there is demanded rigorous discipline of the powers of observation, of the reason, and es- pecially of the will. It is possible greatly to improve heredity: (a) By weeding out from the possibility of reproduction human stocks bearing serious defects. (b) By cultivating pride mn good heredity and by discouraging voluntary in- fertility on the part of those who have a goodly heritage. (c) By increasing opportunities for early and favorable marriages. (d) By care- fully conserving the best human mutations or inherited variations. In this way if in any GENETICS AND ETHICS 507 way the better race will be produced. ‘The possible improvements of heredity are great, the possible improvements of environment and training are great, but whether men of the fu- ture will be better than those of the past or present is a question not only of genetics but also of ethics. ' How better can I close this course of lec- tures than with the words of Francis Galton, one of the greatest students of human he- redity and the founder of the science of Ku- genics? “The chief result of these inquiries has been to elicit the religious significance of the doctrine of evolution. It suggests an alteration in our mental - attitude and imposes a new moral duty. ‘The new mental attitude is one of a greater sense of moral freedom, responsibility and opportunity; the new duty which is supposed to be exercised concurrently with, and not in opposition to the old ones upon which the social fabric depends, is an endeavor to further evolution, especially that of the human race.” REFERENCES TO LITERATURE The following list of books and publications in- cludes only those works which are referred to most frequently in the preceding pages. Several of the books cited, particularly those by Plate and Morgan, contain extensive bibliographies. ‘Those desiring to become more fully acquainted with books and articles dealing with the subjects of heredity and develop- ment are referred to the larger works which are listed here. Books and Larger Works Bateson, W. Materials for the Study of Variation, London, 1894. Bateson, W. Problems in’ Genetics. Yale Univ. Press. 1913. Bateson, W. Mendel’s Principles of Heredity. 3rd Impression, Cambridge, 1913. Baur, E. Ejinfiihrung in die experimentelle Verer- bungslehre. Berlin, 1911. Castle, W. Heredity in Relation to Evolution and Animal Breeding. New York, 1912. Castle, W. E.; Coulter, J. M.; Davenport, C. B.; East, E. M.; Tower, W. L. Heredity and Eu- genics. Chicago, 1912. Correns, C. Die Neuen Vererbungsgesetze. Berlin, 1912. Darbishire, A. R. Breeding and Mendelian Discov- ery. London, 1911. 509 510 HEREDILY AND ENVIRONMENT Darwin, C. Animals and Plants under Domestica- tion. New York, 1887. Davenport, C. B. Heredity in Relation to Eugen- ics. New York, 1911. De Vries, H. Intracellular Pangenesis. Jena, 1889. De Vries, H. Die Mutationstheorie. Leipzig, 1901. De Vries, H. Plant Breeding. Chicago, 1907. Doncaster, L. Heredity in the Light of Recent Re- search. Cambridge, 1911. Driesch, H. The Science and Philosophy of the Or- ganism. Gifford Lectures, London, 1908. Ellis, H. The Task of Social Hygiene. London, 1912. Ellis, H. The Problem of Race Regeneration. New York, 1911. Forel, A. The Sexual Question. New York, 1908. Galton, F. Inquiries into Human Faculty. New York, 1883. Galton, F. Natural Inheritance. London, 1889. Galton, F. Hereditary Genius. London, 1892. Galton, F. Essays in Eugenics. London, 1909. Goddard, H. H. The Kallikak Family. New York, 1912. Goldschmidt, R. Einfiihrung in die Vererbungswis- senschaft. Leipzig, 1911. Hicker, V. Allgemeine Vererbungslehre. Braun- schweig, 1912. Hertwig, O. Allgemeine Biologie. Jena, 1909. Johannsen, W. Elemente der exakten Erblichkeits- lehre, 2d Auf. Jena, 1913. Kellicott, W. E. The Social Direction of Human Evolution. New York, 1911. Lock, R. H. Variation, Heredity and Evolution. New York, 1911. Se REFERENCES 511 Loeb, J. Comparative Physiology of the Brain and Comparative Psychology. New York, 1900. Loeb, J. The Dynamics of Living Matter. New York, 1906. Loeb, J. The Mechanistic Conception of Life. Chicago, 1911. Loeb, J. Artificial Parthenogenesis. Chicago, 1913. Metchnikoff, E. The Nature of Man. Chicago, 1903. Morgan, T. H. MHeredity and Sex. New York, 1913. Morgan, Sturtevant, Muller and Bridges. The Mech- anism of Mendelian Heredity. New York, 1915. Mott, F. W. Heredity and Eugenics in Relation to Insanity. London, 1912. Nageli, C. Mechanische-Physiologische Theorie der Abstammungslehre. Miinchen, 1884. Plate, L. Vererbungslehre. Leipzig, 1913. Problems in Eugenics. Papers communicated to Ist International Eug. Cong. London, 1912. Punnett, R.C. Mendelism. London, 1911. Rignano, E. The Inheritance of Acquired Charac- ters. Chicago, 1911. Romanes, G. J. Darwin and After Darwin. Chi- cago, 1892. Saleeby, C. W. Parenthood and Race Culture. New York, 1909. Semon, R. Das Problem der Vererbungslehre er- worbener Eigenschaften. Leipzig, 1912. Spencer, H. Principles of Biology. New York, 1883. Thompson, J. A. Heredity. Edinburgh, 1908. Thorndike, E. L. Animal Intelligence. New York, rou 512 HEREDITY AND ENVIRONMENT Walter, H. E. Genetics. New York, 1913. Weismann, A. The Germ Plasm. New York, 1893. Weismann, A. Essays on Heredity. Oxford, 1889. Wilson, EK. B. The Cell in Development and In- heritance. New York, 1900. Woods, F. A. Heredity in Royalty. New York, 1906. Monographs and Papers Bailey, L. H. The Modern Systematist. Science, 46, Dec. 28, 1917. Bardeen, C. R. Abnormal Development of Toad Ova fertilized by Spermatozoa exposed to the Roentgen Rays. Jour. Exp. Zool., 4, 1907. Bateson, W. Presidential Address. British Ass’n Adv. Science, Australia, 1914. Baur, E. Vererbungs und Bastardierungsversuche mit Antirrhinum. Zeit. f. induk. Abstam. 3, 1910. Boveri, Th. Zellen Studien. Die Entwicklung di- spermer Seeigel-Eier, etc. Jena, 1907. Broman, I. Ueber geschlechtliche Sterilitat. Woies- baden, 1912. Brooks, W. K. Are Heredity and Variation Facts? Address before 7th Internat. Zoological Congress, 1907. Castle, W. E. Studies of Inheritance in Rabbits. Carnegie Inst. Wash. Publ. 114, 1909. Castle, W. E. and Phillips, J. C. On Germinal Transplantation in Vertebrates. Carnegie Inst. Wash. Pub. No. 144, 1911. Castle, W. E. and Phillips, J. C., Piebald Rats and Selection. Carnegie Inst. Wash. Pub. No. 195, 1914. Cattell, J. McK. The Causes of the Declining Birth REFERENCES 513 Rate. Proc. First National Conference on Race Betterment. Battle Creek, 1914. ‘Conklin, E. G. Embryology of Crepidula. Jour. Morph. 13, 1897. Conklin, E. G. The Cause of Inverse Symmetry. Anat. Anz., 1903. Conklin, E. G. The Organization and Cell-Lineage of the Ascidian Egg. Jour. Acad. Nat. Sci. Phila., 1905. Conklin, E. G. Experimental Studies on Nuclear and Cell Division. Ibid., 1912. Conklin, E. G. The Mutation Theory from the Standpoint of Cytology. Science, 21, 1905. Conklin, E. G. The Mechanism of Heredity. Sci- ence, 27, 1908. Conkln, E. G. The Share of Egg and Sperm in Heredity. Proc. National Acad. Sciences, 2, 1917. Correns, C. 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Theoria Generationis. 