BERKELEY LIBRARY UNIVERSITY OF CALIFORNIA SCIENCES IJBRARY MATTHEW LIBRARY THE N. W. HARRIS LECTURES FOR 1914 N. W. partis 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 before the students and friends of Northwestern University, 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 limi- tation 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." 1907 Personalism. Bordon P. Browne 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 1913 The Constitution of Matter. Joseph S. Ames HEREDITY AND ENVIRONMENT IN THE DEVELOPMENT OF MEN BV EDWIN GRANT CONKLIN PROFESSOR OF BIOLOGY : PRINCETON UNIVERSITY FIFTH EDITION REVISED PRINCETON PRINCETON UNIVERSITY PRESS LONDON: HUMPHREY MILFORD OXFORD UNIVERSITY PRESS 1922 FIRST EDITION Copyright, 1915, by PRINCETON UNIVERSITY PRESS Published February, 1915 Second Printing, June, 1915 REVISED SECOND EDITJON Copyright, 1916 Published May, 1916 Second Printing, August, 1917 Third Printing, March, 1918 REVISED THIRD EDITION Copyright, 1919 .. Published September, ,1919 -Second Prin'tiKg, M9'ij:H^ 1020 ' »EVI,SKD F.OURXH EDITION' ' ' '. ' t ip Jfriglii: 1922 «V / Published January', l9'22 REVISED FIFTH EDITION Copyright, 1922 Published September, 1922 TRANSLATIONS Japanese, from 2d English Ed. Published September 15, 1916, Tokyo. French, from 3d English Ed. Published, December 1920, Paris. MATTHEW LIBRARY Printed in the United States of America PREFACE TO FIRST EDITION The origin of species was probably the greatest biological problem of the past century ; the origin of individuals is the great- est biological subject of the present one. The many inconclusive attempts to determine just how species arose led naturally to a renewed study of the processes by which individuals came into existence, for it seems probable that the principles and causes of the development of individuals will be found to apply also to the evolution of races. As the doctrine of evolution wrought great change in prevalent beliefs 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 theoretical signifi- cance, the phenomena of development are of the greatest practical importance ; indeed there is probably no other subject of such vast importance to mankind as the knowledge of and the control over heredity and development. Within recent years the experimental 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 comprise this volume were given at North- western University in February, 1914, on the Norman W. Harris Foundation and were afterward repeated at Princeton University, f gladly take this opportunity of expressing to the faculties, stu- dents and friends of both institutions my deep appreciation of their interest and courtesy. In attempting to present to a general audience the results of recent studies on heredity and develop- ment, with special reference to their application to man, the au- thor has had to choose between simplicity and sufficiency of state- ment, between apparent dogmatism and scientific caution, between a popular and a scientific presentation. These are hard alterna- M 2714 vl Preface tives, but the first duty of a lecturer is to address his audience and to make his subject plain and interesting, if he can, 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 objects, 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 glossary at the end of the volume may rob these unfamiliar terms bf many of their terrors. The first three chapters of this book appeared in Popular Sci- ence Monthly, June to November 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 illus- trations are from many sources: *Figures 9, 10, 19, 20, 22, 23, 26-29, 35, 40-43, 51-54, 58, 72-75, 77, 83 are original; 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 remaining figures, namely Figures 48, 50, 63-69, 71, 76, 78-80, 84-96 were copied with little or no modification from various papers, photographs and books, the original source being given in each case. The writer wishes to express 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. E. Castle ond Dr. J. H. McGregor for the use of photographs which are reproduced in Figures Si, 82 and 96; and I wish especially to thank my as- sistant, Marguerite Ruddiman, for her aid in preparing figures and manuscript for publication. Princeton, December, 1914 * These figure numbers apply to the first and second editions but not to later ones. 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 expressed and the introduc- tion of considerable new material which will serve, it is hoped, to further elucidate some of the questions discussed, as well as to give results of more recent work. In particular Figs. 4, 5, 9, 10, 1 8, 19 have been added and besides minor corrections and addi- tions to all the chapters considerable changes have been made in Chapter III, section on Maternal Inheritance, Chapter IV, sec- tion on Inheritance or Non-Inheritance of Acquired Characters, and Chapter V, sections on Artificial Selection, Origin of Muta- tions, Past Evolution of Man, and Eugenics. January, 1916. PUBLISHER'S NOTE. — In the second edition, second printing, August, 1917, the text was corrected by the author so as to embody some of the results of the latest investigations in this subject. PREFACE TO THIRD EDITION The exhaustion of previous editions of this book and the necessity of re-setting the whole of it presents the opportunity for a rather thorough revision of the entire work. In addition to minor changes which have been made in all the chapters there has been a rearrangement and enlargement of the material of the chapter on the "Cellular Basis of Heredity and Development" and since this is the most technical chapter in the book it is placed after the chapter on "Phenomena of Inheritance" which deals with subjects which are more familiar to the average reader. Although the cellular phenomena of heredity are less familiar and more technical than other aspects of this subject they cannot be omitted or slighted if one wishes to understand the most recent and sig- nificant discoveries in this field. Figures and descriptions have been added of typical cells, of cell division, of the origin and ma- turation of the germ cells, of sex determination and especially of the mechanism of heredity in that most famous of all objects for the study of inheritance, the fruit fly Drosophila melanog aster. The author is indebted to Professor Morgan and his associates for permission to use certain figures from their books and papers on this subject. In this edition a much stronger position has been taken for the "chromosomal theory" of heredity than in former editions, for this theory is now so well established that it deserves a promi- nent place in even an elementary book. In spite of these additions the object of keeping the presenta- tion as simple as possible has been adhered to and those who de- sire a more complete account should consult the "Mechanism of Mendelian Inheritance" by Morgan and his associates, or "Ge- netics in Relation to Agriculture" by Babcock and Clausen. September, /p/p. PUBLISHER'S NOTE. — The second printing of this edition (March 1920) has been corrected and revised in a few places. viii PREFACE TO FOURTH EDITION I had intended that the previous revision of this book should have been the last, but I cannot resist the opportunity, which is again offered me by the necessity of reprinting it, to make cer- tain corrections and alterations, and in particular to refer to some recent discoveries of great importance. Knowledge is advancing so rapidly in the field of Genetics that it is not possible to keep such a book as this strictly up to date, but at least it should record the golden milestones that are passed. Most of these additions will be found in the chapter on the Cellular Basis of Heredity and Development; others deal with the Phenomena of Heredity, In- ternal Secretions, the Inheritance of Acquired Characters, and Mutations ; while minor changes occur in all the chapters. I am especially indebted to Dr. Theophilus S. Painter for the drawing of human chromosomes shown in Figure 56, and to him and to Professor Michael F. Guyer for information regarding the number of chromosomes in man. To Dr. Calvin B. Bridges and to Professor T. H. Morgan I am under obligations for infor- mation embodied in the new map of Drosophila chromosomes shown in Figure 67, and I am particularly indebted to Professor Morgan for his kindness in loaning me the figures of Drosophila mutants which are reproduced in Figures 101-103. Finally I am glad to thank publicly my colleague Professor George H- Shull for many valuable criticisms and suggestions. Princeton, December, 1921. PREFACE TO FIFTH EDITION The present edition of this book follows so soon after the previous one that few changes have been made except in the figures illustrating the text. Figures 26, 28, 35, 36, 37, 38, 39, 40, 4ia 47, 61, 62, 66, 74, 82, 86, 87, 95, 96, 97, 98, 99, 104 of the previous edition have been remade of replaced by new figures. Again I am especially indebted to Prof esor Morgan for his kind- ness in furnishing figures 61, 62, and 66, and for many valuable suggestions. Woods Hole, August, 1922. CONTENTS CHAPTER I. FACTS AND FACTORS OF DEVELOPMENT INTRODUCTION A. PHENOMENA OF DEVELOPMENT I. DEVELOPMENT OF THE BODY 1. The Germ Cells 2. Fertilization 3. Cleavage 4. Embryogeny 5. Organogeny 6. Oviparity and Viviparitjr 7. Development of Functions II. DEVELOPMENT OF THE MIND 1. Sensitivity 2. Reflexes, Tropisms, Instincts 3. Memory 4. Intellect, Reason 5. Will 6. Consciousness 7. Parallel Development of Body and Mind B. FACTORS OF DEVELOPMENT 1. Preformation 2. Epigenesis 3. Endogenesis and Epigenesis 4. Heredity and Environment CHAPTER II. PHENOMENA OF INHERITANCE A. OBSERVATIONS ON INHERITANCE Individuals and their Characters Hereditary Resemblances and Differences I. HEREDITARY RESEMBLANCES I. Racial Characters xii Contents 2. Individual Characters a. Morphological Features b. Physiological Peculiarities c. Teratological and Pathological Peculiarities d. Psychological Characters II. HEREDITARY DIFFERENCES 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 Inheritance" 2. Galton's "Law of Filial Regression" C. EXPERIMENTAL STUDY OF INHERITANCE I. MENDELISM 1. Results of Crossing Individuals with one pair of contrast- ing characters Other Mendelian Ratios 2. Results of Crossing Individuals with more than one pair of contrasting characters Dihybrids and Trihybrids 3. Inheritance Formulae 4. Presence and Absence Hypothesis 5. Summary of Mendelian Principles a. The Principle of Unit Characters b. The Principle of Dominance c. The Principle of Segregation II. MODIFICATIONS AND EXTENSIONS OF MENDELIAN PRINCIPLES 1. The Principle of Unit Characters and Inheritance Factors 2. Modifications of the Principle of Dominance 3. The Principle of Segregation "Blending" Inheritance Maternal Inheritance III. MENDELIAN INHERITANCE IN MAN Contents xiii CHAPTER III. CELLULAR BASIS OF HEREDITY AND DEVEL- OPMENT A. INTRODUCTORY 1. Definitions 2. The Transmission Hypothesis 3. Germinal Continuity and Somatic Discontinuity 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 c. The Maturation Period C. SEX DETERMINATION 1. Chromosomal Determination 2. Environmental Influence D. THE MECHANISM OF HEREDITY I. THE SPECIFICITY OF GERM CELLS II. CORRELATIONS BETWEEN GERMINAL AND SOMATIC ORGANIZATION 1. Nuclear Inheritance Theory 2. Linkage of Characters and Chromosomal Localization a. Sex Linked Inheritance b. Other Cases of Linkage 3. Cytoplasmic Inheritance a. Polarity b. Symmetry c. Inverse Symmetry d. Localization Pattern E. THE MECHANISM OF DEVELOPMENT 1. The Formation of Different Substances 2. Segregation and Isolation of Substances in Cells a. By Protoplasmic Movements b. By Differential Cell Divisions xiv Contents CHAPTER IV. INFLUENCE OF ENVIRONMENT A. RELATIVE IMPORTANCE OF HEREDITY AND ENVIRON- MENT 1. Former Emphasis on Environment 2. Present Emphasis on Heredity 3. Both Indispensable to Development B. EXPERIMENTAL MODIFICATIONS OF DEVELOPMENT I. DEVELOPMENT STIMULI 1. Physical Stimuli 2. Chemical Stimuli II. DEVELOPMENTAL RESPONSES Dependent upon (a) Nature of Organism, (b) Nature of Stimulus, (c) Stage of Development 1. Modifications of Germ Cells before Fertilization 2. During Fertilization 3. After Fertilization C. FUNCTIONAL ACTIVITY AS A FACTOR OF DEVELOP- MENT D. INHERITANCE OR NON-INHERITANCE OF ACQUIRED CHARACTERS E. APPLICATIONS TO HUMAN DEVELOPMENT: EUTHEN- ICS CHAPTER V. CONTROL OF HEREDITY : EUGENICS A. DOMESTICATED ANIMALS AND CULTIVATED PLANTS I. INFLUENCE OF ENVIRONMENT IN PRODUCING NEW RACES II. ARTIFICIAL SELECTION 1. The Methods of Breeders 2. How has Selection acted? III. METHODS OF MODERN GENETICS 1. Mendelian Association and Dissociation of Characters 2. Mutations 3. Causes of Mutation B. CONTROL OF HUMAN HEREDITY I. PAST EVOLUTION OF MAN II. CAN HUMAN EVOLUTION 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 Contents xv III. EUGENICS 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. THE VOLUNTARISTIC CONCEPTION OF NATURE AND OF HUMAN RESPONSIBILITY II. THE MECHANISTIC CONCEPTION OF NATURE AND OF PERSON- ALITY 1. The Determinism of Heredity 2. The Determinism of Environment III. DETERMINISM AND RESPONSIBILITY 1. Determinism not Fatalism 2. Control of Phenomena and of Self 3. Birth and Growth of Freedom 4. Responsibility and Will 5. Our Unused Talents 6. Self Knowledge and Self Control IV. THE INDIVIDUAL 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 Hicrhest Ethical Obligation REFERENCES GLOSSARY INDEX CHAPTER I FACTS AND FACTORS OF DEVELOPMENT CHAPTER I FACTS AND FACTORS OF DEVELOPMENT INTRODUCTION Man's Place in Nature. — One of the greatest results of the doc- trine 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 developed, 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 mammae, 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 special divine fiat, it was possible to main- tain that man was absolutely distinct from the rest of the animal world and that he had no kinship to the beasts, though undoubt- edly he was made in their bodily image. But with the establish- ment of the doctrine of organic evolution this resemblance be- tween man and the lower animals has come to have a new sig- nificance. The almost universal acceptance of this doctrine by scientific men, the many undoubted resemblances 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. Oneness of All Life- — But wholly aside from the doctrine of 3 4 Heredity and Environment - 'evolution,' tk£ fact that essential and fundamental resemblances exist among all kinds of organisms can not fail to impress thoughtful men. Life processes are everywhere the same in prin- ciple, though varying greatly in detail. All the general laws of life which apply to animals and plants apply also to man. This is no mere logical inference from the doctrine of evolution, but a fact which has been established by countless observations and experiments. The essential oneness of all life gives a direct hu- man interest to all living things. If "the proper study of man- kind is man," the basic 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 experi- mentation. Upon this fundamental likeness between the life processes of man and those of other animals are based the won- derful advances in experimental medicine, which may be counted among the greatest of all the achievements of science. Control of Development and Evolution. — The experimental study of heredity, development and evolution in forms of life be- low man must certainly increase our knowledge of and our con- trol 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 better- ment and race improvement can compare for thoroughness, per- manency of effect, and certainty of results, with that which at- tempts 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 nature, 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 toward the improve- ment of the human race is consciously and purposively to apply to man those great principles of development and evolution which have been at work, unknown to man, through all the ages. Facts and Factors of Development A. PHENOMENA OF DEVELOPMENT Ontogeny and Phytogeny. — One of the greatest and most far- reaching themes which has ever occupied the minds 'of men is the problem of development. Whether it be the development of an animal from an egg, 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 same in fundamental principles, and knowledge gained in one of these fields must be of value in each of the others. Ontogeny and phylogeny are not wholly distinct phenomena, but are only two aspects of the one general process of organic development. The evolution 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 familiarity with the fact of development does not remove the mystery which lies back of it, though it may make plain many of the processes 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 apparent; its principal steps or stages are known for thousands of animals and plants; even the precise manner of development and its factors 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 human development to show that in all essential respects it resembles that of other animals, and that the problems of heredity and differentiation are fundamentally the same in man as in other animals. 