1759. Woltereck, R. Beitrag zur Analyse der Vererbung erworbener Ejigenschaften; Transmutation und Priinduktion bei Daphnia. Verh. D. Zool. Ges., JUS EG be GLOSSARY AccEssorY cHRO’-Mo-somMe. An odd chromosome which is found in only half of the spermatozoa of certain animals; see “sex-chromosome.” A-cHro’-ma-t1n. The non-staining substance of the nucleus as contrasted with the chromatin. A-cHon’-pRo-pLa-sy. A condition in which the long bones cease to grow in length at an early age thus producing a dwarf with large body and head but short limbs. Acaurrep CuHaracter. A character, the differential cause of which is environmental. ALTERNATIVE INHERITANCE. Galton’s term for a doubtful kind of inheritance in which all characters are derived from one parent. In present use, Mendelian inheritance. Am’-ni-on. One of the embryonic membranes of higher verte- brates. Am-pHI-ox’-us. One of the lowest and simplest animals hay- ing a notochord (backbone). AN-EN-CEPH’-a-Lty. The condition of a brainless monster. Anima. port. That pole of an egg at which the polar bodies are formed. Awn’-1a-ce. The embryonic basis of any developed part. A-or’-ta. The great artery arising from the heart. Ar’-cu1-pLasm. The deeply staining plasm surrounding the centrosome. As’-ca-ris. A genus of round worms which are intestinal parasites. As’-ca-R1s meg-a-lo-ceph’-a-la. A parasite in the intestine of the horse. As-cip’-1-an. A “sea-squirt”; one of the lowest types having a notochord, or elementary backbone. 519 520 GLOSSARY As'-rer. The radiating figure surrounding the centrosome in a cell. As-sIM-I-LA’-T10N. Conversion of food substances by an or- ganism into its own living substance. A-syM’-ME-TRY. ‘lhe condition where opposite sides are unlike. At’-a-vism. The condition in which an individual resembles a grandparent, or a more distant ancestor, more than one of the parents. Bi'-o-pHores. ‘The ultimate units of life (Weismann). Bi/-vA-LENT CHRO’-Mo-someEs. A pair of chromosomes, one ma- ternal the other paternal, temporarily united. Bras’-ro-corL. The cavity within a blastula. Buas’-T0-DER’-MIc VeEs’-1-cLE. A hollow sphere, formed from the segmented egg of a mammal, which becomes attached to or embedded within the wall of the uterus. Buas’-ro-pore. ‘The mouth of a gastrula. Buas’-ru-La. A mass of cells, usually in the shape of a hollow sphere, formed by repeated divisions (cleavages) of an egg. Bienpinc INueERITANCE. Galton’s term for that kind of in- heritance in which the characters of the parents seem to blend in the offspring. BracH-y-pac’-ry-LisM. The condition of having abnormally short fingers or toes. Cert. The fundamental unit of structure and function in all living things. Cen’-rro-some. The body at the center of radiations in a dividing cell. CrpH’-a-Lo-pops. A class of mollusks which includes the squid, — cuttle-fish and devil-fish. Crer’E-BraL Gano’-t1-on. The brain of an_ invertebrate animal. Cuaracter. Any feature or property of an organism. Cyor’-pa. A cellular rod in vertebrate embryos which forms the basis of the backbone. Cuor’-pate. A member of the highest phylum of the animal kingdom, including all animals having a chorda or back- bone. GLOSSARY 521 Cuo’-r1-on. A tough membrane around an egg secreted by surrounding cells. Curo’-ma-TIN. The deeply staining substance of the nucleus. Curo’-mo-somes. Deeply staining bodies found in the nucleus at the time of indirect division. Cit’-1-a. Minute protoplasmic threads on the surface of a cell which produce movements in the surrounding medium by waving back and forth. . Crass. The chief sub-division of a phylum. Cireav’-acGE. ‘lhe division of the egg cell after fertilization into many cells. Crep-si’/-NE. A genus of leeches. Coxr’-tom. The body cavity. ConTINvous variation. A series of minute variations. CoRRELATIVE DIFFERENTIATION. Differentiation due chiefly to the interaction of different parts of an organism. CrE-Pip’-u-La. A genus of marine gastropods. “Criss-Cross” INHERITANCE. Morgan’s term for that kind of inheritance in whicn maternal characters are transmitteu to sons and paternal ones to daughters. Cren’-o-pHorE. A jelly-sphere; a member of a phylvm of marine animals standing above the jelly-fishes. Cy-cto’-p1-a. A monstrosity in which both eyes have fusec into a single one. Cy-tou’-o-cy. The science which treats of cells. Cy’-ro-ptasm. The protoplasm of a cell outside of the nucleus. Dat’-ron-1smM. That form of color-blindness in which one is unable to distinguish red and green; usually limited to ° males. Dar’-win-1sm. The doctrine that evolution takes place through natural selection or the survival of the fittest. Determinants. The units of heredity (Weismann). Dererminer. The differential cause or factor in a. germ cell which determines the development of a character. Dex’-TRAL snarL. The usual type of snail in which the shell coils from base to apex in a clockwise direction. DirrERENTIATION. The process of producing specific parts or substances from a general part or substance. 522 GLOSSARY Di-ny’-sripv. The offspring of parents differing in two char- acters. Di-o-war’-a. An insect-catching plant, the “Venus Fly-trap.” Die’-Lomw. The full number of chromosomes found in the ferti- lized egg and in all cells derived from this, except the mature germ cells. DoMINANT CHARACTER. A character inherited from one parent which develops to the exclusion of a contrasting character of the other parent. Dros-opH’-1-1a. A genus of fruit-flies. Du’-PLEX FACTORS Or CHARACTER. A condition where the de- terminers for a character are derived from both parents. Ei-cu1/-No-perms. A phylum of marine animals which includes star-fishes and sea-urchins. E-cot’-o-cy. The science which deals with the relations of organisms to one another and to environment. Ec’-ro-perm. ‘The outer layer of cells of an embryo which gives rise to epidermis, sense organs and nervous system. Em-pry-oc’-E-Ny. Early development of an egg leading to the formation of an embryo. En’-po-perM. The inner layer of cells of an embryo, which gives rise to the digestive cells of the alimentary system. E\N-DO-GEN’E-sIs_ (development from within). The theory that the differential causes of development are within the germ cells, which are therefore complex. Ep-1-Gen’E-sis (—development from without). The doctrine that the germ is simple and homogeneous and that the differential causes of development are in the environment. Eavation-Division. An ordinary nuclear division in which each chromosome divides equally. Eu-cen’-1cs. The system of improving races by good breeding. Evu-rHen’-1cs. The system of improving individuals by good environment. Ex-o-cas’-Tru-LA. A gastrula with the endoderm turned out instead of in. Facror. A specific germinal cause of a developed character. GLOSSARY 523 Fertiization. The union of male and female sex cells. Fra-cet’-tum. A vibratile thread of protoplasm which serves as an organ of locomotion. Friucruations. Variations which are not inherited. Fou’-11-cte Cetts. Nutritive cells surrounding an ovarian egg. FraterNaL Twins. Twins produced from different eggs and showing different hereditary characters. Functional activiry. Use. Gam’-reteE. ‘he mature male or female sex cell. Gane’-L1-on. A group of nerve cells. Gas’-rro-corL. ‘The digestive cavity of the gastrula. Gas’-TrRu-LA. A stage in development following the blastula, in which the embryo consists of an outer (ectoderm) and an inner (endoderm) layer of cells. Genes. Factors, units, elements of germ cells which condition the characters of developed organisms (Johannsen). Gr-neET’-1cs. ‘The science which deals with the origin of indi- viduals and particularly with heredity. Gr’-no-tyPE. The germinal type with all its hereditary pecu- liarities, ‘The fundamental hereditary constitution of an organism” (Johannsen). Gererm-Priasm. The material basis of inheritance. Grerm-Track. The cell-lineage of the germ cells in a develop- ing animal. Germinat Units. Hypothetical parts of germ cells which are supposed to have certain specific functions in development. Haer-mo-pHit’-1-a. An abnormal condition in which the blood clots slowly. Hap’-tor. The reduced number of chromosomes in the gametes. Hereprry. The appearance in offspring of characters whose differential causes are in the germ cells. Herirace. The sum of those characters which are inherited by an individual. Het-ER-0-zy-co’-sis. Hybridization; cross-breeding. Het-Er-o-zy’-cotes. Hybrids resulting from the union of gametes which are hereditarily dissimilar. 