6 Heredity and Environment I. DEVELOPMENT OF THE BODY The entire individual — structure and functions, body and mind — develops as a single indivisible unity, but for the sake of clarity it is desirable to deal with one aspect of the individual at a time. For this reason we shall consider first the development of the body, and then the development of the mind. I. The Germ Cells. — In practically all animals and plants in- dividual development begins with the fertilization of a female sex cell, or egg, by a male sex cell, or spermatozoon. The epigram of Harvey, "Omne v'wum ex ovo" has found abundant confirma- tion in all later studies. Both egg and spermatozoon. are alive and manifest all the general properties of living things. How little this fact is appreciated by the public is shown by the repeated announcements of the newspapers that someone who has made an egg develop without fertilization "has created life." An egg or a spermatozoon is as much alive as is any other cell; as character- istically alive as is the adult animal into which it develops. What is Life? — It is difficult to define life, as it is also to define 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 complex animal, from the microscopic egg or spermatozoon to the adult man, manifests the following distinctive properties : (a) Protoplasmic and Cellular Organization. — It contains pro- toplasm, "the material basis of life," which is composed of the most 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 being the nucleus and the cell-body (Fig. I.) The nucleus is a central rounded body usually denser than the .surrounding cytoplasm from which it is separated by a thin mem- brane. It contains granules or threads of a substance which has Facts and Factors of Development B FIG. i. TYPICAL TISSUE CELLS FROM DIFFERENT ORGANS. A, Epithelial cells from intestine of duck embryo, showing nuclei with dark chromatic masses and clear achromatin surrounded by nuclear membrane, centrosomes as two or three dark granules at the free border of the cells, and cytoplasm filling the cell body. B. Two nerve cells, a. of a ringed worm, b. of a fish, showing cell-body containing nucleus and nucleolus, many neurofibrils and in a one nerve fiber, in b many processes, one of which (-J-) is the nerve fiber. C. Muscle cell from a round worm showing nucleus and cytoplasm in upper part of cell and contractile fibrils in lower part. 8 Heredity and Environment a strong chemical affinity for certain dyes and hence is called chromatin; it also frequently contains one or more rounded bodies which look like little nuclei and are called nucleoli. The chromatin and nucleoli are imbedded in a substance which does not stain readily with dyes and which is therefore called achro- matin. Surrounding the nucleus is the substance of the cell-body or cytoplasm and in this the various products of differentiation such as muscle or nerve fibrils, secretion products and food sub- stances are found. The cytoplasm often contains also a centro- some which is a deeply-staining granule surrounded by radiating lines and which is an organ for causing intra-cellular move- ments, especially in connection with the division of the nucleus and cell body. The nucleus and cytoplasm also contain more or less water and inorganic salts, and all of these things taken to- gether constitute what is known as protoplasm (Fig. i). 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 that are fixed and stable in character, but they are systems that are undergoing continual change. They are like the river, or the whirlpool, or the flame, which are never at two consecutive 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 equilibrium are : (b) Metabolism, or the transformation of matter and energy within the living thing in the course of which some substances are oxidized into waste products, with the liberation of energy, while other substances are built up into protoplasm, each part of every cell converting food substances into its own particular substance by the process of assimilation. Facts and Factors of Development 9 (c) Reproduction, or the capacity of organisms 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. Memb FlG. 2. A NEARLY RlPE HUMAN OVUM IN THE LlVING CONDITION. The ovum is surrounded by a series of follicle cells (FC) inside of which is the clear membrane (Memb.) and within this is the ovum proper containing yolk granules (Y) and a nucleus (N) embedded in a clear mass of proto- plasm. Magnified 500 diameters (x 500). (From O. Hertwig.) B, two human spermatozoa drawn to about the same scale of magnification. (After G. Retzius.) io Heredity and Environment Germ Cells Alive. — 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 organism, they differ from other cells in that they are capable under suitable conditions of pro- ducing these structures and functions by the process of develop- ment or differentiation, in the course of which the general struc- tures and functions of the germ cells are converted into the spe- cific structures and functions of the mature animal or plant. Gametes and Zygotes. — In both plants and animals the sex cells are alike in many respects, though they differ greatly in ap- pearances. The female sex cells of animals are called ova, the male cells spermatozoa. Corresponding male -and female sex cells are found in plants also. 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 con- tains more or less food substance 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 microscopic eggs of oysters, worms, etc., which may be no more than a few thou- sandths of a millimeter in diameter. The human ovum (Fig. 2) is microscopic in size (about 0.2 mm. in diameter) but it is not as small as is found in many other animals. It has all the char- acteristic parts of any egg cell, and can not be distinguished micro- scopically from the eggs of several 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 pro- duced by any other individual. Facts and Factors of Development 1 1 The sperm, or male gamete, is among the smallest of all cells and is usually many thousands 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 spermatozoon swims forwards 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 appearance, and there is every reason to believe that the spermatozoa of each species are peculiar in cer- tain respects even though we may not be able to distinguish any structural difference under the microscope. The human sperma- tozoa (Fig. 3) 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 usually broken off and left outside (Fig. 4). About the time of the entrance of the spermatozoon into the egg the latter divides twice, giving off two minute cells known as polar bodies which lie at the upper or animal pole of the egg. The nucleus in the head of the sperm, after it has entered the egg, begins to absorb material from the egg and to grow in size and at the same time a minute granule, the H FIG. 3. Two HUMAN SPERMATOZOA. A, showing the side of the flattened head; B, its edge; H, head; M, middle-piece; T, tail. (After G. Retzius.) 12 Heredity and Environment FIG. 4. FERTILIZATION OF THE EGGS OF STAR-FISH AND SEA-URCHIN. A-C, Successive stages in the entrance of a spermatozoon into the egg of the star-fish, Asterias glacialis. Only one sperm has penetrated the jelly layer (//) which surrounds the egg and the peripheral protoplasm (pp) of the egg protrudes as an entrance cone (ec) to meet it. (After Fol.) D, Mature spermatozoon of the sea-urchin, Toxofrneustes, showing head (A) ; middle-piece (m) ; and tail (f). E-H, Successive stages in the pene- tration of the sperm nucleus ( $ N) and centrosome (#C) into the egg of Toxopneustes. I-L, Stages in the approach of the sperm nucleus ($N) to the egg nucleus ($AO, and in the division of the sperm centrosome ( $ C) and the formation of the first cleavage spindle. (D-L after Wilson.) Facts and Factors of Development 13 . \ 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 cen- trosome then approach the egg nucleus and ultimately the two nuclei come to lie side by side (Fig. 4). Usually when one spermatozoon has entered an egg all others are barred from en- tering, probably by some change in the surface layer of the egg or in the chemical substances given out by the egg. Oosperm or Zygote a Double Being. — 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 begins, immediately after fertilization, a long series of transformations and differentiations of the ferti- lized egg which leads to the development of a complex animal, even of a person. In the fusion of egg and sperm a new individual, 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 of every cell, caused by the union of egg and sperm, is never lost. A New and Distinct Individual. — 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 practi- cally 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 evident that the new individual comes into being with the fertilization of the egg; rather the moment of birth, or separation from the mother, is generally looked upon as the beginning of the individual's existence. And yet in all cases Heredity and Environment FIG. 5. FIRST CLEAVAGE OF THE EGG OF THE SEA-URCHIN, Echinus micro tubcrculatus. A, Nuclear division figure, or mitotic spindle, with a centro- some at each pole and with the chromosomes from the egg and sperm nuclei at the equator of the spindle. B and C, Later stages showing the separation of the daughter chromoses from one another and their move- ment toward the two poles of the spindle. D and E, Still later stages showing the swelling of the chromosomes and their fusion to form nuclear vesicles. F, Complete division of egg into two cells, each containing one daughter nucleus and centrosome. (After Boveri.) Facts and Factors of Development 15 the new individual is always distinguishable 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 protoplasm, not a nerve fiber, not a blood vessel passes over from the mother to the embryo ; the latter is from the mo- ment of fertilization onward a distinct individual with particu- lar 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 development begins. •The fertilized egg of a star-fish or frog or man is not a dif- ferent 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 protoplasm from food substances ; it receives food and oxygen from without and it gives out carbonic acid and other waste products; it is sensitive to certain alterations in the environment such as ther- mal, chemical and electrical changes — it is, in short, a distinct living thing, an individual or person. Under proper environmen- tal conditions this fertilized egg cell develops, step by step, with- out 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, or a man. 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 activi- ties, reactions, instincts, habits, and intelligence of the mature animal. Is not this miracle of development more wonderful than any possible miracle of creation? And yet as one watches this mar- vellous process by which the fertilized egg grows into the embryo, and this into the adult, each step appears relatively simple, each 16 Heredity and Environment perceptible change is minute ; but the changes are innumerable and unceasing and in the end they accomplish this miracle of trans- forming 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. Cell Division. — After fertilization the first step in devel- opment is the cleavage or division of the egg. This is in the main like any typical cell division and since the details of this process are of extraordinary interest in the study of the mechan- ism of heredity and development it is desirable to give at once a rather detailed account of the way in which the nucleus and cell-body divide. a. Mitosis or Indirect Division of the Nucleus. — It was once supposed that both the nucleus and the cell-body divide by a simple process of constriction or direct division, but it is now known that the nucleus rarely divides in this manner, and that the nuclei of germ cells never do so. On the contrary the nu- cleus almost always divides by a complex process known as mitosis or indirect division (Figs. 6 and 7). During this process the chromatin granules of the "resting" nucleus become arranged in lines like beads on a thread (Fig. 8) ; these threads, which are called chromosomes, are at first long and slender and much coiled, but afterwards they grow shorter, thicker and straighter and it can then be seen that in each species of animal or plant there is a definite and constant number of these threads or chro- mosomes (Fig. 6, A-D) ; this number varies from 2 to 200 in dif- ferent species of animals, the most usual number being some- where between 10 and 30, but so far as is known each species has a constant number of chromosomes in every cell of the body. The nucleolus and the nuclear membrane then disappear, the chromosomes move into the equator of the cell forming the equa- torial plate (Fig. 6, F) and each one splits lengthwise into two Facts and Factors of Development 17 daughter chromosomes which move apart toward the two poles of the cell (Fig. 7, G, H) where all the daughter chromosomes come together to form the two daughter nuclei. The cell body then divides by a process of constriction into two daughter cells (Fig. 7, /, /)• The formation, splitting and separation of the chromosomes is the most constant and characteristic feature of indirect nuclear division, but there are other important features which must now be mentioned. In all animals and in many of .the lower plants there is present in the cell-body just outside the nuclear mem- brane a small deeply-staining granule, the centrosome, which is usually surrounded by radiating lines. In the early stages of mitosis this granule divides into two which move apart until they come to lie on opposite sides of the nucleus (Fig. 6, A-C). When the nuclear membrane dissolves the radiating lines which sur- round these two centrosomes increase greatly in length forming two asters and those rays which run through the nuclear area constitute a spindle with the chromosomes in its equator and the centrosomes at its two poles (Fig. 6, D-F). Later the chromo somes move along the spindle toward its poles where the daugh- ter nuclei are formed. The centrosomes, asters and spindle, known collectively as the, amphiaster, constitute an apparatus for the accurate separation of the daughter chromosomes and for the division of the cell-body. The chromosomes are most compact and deeply-staining at the metaphase or equatorial plate stage of division; after they have moved to the poles of the spindle they begin to absorb achromatin from the surrounding plasma thus swelling up and becoming chromosomal vesicles with clear contents and chromatic walls (Fig. 8, E, F). These vesicles then continue to enlarge and their chromatin takes the form of threads or granules. After the formation of the daughter nuclei the different vesicles are so closely pressed together that it is usually impossible to see the partition walls between them; however in several different ani- i8 Heredity and Environment mals and plants the chromosomal vesicles are recognizable even in the resting nucleus (Fig. 8, G), and in every organism the same number of chromosomes, having the same relative shapes and FIG. 6. FIGS. 6, 7. DIAGRAMS OF SUCCESSIVE STAGES IN THE DIVISION OF A CELL BY MITOSIS. A, Cell with "resting" nucleus and centrosome (c) ; B-E, Early stages of division during which the chromatin takes the form of short thick threads, the chromosomes (C, D, E) while the centrosome gives 'Facts and Factors of Development 19 sizes, come out of a nucleus at the time of division as went into it at the preceding mitosis, each new chromosome coming out of a chromosomal vesicle (Fig. 8, A\ B, C). Whenever one can trace FIG. 7. rise to a spindle (a) with astral radiations at its poles; F, Middle stage of division in which the chromosomes lie in the equator of the spindle forming. the equatorial plate (ep) ; G, H, Stages showing the splitting of each chromosome and the movement of the halves toward the poles of the spindle; ep, Equatorial plate; if, Interzonal filaments; n, Nucleolus: I, Complete separation of daughter chromosomes and formation of daugh- ter nuclei; beginning of division of cell body; /, Complete separation of daughter cells and return of nucleus and centrosome to resting condi- tion. (After Wilson.) 20 Heredity and Environment FIG. 8. SUCCESSIVE STAGES OF MITOSIS IN THE CLEAVAGE OF THE EGG OF A FISH (Fundulus) showing that new chromosomes are formed inside of old ones (chromosomal vesicles), that chromosomes or chromosomal vesicles persist from one division to the next and that even the "resting" nucleus is composed of chromosomal vesicles. A, nu- cleus at beginning of mitosis, having shrunk from dotted outline and showing chromo- somal vesicles containing chromatin granules; B, each chromosomal vesicle contains granules or chromosomeres which are condensing to form a chromosome; C, amphiaster showing faint outlines of the chromosomal vesicles with their cntained chromosomes-; D, amphiaster showing each chromosome beginning to split and the chromosomes divid- ing; E, late phase of division showing daughter chromosomes at the poles of the spindle and each chromosome becoming vesicular; F, still later phase, each chromosome a vesicle containing chromatin granules; G, daughter nucleus showing chromosomal vesicles containing scattered chromatin granules. (After Richards.) Facts and Factors of Development 21 individual chromosomes or chromosomal vesicles through the resting stage it is certain that every chromosome preserves its individuality or identity, and even where this cannot be done the fact that the same number of chromosomes, having the same peculiarities of shape and size come out of a nucleus as went into it, is evidence that here also each chromosome has preserved its identity. b. Cleavage of the Egg. — After the, entrance of the spermato- zoon into the egg the sperm nucleus moves toward the egg nu- cleus until the two meet when they divide by mitosis (Figs. 4 I-L, 5 A-F). 