524 GLOSSARY Ho-mo-zy'-cores. Pure-breds resulting form the union of gametes which are hereditarily similar. Hy’-srip. The offspring of parents which differ in one or more characters. — IDENTICAL Twins. Twins which have come from a single egg and which show identical hereditary characters. Ip’-1-o-pLrasmM. The germ-plasm or inheritance material. Inpuction. A modification of the first filial generation caused by the action of environment on the germ cells of the parental generation. (Woltereck). INHERITED CHaractEeR. A character the differential cause of which is in the germ. Instincrs. Complex reflexes involving nerve centers. InverRsE SyMMetRY. Having the right half of one asymmetri- cal individual equivalent to the left half of another. IrrirabiLiry. Capacity of receiving and responding to stimuli. La-marcxk’-1sm. The doctrine that evolution takes place through the inheritance of acquired characters. Locatization. The gathering together of particular sub- stances in definite parts of an egg or embryo. Lor’-1-co. The squid, a genus of cephalopod mollusks. Mar-sv’-pi-ats. A primitive group of mammals, including op- posums and kangaroos, which carry the young in a pouch. Mart-u-ra’-t10n. The final stages in the formation of sex cells, characterized by two peculiar cell divisions. Me-ris’-t1c Vartation. Variation in the number of parts. Mes’-EN-cHyME. Loosely scattered cells of the mesoderm. Mes’-o-pERM. A layer or group of embryonic cells lying be- tween ectoderm and endoderm. Me-rTax’-o-t1ismM. ‘Transformations of matter and energy within a living thing. | Mr’-cro-pyte. The minute opening in an egg membrane through which the spermatozoon enters. Mr-to’-sts. Indirect nuclear division in which the nucleus is transformed into a spindle and chromosomes; the latter split and the halves move to the poles of the spindle where they form the daughter nuclei. GLOSSARY 525 Mon-o-Hy’-Brip. The offspring of parents differing in one character. Mon’-o-rremes. The lowest group of mammals, including the duck-bill and the spiny anteater. Mor-pHot’-o-Gy. The science which deals with structure and form. Movs’-ca. A genus of flies including the house-fly. Mov’-rant. A sudden variation or sport which breeds true. Mu-ta’-t1ons. Inherited variations which are more or less striking; “sudden variations,” “sports.” Nec-ru’-rvus. A large salamander; the mud-puppy. Nem’-a-topE. A round-worm or thread-worm. Ne’-rE-1s. A marine annelid, or ringed worm. Nevurat Groove. The groove on the dorsal surface of the embryo of a vertebrate which develops into the brain and spinal cord. Nevrat Tuse. A tube formed from the neural groove and giving rise to brain and spinal cord. No’-ro-cHorp. The cellular rod which forms the basis of the backbone. Nou’-ctz-us. The central organ of a cell, composed of chro- matin and achromatin. Nou irpLex Factors or CuHaracrer. A condition in which a character is absent because its determiner is found in neither parent. On-toe’-E-Ny. Development of an individual. O’-o-cytr. The ovarian egg before maturation (formation of polar bodies). Q-0-cENn’-E-sis. The development of an ovum from a primitive sex-cell. O-0-co’-n1-a. The earliest generations of cells which produce ova; primordial egg cells. O’-o-spermM. The fertilized egg after union of egg and sperm. Orver. The chief sub-division of a class. Orcanization. Differentiation and integration, i.e. different parts united into one whole. 526 GLOSSARY Or-can-oc’-E-Ny. The formation of various organs of the body. Or-rHO-GEN’-E-sIis. The doctrine that the course of evolution is definitely directed by intrinsic causes. O-vi-par’-1-Ty. Young brought forth as eggs, ie., in an early stage of development. O’-vutes. The female sex cells of flowering plants with the immediately surrounding parts. O’-vum. The female sex cell. Ox-y-cHRo’-mMa-tT1iIn. That portion of the chromatin which does not form chromosomes. Pan-GEn’-E-sis. The hypothesis proposed by Darwin that every cell of the body gives off minute germs, “gemmules,” which then collect in the sex cells. Par-a-ME’-c1-uM. A ciliated protozoan. PaR-THE-NO-GEN’-E-sIs. Development of an egg without pre- vious fertilization. PARTICULATE INHERITANCE. Galton’s term for that kind of in- heritance in which certain characters are derived from one parent and others from the other parent, i.e. Mende- lian Inheritance. Pa-ruHo.u’-o-cy. ‘The science which deals with disease. Pue’-no-ryre. The developed type in which some of the her- editary possibilities are realized while others remain un- developed. “Developed, measurable realities” (Johannsen). Puy-toc’-E-Ny. Evolution of a race or species. PuHyu-Lox’-E-RA. A genus of plant lice. Puy’-tum. One of the chief sub-divisions of the animal king- dom. Puys-1-oL’-o-cy. The science which deals with function. Puas’-ro-somes. Threads or granules in the cytoplasm which are colored by certain dyes. Potar Bopies. Two minute cells which are separated from the egg in its two maturations divisions. Po-tar’-1-ty. The condition where two poles of a body differ; in eggs the two poles are the animal (formative) and the vegetative (nutritive). ’ GLOSSARY 527 Pou'-tEN. ‘The male sex cells of flowering plants. Pot-y-pac’-ryt-ismM. The condition of having more than the normal number of digits on hands or feet. Pot-y-Hy’-Brip. The offspring of parents differing in more than three characters. Pre-For-Ma’-T10N. The doctrine that the fully formed organ- ism exists in the germ, and that development is merely its unfolding. Pre-1n-puc’-tT1ion. A modification of the second filial gener- ation caused by the action of environment on the germ cells of the parental generation. (Woltereck). PreE-IN-HER’-I1-ANCE. The transmission of characters developed in a previous generation. Pre-po’-tEN-cy. The preponderance of one parent over the other in the transmission of hereditary characters. Pri’-mates. The highest order of mammals, including monkeys, apes, and man. Primitive Sex Ceizs. The earliest recognizable progenitors of the sex cells in development. Pro’-rE-IN. Complex organic substances containing nitrogen, e.g. white of egg. Pro-tr’-nor. A genus of the true bugs. Pro’-ro-pLasm. The living material of an organism. Pro-ro-zo’-a. The simplest animals, usually consisting of a single cell. Py-to’-rus. The narrow opening between stomach and intestine. ReEcESSIVE CHARACTER. An inherited character which remains undeveloped when mated with a dominant character. Repuction-Division. That maturation division in which the number of chromosomes is halved. Reriexes. Relatively simple, automatic responses. Response. Any activity of an organism called forth by a stimulus. Reversions. The sudden reappearance of long-lost racial characters. SrcrecaTion. The separation of contrasting parental char- acters in the offspring of hybrids. 528 GLOSSARY SetF DirrerentiatTion. Differentiation due chiefly to intrinsic causes. Sensitivity. Capacity of receiving and responding to stimuli. Sex Curo’-mo-some. The “odd” or accessory chromosome which is supposed to determine sex. Sex-LimireD. Any character which is found in one sex only. SEXx-LINKED. Any character, the determiner of which is as- sociated with the determiner of sex. Simprex Factors or Cuaracter. A condition where the de- terminer for a character is derived from one parent only. S1n’-1s-TRAL sNAIL. A type of snail in which the shell coils from base to apex in an anti-clockwise direction. So’-ma. The body as contrasted with the germ cells. So-mar’-1c. Pertaining to the body, as contrasted with “germi- nal” pertaining to the germ cells. So’-ma-tTo-pLasM. The body-plasm as contrasted with the germ-plasm. So/-mire. A segment of the body of a segmented animal. Sper-Ma’-ro-cytes. The mother and grandmother cells of spermatozoa. SPER-MA-T0-GEN’-E-sIs. The development of a spermatozoon from a primitive sex cell. SPER-MA-TO-GO’-NI-A. Primordial sperm cells. Sper-Ma-To-zo’-on. The mature male sex cell. Sprnpie. The nuclear division figure. Spi’-remeE. A coiled thread of chromatin which appears in the nucleus at the beginning of division. Spr-rit’-ta. A spiral type of bacteria. Sten’-ror. A ciliated protozoan. SreR-E-o-1 -so-mMeERES. Molecules having the same composition but different properties dependent upon varying spatial relations of their constituent atoms. Srim’-u-Ltus. Anything acting on an organism which calls forth a response. Sry-r£’-ta. A genus of Ascidians. Sym’-me-try. The condition where opposite sides or poles are alike; bilateral, having equivalent right and left sides. GLOSSARY 529 Syn-ap’-sis. The conjugation of maternal and paternal chro- mosomes preceding the maturation divisions. Syn-pac’-ryt-1sm. The condition of having webbed fingers or toes. Tr-neEB’-RI-0. A genus of beetles, the larva of which is the common meal worm. Ter-a-Tou’-o-cy. The science which deals with monstrous or abnormal forms. Tet’-raps. Bivalent chromosomes which appear 4-parted in the maturation divisions. To-t1r’-o-TENCE. The capacity of a cleavage cell to give rise to a whole animal. Tox’-1n. A poisonous substance particularly such as is pro- duced by bacteria. Tri-Hy’-Briw. The offspring of parents differing in three char- acters. TrorH’-o-Biast. The outer layer of the blastodermic vesicle of a mammal. : Tro’-risms. Automatic movements of organisms toward or away from a source of stimulus. Unir Cuaracter. A character which is inherited as a whole and cannot be sub-divided. VEGETATIVE PoLE. The pole of an egg opposite the polar bodies. Vir’-11. Processes which grow out from the embryonic mem- branes of a mammal and connect it to the walls of the uterus, VI-TEL’-LINE MEMBRANE. A delicate membrane around an egg secreted by the egg itself. Viv-1-par’-1-Ty. Young brought forth “alive,” ie, in an ad- vanced stage of development. Zy'-cotr. The product of the union of male and female sex cells. ores sant hadilewh pene — Aottebae iuere shh ea i key CR AES TO Bo ae guys $26 sagae sete enteos ieinaae phate Pasettad Oe » INDEX Proper names and titles of sections are in small capitals; page references to illustrations in italic numerals. Aborigines of Australia 413 New Zealand 413 North America 413 Pacific Islands 413 South America 413 Tasmania 413. West Indies 413 “Accessory” chromosome, 151, 171-173, 277-284 Achondroplasy, 206, 302 Achromatin, 115, 119 differential distribution, 191, in cell body, 188 Acxkert, Effects of Selection on Paramecium, 390: AcauirepD CHARACTERS, INHERI- TANCE OF, 342-360 Darwin on, 343 Lamarck on, 342 Weismann on, 343 definition of, 345 general. objections to, 346, 347 specific objections to, 347- 356 Adaptive responses, 60, 83 Adrenal gland, fed to tadpoles, 334 African negro, 413, 419 in Jamaica, 433 in U. S..433 Age of human race, 410 Albinism, 298, 301 Alcoholism, 212, 303, 466 cause of sterility, stillbirths, malformation, dwarfs, 319, 320, 321 146, | 531 induction effect on rotifers, 358 influence on germ cells, 319 | Alkaptonuria, 302 | Alpine plants, non -inheritance of acquired characters, 355 “Alternative” inheritance, 274 Alternatives in development, 478, 479 Amalgamation of races, 413, 432-435 American men of science, fami- lies of, 451 Ampuioxus, cleavage and differ- entiation, 125 cleavage and gastrulation, 24, larvae, 24, 25 isolated cleavage cells, 322, 328 Ancestors, number of, 222-224 Ancestry, pride of, 445 ANCYRACANTHUS, oogenesis, 140 spermatogenesis, 138 Andalusian fowl,:Blue, 272: Anencephaly, 333 Animal pole of egg, 174 Annelid type of egg, 182 Ants and bees, 456, 499 Artificial limitation of families, 454, 455 Artificial parthenogenesis, 322 ARTIFICIAL SELECTION, 384 chief factor in production of domestic races, 384 creates nothing, 384-392 . isolates pure lines, 386-391 lacking, 422, 424, 425 532 Ascaris, fertilization of, 478 “germ track,’ 132, 133 sex differentiation in, 150 Ascidian egg, 125, 126, 130, 176 825, 827, 828, 830 type, 182 AssHETON, Cleavage of egg of sheep, 29 Aster, 115, 117 Astertas, fertilization of, 20 Astral radiations, 116 Atavism, 215 ATHENS, 419, 420 Athletes, prize, 431 Athletics, educational value of, 492, 493 Attica, illustrious men of, 418- 420 Automaton, 475, 477 Backbone, development of, 25, 27, 32 “Back cross,” ratios of, 243, 244 Bacteria, reactions to light, 52) to salt, 55 Baldness, inherited, 209 Batzac, heredity a maze, 306 Barbarism, 372, 408, 424, 502 BaRDEEN, X-rayS on sperma-) tozoa, 319 | Bateson, Blue Andalusian, 272! brachydactyl hand, 300 “homozygote and heterozy- gote,” 234 on Mendelism, 231 on evolution, 404 Bateson and PunNETT?, on sweet pea hybirds, 262, 263, 264 Baur, factors for flowers of) Antirrhinum, 265 on leaf colors, 295 induction effects of poor soil, 357 | Beans, pure lines, 203, 211, 385,| Bees, and Ants, 456, 499 influence of food on devel- opment, 334 | | “BLENDING” INDEX workers, 3834 BEETHOVEN, 489 Behavior, dogs, cats, monkeys, queens, drones, fish and frog, 481 lower organisms, 47 modifiability of, 73 Paramecium, 66-68, 71 plasticity of, 75, 480 rigidity of, 76 test of psychical processes, 51 worms, star-fish, crustacea, vertebrates, 68-76 Biglow Papers, on Lamarck- ism, 343 | “Biophores” of Weismann, 106 | Birthrate and deathrate, nor- mally equal, 447 both decreasing, 448, 449 decreasing most in best fam- ilies, 450 in Massachusetts, 451 Bivalent chromosomes, 139 ~ Blastula, 24, 27, 30 INHERITANCE, 286, 288-293. in length of ears in rabbits, 291 in length of skull in rabbits, 292 in skin color of mulatto, 288-291, 297 “Blood lines,” 387 Bonp, negro X white cross, 297 Bovutr, Chapelle - aux - Saints skull, 409 Boveri, chromosomes differ in value, 162, 171 dispermic eggs, 321 Brachydactylism, 205, 300, 302 21h | Brain, development, 31 size and weight, 488, 489 Breeder, methods of, 432 Broman, death of families, 452 Brooxs, 220 INDEX Buddhistic belief in transmigra- | tion, 44 BurBAaNnxk, new combinations of | characters, 393 | Butter, BisHop, 462 | Canary birds, fed on red pep- | per, 334 | Capacity, greater than realiza-_ tion, 487, 488 | Captivity, cause of infertility, | 452 | Castie, factors for coat colors | of rabbits, 265 recombinations of charac- ters, 392-395 size in rabbits, 292, 293 value of selections, 387 CasttE and Puitiies, trans- planted ovaries of guinea- pigs, 350-352, 354 Cataract, hereditary, 205, 303 Catrett, birthrate of college graduates, 446, 417 families of scientists, 451 Cause and effect, universality of, 462, 463 Causes, natural vs. Celibacy, 422, 447 Cell characters inherited, 2 Cell division, 22, 23-28, 116, 118, was final, 173 119, 120, 122, 126, 128, differential, 125, 128, 190-192 non-differential, 128, 190- 192 significance of, 128, 129, 131, 132 Cell-Lineage, diagram of 100 CerLutarR Basis or HEREDITY, 95 Centrifuged eggs, 329, 330 Centrosomes, 115 equal division of, 129 Chances, definition of, 478, 479 infinity of in development, 441 ‘Civilization, 533 Cuaracrers, developed not transmitted, 359 individual, 202 inheritance of aeduited 342-360 inherited, definition of, 345, 346 latent, 215 new in evolution, 397, 400, 404, 406, 407 not independent, 199 patent, 215 racial, 201 Cuitp, on chromosomes, Choice of alternatives, 482 Chorea, 303 Chromatin, 28, 115 granules, 116 Chromomeres, equal division of, 190 Chromosomes, 28, 117, 119 abnormal distribution, 321, 403, 404 accessory, 146 bivalent, 139 conjugation of, 137 daughter, 119 diploid, 143 distribution, 121 division, 117, 778, 190 haploid, 143 identity, 119 individuality, 121 maternal and paternal, 123 number of, 117, 151-153 “odd,” 147 reduction of, 142-143 seat of factors, 170-174 shuffle and deal of, 164-166 tetrads, 141 X and Y, 149, 171 means good _ vironment, 312 vs. heredity, 371 173 SO) 129, 131, en- 534 will it endure, 407, 408, 412, 413, 502 Classes, hereditary, 428-430 exclusive, 