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 (Fig. 4). The chromatin, or stainable substance of the egg and sperm nuclei, takes the form of threads or chromosomes (Fig. 5). Each chromosome then splits lengthwise, its two halves moving to opposite ends of the spindle, where the daughter chro- mosomes 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 divisions, each of them essentially like the first, by which the egg is sub- divided successively into a constantly increasing number of cells. During the earlier divisions there is little or no increase in the volume of the egg, consequently successive generations of cells continually grow smaller (Figs. 9-11). This process is known as the cleavage of the egg, and by it the egg is not only split up into a considerable number of small cells, but a much more im- portant 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 different kinds of substance, and thus to differ from one another. The differentiations of the cleavage 22 Heredity and Environment A B FIG. 9. SUCCESSIVE STAGES IN THE CLEAVAGE AND GASTRULATION OF Am- phioxus. A, one cell;; B, two cells; C and D, four cells; E, eight cells; F, sixteen cells; G, blastula stage of about ninety-six cells; H, section through the same showing the cleavage cavity; /, 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; /, section through the same showing these three types of cells; K and L, successive stages in the infolding of the endo- derm ; cells indicated as in the preceding figure. The polar body is shown at the upper pole, a, anterior ; p, posterior ; v, ventral ; d, dorsal ; be, blas- toccel; gc, gastroccel. Facts and Factors of Development 23 cells appear much earlier in some forms than in others, but in all cases such differentiations appear during early or late cleavage (Figs. 9-1 1 ). 4. Embryogeny. — From this stage onward the course of de- velopment 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. 9, u, //), and this in turn be- comes a gastrula (Figs. 9, n, 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 nearest the lower pole ; a middle layer, or group of cells, the mesoderm, is formed from cleavage cells which in vertebrates lie between the upper and lower poles (Fig. u, 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 appear. From the ectoderm are formed the outer layer of the skin and the whole nervous system ; from the endoderm arise the lining of the alimentary canal and its outgrowths ; from the mesoderm come, in whole or in part, the skeletal, muscular, vascular, excretory, and reproductive systems. In vertebrates the nervous system appears as a plate of rather large ectoderm cells (Fig. n, n) ; this plate rolls up at its sides to form a groove and then a tube ; and by en- largement of certain portions of this tube and by foldings and thickenings of its walls the brain and spinal cord are formed (Fig. n, K, L; 13, C, D). The retina or sensory portion of the eye is formed as an outgrowth from the fore part of the brain (Fig. 13, D) ; the sensory portion of the ear comes from a cup- shaped depression of the superficial ectoderm which covers the hinder portion of the head (Fig. 13, E and F). The back-bone begins to appear as a delicate cellular rod (Fig. n, c), which then in higher vertebrates becomes surrounded successively by a Heredity and Environment .V FIGS. 10, ii. DIAGRAMS OF FROG'S EGGS SHOWING THE RELATIONS OF THE AXES AND SUBSTANCES OF THE EGG TO THE AXES AND PRINCIPAL ORGANS OP THE EMBRYO. All eggs viewed from right side, polar bodies above; A, anterior; P, posterior; D, dorsal; V, ventral; s, spermatozoon; $N, sperm nucleus, $N, 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; area around polar bodies will form ecto- derm of skin ; sc, segmentation cavity ; bp, blastopore ; E, enteron ; M, region of mouth. Facts and Factors of Development FIG. 10. SURFACE VIEWS OF ENTIRE EGGS. A, before entrance of sper- matozoon; B, just after entrance of sperm; C, union of egg and sperm nuclei ; D, 2-cell stage ; £, 4-cell stage ; F, 8-cell stage. FIG. ii. SECTIONS IN MEDIAN PLANE OF EMBRYOS. G, i6-32-cell stage; H, blastula; /, early gastrula; /, late ga&trula; K, early embryo, L, late embryo. 26 Heredity and Environment fibrous, a cartilaginous, 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 gradually become more complicated by a process of growth and differentiation, until these embryonic organs assume more and more the mature form. 6. Oviparity and Viviparity. — This very brief and general statement of the manner of embryonic development applies to all vertebrates, man included. There are many special features of human development which are treated at length in works on em- bryology, but which need not detain us here since they do not affect the general principles of development already outlined. In one regard the development of the human being or of almost any mammal is apparently very different from that of 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 eggs are laid, and there are mammals (monotremes) which lay eggs, while in others (marsupials) the young are born in a very imperfect condition. Mother always Distinct from Child. — These facts indicate that Facts and Factors of Development 27 there is no fundamental difference between oviparity and vivi- parity. 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 sup- FIG. 12. DIAGRAMS SHOWING THE EARLY DEVELOPMENT OF THE HUMAN OOSPERM. A, cleavage stage which has just come into the uterus; B and C, blastodermic vesicles embedded in the mucous membrane of the uterus ; D, E and F, longitudinal sections of later stages, the anterior and poster- ior poles being marked by the axis a p. In C cavities have appeared in the ectoderm, endoderm and mesoderm. D, villi forming from the trophoblast (nutritive layer, tr} ; black indicates ectoderm (ect) ; oblique lines, endoderm; few stipples, mesoderm; V, villi; am, amnion; ys, yolk sac; n, neurenteric canal; x 25. (After Keibel.) 28 Heredity and Environment FIG. 13. A-H, successive stages in the early development of the human embryo; A, blastodermic vesicle showing primitive axis in embryonic area, age unknown; By blasto- dermic vesicle attached to uterine wall at the posterior pole, showing neural groove, age unknown; C, later stage in which the neural folds are closing and five pairs of somites have appeared, age ten to fourteen days; D, stage of fourteen somites showing enlarge- ments of the neural folds at the anterior end which will form the brain, age fourteen to sixteen days; E 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, em- bryo >of thirty-six somites showing nasal pit, eye, branchial arches and clefts, limb buds and heart. (After Keibel.) Facts and Factors of Development A B 29 FIG. 14. A, human embryo of forty-two somites, age twenty-one days; B, embryo of about four weeks; C, still older embryo showing the begin- nings of the formation of digits; D, embryo of about two months; C and D are drawn on a smaller scale than A and B. (After.Keibel.) 30 Heredity and Environment posed "maternal impressions" of the physical, mental, or emo- tional 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 influences 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 are the same in oviparous and in viviparous animals — in fishes, frogs, birds and men. Summary. — This is a very brief and incomplete 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 struc- tures of the developed 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 division many cells which dif- fer from one another more and more as development proceeds, until finally the adult animal results. A specific type of develop- ment is due to a specific organization of the germ cells with which development begins, but the earlier differentiations of the egg are relatively few and simple as compared with the bewildering com- plexities of the adult, and the best way of understanding 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 development 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., (i) Construc- tive 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 gen- eral functions of living things are manifested by germ cells, but Facts and Factors of Development 31 as development advances each of these functions becomes more specialized, more complicated and more perfect. A cell which at an early stage was protective, locomotor and sensory in function may give rise to daughter cells in which these functions are dis- tributed to different cells; cells which at an early stage were sensitive to many kinds of stimuli give rise to daughter cells which are especially sensitive to one particular kind of stimulus, such as vibration, light, or chemicals. Differentiation of Functions and Structures. — Functions de- velop from a generalized to a specialized condition by the process of "physiological division of labor" which accompanies morpho- logical division of substance. But just as in the union of hydro- gen and oxygen a new substance, water, appears which was not present before, by a process of "creative synthesis," and as in the development of structures new parts appear, which were not present in the germ, so new functions appear in the course of de- velopment, which are not merely sorted out of the general func- tions present at the beginning, but which are created by the in- teraction and synthesis of parts and functions previously present. For example, Lane has shown that young rats are quite insensitive to light until several days after birth although the eye begins to form at a very early stage of development. Doubtless every part of the eye is functioning in one way or another during the entire development but not until all parts are formed and connected and all their functions are synthesized 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 present and integrated, when they sud- denly appear. Living Functions and Structures Inseparable. — Much less at- tention has been paid to the development of functions than to the development 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 de- velopment of particular functions the general fact of such devel- 32 Heredity and Environment opment is well established. 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 depends the fate of certain important theories, for example Lamarckism; but this problem can be solved only by thorough- going experimental and analytical work. In the meantime it seems safe to conclude that living structures and functions are insep- arable and that anything 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 morpholo- gists and physiologists only as a matter of convenience. 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 modern biological work such as bacteriology, cytology, and ge- netics, many functional distinctions are recognizable between or- ganisms that are 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 micro- scopist" could discover a corresponding change of structure. II. DEVELOPMENT OF THE MIND The development of the mind parallels that of the body : what- ever 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 is surely no more disturbing to philosophical and religious beliefs than the germinal origin of the individual, and yet the latter is a fact of Facts and Factors of Development 33 universal observation which can not be relegated to the domain of hypothesis or theory, and which can not be successfully denied. If we admit the fact of the development of the entire individual, surely it matters little to our philosophical or religious beliefs to admit the development or evolution of the race. Ancient Speculations. — The origin of the mind, or rather of the 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 Pythagoras and Plato. According to these teachings, 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 gener- ally 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 Tertul- lian taught that souls of children are generated from the souls of parents as bodies are from bodies. This doctrine, which is known as traducianism, has been defended by certain modern theolo- gians, 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 doctrines named, but it is based upon the prevalent 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 34 Heredity and Environment p. 125). Consequently it is not possible to hold that bodies gen- erate 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. Certainty of Mental Development. — No fact in human exper- ience is more certain than that the mind develops by gradual and natural processes from a simple condition 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, embryos 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 Shakes- peare 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. Wonders of this Development. — The development of the mind from the activities of the germ cells is certainly most wonderful 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 supernatur- ally created than that the body is. Indeed, we know that the mind is formed by a process of development, and the stages of this de- velopment are fairly well known. There is nowhere in the en- tire course of mental development a sudden appearance of psychi- cal 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 gen- Facts and Factors of Development 35 erally similar to the reactions of the germ cells and embryos of other animals, and to the behavior of many lower organisms. Matter and Mind. — A few years ago such a statement would have been branded as "materialism" and promptly rejected with- out 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 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 individual, mind and body, with the devel- opment 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 for- gotten 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 beginnings of mind in the germ are associated with the beginnings of the body, and both of these tenets are incontrovertible. Body and Mind. — It seems to me that the mind is related to the body as function is to structure; there are those who main- tain that structure is the cause of function, that the real problem in evolution or development is the transformation of one struc- ture into another, and that the functions which go with certain structures are merely 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 corre- sponding transformations of structure. Among adherents of the former view may be classed many morphologists and Neo-Darwin- ians ; 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 entire organism, 36 Heredity and Environment structure as well as function; that neither of these precedes the other as cause precedes effect, though each may modify or condi- tion 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 organization 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 be traced back to certain fundamental structures 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 necessary characteristics of living things. In the simplest proto- plasm 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 germinal or in its adult stage. GERMINAL BASES OF MIND 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 comparisons between human development and 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 psy- chical processes of organisms other than ourselves we are com- pelled to rely upon a study of their activities, their reactions to Facts and Factors of Development 37 stimuli, since we can not approach the subject in any other way. The reactions and behavior of organisms under normal and ex- perimental conditions give the only insight which we can get into their psychical processes ; and this applies to men no less than to protozoa. i. Sensitivity. — The most fundamental phenomenon in the be- havior of organisms is irritability or sensitivity, which is the abil- ity of receiving and responding to stimuli : this is one of the fun- damental properties of all protoplasm. But living matter is not equally sensitive to all stimuli, nor to all strengths of the same stimulus. Many of the simplest unicellular plants and animals show that they are differentially sensitive ; they often move toward weak light and away from strong light, away from extremes of heat and cold, into certain chemical substances and away from others; in short, all organisms, even the simplest, may respond differently to different kinds of stimuli or to different degrees of the same stimulus. This is what is known as differential sensitiv- ity (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 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 distinguishing between different kinds of stimuli is far from perfect. Sensitivity of Germ Cells. — Egg cells and spermatozoa show this property of sensitivity. The egg is generally incapable of lo- comotion, and since the results of stimulation must usually be detected by movements it is not easy to determine to what extent the egg is sensitive ; but though the egg lacks the power of locomo- tion, it possesses in a marked degree the power of intra-cellular movement of the cell contents. When a spermatozoon comes into contact with the surface of the egg the cortical protoplasm of the egg flows toward that point and may form a cone or pro- toplasmic prominence into which the sperm is received (Fig. Heredity and Environment 4, ec). It is an interesting fact that the same sort of response 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; spermatozoa are strongly stimulated by weak alkali and alcohol, they gather in certain chemical substances and not in others, they collect in great numbers around fertilizable egg cells, etc. Sensitivity of Oosperm and of Embryo. — 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 internal stimuli and by non- localized external ones. Modifications of the external stimuli often lead to modifications of these intracellular movements and to abnormal types of cleavage and development — in short, these movements show that the fertilized egg is differentially sensitive. In the further course of development particular portions of the embryo become especially sensitive to some kinds of stimuli, while other portions become sensitive to others. In this way the differ- ent sense organs, each especially sensitive to one particular kind of stimulus, arise from the generalized sensitivity of the oosperm, and thus general sensitivity, which is a property of all protoplasm, *B 0 1> tf&J1 9 FIG. 15. DISTRIBUTION OF BACTERIA IN THE SPECTRUM. 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 Jennings, after Engelmann.) Facts and Factors of Development 39 becomes differential sensitivity and special senses in the process of embryonic differentiation. Such sensitivity is the basis of all psychic processes ; sensations are the elements of the mind. 2. Reflexes, Tropisms, Instincts— A\\ the responses of germ FIG. 16, a, b, c. REPULSION OF Spirilla BY COMMON SALT, a, condition immediately after adding crystals ; b and c, later stages in the reaction. x, y, z, repulsion of Spirilla by distilled water. The upper drop consists of sea-water containing Spirilla, the lower drop, of distilled water. At x 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.) Heredity and Environment cells, and of the simplest organisms, to stimuli are in the nature of reflexes or tropisms, that is, relatively simple, machine-like responses. "Reflex motions" originally referred to those re- sponses of higher animals in which the peripheral stimuli were reflected, as it were, from the spinal cord to the appropriate muscles without the participation of the brain. But at present the word "reflex" has come to have a much broader application and is used for all simple, automatic responses, even though there are no nerves and even when the response is not movement but secretion, metabolism or any other activity. "Tropisms," on the other hand, is a more specific term and refers to the movements of organisms 19 19* 38 Q 10* 25* FIG. 17. REACTIONS OF Paramecium TO HEAT AND COLD. At a the in- fusoria 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 Jennings, after Mendelssohn.) Facts and Factors of Development 41 toward or away from a source of stimulus, the former being known as positive and the latter as negative^ tropism. When re- sponses are very complex, one response calling forth another and involving many complex reflexes or chains of reflexes, as is fre- FIG. 18. PHOTOTROPISM OF SEEDLING OF WHITE MUSTARD supported by a sheet of cork (K, K} floating on the water. The direction from which light comes is shown by the arrows ; the stem and leaves are turned toward the light (positive phototropism), the root away from the light (negative phototropism). (After Strasburger.) 42 Heredity and Environment quently the case in animals, they are known as "instincts." Re- flexes and tropisms occur in the simplest organisms, such as bac- teria, protozoa, ancf single cells, as well as in higher plants and animals (Figs. 15-19), but instincts are limited to animals with a nervous system. Reflexes and Tropisms of Germ Cells and Embryos are seen in movements of spermatozoa, movements of the protoplasm in egg cells and embryonic cells, movements of cells and cell masses in the formation of the gastrula, alimentary canal, nervous system and other organs. Indeed the entire process of development, whether accompanied by visible movements or not, may be re- garded as a series of automatic responses to stimuli. When the embryo becomes differentiated to such an extent as to have spe- cialized organs for producing movement its capacity for making responsive movements to stimuli becomes much increased. In the embryo the rhythmic contractions of heart, anmion and intestine are early manifestations of reflex motions. These appear chiefly in the involuntary muscles before nervous connections are FIG. 19. GEOTROPISM OF SEEDLING OAK. After starting to grow with the axis A-A 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 (positive geotropism) as indicated by dotted outlines. Facts and Factors of Development 43 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 impulses. Tropisms are manifested only by organisms capable of considerable free move- ment and hence are absent in the foetus though present in many free-living larvae. Development of Instincts. — Some instincts are present imme- diately after birth, such as the instinct of sucking or crying in the human infant, 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 consequently we might expect that they would in general make for the preservation of the species ; as a matter of fact we usually find that they are remarkably adapted to this end. For instance the instincts of the human infant to grasp objects, to suck things which it can get into its mouth, to cry when in pain, are compli- cated reflexes which have survived in the course of evolution, probably becauSfe they serve a useful purpose. Very much has been written on the nature and origin of in- stincts, 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 ontogenetically and phylo- genetically, under the stress of the elimination of the unfit, so that they are usually adaptive. 3. Summation of Stimuli, 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 stimulus still persists, the response to the second stimulus may be 44 Heredity and Environment 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 is called forth, but if they be stroked a second time within three minutes the leaf in- stantly closes. If a longer period than three minutes elapses after the first stimulus and before the second no visible response fol- lows, i.e., two successive stimuli are necessary to cause the leaves to close, and the two must not be more than three minutes apart; the effects of the first stimulus are in some way stored or registered in the leaf for this brief time. This kind of phenome- non is widespread among living things and is known as "summa- tion of stimuli." In all such cases the effects of a former stimu- lus 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 formation FIG. 20. Dionaea muscipula (VENUS' FLY-TRAP). Three leaves showing marginal teeth and sensitive hairs (SH). The leaf at the left is fully expanded, the one at the right is closed. Facts and Factors of Development 45 of some chemical substance which remains in the protoplasm for a certain time, during which iime 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. Organic Memory. — 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 accurately than an untrained muscle; and the nervous mechanism through which the stimulus is transmitted also becomes trained in the 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 difficult art of talking, is probably due to the persistence in mus- cles and nerves of the effects of many previous activities. All such phenomena were called by Hering "organic memory," to in- dicate 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 com- monly call memory. Associative Memory. — It seems probable that this ability of pro- toplasm 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, develops associative and conscious memory in the ma- ture organism. 4. Intellect, Reason. — Even the intellect and reason which so strongly characterize man have had a development from rela- 46 Heredity and Environment tively simple beginnings. All children come gradually to an age of intelligence and reason. In its simpler forms at least intelligence is the capacity of consciously profiting by experience, while reas- oning consists in the comparison of past experiences with new and more or less different phenomena. In the absente of indi- vidual experience young children have none o*f this power, but it comes gradually as a result of remembering past experiences and of fitting such experiences into new conditions. Useful Responses.— Young infants and many lower animals lack intelligence or reason, though their behavior is frequently of such a sort as to suggest that they are reasoning. Even the low- est 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 intelligent and pur- posive behavior has been shown to be due to the gradual elimina- tion of all sorts of useless activities, and to the persistence 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 ani- mal forward in a spiral course. However, when it is strongly ir- ritated, the normal forward movement is reversed; the cilia beat forward instead of backward and the animal is driven backward for some distance (Fig. 21, I, 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 (Fig. 21, 3, 4, 5, 6). These movements 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 modified by experience or physiological states. Para- mecium behaves as it does by virtue of its constitution, just as an egg develops in a particular way because of its particular organi- zation. "Trial and Error" — But although limited in its behavior to Facts and Factors of Development 47 these relatively simple motor reactions, Paramecium does many things which seem to show intelligence and purpose. It avoids many injurious substances, such as strong salts or acids, and it collects in non-injurious or beneficial substances, such as weak acids, masses of bacteria upon which it feeds, etc. It avoids ex- tremes of heat and cold and if one end of a dish containing Para- mecla 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 the behavior of sin- gle individuals, established the fact that this apparently intelli- gent action is due to differential sensitivity and to the single motor reaction of the animal. If in the course of its swimming a Para- mecium comes into contact with an irritating substance or con- dition, 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 in- definitely until finally a path is found in which the cause of irri- tation is avoided altogether. In short, Parameciutn continually tries its environment, and backs away from irritating substances or conditions. Its apparently intelligent reactions are thus ex- plained as due to a process of "trial and error."* The behavior of worms, star-fishes, crustaceans, mollusks, as well as of fishes, frogs, reptiles, birds and mammals, has been studied 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 * 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 responses 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." 48 Heredity and Environment gradual elimination of useless responses and the preservation (or remembering) of useful ones the behavior may come to be pur- posive and intelligent. Intelligence Develops from Trial and Error. — Thorndike found that when dogs, cats or 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. Thereafter 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 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. FIG. 21. DIAGRAM OF THE AVOIDING REACTION OF Paramecium. A is a solid object or other source of stimulation. 1-6, successive positions oc- cupied by the animal. The rotation on the long axis is not shown. (After Jennings.) Facts and Factors of Development 49 There is every evidence that human beings arrive at intelli- gence 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 Parameciuni ? Is not intelligence and reason in all of us, and upon all subjects, based upon the same processes of trial and error, memory of past experiences and application of these to new conditions? Surely this is true in all experimental and scientific work. Indeed the scientific method is the method of trial and error, and finally trial and success — the method recom- mended by St. Paul to 'prove all things and hold fast that which is good/ Learning by Experience. — 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 associa- tive memory is but slightly developed and the animal learns but little by experience and can make no application of past ex- periences 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 experiences to new conditions. In adult man all of these processes are fully devel- oped and particularly 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 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 appli- 50 Heredity and Environment cation of past experience to new conditions, 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 undergoes development and from relatively simple beginnings. The will of the child has developed out of something which is far less perfect in the infant and embryo than in the child. Obser- vations and experiments on lower animals and on human beings, as well as introspective study of our own activities, appear to justify the following conclusions: (i.) No Activity Without Stimuli. — Every activity of an organ- ism 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 pos- sible number of responses are few and prescribed owing to their relative simplicity, and the response follows the stimulus direct- ly. In more complex organisms 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.) Inhibitions. — The response to a stimulus may be modified or inhibited in the following ways: (a) Conflicting Stimuli. — 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 con- flict they may nullify one another or the organism may respond to the strongest stimulus and disregard the weaker ones. When an organism has begun to respond to one stimulus it is not easily diverted to another. Jennings found that the attached infusor- ian, Stentor, which usually responds to strong stimuli by closing up, may, when repeatedly stimulated, loosen its attachment and swim away, thus responding in a wholly new manner when its physiological state has been changed by repeated stimuli and re- Facts and Factors of Development 51 sponses. Whitman found that 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 aqua- rium with leeches, they at once leave the shade and attach them- selves to the turtle. They prefer shade to bright light but they prefer their food to the shade. The tendency to remain 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) Compulsory Limitations. — Responses may also be modi- fied through compulsory limitation of many possible responses to a particular one, and the consequent formation of a habit. This is the method of education employed in training all sorts of ani- mals. 