430-431 CreavacE, of egg, 22-30 AND DIFFERENTIATION, 132 differential, 128, 129 non-differential, 128 significance of, 128, 129, 132 116- Cleavage cells, differentiation of, 30 isolated, development of, 322-330 CiepsINE, behavior of, 74 Coeducation, 445-446 Cold, induction effect on Daph- nia, 358 induction effect on mice, 358 Coloboma, 206, 304 Color, of skin, hair, eyes, 202,, 301, 297 Color-blindedness, 281-285 Consciousness, 78-81 continuity of, 79 loss of, 80, 81 subconscious, 78 Controt or, alternatives, 482 heredity and development, DoD HUMAN EVOLUTION, 416 meaning of, 477 nature, 6 phenomena, 375, 477 self, 477 CoRRELATIONS OF GERM AND soma, 168-185 “Correlative differentiation,” 339, 340 CorrEens, rediscovery of “Men- del’s Law,” 230 on Mirabilis, 237, 238 256, Q71 leaf colors, 295 ‘creationism, 44 INDEX Creative Synthesis, 40, 89, 187 CrEPIDULA, Maturation and fer- -tilization, 772, 113 individuality of germ nuc- lei, 122 exogastrula of, 332 Cretin, 305, 339 Criminality, 303, 461 CrENOPHORE, egg, 182 CULTIVATED PLANTS, 375 number of species, 375 Culture, grades of, 408 Cuvier, 489 Cyclopia, 333 CYTOPLASMIC .CoRRELATIONS, 174 differentiations, 188 inheritance, 176-185 localization, 129 | movements, 123 Daltonism, sex-linked, 282, 283 Dapunta, effect of cold on, 358 DarBysuHirE, 509 Darwin, hypothesis of pangen- esis, 98 on domestic pigeons, 376, B77, 878 inheritance of acquired characters, 343 prepotency, 229 reversion, 229 “sports,” 216 | zeal, 488 organism a microcosm, 194 theory of Natural Selec- tion, 385 Daughter nuclei, 28, 119 Davenport, degrees of relation- ship, 223 extra toe in fowls, 274 inheritance of skin color, 289-291 Mendelian man. 299 transplanted ovaries, 350 inheritance in INDEX “weakness with strength,” 442 white < black leghorns, 273 DaveNPoRT AND WEEKs, epi- lepsy inherited, 212 Deaf-mutism, 206, 304 Death, of families, 450-453 Deathrate, declining, 447-451 Declaration of Independence, 311 Decline of families and nations, 503 Defectives, growing burden of, 494 alarming increase of, 436 Defects, educational influence of, 365 Democracy and human equality, 312 Descartes, 311 “Determinants” 106, 261 Determiners, 107 combinations of, 261 differential causes, 260 DETERMINISM, and ResponsIsBinity, 475 definition of, 476 not Fararism, 476 not predeterminism, 476 of ENVIRONMENT, 469-475 of Hereprry, 463-469 of personality, 475 scientific, 476 Development, a_ series sponses, 335, 473 alternatives in, 478 definition of, 109 is transformation not new formation, 168 mosaic, 341 not reversible, 478 of function, 39 of personality, 7, 82, 476 potentialities of, 470, 471 physiology of, 315, of Weismann, of re- 535 various aspects of, 81, 82 viviparous, 19 DEVELOPMENT OF Bopy, 8 or Minn, 43-45 DEVELOPMENTAL Regponsss, 318- 335 AFTER FERTILIZATION, 322 BEFORE FERTILIZATION, 321 DURING FERTILIZATION, 321 Vries, action of selection, 385, 391 fluctuations, 217, 218 induction effects of poor soil, 357 intra-cellular 188 “mutation theory,” 217 mutations, 217, 218 Oenothera mutants, 403, 405 on nuclear control of differ- entiation, 188 pangenes, 106 rediscovery of Law,” 230, 510 Diabetes, 302 Differential division, of Cyto- plasm, 132 of cells, 125, 128, 190-192 Differentiation, 37 “correlative,” 339, 340 definition of, 109 due to interaction of cell parts, 169, 187, 188, 189 measure of, 192 nuclear control of, 188, 189 “self,” 341 Dihybrid, 247, 251 |Dimples, inheritance of, 203 Dionakra, reactions of, 61, 62 Diploid number of chromo- somes, 143 Disease, inheritance of, 206-208 slight resistance to, 207 Dislocation of organs in centri- fuged eggs, 329, 330 pangenesis, 357, “Mendel’s 536 Dispermic eggs, 321 Divines, poor health, 366 Division period, 134 Dogs, different races, 376 psychological characters in- herited, 210 Domestic AnriMats, 375-382 degree of change, 376 how produced, 383 number of species, 375 progenitors, 375, 376 regressive mutants, 402 DOMINANCE, MODIFICATIONS OF, "O71 Blue Andalusian 272 echinoderm hybirds, 274 extra toe in fowls, 274 in red x white Mirabilis, 971 nature of, 285 not fundamental, 285 plain > banded snails, 273 | red < white cattle, 272 | sex-limited characters, 274) sex-linked characters, 277-| 285 | white black leghorns, 273 Dominant characters, 233 “extracted,” 236 ratio to recessive, 239 races, 412, 414 Doncaster, 510 Double monsters, 323, 324, DriescH, 341, 510 DrosopHita, rapid breeding of,| 299 | sex-linked characters, 172, 277-281 | Dryvden, 212 | Duplex Characters, 255, 256 | Duty, 459, 475 of science, 496 Dwarfs, true, 302 caused by alcohol, 320 | Dynamic equilibrium, 10 | | | 331 INDEX Ear, development, 31 East, heterozygosis, 396, 398, 399, 182 EcHInopERM type of egg, 182 Ecuinvs, first cleavage, 22 Ectoderm, 24, 25, 30, 31 Education, and heredity, 444 definition, 363 good and bad, 363, 365, 366 habit formation, 473 limiting activities, 365, 366 more potent in man, 363 needs of 491 possible improvements, 505 Egg and sperm, hereditary in- equality of, 184 Egg nucleus, 17, 23 Egg organization, 182 types of, | Etssere, plastidules, 106 Emboitment, 86, 406 EmpryoLocy, 30 Embryology, experimental, 315 Embryonic differentiation, pro- cesses in, 186 Embryos, double, 330,. 831 dwarf, 323 half and three quarter, 325, 3827 Endoderm, 24, 25, 80, 31 ENDOGENESIS AND EPIGENESIs, 88 “Energies of Men,” 488 ENGLEMANN, 52 English sparrow in U. S., 448 Engrammes, 356 Environment, acting at sensi- tive period, 383 definition, 315 direct action on germ cells, 356, 359, 360 and education, 369 good and bad, 364, 365, 366, 491, 492 823, 316, INDEX influence in producing new races, 383, 384 influence on ontogeny, 311, 313 influence on phylogeny, 310, 313 possible improvements, 505 social institutions and, 312 Epidermolysis, 302 Epigenesis, 86, 88, 167, 187, 468 Epilepsy, 211, 212, 303 Equality of Man, 311, 464 Eruicat Osrication, 500 Ethics, 507 Eugenicist, methods of, 432 Eugenical rules as to defects, recessive, 437, 438 serious, 442-444 slight, 442 Eucentcs, 423, 456 contributory, 444-447 declining birthrate, 447-456 definition of, 425 ideals, 428-435 negative, 435-439 only hope, 465 positive, 439-444. EvtuHenics, 361 Evolution, control of, 416, 417 experimental, 313, 407 progressive, 412 promotion of, 416, 504, 507 requires new characters, 397, 400, 404, 406 retrogressive, 411 Evotution oF Man, contemporary, 410 control of, 416, 417 future, 410-412, 425 intelligence in, 415 natural selection in, 413 prehistoric 410 Exogastrula, 332 Experience, factor in behavior, 480 learning by, 70, 481 407-416 537 Experimental medicine, 5 EXPERIMENTAL, Srupy oF In- HERITANCE, 230 Eyes, development, 31 color, 203, 301 lacking, 333 fusea together, 333 Facial features, inheritance of, 203 Hapsburg type, 301 FACTORS OF DEVELOPMENT, 84-86 Factors, added in progressive mutations, 406 chemical comparisons, 406 definition of, 107 differential, 270 distribution in maturation and fertilization, 268 dominant and recessive, not modified by union, 353 drop ‘out in regressive mu- tations, 404 extrinsic and intrinsic, 90 for color developers, 262 of rabbits and mice, 265 of sweet peas, 262, 263, 264 for pigment, 262 location in cell, 267, 268, 269, 270 Mendelian, 270, 271 multiple, 287-293 nature of, 266 no formation de novo, 407 origin of new, 406, 407 relations to characters, 261, 262 sex determining, 271 lf AHLENBECK, noble families of Sweden, 451 Farapay, 490 Fat stains, effects on next gen- eration, 296, 359 “Fate of part, function of po- sition,” 341, 342 of organization, 342 538 Fecundity, inherited, 210 Feeble-mindedness, 211, 212, 303 Feminist movement, 500 Fertility, of lower types, 424 Fertiization, 15, 17, 113-116 heterogeneous, 321 “Fewer and better children,” 453 FiscHer, mutations of insects, 357 Fluctuations, 217, 218, 400 Food, influence on development in tadpoles, canaries, bees, 334, 335 Foor