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. (3) Fixed and Plastic Behavior. — Responses which have be- come fixed and constant through natural selection or other means of limitation may become more varied and general when the com- pulsory limitation is relaxed. Behavior in the former case is fixed and instinctive, in the latter more varied and plastic. Thus Whit- man found that the behavior of domesticated pigeons is more var- iable and their instincts are less rigidly fixed than in wild species. If the eggs of the wild passenger pigeon are removed to a little distance from the nest the 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 52 Heredity and Environment a few minutes without heeding the eggs. The ring-neck pigeon also misses the eggs and sometimes 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 uni- formity of action, with little or no choice, to a stage of less rigid uniformity .... Under conditions of domestication the action of natural selection 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 opportunities and provocations arise, and thus the in- ternal 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 encapsuled in the brain. . . . But increased plasticity invites great- er interaction of stimuli and gives more even chances for con- flicting impulses. (4) Conscious Choice or Will. — Finally in all animals behavior is modified through previous experience, just as structure is also. Where several responses to a 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 compulsion 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 com- pulsory education. The teleological alternatives are organically provided; one is taken and fails to give satisfaction, another is tried and gives contentment. This little freedom is the dawning Facts and Factors of Development 53 grace of a new dispensation, in which education by experience comes in as an amelioration of the law of elimination. . . . Intelli- gence 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 intelli- gence as additional stimuli. The activities which in lower animals are "cabined, cribbed, confined," reach in man their fullest and freest expression ; but the enormous difference between the rela- tively fixed behavior of a protozoan or a germ cell and the rela- tively free activity of a mature man is bridged not only in the process of evolution, but also in the course of individual develop- ment. 6. Consciousness. — The most complex of all psychic phenomena, indeed the one which includes many if not all of the others, is consciousness. Like every other psychic process this has under- gone development in each of us ; we not only came out of a state of unconsciousness, but through several years we were gradually 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 unconscious or sub- conscious states, and complete consciousness must depend upon the complete development and activity of the other faculties, par- ticularly associative memory and intelligence. Germ Cells Not Conscious. — The question is sometimes asked whether germ cells, and indeed all living things, may not be con- scious in some vague manner. One might as well ask whether water is present in hydrogen and oxygen. Doubtless the elements out of which consciousness develops 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 elements or fac- 54 Heredity and Environment tors, which by a long series of combinations and transformations, due to interactions with one another and with the environment, give rise to the fully developed condition. Continuity of Consciousness. — Finally there seems good rea- son for believing that the continuity of consciousness, the con- tinuing sense of identity, is associated with the continuity of or- ganization, for in spite of frequent changes of the materials of which we are composed our sense of identity remains undis- turbed. However, the continuity of protoplasmic and cellular organization generally remains undisturbed throughout life, and the continuity of consciousness is associated with this continuity of organization, especially in certain parts of the brain. It is an interesting fact that in man, and in several other animals which may be assumed to have a sense of identity, .the nerve cells, espe- cially those of the brain, cease dividing at an early age, and these identical cells persist throughout the remainder of life. If nerve cells continued to divide throughout life, as epithelial cells do, there would be no such persistence of identical cells, and one is free to speculate that in such cases there would be no persistence of the sense of identity. Organization includes both structure and function, and con- tinuity of organization implies not only persistence of protoplas- mic and cellular structures but also persistence of the functions of sensitivity, reflexes, memory, instincts, intelligence, and will ; the continuity of consciousness is associated with the continuity of these activities as well as with the structures of -the body in gen- eral and of the brain in particular. It is well known that things which interrupt or destroy these functions or structures interrupt or destroy consciousness. Lack of oxygen, anesthetics, normal sleep cause in some way a temporary interruption of these func- tions and consequently temporary loss of consciousness; while certain injuries or diseases of the brain which bring about the destruction of certain centers or association tracts may cause permanent loss of consciousness. Facts and Factors of Development 55 7. Parallel Development of Body and Mind. — The development of all of these psychical faculties runs parallel with the develop- ment of bodily structures and apparently the method of develop- ment in the two cases is similar, viz., progressive differentiation of complex and specialized structures and functions from relative- ly 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 phenomena, may be con- sidered from several different points of view, such as (i) physico- chemical events involved, (2) physiological processes, (3) mor- phological features, (4) ecological correlations and adaptations, (5) psychological phenomena, (6) social and moral characteris- tics. 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 belief that each of the succeeding aspects of devel- opment named above may be causally explained in terms of the preceding ones, and hence all be reducible to physics and chem- istry. 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 development 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. The phenomena of mental development in man and other ani- mals may be summarized as follows: Heredity and Environment DEVELOPMENT OF PSYCHICAL PROCESSES IN ONTOGENY AND PHYLOGENY OUT OF GENERAL IRRITABILITY OR SENSITIVITY — CAPACITY OF RESPONDING TO STIMULI ALL LIVING THINGS, INCLUDING GERM CELLS AND EMBRYOS, SHOW: 1. Differential Sensitivity = Different Responses to Stimuli differing in Kind or Quantity. 2. Reflexes, Tropisms = Relatively Simple, Mechanical Responses. 3. Organic Memory = Results of Previous Experience registered in General Proto- plasm. 4. Adaptive Responses = Results of Elimination of Useless Responses through Trial and Error. 5. Varied Responses = Dependent upon Conflicting Stim- uli and Physiological States. 6. Identity = Continuity of Individual Organi- zation. 7. Subjective Phenomena, if any, Accompanying preceding pro- cesses. MATURE FORMS OF HIGHER ANI- MALS SHOW: i. Special Senses and Sensations = Differentiated out of General Senses and Sensations. 2. Instincts (Inherited), Habits (Acquired) = Complex Reflexes, involving Nerve Centers. 3 Associative Memory •= Results of Experience registered in Nerve Centers and Associa- tion Tracts. 4. Intelligence, Reason = Results of Trial and Error plus Associative Memory, i.e., Ex- perience. 5. Inhibition, Choice, Will = Dependent upon Associative Memory, Intelligence, Reason. 6. Consciousness = Continuity of Memory, Intelli- gence, Reason, Will. 7. Feelings, Emotions — Accompanying one or more of pre- ceding processes. B. FACTORS OF DEVELOPMENT These are some of the facts of development, — a very incom- plete resume 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 consciousness develop out of Facts and Factors of Development 57 the relatively simple psychical elements of the germ? These are some of the great problems of development — one of the greatest and most far-reaching themes which has ever occupied the minds of men. i. Pre formation. — When the mind is once lost in the mystery of this ever-recurring miracle it is not surprising to find that there have been those who have refused to believe it possible and who have practically denied development altogether. The old doc- trine 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 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 supposed to be due to the smaller size of the parts and to their greater transparency, and with poor micro- scopes and good imaginations some observers thought they could see the little animal 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 development, 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 natural develop- ment rather than by supernatural creation. If every individual 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 individuals was created at once. Thus arose the theory of "emboitement," or infinite encasement, the absurdities of which * The little plant in the seed is itself the product of the development 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. 58 Heredity and Environment contributed to the downfall of the entire doctrine of preformation, which, in the form given it by many naturalists of the eighteenth century, is now only a curiosity of biological literature. 2. Epigenesis. — 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 di- rect observation of development, and which maintained that the germ contains none of the adult parts, but that it is absolutely simple and undffferentiated, and that from these simple begin- nings the individual gradually becomes complex by a process of differentiation. We owe the "theory of epigenesis at least so far as its main features are concerned, to William Harvey, the dis- coverer of the circulation of the blood, and to Caspar Friedrich Wolff, whose doctoral thesis, published in 1759 and entitled "Theoria Generationis" marked the beginning of a great epoch in the study of development. Wolff demonstrated 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, homo- geneous and undifferentiated, and that differentiation and or- ganization gradually appear in this undifferentiated substance. How to get differentiations 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 develop- ment altogether; epigenesis recognized the fact of development, but attributed it to mysterious and purely hypothetical external forces ; the one placed all emphasis upon the germ and its struc- tures, the other upon outside forces and conditions. 3. Endogenesis and Epigenesis. — Modern students of develop- ment recognize that neither of these extreme views is true — adult Facts and Factors of Development 59 parts are not present in the germ, nor is the latter homogeneous — but there are in germ cells many different structures and func- tions which are, however, very unlike those of the adult, and by the transformation and differentiation of this germinal organi- zation the complicated organization of the adult arises. Develop- ment is not the unfolding of an infolded organism, nor the mere sorting of materials already present in the germ cells, though this does take place, but rather it consists in the formation of new materials and qualities, of new structures and functions — by the combination and interaction of the germinal elements present in the oosperm. In similar manner the combination and interaction of chemical elements yield new substances and qualities which are not to be observed in the elements themselves. Such new sub- stances and qualities, whether in the 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 "crea- tive 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 substances" 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. Increasing complexity, which is the essence of development, is caused by the combina- tion and interaction of germinal substances under the influence of the environment. The organization of the oosperm may be compared to the arrangement of tubes and flasks in a complicated chemical operation ; they stand in a definite relation to one another and each contains specific substances. The final result of the operation depends not merely upon the substances 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 condi- 60 Heredity and Environment tions represented by temperature, pressure, moisture or other extrinsic factors. 4. Heredity and Environment. — Unquestionably 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 en- vironmental factors exercise chiefly a stimulating, inhibiting or modifying influence on development. In the same dish and un- der similar environmental conditions, one egg will develop into a worm, another into a sea urchin, another into a fish, and it is certain that the different fate of each egg is determined by con- ditions 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 function is determined by the organism, though it may be modified by environment, so the character of development is determined by heredity, i.e., by the organization of the germ cells, though the course and results of development may be modi- fied by environmental conditions. SUMMARY In conclusion, we have briefly reviewed in this chapter the well known fact that every living thing in the world has come into existence by a process of development ; that the entire human per- sonality, mind as well as body, has thus arisen; and that the factors of development may be classified as intrinsic in the organi- zation of the germ cells, and extrinsic as represented in environ- mental forces and conditions. The intrinsic factors are those which are commonly called heredity, and they direct and guide development in the main ; the extrinsic or environmental factors furnish the conditions in which development takes place and they modify, more or less, its course. CHAPTER II PHENOMENA OF INHERITANCE CHAPTER II PHENOMENA OF INHERITANCE A. OBSERVATIONS ON INHERITANCE The observations of men for ages past have established the fact that in general "like produces like," and that, in spite of many exceptions, 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 gen- eral 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 constit- uent characters; either aspect is a true one and yet neither is complete in itself. Formerly in discussions on heredity the individual was regarded 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 dif- ferences led to endless confusion. In heredity no less than in anatomy it is necessary to deal with the constituents of organ- isms; in short, the organism must be analyzed and each part studied by itself. Method of Gallon and Mendel. — Francis Galton was one of the 63 64 Heredity and Environment first to bring order out of chaos by dealing with traits or char- acters singly instead of treating all together. He made careful studies on the inheritance 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 Galton was thus laying the foundations for a scientific study of heredity by dealing with characters separately, another and an even greater student of heredity, Gregor Mendel, 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 supposed that such characters could exist independently of other characters and apart from the entire organism. This is such a self-evident 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 analyze or- ganisms 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 organism may be analyzed show a greater or smaller degree of resemblance to the corre- sponding characters of its parents. Whenever the differential cause of a character is a germinal one the character is, by defini- tion, inherited ; on the other hand, whenever this differential cause is environmental the character is not inherited. While it J*s 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 plainly seen when, in any character, a child resembles a grandparent or a Phenomena of Inheritance 65 more distant ancestor more than either (parent. Sometimes actually new characters arise in descendants which were not present in ascendants, but which are thereafter inherited. Ac- cordingly inherited characters may be classified as resemblances and differences, though both are determined by germinal organi- zation, or heredity. There is therefore no fundamental difference between inherited similarities and dissimilarities. Heredity and variation are not opposing nor contrasting tendencies which make offspring like their parents in one case and unlike them in an- other; really inherited characters may be like or unlike those of the parents. 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 dis- tinguish between hereditary and environmental resemblances and differences, but among men where experiments are generally out of the question it is often difficult or impossible to make this dis- tinction. I. HEREDITARY RESEMBLANCES 1. Racial Characters. — All peculiarities which are characteris- tic of a race, species, genus, order, class and phylum are of course inherited, otherwise there would be no constant characteristics of these groups and no possibility of classifying organisms. The chief characters of every living thing are unalterably 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 known spe- cies of animals and plants differ from one another because of inherited peculiarities, because they have come from different kinds of germ cells or protoplasm. 2. Individual Characters. — Many characters which are pecu- liar to certain individuals are known to be inherited, and in gen- 66 Heredity and Environment eral use the word "inheritance" refers to the repetition in suc- cessive generations of such individual peculiarities. Among such individual characters 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, etc., of each and every part. The number of such individual peculiarities which are inherited is innumerable and only a few of the more striking of these can be mentioned. It is a matter of common knowledge that unusually great or small stature runs in certain 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 (Fig. 25). However, his statistical and mathematical formulae 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 chin 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 occur, and these trifling peculiarities are inherited 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 each race and were of course inherited. Jennings has found races of Parame- cium which show hereditary differences of .005 mm. in average length (Fig. 22). Nettleship says that the lens of the human eye Phenomena of Inheritance 67 weighs only 175 milligrams, or about one three-millionth part of the body weight, and in hereditary cataract only about one twen- tieth part of the lens becomes opaque, and yet this minute frac- tion of the body weight shows the influence of heredity. Even the size, shape and number of the cells in certain organs, and in given embryonic stages, may be repeated generation after gener- ation; and if our analysis were sufficiently complete we should doubtless find that even the minute parts of cells, such as nuclei, chromosomes and centrosomes, show individual peculiarities which are inherited. (b) Physiological peculiarities are inherited as well as morpho- 105 43 FIG. 22. DIAGRAM OF EIGHT DIFFERENT RACES OF Paramedum, each hori- zontal row (A-H) representing a single race. The individual showing the mean size in each race is indicated by -\- ; the mean of all the races is shown by the line X-X. The numbers are the lengths in micra (thousandths of a millimeter), X 43- (After Jennings.) 68 Heredity and Environment logical ones; indeed function and structure are only two aspects of one and the same thing, namely organization. For all morpho- logical characters there are functional correlatives, for functional characters morphological expressions, and if the one is inherited so is the other. But there are certain characters in which the physiological aspect is more striking than the morphological one. Longevity. — 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 inherited 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 squash bug, are as surely the result of inherited causes as are the structural differences be- tween these animals. Within the same species different races or lines show characteristic 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. Other Functional Characters. — Obesity is another physiological characteristic 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. Haemophilia, or excessive bleeding after an injury, which is due to a deficiency 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-handed- ness, a peculiar lack of resistance to certain diseases, and many other physiological peculiarities are probably inherited. Phenomena of Inheritance 69 (c) Pathological Peculiarities are really only unusual or abnor- mal anatomical or physiological characters, but they are of such interest and importance as to deserve special mention. Many such abnormalities are undoubtedly inherited, among which are the following : polydactylism, in which more than the normal number of digits are present (Fig. 38) ; syndactylism, or a condition of webbed fingers and toes ; brachydactylism, in which fingers are short and stumpy and usually contain less than the normal num- ber of joints (Fig. 39) ; achondroplasy, or short and crooked limbs, such as occur in certain breeds of dogs and sheep and in certain human dwarfs ; myopia, in which the eyeball is elongated ; glaucoma, or pressure within the eyeball ; coloboma, or open su- ture 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 monstrous conditions are due to abnormal environment and are not inherited. Are Diseases Inherited. — The question of the inheritance of diseases may be briefly considered here. If a disease is due to some defect in the hereditary constitution, it is inherited ; other- wise, according to our definition of heredity, it is not. Of course no disease develops without extrinsic causes but when one indi- vidual 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 certain diseases run in families and have the ap- pearance of being inherited, but in this case as in many others it is extremely difficult in the absence of experiments to distinguish between effects due to intrinsic causes and those due to extrinsic ones. Where the specific cause of disease is some micro-organism the individual must have been infected at some time or other, al- most invariably after birth. In few instances is the oosperm itself infected, and even when it is, this is not, strictly speaking, a case 70 Heredity and Environment of inheritance, but rather one of early infection. Leo Loeb has shown that cancer is inherited in mice and Little finds that there is inheritance of a predisposition to cancer in man. Pearson has found that there is a marked correlation (represented by the num- ber .55 when complete correlation is I.) between tuberculous parents and tuberculous 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, 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 predisposing causes, are hereditary. (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 morphological and physiological characters applies also to psychological ones. No one doubts that particular instincts, aptitudes and capacities 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 ter- rier, the collie and many others are characterized by peculiarities of temperament, affection, intelligence and disposition. No one who has much studied the subject can doubt that different human races and families show characteristic differences in 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. 22). The fact is that racial characteristics are not determined by excep- Phenomena of Inheritance 71 tional and extreme individuals but by the average or mean quali- ties of the race; and measured in this way there is no doubt that certain types of mind and disposition are characteristic of certain families. There is no longer any question that some kinds of f eeble-mind- edness, epilepsy and insanity 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 denied that strength or weakness of will, a tendency to moral obliquity or rectitude, capacity or incapacity for the highest in- tellectual pursuits, occur frequently in certain families and ap- pear to be inherited. In spite of certain noteworthy exceptions, which may perhaps be due to remarkable variations, statistics col- lected by Galton show that genius runs in certain families ; while the work of some recent investigators, particularly Goddard, Davenport and Weeks, proves that f eeble-mindedness and epilepsy are also inherited; and the careful work of Mdtt and of Rosanoff indicates that certain types of insanity are hereditary. On the other hand, Cotton maintains that mental disorders are not di- rectly inherited, but that "there is probably a constitutional lack of resistance to various toxins and poisons, and not an inherited mental instability, which causes the mind to break down under mental stress and strain." It frequently happens that families in which hereditary insanity occurs also have other members afflicted with epilepsy, hysteria, alcoholism, etc., which seem to indicate that the thing inherited is an unstable condition of the nervous system which may take various forms under slightly different conditions. Indeed there is a good deal of evidence that extraordinary ability, or genius is frequently associated with an unstable nervous organization which sometimes takes 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." 72 Heredity and Environment Woods has collected data concerning "Heredity in Royalty" which seem to show that very high or low grades of intellect and morality may be traced through the royal families of Europe for several generations. Extensive study of certain families in which an extraordinary number of feeble-minded, degenerate, and crim- inal individuals have appeared, seems to demonstrate that moral and social qualities are also inherited. One recalls in this con- nection the famous, or rather infamous, "Jukes", "Kalikaks", "Nams", and "Ishmaels", — these names being pseudonymns for notoriously bad families whose traits have been followed through several generations. The general tendency of recent work on heredity is unmistak- able, whether it concerns man or lower animals. The entire or- ganism, consisting of structures and functions, body and mind, develops out of the germ, and the organization of the germ deter- mines all the possibilities of development of the mind no less than of the body, though the actual realization of any possibility is dependent also upon environmental stimuli. II. HEREDITARY DIFFERENCES There are many exceptions to the general rule that children resemble their parents ; indeed no child is ever exactly like a par- ent and the points in which they differ are known generally as variations. These variations are of two kinds, those which are caused by a different germinal constitution and are therefore in- herited and those due to environmental differences which are not inherited. Sometimes inherited variations are due to new com- binations of ancestral characters, sometimes they are actually new characters not present so far as known in any of the ancestors, though even such new characters must arise from new c'ombina- tions of the elements of old characters, as we shall see later. i. New Combinations of Characters. — In all cases of sexually produced organisms new combinations of ancestral characters are evident. Usually a child inherits some traits from one parent Phenomena of Inheritance 73 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 de- scribed by Galton as "particulate" (Fig. 23) and is known today as "Mendelian." As we shall see later (p. 88 et seq.) this is prob- ably the only kind of inheritance. 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 improb- able; such inheritance he called "alternative"* (Fig. 23). In other cases the traits of the parents appear to blend in the offspring, as for example, in the skin color of mulattoes; such BLENDING ALTERNATIVE PARTICULATE FIG. 23. DIAGRAM TO ILLUSTRATE THREE KINDS OF INHERITANCE described by Galton. Only the last of these (particulate) really occurs. (After Walter.) cases were called by Galton "blending" inheritance (Fig. 23). Such cases of blending inheritance are now known to be the result of particulate inheritance of many factors (p. 108). Some- times characters appear in offspring which were "latent" in the parents but were "patent" in one or more of the grandparents ; * It is necessary to distinguish between alternative inheritance of a single character (Mendel) and this supposed alternative inheritance of all char acters (Galton), 74 Heredity and Environment such skipping of a generation, during which a character re- mains "latent," has long been known as "atavism." At other times characters which were present in distant ancestors, 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 appear only in the male sex, others only in the female, this being called "sex-limited" in- heritance ; 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 distribution and combination of ancestral characters new and unexpected characters sometimes develop 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 variations have been called by Bateson "meristic." However, sudden variations may include any marked departure from the normal type, in color, shape, size, chemical composition, 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 in- clined to assign much importance to them in the general process of evolution. Galton, on the other hand, maintained that varia- tions, or what would now be called "continuous variations," can- not be of much significance in the process of evolution, but that the case is quite different with "sports" ("Hereditary Genius," prefatory chapter). More recently the entire biological world has been greatly influ- *See p. 187. Phenomena of Inheritance 75 enced by the "Mutation Theory" of deVries, which has placed a new emphasis upon the importance of sudden variations in the process of evolution. At first deVries was inclined to emphasize the degree of difference, that is the discontinuity, in these varia- tions, 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 Flucttuftionis. — This clear cut distinction be- tween mutations and fluctuations marks one of the most impor- tant advances ever made in the study of development and evolu- tion. Thousands of fluctuations occur which are purely somatic in character and which do not affect the germ cells, for everv 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 environment ( fluctuations )^ and those due to hereditary causes (mutations) was recognized by Weismann and many of his followers, but the actual demon- stration 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 organization of the germ cells, just as do in- herited resemblances. Heredity is not to be contrasted with var- iation, nor are hereditary likeness and unlikeness due to con- flicting principles; both are the results of germinal organization and both are phenomena of heredity. 4. Every Individual Unique. — As a result of the permutations of ancestral characters, the appearance of mutations, and the 76 Heredity and Environment fluctuations of organisms due to environmental changes, it hap- pens that in all cases offspring differ more or less from their par- ents 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 appears 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 constant, and even the compounds fall into definite categories which have constant char- acteristics. 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, — indeed lack of constancy is evidence in itself of lack of analysis into real ele- ments or of lack of uniform conditions, — but whatever its cause the extraordinary fact remains that every living being appears to be unique. • "Reproduction is the generation of unique beings that 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 organisms show, as well as all of their resem- blances, 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 attempted to reduce the mass of conflicting observations on heredity and variation to some system and to establish certain principles as a result^) f sta- tistical study. He was the real founder of the scientific study of inheritance ; he studied characters singly and he introduced quan- titative measures. Gallon's researches, which were published in several volumes, consisted chiefly in a study of certain families * See p. 229. Phenomena of Inheritance 77 with regard to several selected traits, viz., genius or marked in- tellectual capacity, artistic faculty, stature, eye color and disease. As a result of his very extensive studies two main principles ap- peared to be established : i. The Law of Ancestral Inheritance which he stated as fol- lows: 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 oh, the sum of the series 1/2 + 1/4 + 1/8 + 1/16 . . . being equal to I, 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 + !/8 + 1/16 + ...,1/4=1/8+ 1/16 + . . v and so on. 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 wiith respect to different qualities are also presumably eliminated. The average contribution of each ancestor was thus stated definitely, the contribution diminishing with the remoteness of the ancestor. This Law of Ancestral Inheritance is represented graphically in the accompanying diagram (Fig. 24). Pearson has proposed a Law of Reversion according to which the average re- version of offspring to each ascending generation of ancestors is represented by the series .3, .15, .075, .0375, etc. Ancestors and Contributors. — Theoretically the number of an- cestors doubles in each ascending generation ; there are two par- ents, four grandparents, eight great-grandparents, etc. If this continued to be true indefinitely the number of ancestors in any ascending generation would be (2)°, in 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)57 or about 120 quadrillions, — a number far greater than the entire human population of the globe since that time. As a matter of fact, Heredity and Environment owing to the intermarriage of cousins of various degrees the actual number of ancestors is much smaller than the theoretical number. For example, Plate says that the late Emperor of Ger- many, Wilhelm II, had only 162 ancestors in the loth ascend- ing generation, instead of 512, the theoretical number. Neverthe- less this calculation will serve to show how widespread our an- cestral 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 3Oth cousins, while most people are much more closely related than that. If we allow three genera- tions to a century, and calculate that the degree of cousinship is determined by the number of generations less two, since/ first cousins appear only in the third generation, the first being that of the parents and the second that of the sons and daughters, we find that 3Oth cousins at the present time would have had a com- mon ancestor about one thousand years ago or approximately at the time of William the Conqueror. As a matter of fact most per- cT 9 cT 9 d* 9 c r 9 c ? 9 c r 9 c? 9 d1 9 c? 9 c? 9 d- 9 J 9 j 9 c? 9 d1 ? rlr 4r s 4r £ 33 X I I -1 1 1 1 1 1 i Parents Grand Pta Gt Gd Pts (ft G\ Gd P\s FIG. 24. DIAGRAM OF GALTON'S "LAW OF ANCESTRAL INHERITANCE." The whole heritage is represented by the entire rectangle; 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.) Phenomena of Inheritance 79 sons 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. On the other hand we now know that we do not inherit equally from all our ancestors; on the average we inherit about as many traits from our fathers as from our mothers, but inheritance from 63 F.c. 25. SCHEME TO ILLUSTRATE GALTON'S "LAW OF FILIAL REGRESSION" 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 grpups 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.) 