and SrroBexL, on chromo- somes, 173 on sex-limited characters, | 280 Foret, effect of alcohol on germ | cells, 321 ForMATION OF SUBSTANCES IN | CELLS, 186 ForMULAE, INHERITANCE, 251- 254 | Fowls, races of, 379, 880 transplanted ovaries, 350 Frreepom, and determinism, 476 | birth and growth of, 480 definition of, 482 development of, 481 from reproduction, 501 greatest in man, 481 not absolute, 4:75 not uncaused activity, 482 of action, 77 of individual, 430, 459, 498 of society, 498 Friedrich’s Disease, 303 Frog, behavior of, 481 double embryos, 322, 324 early development, 26, 27 Function and structure, corre- lated, 41, 48, 208 INDEX Funcrionat Activity, 335 in human development, 362 GacER, radium on nuclear di- vision, 319 Gatton, age of marriage, 435, 447 “Ancestral 991, 222, 229 artistic faculty, 198, 221 characters, 198 definition of eugenics, 425, diseases, 198, 221 eugenical policy, 435, 436 eye-color, 198, 221 “Filial Regression,” 226, 229 genius inherited, 212, 221 heredity vs. civilization, 370 intermarriage of scholars, 431 kinds of inheritance, 274 | nature and nurture, 309 | on Ancient Greeks, 418, 419 on identical twins, 367 | pioneer in heredity, 199 poor health of divines, 366 | religious significance of evo- lution, 507 | on “sports,” 216, 217 statistical study of inheri- tance, 220-229 stature, 198, 202, 221 | weight of seeds, 198 Gamete, 13 Gardener, methods of, 432 |Gastrula, 24, 30 Gates, Oenothera chromosomes, | 403 Gauss, 489 “Gemmules,” 106 Generalized types, 429, 430, 431 |Generations, parental and fil- | ial, 234, 236 | symbols of, 237 Inheritance,” Q24— of Darwin, 98, INDEX Genes, 107 Genius, and physical defects, 366 hereditary, 212 unstable nervous organiza- tion, 212 Genotype, 102, 247, 254, 345, 392 Geographical isolation, 432 Geotropism, of seedling, 59 GerMAN Emperor, number of, ancestors, 223 GerM cELLs, 8, 110 alive, 9 complexity of, 194 possibilities determined in, 464 potential personalities 455 reactions of, 480 Germ nuclei, 28 individuality of, 121, 122 Germ-plasm, in nucleus, 129 Theory of Weismann, 101- 103, 343, 344. Germ ys. Soma, 101 249, 250 in, “Germ track,” diagram of, 101 GrerMINAL ContTINuITy, 98-103 Glandular secretions, effects of, 337-339 Glaucoma, 304 Gopparp, feeble-mindedness in- herited, 212 GorpscHmiIptT, 157, 510, 513 Gonia, 134 Grafts, not modified by stock, S505 Go Great men, in crises, 490 GREECE, decay of, 501, 502 GREEKs, ancient, 418-421 Growth period, 135 GUDERNATSCH, effects of food on tadpoles, 334 | Guinea-pigs, recombinations of | characters, 392-395 | | | | | | | | 539 transplanted ovaries, 350- 354 Guturig, transplanted ovaries, 350 Guyer, on chromosomes of man, 153 Hastirts, definition, 363 good and bad, 363-365, 480 Hacker, 510 HaEckeEL, plastidules, 106 Haremopuitia, 209, 281, 304 Hair, color, 301 form, 297, 301 Half castes, of Australia 433 of New Zealand, 433 Hansen, mutations of yeast, 357 Haploid number of chromo- somes, 143 Hardship, educational value of, 364, 491 ° Harris, induction effects of poor soil, 357 PiaRrRIson, on transplanted limbs, 341 graft of tadpoles, 350, 351 Harvey, epigram, 9 epigenesis, 86 HarscHexK, cleavage and gas- trulation of AmMPpHIoxus, 24 larvae of AmpuHtoxus, 25 | Hereditary lines, interwoven, 504 | Hereprrary REesEMBLANCES AND DIFFERENCES, 199, 200, 213-220 | Heredity, and memory, 356 and variation, 200, 218 definition of, 96, 109 includes assimilation, etc., 169 mechanism of, 111 more potent than environ- ment, 454 540 possible improvements, 506 theories of, 111 usually unchanged by en- vironment, 450, 454-463 HereDItry AND DEVELOPMENT, 108-110 HerepDiry AND ENVIRONMENT 90, 91, 310-314 HEREDITY, ENVIRONMENT, TRAINING, 366-368 Herinc, Organic memory, 64 Heritage, definition of, 109 Hertwic, O., discovery of ferti- lization, 113 human ovum, // idioblasts, 106 influence on germ cells of) X-rays, radium, chemi- cals, drug habit, 319-321 | Herrwic, R., modification of) sex ratio, 154-157 Heterozygosis, 396-399 Heterozygotes, 234, 249, 250 Hindu Dwarfs, 305 Hippocrates, 97 Homo sapiens, 408, 413, 428 NEANDERTHALENSIS, 409 Homozygotes, 234, 236, 247-250 “Homunculus,” 85 Horrr, Effect of alcohol on germ cells, 320 Human embryo, development, 36, 88 HuMAN EVOLUTION, CONTROL OF, 416 slow, 455 Human faculties, definition, 363 Human Heredity, no improve- ment in, 417-423 Human oosperm, early develop- ment, 34 ovum, 77, 14 spermatozoa, 14 Humidity, influence on muta- tion, 318 236, 247, | INDEX Huntington’s Chorea, 303 Hoxtry, Evolution and Ethics, 370 Hybrid races, quality of, 434 Hybridization, human, 432-434 Hybrids, increased vigor, 396 Hypertrophied heart, not in- herited, 347, 349 Hypophysis, effects on de- velopment, 339 Hypotrichosis, 302 Hysteria, 212, 303 - Ideals, individual and _ social, 428-430 Identity, sense of, 80 “Tdioblasts” of Hertwig, 106 Idioplasm, of Niageli, 103 Immigration, 419, 434, 435 laws, 437 Immunity, transmission of, 296 | Impulses, conflicting, 481 Inbreeding, 430-432 | INpIvipuaL AND Race, 498 minor unit, 504, 505 | InprvipuaL CHaracrers, 202 Morphological, 202 Physiological, 208 Psychological, 210 Teratological, 205 Individuals and their charac- ters, 197 “Induction,” effect of colored soil, 358 poor soil, cold, alcohol, 357- 358 not inherited, 359-360 Inequality of all men, 464 Infancy, prclonged in man, 362, AT2 Infertility, causes of, 452 Inheritance, “alternative,” 214 “blending,” 214 of baldness, 209 cell characters, 205 dimples, 203 INDEX facial features, 203 fecundity, 210 genius, 212 instincts, 210 intellectual capacity, 211 left-handedness, 210 longevity, 208, 209 moral tendency, 211 obesity, 209 “particulate,” 214 pathological characters, 205-208 physiological characters, 208 psychological characters, 210-213 | sex-limited and sex-linked, | We | stature, 202, 224-226 temperament, 211 teratological characters, 205 through cytoplasm, 183-185 | tuberculosis, 207, 208 will, 211 INHERITANCE Factors, 259 Inheritance material, 103 seat of, 169 Inheritance units, 105, 108 Inhibition, 73, 480 Insanity, 211, 212, 303 Instincts, 56-61, 83 altruistic, 483 inherited, 210 origin of, 61 reproductive, 500 INTELLECT, 65-72 Intellectual capacity, inherited, 911 genius, 212, 301 mediocrity, 301 Intelligence, factor in control, ATT in evolution of man, 415 Interaction of parts, 336-342 Intra-cellular pangenesis, 188 InversE SyMMetRY, 176-181, 294 Ivritability, 12, 39 | 541 IsOLATION OF SUBSTANCES IN cells, 189-191 in protozoa. 191 James, WiLttaM, 488 JENNINGS, action of selection, on Paramecium, 386 on Difflugia, 390 behavior of Paramecium, 67 behavior of Stentor, 73 inheritance of size in Para- mecium, 204 on Galton’s law, 227 on potential personalities, 490 rapid breeding mecium, 299 training of star-fish, 75 JEROME, St., 44 Jews, mixture with Gentiles, 433 JOHANNSEN, action of selection, of Para- 385 genotype and_ phenotype, 102 inherited weight of seeds, 203 “pure lines,” 385, 388 JoHNson, marriages of college women, 446, 447 Kammerer, effects of colored soil on salamanders, 358 KerpeL, development of human embryo, 34, 36, 38 Ketzicorr, 510 Keratosis, 302 Kine, modification of sex ratio, 155, 157 Korscuerr and Heer, Sym- metry of egg of Musca, 175 Lane, development of vision, 40 Lamarck, on inheritance of ac- quired characters, 342, 343 542 LAMARCKIAN HyporHeEsis, 356 Lamarckism, 41 Lane, snail hybrids, 273, 294 Laws on Eugenics, 436, 437 Learning by experience, 70, 481 Left-handedness, inheritance of, 210 Lens, cataract, 205, 304 development of, 340 displaced, 304 weight of, 205 Leptinorarsa, selection in 387 Life, artificial production of, 310, 312 conditions limited, 469 definition of, 9 maze of, 479 Liu, fertilization 16 | on fertilizin, 160 sex determination, 157 Limbs, transplanted, 341 | Lincotn, 440 LocatizaTion Pattern, 181 in eggs of ctenophore, flat-| worm, echinoderm, anne-| lid-mollusk, chordate, 