8o Heredity and Environment the four grand-parents is usually unequal and the farther back we go the more ancestors we find who have contributed nothing to our inheritance. Of all the thousands or even millions of an- cestors that each of us has had, only a relatively small number have contributed anything to our inheritance; although we are descended from all the others we are not related to them bio- logically and have received none of their traits. Those who have contributed to our . inheritance may be called "contributing an- cestors" or merely "contributors"* to distinguish them from non- contributing ones, and the fact that ancestors do not contribute equally to heredity disproves Galton's "law of ancestral inherit- ance." 2. The Law of Filial Regression is the second principle which Galton deduced from his statistical studies, or it may be called the tendency to mediocrity. He found that, on the average, ex- treme peculiarities 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 good fortune if he begets a son who is as richly en- dowed as himself." This so-called law of filial regression is represented graphically in Fig. 25 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. Statistical vs. Physiological Methods. — One of the chief aims and results of statistical 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 * I have adopted this term proposed by Dr. H. H. Laughlin in prefer- ence to "transmitters" which I had previously used. Phenomena of Inheritance 81 physiological, and averages are of less value in finding the causes of such phenomena than is the intensive study of individual families. By observation alone it is usually impossible to distinguish be- tween inherited and environmental resemblances and differences, and yet this distinction is essential to any study of inheritance. If all sorts of likenesses or unlikenesses are lumped together, whether inherited 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. Statistical Studies Insufficient. — Unfortunately Galton and Pearson, as well as some of their followers, have not always carefully distinguished between hereditary and environmental characters. Furthermore much of their material was drawn from a general population in which were many different fam- ilies and lines not closely related genetically. Consequently their statistical studies are of little value in discovering the physio- logical 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 is "hereditary or environ- mental, 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 (i) experiments in which the environment is kept as uniform as possible while the hereditary factors differ, and (2) experiments in which, in a series of cases, the hereditary factors are fairly constant while the environment differs. In this way the differential cause or causes of any character may be located in heredity, in environment or in both. 82 Heredity and Environment The observational and statistical study of inheritance helped to outline the problem but did little to solve it. Certain phenom- ena of hereditary resemblances between ascendants and descen- dants were made intelligible, but there were many peculiar and apparently irregular or lawless phenomena which could not be predicted before they occurred nor explained afterward. For example when Darwin crossed different breeds of domestic pigeons, no one of which had a trace of blue in its plumage, he sometimes obtained offspring with more or less of the blue color and 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 "reversions." He observed many cases in which certain characters of one parent prevailed over corresponding characters of the other parent in the offspring, this being known as "pre- potency"; but there was no satisfactory explanation of these curious phenomena. They did not come under either of Galton's "laws," and their occurrence was apparently so irregular that every such case seemed to be a law unto itself. C. EXPERIMENTAL STUDY OF INHERITANCE I. 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 independently an important principle of heredity and at the same time brought to light a long neglected and forgotten work on "Experiments in Plant Hybridization" by Gregor Mendel, in which this same principle was set forth in detail. This principle is now generally known as "Mendel's Law." Mendel, who was a monk, and later abbot, 0* the Konigskloster, an Augustinian monastery in Briinn, Moravia, published the results of his experiments on hybridization Phenomena of Inheritance 83 in the Proceedings of the Natural History Society of Briinn in 1866. The paper attracted but little attention at the time al- though it contained some of the most important discoveries re- garding inheritance which had ever been made, and it remained buried and practically unknown for thirty-five years. Plant hy- bridization jiad 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 re- sults of his work are so well known through the writings of Bate- son, Punnett, 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 count- ing the number of offspring in successive generations showing one or the other of these characters. Mendel's Experiments on Peas. — During the eight years pre- ceding the publication 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 the different varieties are distinguished by constant differentiating characters. Mendel devoted his attention to seven of these contrasting char- acters, which he followed through several generations of hybrids, viz., 1 i ) 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 yellow, orange or green. (3) Differences in the color of the seed coats (and in some oases 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. 84 Heredity and Environment (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. . (7) Differences in the length of the stem, whether tall or short. i. Results of Crossing Individuals with one Pair of Contrast- ing Characters. — Having determined that these characters were constant for certain varieties Mendel then proceeded to cross one variety with another, by carefully removing the unripe sta- mens, with their pollen, from the flowers of one variety and dust- ing 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 mentioned above, and then studied the offspring of several successive generations with respect to this character. Dominant and Recessive Characters. — In every case he discov- ered that the plants that developed from such a cross showed only one of the two contrasting characters of the parent plants, i.e.} all were round-seeded, yellow-seeded, or tall, etc., although one of the parents had wrinkled seeds, green seeds, or short stem, etc. "Those characters which are transmitted entire or almost unchanged in the hybridization are termed dominant, and those which become latent in the process, recessive!' Ratio of Dominants to Recessives. — These hybrids* when self- fertilized gave rise to a second filial generation of individuals some of which showed the dominant character and others the re- cessive, the relative numbers of the two being approximately three to one. Thus the hybrids produced by crossing yellow- * Bateson introduced the term "homo-zygote" for pure-bred individuals resulting from the union of gametes which are hereditarily similar, and "hetero-zygote" for hybrids resulting from the union of hereditarily dis- similar gametes. The gametes formed from a homo-zygote are all of the same hereditary type, those formed from a hetero-zygote are of two dif- ferent types for every unit difference of the parents. The members of a pair of contrasting characters are called "allelomorphs"; eacsh member of such a pair is "allelomorphic" to the other member. Phenomena of Inheritance 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. 26). The hybrids produced by crossing round and wrinkled seeded varieties yielded in the second filial genera- tion 5,474 round and 1,850 wrinkled seeds, or approximately three round to one wrinkled (Fig. 30). The hybrids from tall-stemmed and short-stemmed parents produced in the second filial genera- oo • • Parents FIG. 26. DIAGRAM SHOWING THE RESULTS OF CROSSING YELLOW-SEEDED (LIGHTER COLORED) AND GREEN-SEEDED (DARKER COLORED) PEAS. (From Morgan after Thompson.) 86 Heredity and Environment tion 787 long-stemmed and 277 short-stemmed plants, or again approximately three 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. "Extracted" Dominants or Recessives. — These recessives derived from hybrid parents are pure and are known as "ex- tracted" recessives; when self-fertilized they produce only re- cessives. One-third of the dominants are also pure homozy- gotes, or "extracted" dominants, and when self-fertilized pro- duce only pure dominants. On the other hand two-thirds of the dominants are heterozygotes and when self-fertilized give rise in the next generation to pure dominants, dominant-reces- sives and pure recessives in the proportion of 1:2:1. These general results are summarized in the accompanying diagram (Fig. 27) in which dominant characters are indicated by the letter D, recessive characters by R, and dominant-re- cessives, with the recessive character unexpressed, by D (R) ; while DD or RR indicate extracted dominants or recessives, that is, pure dominants or recessives which have separated out from dominant-recessives, D (R). The parental generation is usually indicated by the letter P, and the successive filial generations by Flf F2, F3, etc. Parent Generation r* • Homozygotea Helerozygotes Fa • FIG. 27. 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 dominant-recessives (heterozygotes) are represented by circles half white and half black. Successive generations are marked F,, F2, Flf etc. Phenomena of Inheritance 87 Incomplete Dominance. — In the case of the peas studied by Mendel the hybrids of the Ft generation show only the domi- nant 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 gener- ations split up into both parental types and into the hybrid type ; thus Correns found that when a white-flowered variety of Mira- bilis, the "four o'clock," was crossed with a red-flowered variety all of the hybrids in the FA generation had pink flowers and from these in the F2 generation there came white-flowered, pink-flow- FIG. 28. RESULTS OF CROSSING WHITE-FLOWERED AND RED-FLOWERED RACES OF Mirabilis Jalapa ("four o'clocks") giving a pink hybrid in Flf which when inbred gives in F0 i white, 2 pink, i red. The gametes bear- ing white or red are indicated by white or black circles. (From Woodruff, after Correns.) 88 Heredity and Environment ered and red-flowered forms in the proportion of I white : 2 pink: I red, as shown in Fig. 28. This is a better illustration of Mendel's principle of splitting than is offered by the peas, since in this case the heterozygotes D(R) are always distinguishable from the pure dominants DD. Results in Later Generations. — In the F2 generation and in all subsequent ones the pure dominants and the pure recessives al- ways breed true when self-fertilized, whereas the heterozygotes continue to split up in each successive generation into pure domi- nants, heterozygotes and pure recessive in the proportion of 1:2: i. The result of this is that with continued self-fertiliza- tion the relative number of dominants and recessives increases in successive generations, whereas the relative number of hetero- zygotes decreases, and in a few generations a hybrid race will revert in large part to its parental types if continued hybridiza- tion 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 generations; an equal number of pure dominants and pure recessives is produced in each generation (Fig. 27). "Pwity" of Germ Cells.— With remarkable insight Mendel recognized that the real explanation of the splitting of pure recessives and pure dominants from hybrid parents must be found in the composition 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 between 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 hy- brid. But the fact that one-quarter of the progeny of this hy- brid 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. Furthermore the fact that one-half the progeny of this hybrid are themselves hybrid Phenomena of Inheritance 89 * 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 al- ways 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. 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 individual 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 ex- plains not only the segregation of dominant and recessive charac- ters from a hybrid in which both are present, but also the relative numbers of pure dominants, pure recessives and heterozygotes in each generation. For if all germ cells are pure with re- spect to any particular character the hybrid offspring of any two parents with contrasting characters will produce in equal numbers two classes of germ cells, one bearing the dominant and the other the recessive factor, and the chance combination of these two classes of male and female 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, iDD : 2D(R) : iRR, as shown in the accompanying diagram and in Fig. 29 b. 9 germ cells D $ germ cells D Possible combinations I DD : 2D(R) : I RR. Heredity and Environment Other Mendelian Ratios. — When a pure dominant is crossed with a hybrid dominant-recessive (Fig. 29 r) all of 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, R repre- senting the factor for round seed and W that for wrinkled seed : $ germ cells R^ .R 1X1 a germ cells R ^W Possible combinations p a 2RR: Fi FIG. 29. DIAGRAM OF MENDELIAN INHERITANCE, in which the individual is represented by the large circle, the germ cells by the small ones, domi- nants being shaded and recessives white, a, Pure dominant X pure re- cessive — (yields) all dominant-recessives ; bf Dominant-recessive X domi- nant-recessive = i pure dominant : 2 dominant-recessives : I pure recessive ; c, Dominant-recessive X pure dominant = 2 pure dominant : 2 dominant- recessive; d, Dominant-recessive X pure recessive = 2 dominant-reces- sive : 2 pure recessive. Phenomena of Inheritance gi In subsequent generations the progeny of the pure round (RR) breed true and produce only round-seeded peas, whereas the pro- geny of the hybrid round-wrinkled (RW) split up into pure round, hybrid round-wrinkled, and pure wrinkled in the regular Mendelian ratio of i RR : 2 R(lfi") : I WW (Fig. 30). When a pure recessive is crossed with a hybrid dominant- recessive (Fig. 29, d) another typical ratio results. Thus if a wrinkled-seeded variety of pea is crossed with a hybrid between a round-seeded and wrinkled-seeded one, round-seeded and wrinkled-seeded peas are produced in the proportion of I : I 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: $ germ cells W. W 1X1 $ germ cells A W Possible combinations 2 R ( W) : 2 WW. This ratio of 2 : 2 or I : i is approximately the ratio of the two sexes in many animals and plants, and there is good reason to be- lieve that sex is a Mendelian character of this sort, in which one parent is heterozygous for sex and the other homozygous (See p. 167). 2. Results of Crossings where there is more than one Pair of Contrasting Characters. — It rarely happens that two individuals differ in a single character only; more frequently they differ in many characters, and this leads to a great increase in the number of types of offspring in the F2 generation. But however many pairs of contrasting characters the parents 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 accord- ing to the regular Mendelian formula given above. When the parents differ in one unit character only, the offspring formed by their crossing are called mono-hybrids, when there are Heredity and Environment two contrasting characters in the parents the offspring are di-hy- brids, when three, tri-hybrids, and when the parents differ in more than three characters the offspring are called poly-hybrids. 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 increases greatly when more than one or two characters are considered at the same time, it is cus- tomary to deal simultaneously with only one or two characters of hybrids, even though the parents may have differed in many characters. The different types of developed organisms are called by Johannsen "phenotypes" whereas the different hereditary types whether patent or latent are called "genotypes-" Dihybrids. — When two or more contrasting characters of the w FlG. 3O. MONOHYBRID DIAGRAM SHOWING RESULTS OF CROSSING ROUND- (/?) SEEDED WITH WRINKLED- (W) SEEDED PEAS. Large circles represent zygotes, small ones, or single letters, gametes. In Fl all individuals are round but contain round and wrinkled gametes. In F2 the $ gametes are placed above the square, the $ ones to the left, and the possible com- binations of $ and 9 gametes are shown in the small squares, the relative number of different genotypes being I RR : 2 R(W) :i WW. Phenomena of Inheritance 93 parents are followed to the F2 generation all possible 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 three genotypes and usually but two phenotypes in the GW FIG. 31. DIHYBRID DIAGRAM SHOWING RESULTS OF CROSSING PEAS HAV- ING YELLOW ROUND (YR) SEEDS WITH OTHERS HAVING GREEN WRINKLED (GW) ONES. The hybrids of the first filial generation (FJ are all yellow and round since these characters are dominant while green and wrinkled are recessive, YR(GW}. Four types of germ cells are formed by such a hybrid, viz., YR, YW , GR, GW, and the 16 possible combinations of these $ and $ gametes are shown in the small squares. Of these 16 combina- tions 7 contain the same letters (factors) so that there are only 9 differ- ent genotypes, and since recessive characters do not appear when mated with dominant ones these 9 genotypes produce only 4 phenotypes in the following relative numbers: 9 YR : 3 YW ': 3 GR : I GW. There is I pure dominant (upper left corner), I pure recessive (lower right corner) ; 4 homozygotes in the diagonal line between these corners, and 12 heterozy- gotes. 94 Heredity and Environment F2 generation, viz., dominants and recessives in the ratio of 3 : i (Fig. 30) ; where there are two pairs of contrasting characters in the parents there are nine genotypes (32) and usually four phenotypes in the F2 generation in the ratio of (3: i )2 = 9 : 3 : 3 : i. 