181, | 182 Localization of substances in} cells, 191 | Locx, 510 Lorn, J. Artificial partheno-| genesis, 114, 159, 511 reflexes, 60 tropisms, 60 Logic, as test of truth, 467 Lorico, symmetry of egg, 175 Longevity inherited, 208, 209 Lorsy, source of variations, 393 Lowe tt, Bictow Papers, 343 Luxury, cause of infertility, 452, 453 in education, 491, 492 MacFaARLANE, on Dionaea, 61 INDEX McCienpon, production of one- eyed monsters, 159 McCiune, on sex determina- tion, 147, 171 MacDovucat, influence of chem- icals on ovules, 319 MacDowet., size in rabbits, 292 Selection in Drosophila, 389 McGrecor, Restoration of Pithecanthropus skull, 409 Male babies, greater mortality, 156 Matruus, theory of, 436 Man, controls destiny, 410 dominant races of, 412 evolution of, 407-415 extermination of, 413, 414 extinct types of, 408, 409 freer than animals, 481 mongrel race, 441 place in nature, 3 prehistoric, 410 races of, 412 species of, 408, 409 value of races of, 412 Maorts of New Zealand, 413, Marriage, age of, 435, 446 selection, 442-444 Marsupials, 35 Massart, reactions of SPiriLua, 55 Materialism, 47 '“Maternal impressions,” 35 Matter and mind, 48 MaruRATION PERIOD, 141 divisions, 141-144 Maze, of heredity, 220, 306 life, 479 MECHANISM 86 MecHANIsM OF Herepiry, 157 Mechanistic hypothesis, 495 Mediocrity, tendency to, 224 MeiscHer, On stereoisomeres of albumin, 161 OF DEVELOPMENT INDEX Memory, 61-64 Menpet, abbot of Briinn, 330 dominant and _ recessive characters, 233, 234 dominant: recessive ratios, 234, 236 experiments on peas, 199, 931, 233, 285 inheritance formulae, 251- 253 inheritance units, 261 method of work, 231 neglect of discoveries, 230 purity of germ cells, 240 MeENDELIAN ASSOCIATION AND DissoctaTIon, 392-396 Mendelian factors and chro- mosomes, 170-173, 268, 269, 271 MeENDELIAN INHERITANCE IN Man, 297-305 Table of, 301-304 MENDELIAN PRINCIPLES, 256 DoMINANCE, 257 MopiFIcaTIONS AND EXTEN- sions, 258 SEGREGATION, 257 Unit CHARACTERS, 257 Mendelian ratios, simple, 234, 235, 238-250 “back cross,” 243 monohybrid, 246, 251 dihybrid, 245, 247, 251 trihybird, 245, 249, 250 dominant-recessive, 236 departures from, 287, 288, | MENDELIsM, 230-258, 306 MENDELSSOHN, reactions Paramecium, 57 Meniere’s disease, 303 of Mentality, influence of educa-| tion, 311 Mesoderm, 31 Metabolism, 12, 39 METCHINIKOFF, disharmonies in man, 370 543, Metempsychosis, 44 Microscopic particles, smallest visible, 105 MippietTon, Effects of Selection on Stylonychia, 390 Mind and body, 48 Mind, development of, 45, 46 Mirasitis, white-red cross, 237, 238, 240, 256, 271 Mitosis, 16, 28, 116, 118-122, 126 significance of, 128, 129, 131, 132 “Mneme” theory, 128-132 Modifiability of behavior, 73 Molecular constitution, stere- oisomeres, 161 Molecules, largest known, 105 Monasticism, 422, 445, 446 Monohybrid, 234, 242, 246, 251 Monotremes, 35 Monstrous development, A470, 471 cause of, 318 MontcomMery, on Chromosomes of man, 153 Moral qualities inherited, 211 Morecan, mutations of insects, 357 sex chromosome, 172, 173 sex determination in Phyl- loxera, 156 sex-linked inheritance, 277, 279-285 rapid breeding of Droso- phila 299 Morphological characters, 202 tests, 42 Mosaic development, 326 Moth: and flame, 480 Morr, insanity inherited, 212 Mouse, maturation and fertili- zation, 120 Movements, within cleavage cells, 116, 123, 189 effects of stopping, 336 317, eggs and 54, 112, 544 INDEX random, 60 | of spermatozoa, 53, 54 Mulattoes, skin color, 288-291 in Jamaica, 433 in U. S. 433 Miitter, JoHANNEs, 494 Mustow, 138-141 Multiple factors, in oats and) wheat, 287 skin color, 288-291 size, 291-293 Multiple sclerosis, 303 Musca, symmetry of egg, 175 Muscular atrophy, 303 Mutation Theory, 313 Mourations, 215-217 AND Friucruations, 217, 218, 400, 402 progressive and regressive, 4.04 origin of, 318-322, 356, 357, 400-404 Mutilations, not inherited, 347 Myopia, 206 N&AGELI, idioplasm, 103 non-inheritance of habit, 355 Natural selection, 385, 392, 415, 354 nullified, 421-424 Nature, definition of, 462 man part of, 463 mechanistic conception of, 462 vs. nurture, 309 stability of 411 voluntaristic conception of, 460 Necrurus, behavior of, 74 Neo-Darwinism, 49 Neo-Lamarckism, 49 NEREIs, spermatozoon of, 16 maturation and _ fertiliza- tion of, 16 alpine | NETTLESHIP, hereditary cata- ract, 204, 205 Neural plate, groove, tube, 25, 31 _ Neuritis optica, sex-linked, 304 Neuropathy, 303 New England families dying out, 450 Newcoms, practical eugenics, 427 Newton, 490 Night blindness, sex-linked, 304 Nitsson-Ente, multiple factors, 287, 288 Non-MENDELIAN 294, Notochord, 25 NUCLEAR CORRELATIONS, 169 Nuclear division, indirect, 16, 24, 116, 118-122, 126-132 Nuclear inheritance theory, 1u3- 174 Nucleus, 10, 115-120 and cytoplasm concerned ia heredity, 185 Nulliplex character, 255 INHERITANCE, Obesity inherited, 209 OBSERVATIONS ON INHERITANCE, 197 “Odd” chromosome, 147 OENOTHERA, mutants, 403, 405 chromosomes of, 403 Oneness of life, 5, 51 Ontogeny and Phylogeny, 7, 313 Oocytes, 135 of rabbit, 736 Oogenesis of Ancyracanthus, 140 Oogonia, 134 Oosperm, 13, 18 double cell, 18 individuality of, 18, 19, 23 infection of, 207 Organ-forming substances, &8$ ) ai INDEX in Styela, Amphioxus, frog, 125 Organism of humanity, 505 Organization, 10 OrGANOGENY, 31 Orictn oF Sex CEtts, 132-144 Division PERIOD, 134 Primitive sex cells, 134 Oogonia, 134 Spermatogonia, 134 GrowTH PERIOD, 135 Oocytes, 135 Spermatocytes, 135 MatTuRATION PERIOD, Orthogenesis, 313 Oszorn, Cartwright Lectures, 408 Otosclerosis, 206, 304 | Ovaries, transplanted, 351-355 | Oviparity, 32 Oviparous development, 18, 19, Ovules, 13 Oxychromatin, differential dis- tribution, 191 in cell body, 188 141 “Pangenes” of deVries, 106 Pangenesis, hypothesis of, 98, 343 PaRaMECIUM, avoiding reaction, 67 behavior of, 66 reactions to heat and cold, 57 races differing in size, 204 races of, 211 rapid breeding of, 299 selection in, 386 trial and error, 68 Parthenogenesis, 114 “Particulate” inheritance, 214 Partition walls between cells, 191 Pasteur, 490 545 PATHOLOGICAL CHARACTERS IN- HERITED, 205-208 Peart, action of selection, 387 PraRL AND ParsHiLEy, modifica- tion of sex ratio, 155 Pearson, ancestral inheritance, 229 inheritance of tuberculosis, 207 statistics, fault of, 225 Permutations in distribution of chromosomes, 162 Personality, determined by her- edity, 463, 469 development of, 7, 84, 472 infinity of chances in, 440, 441 not predetermined, 469 potential, 490 prediction impossible, 440 PHENOMENA OF DEVELOPMENT, 6 Phenotype, 247, 249, 250, 392 vs. genotype, 102 Phototropism of Seedling, 58 PHyYLLoxera, degeneration, of male-producing sperma- tozoa, 156 Physiological characters, heritance of, 208 division of labor, 40 processes, 10, 12 states, 73, 480 tests, 42, 160 units, 106 Pigeons, behavior of, 75, 76 numerous races, 376, 377, 878 Pineal gland, 170 PITHECANTHROPUS ERECTUs, skull 409 “Plasomes” of Wiesner, 106 Plasticity of behavior, 76 “Plastidules” of Elsberg and Haeckel, 106 Plastosomes, equal division of, 190. in- DAG Puare, ancestors of German Emperor, 223 factors for coat colors of mice, 265 Mendelian inheritance in man, 299-304 “Pseudo-heredity,” 295 Piato, on transmigration, 44, Polar bodies, 773, 144 Polarity, 174, 175 of Styela egg, 123, 125, 18 Pollen, 13 Polydactylism, 205, 302 Polyhybrid, 251 Population, normally ary, 447-450 of Kurope, 449 Poultry, selection for egg pro- duction, 387 PREFORMATION, 84, 86, 167, 468 Epicenesis, 88-90 “Preinduction,” in Daphnia, 358 “Preinheritance,” 295 Prepotency, 229 PRESENCE AND ABSENCE POTIIESIS, 254-256 Primitive sex cells, 700, 134 Principles of good breeding, violation of, 421 Propagation of worst, 422 Prorenor, sex differentiation in, 148 Protoplasm, 9 Psychical Anlagen, 50 Psychial development, of, 83 PsYCHOLOGICAL CHARACTERS IN- HERITED, 210-213 PuUNNET?Y, 23), 264 “Pure lines,’ 386, 388 Puritans and Cavaliers disap- pearing 450 Purity of germ cells, 240, 257, PyrHacoras, on transmigration, 44 station- Hy- table INDEX Race amalgamation, 413, 414 extermination, 413 improvement, 6, 500-507 preservation, 500 Radium, disintegration of atom, 406, 407 influence on spermatozoa, 319 Rana, grafted tadpoles of, 350, 3851 Reactions, of germ cells, 47, 50 machine-like, 481 Reason, 65-72 Reception cone, 20, 116 Recessive characters, 234 “extracted,” 236 ratio to dominant, 239 Reduction of chromosomes, 143 REFLEXES, 56-64 Regeneration, in eggs adults, 326, 329 Reproduction, 12, 39 Responses, varied, 73 Responsibility, 460, 475 definition of, 483 of society, 485, 498 varied, 484, 485 Retinal degeneration, 303 and Rerzius, human spermatozoa, Tiley 1) Reversible changes not in- herited 359 Reversion, 215, 229 Rickets, not inherited, 346 Rippie, Sex determination, 157 Rigidity of behavior, 76 Rienano, “Centro - epigenesis” theory, 356 RomangEs, 377-382 cattle, 382 fowls, 379, 880 swine, 381 | Rome, decay of 501, 502 Rosanorr, insanity inherited, Q12 INDEX Rotifers, induction effect of alcohol, 358 Salamanders, effects of colored soil on, 358 SaLeEeBy, 511 Savagery, 372, 408 Scholars, prize, 431 Scuuttrze, double frog embryos, 324 Science, duty of, 496 Segregation, apparent lack of, ~ 4 fundamental to Mendelism, 286 of Mendelian factors, 241, 957 of substances in cells, 189 SetectivE Breepine, only meth- od of improving race, 416 Spartan method, 417 Selection, value in Evolution, 384 Self-control, 489, 491, 493, 494 “Self differentiation,” 341 Self discovery, 489, 491 Semon, “Mneme” theory, 356 Sensitive periods, 318 Sensitivity, 51 differential, 52 general, 54 of germ cells, 53 Sex, a Mendelian character, 244 274, 276 influence of food and tem- perature, 311 Sex cells, fundamentaly alike, 13 SEX DETERMINATION, 145-157 in human embryo, 145 in man, 151, 752, 153 McCtiune on, 147 Witson on, 147 SzEVENS on, 147 5AT Sex glands, effects on develop- ment, 337-339 SEX-LIMITED INHERITANCE, 215, 274, 280 SEX-LINKED INHERITANCE, 277-285, 304 Sex ratio, modification of, 146, 154-157 Sexual reproduction, value of, 163, 432 Suaw, BernarpD, 454 Sheep, cleavage of egg, 29 SHULL, leaf colors, 295 heterozygosis, 396 Significance of cleavage, 190 of mitosis, 129-132 Simplex character, 255 Skin color, 203, 288-291 mulatto, 288, 297, 433 influence of light on, 311 Slow breeding of man, 299, 455 Sozotta, fertilization of mouse, 215, 128, 120 Social inheritance vs. germinal, 369-372 Social institutions, deal only with environment, 369 Society, highest grade of or- ganization, 498 power of, 423 responsibility of, 498 supreme duty of, 504-507 Soil, poor, induction effect on plants, 357 SoMATIC DISCONTINUITY, 98 Somatoplasm, in cell body, 131 Somites, 25 Sparra, destruction of unfit, 417 Special senses, origin of, 54 Specialized types, 430, 431 | Speciriciry oF Germ Certs, 158- | 168 | SPENCER, 106 physiological units, 548 Sperm centrosome, 17 Sperm nucleus, 17, 28 Spermatocytes, 135 Spermatogenesis of Ancyracan- thus, 138 Spermatogonia, 134 Spermatozoon, 14, 16, 20 formation of, 144 spina bifida, 332 Spinal cord, development, 31 Spindle, mitotic, 22, 28, 117 Spireme, 117 Sprritta, reactions to chemicals, 55 “Sports,” 216, 217 Sranpruss, mutations of insects, 357 Star-fish, isolated cleavage cells, 322 Statistical methods, strength and weakness of, 225, 227 STATISTICAL Srupy oF INHERT- | TANCE, 220-229 Stature, inheritance of, 202, 221, 226 tendency to mediocrity 224 influence of food on, 311 Srenror, modifiable behavior, 73 Sterile insects, 456 Sterility, 422, 451-454 Sterilization, 435-438 Srevens, on sex determination, 148, 149, 171 Stimuli, chemical and physical, 483 conflicting, 73, 480 definition, 316 DEVELOPMENTAL, 315-318 chemical, 316 non-specific, 317 physical, 316 external and 471, 481, 482 range of, 484 rational, social ethical 483 summation of, 63 internal, 72, INDEX SrocKarp, alcohol on spermato- zoa, 319, 320, 358 experimental cyclopia, 159, 333 Structures and functions, recip- rocal relations, 41, 48, 208 StyeLta, anterior half-embryos, 327 dislocated organs, 330 egg substances, 123-127 gastrulation and larva, 130 half and three-quarter em- bryos, 325 maturation, fertilization and cleavage, 123, 126, 127, 130 posterior half-embryos, 328 Summation of Stimuli, 63 SuMNER, effect of cold on mice, 358 Superman, 426, 455 SWEDEN, extinct noble families, 451 Swine, wild and domestic, 381 Symmetry, 176-180 Synapsis, 137 | Syndactylism, 205, 302 Tadpoles, fed on thyroid, thy- mus, adrenal, 334 ‘VYalents, unused, 487 parable of, 489 Temperament, inheritance of, 210, 301 Temperature, influence on mu- tation, 318 influence on cell division, 319 TENEBRIO, sex differentiation in, 149 TENNENT, modified dominance in echinederm hybrids, 274 Teratological characters inheri- ted, 205 | TERTULLIAN, 44 | Tetrads, 141 INDEX TuHompson, cross of yellow x green peas, 235 diagram of Galton’s Law, 222 Thomsen’s disease, 303 THORNDIKE, behavior of dogs, cats, monkeys, 69 “Thoroughbreds,” 430 Thymus gland, fed to tadpoles, 334 Thyroid, effects on development, Ist gland, fed to tadpoles, 334 Totipotence, of cleavage cells, 324, 326, 341 Tower, action of selection, 387 mutations in Leptinotarsa, 318, 357, 401 Toxopneustes, fertilization of egg, 20 Traducianism, 44 Training of animals, 74 Transmission hypothesis, 97 Trial and Error, 68 Trihybrid, 249, 250, 392 Triplets, hereditary, 210 Trophic correlations, 337 ‘Tropisms, 56-60 TscHERMAK, rediscovery of “Mendel’s Law,” 230 Tuberculosis, inheritance of, 207, 208 Turbellarian type of egg, 182 Twins, fraternal, 332 hereditary, 210 identical, 219, 331, 367, 474 Ultra-microscopic units, 105 Uniqueness of every individual, 161, 218-220 Unir CHaracters, 257, 259 UnItTs OF LIVING MATTER, 103-108 ultra microscopic, 105 units of growth and division, 105 549 units of heredity, 105-108, 259-271 Unity of organism, 49 Universal laws, 467 Use and disuse, effects of, 335 effects not inherited, 343 Uterus, attachment of oosperm to, 34 Variability, caused by environ- ment, 400 Variations, continuous, 216 discontinuous, 216 fluctuations, 217, 218 meristic, 216 mutations, 215, 217 “sports,” 216 Varied responses, 74 Vegetative pole of egg, 174 VIVIPARITY, 32 Viviparous development, 18, 19 Watter, diagram of Galtonian inheritance, 214 filial regression, 226 Wars, effects of, 438, 439 shake off social heredity, 372 Wassermann test, 160 Warase, symmetry of egg of Loligo, 175 WEEKs, 212 Weidal test, 160 WEISMANN, determinants biophores, 106, 261 germ plasm theory, 101, 344 hereditary and environment- al variations, 218 on differential division of chromosomes, 131 inheritance of acquired characters, 343 nuclear control of differ- entiation, 187 and 650 reduction of chromosomes, | 143 Wurman, behavior: of cLEP- SINE, 74 Necrurus,: 74 pigeons, 75, 76 freedom and choice, 76, 77 Wuitney, induction effects of alcohol, 358 Sex determination in Roti- fers, 155 Wiesner, plasomes, 106 Wiper, Duplicate twins and double monsters, 331 Witt, 72-78, 475 absolutely free, 461 defined, 486 _ good and evil, 461 inherited, 211 nature, expression of, 460 supreme faculty, 487 training of, 486, 487 Witson, distribution of factors, 268 of chromosomes, 269 dwarf and double Amphi- INDEX oxus embryo, 328 on sex determination, 147, 148, 150, 171, 172 WINIWARTER, ON chromosomes of man, 151, 152 oocytes of rabbit, 136 Wotrr, “Theoria Generationis,” 86 WottERECK, induction effects of cold, 358 preinduction, 358 Women’s Colleges, influence on marriage, 446, 447 Woons “Heredity in Royalty,” 212, 512 X-rays, influence on spermato- zoa, 319 X and Y chromosomes, 149, 171 Yolk influence on size of eg, 13, 14 ZELENY AND Marroon, Selection in Drosophila, 389 Zygote, 12 ‘i ; Mi sae Tieton ih Ba ual Gh eis y iil IT af on or ia” Heredit ae levelopm