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 four types, yellow and round (YR), yellow and wrinkled (YW), green and round (GR), and green and wrinkled (GW) in the proportion of 9: 3 : 3 : i as shown in Fig. 31. In this case also this ratio may be explained by assuming that the germ cells are pure with respect to each of the contrasting characters, round or wrinkled, yellow or 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 combinations of these characters in both male and female cells as follows : Y G | X | i.e. YR, YW, GRf GW. R W Each of these four kinds of male cells may fertilize any one of the same four kinds of female cells, thus giving rise to sixteen com- binations, as shown in Fig. 31. 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. Ac- cordingly the sixteen possible combinations yield seeds of four different appearances and in the following proportions : 9 YR : 3 GR : 3 YW \ i GW. Only one individual in each of these four classes is pure (homozygous) and continues to breed true in successive generations; in Fig. 31 these are found in the diagonal from the upper left to the lower right corner. All these in- Phenomena of Inheritance 95 dividuals are heterozygous and show Mendelian splitting in the next generation. Trihybrids. — When parents differ in three contrasting charac- ters there are twenty-seven genotypes (33) and eight phenotypes (23) in the F2 generation in the proportion of (3: i)3 = 27:9: 9:9:3:3:3: r- Thus if a pea with round (R) and yellow ( F) seeds and with tall (T) stem is crossed with one having wrinkled (IV) 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 F2 gener- ation there are 64 possible combinations (27 genotypes) of these six characters (Fig. 32) ; but since a recessive character does not develop if its contrasting dominant character is present there are only eight phenotypes which come to expression and in the following ratios: 27 RYT:g RYD:g RGT:g WYT:$ RGD: 3 IVYD : 3 WGT: i WGD. Of these sixty- four combinations only eight are homozygous and breed true (those lying in the diagonal between upper left and lower right corners in Fig. 32), while only one is a pure dominant and one a pure recessive (the ones in the upper left and lower right corners of Fig. 32). 3. Inheritance Formulae. — Mendel represented the hereditary constitution of the plants used in his experiments by letters em- ployed as symbols, dominant characters being represented by capi- tals and recessives by small letters. The seven contrasting char- acters of his peas could be represented as follows : Seeds, round (A), or wrinkled (a) ; yellow (B), or green (b) ; with gray seed coats (C), or white seed coats (c). Pods, green (D), or yellow (rf) ; inflated (E), or constricted (0- Habit, tall (F), or dwarf (/). Flowers, axial (G), or terminal (g). It is possible for one plant to have all of these dominant char- acters or all of the recessive ones, or part of one kind and part of the other. The gametic formula of a plant having all seven of Heredity and Environment WGD F, RYT RYD RGT ROD WYT WYD WGT WGD FIG. 32. TRIHYBRID DIAGRAM SHOWING RESULTS OF CROSSING PEAS HAV- ING ROUND YELLOW SEEDS AND TALL STEM (RYT) WITH PEAS HAVING WRINKLED GREEN SEEDS AND DWARF STEM (WGD). Eight types of germ cells are formed by the F1 hybrid, as shown in the $ gametes above the square and the $ ones to the left of it, and the possible combinations of these $ and $ gametes are shown in the 64 small squares of which only I is pure dominant (upper left corner), I pure recessive (lower' right corner) and 8 homozygotes (in diagonal line between these corners). There are 27 different genotypes, all combinations below this diagonal be- ing homologous with the corresponding ones above and all in the other diagonal being of the same genotype, while 12 other combinations on each side of the first diagonal constitute only 6 genotypes. There are 8 phenotypes, resembling the 8 homozygotes, and their relative numbers are 27 RYT : 9 RYD: 9 RGT \ 9 IVY T : 3 ROD : 3 WYD -.3 WGT: i WGD. Phenomena of Inheritance 97 the dominant characters is ABCDEFG ; of one having all of the recessive characters abcdefg. When two such plants are crossed the zygotic formula of the hybrid is AaBbCcDdEcFfGg, and since the dominant and recessive characters (or rather determin- ers of characters) represented by these seven pairs of letters separate in the formation of the gametes; and since each separate determiner may be associated with either member of the six other pairs, the number of possible combinations of these deter- miners in the gametes is (2)* or 128. That is, in this case 128 kinds of germ cells may be produced, each having a different in- heritance 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 combinations of these characters which are pos- sible in the F2 generation is (i28)2 or 16,384, while the number of different genotypes is (3)7 or 2187. Every one of these more than two 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 letter in the name of that character. By this form of shorthand one can show in a graphic way the possible segregations and combinations of heredi- tary units in gametes and zygotes through successive generations, and as a result many modern works on Mendelian inheritance look like pages of algebraic formulae. 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 others have regarded these contrasting charac- ters as due to the presence or absence of single factors: thus round seeds are due to the presence of a factor for roundness (A) while wrinkled seeds were said to be clue to the absence of that factor (a). Round seeds were spoken of as wrinkled seeds plus the factor for roundness. But it is practically certain that 98 Heredity and Environtnent recessive characters are not due to the absence of factors for dominant characters; there are many genetical and philosophical objections to such a view, which leads logically to some strange conclusions, such as Bateson's speculations on evolution (p. 282). Morgan and his associates have found that a given dominant character may have several different kinds of recessive contrast- ing characters or allelomorphs ; thus the dominant eye color of the wild pomace fly, Drosophila melanogaster, is red, but, instead of a single contrasting recessive character, eleven have been found, viz., apricot, blood, buff, cherry, coral, ecru, eosin, ivory, tinged, wine, and white. Such a condition is known as "multiple allelo- morphism." If the red color is due to the presence of a certain factor, all these other allelomorphic colors cannot be due to its absence, since, there can be only one kind of absence. Each of these recessive colors must be due to the presence of a differentia) factor. Therefore the presence-absence hypothesis must be aban- doned. When both gametes carry similar dominant 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 factors, the zygote receives only negative fac- tors and is said to be nulliplex. Thus the union of gametes AB ( 9 ) and AB ( $ ) yields zygote AABB, which is duplex in constitution ; gametes Ab ( $ ) and aB ( $ ) yield zygote AaBb, which is simplex ; gametes ab ( $ ) and ab ( $ ) yield zygote aabb, which is nulliplex. In some instances a character comes to full expression only when it is derived from both parents, thaj: is, when it is duplex; if derived from one parent only, that is, if simplex, it is diluted in appearance and is intermediate between 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 derived Phenomena of Inheritance 99 from both parents, pink flowers when it is derived from one parent, white flowers when it is derived from neither parent (Fig. 28). 5. Summary of Mendelian Principles. — Since the rediscovery in 1900 of Mendel's work many investigators have carried out similar experiments on many species of animals and plants and have greatly extended our knowledge of the principles of inheri- tance 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 sexually produced organisms. In brief the "Mendelian Law of Alternative Inheritance" or of hereditary "splitting" consists of the following principles: (a) The Principle of Unit 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 appears fully developed, while the other is recessive and temporarily drops out of sight. (c) The Principle of Segregation. — Every 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 produced, 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 of its determiners having been derived from the male and the other half from the female sex cell. This double set of determiners again becomes single in the formation of the germ cells only once more to be- come double when the germ cells unite in fertilization. ioo Heredity and Environment II. MODIFICATIONS AND EXTENSIONS OF MENDELIAN PRINCIPLES It is a common experience that natural phenomena are found to be more complex the more thoroughly they are investigated. Nature is always greater than our theories, and with few excep- tions hypotheses which were satisfactory at one stage of knowl- edge have to be extended, modified or abandoned as knowledge increases. This observation is well illustrated 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 phe- nomena of inheritance now known it has become necessary to modify or extend them in many ways. And yet the general and fundamental truth of these principles has been established in a surprisingly large number of cases, and they have been extended to forms of inheritance where at first it was supposed that they could not apply. I. The Principle of Unit Characters and of Inheritance Fac- tors.— There has been much criticism on the part of some biolo- gists of the principle of unit characters. It is said that unit char- acters cannot be independent and discrete things; the organism itself is a unity and every one of its parts, every one of its char- acters, 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 inde- pendent, and yet there are varying degrees of independence in organisms, organs, cells, parts of cells, hereditary units and char- acters 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. But the most serious objection to the doctrine of unit characters is not against their independence but against their unity. Every character is complex, many factors enter into its development, and since the combination of these factors is variable the character itself cannot be constant. Strictly speaking. Phenomena of Inheritance toi characters are not units, and while the conception of "unit char- acters" has served a useful purpose it cannot any longer be re- garded as wholly accurate. Inheritance Factors are Differential Causes. — Of course char- acters of adult individuals do not exist as such in germ cells, but there is no escape from the conclusion thaUin the case of inherited differences between mature organisms there must haVe been dif- ferences 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, what- ever 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 com- bination of causes is that which is peculiar to the development of each particular character. Of course Mendelian factors are not the only factors of development but merely the differential factors which cause, for example, one guinea-pig to be white and its bro- ther to be black. Very many factors are involved in the produc- tion of white or black color but there is at least one differential factor for every unit character and this alone is the Mendelian factor. Factors Are Not Undeveloped Characters. — 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 con- tains a peculiar chemical element; but it is necessary to suppose that each hereditary character is caused by some particular com- bination of inheritance units and that each compound is produced by some particular combination of chemical elements. An enor- mous number of chemical compounds exists as the result of var- ious combinations of some eighty different elements, and an almost endless number of words and combinations of words — indeed IO2 Heredity and Environment whole literatures— may be made with the twenty-six letters of the alphabet. It is quite probable that the kinds of inheritance units are few in number as compared with the multitudes of adult char- acters, and that different combinations of the units give rise to different adult characters ; but it is certain that inherited dif- ferences 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 conceived that there was something in the germ which corresponded to each character of the plant. Weis- mann postulated a determinant in the germ for every character which is independently 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 and adult characters. Many different characters may be determined by a single unit or factor; for example, all the numerous secondary sexual characters which distinguish males from females may be determined by the original factor which determines whether the germ cells shall be ova or spermatozoa. Multiple Factors. — 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. (i) a pig- ment factor for each particular color, and (2) a color developer. 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 rab- bits as well as for several species of plants. Bateson and Pun- nett found two varieties of white sweet peas which were appar- ently alike in every respect except the shape of their pollen grains, one of them having long and the other round pollen. But when these were crossed a remarkable thing occurred for the progeny "instead of being white were purple like the wild Sicilian plant Phenomena of Inheritance 103 from which our cultivated sweet peas are descended." 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 pigment factor R and a color devel- oper C. One of these was lacking in each of the white parents, their gametic formulae being Cr and cR respectively, but when these two factors came together in the offspring a purple-flowered type was produced with the zygotic formula CcRr. These Fl plants produced colored and white F2 plants in the proportion of 9 colored to 7 white and the colored forms were of six different kinds (Fig. 33). For the production of these six colored forms five different factors must be present in the gametes, according to Punnett, viz.: (i) 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 /. When all of these factors are present the result is the purple wild form with blue 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 F2 generation. Castle found that eight different factors may be involved in producing the coat colors of rabbits : these are : C a common 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, its middle yellow, its basal part gray. E a factor for the extension of black or brown but not of yellow. Plate found that all of these factors except the last, E, are also involved in the production of the coat colors of mice. Baur has recognized more than twenty different factors for the color and form of flowers in the snapdragon, Antirrhinum. 1O4 Heredity and Environment I 1 WHITE ll^-Vj VERY PALt PIMPLE $M3 PINK PM.E PURPLt FED PUKPLB Butt fiEB DEEP PURPLE FIG. 33. RESULTS OF CROSSING Two DIFFERENT RACES (A AND 5) OF WHITE SWEET PEAS; all the F^ hybrids (C) are purple with blue wings like the wild ancestral stock; in F2 six colored varieties are formed rang- ing from purple with blue wings (D) to tinged white (/) and several kinds (genotypes) of white varieties (K). (After Punnett). Phenomena of Inheritance 105 Modifying Factors. — Morgan and Bridges have found that the effects of many factors may be modified by other factors. Thus the eye color of Drosophila known as "eosin" may be modified by six or seven different factors, occupying different loci in the chromosomes, one of which intensifies "eosin" while the others dilute it. These modifying factors are undoubtedly like other Mendelian factors in their behavior and they show that an adult character may be the result of several different inheritance fac- tors. Indeed Morgan says "that an overstatement that each fac- tor may affect the entire body is less likely to do harm than to state that each factor affects only a particular character." And again he says, "It cannot too insistently be urged that when we say a character is the product of a particular factor we mean no more than that it is the most conspicuous effect of the factor" (Morgan, 1916, p. 117). Lethal Factors. — Morgan and his associates have also demon- strated the existence of a considerable number of lethal factors in Drosophila that cause the early death of those gametes or zygotes in which this factor is not balanced by a normal one. This phenomenon greatly modifies expected Mendelian ratios for only heterozygotes survive, and all individuals that are homo- zygous for a lethal factor usually die so early that they are never seen. Nevertheless their existence can be determined by indirect methods that will be mentioned in the next chapter under "link- age." Such lethal factors greatly complicate the study of genetic? but they do not destroy its fundamental principles. What are factors? — Inheritance factors are probably complex chemical substances which preserve their individuality in various combinations, just as groups of atoms or radicals do in chemical reactions ; they may be dropped out or added, substituted or trans- posed, 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 environmental changes, so that the characters of io6 Heredity and Environment the developed organism are the resultants of all these reactions and interactions. Some progress has been made, in identifying certain structures of the germ cells with certain hereditary units, but quite irrespec- tive of what these units may be and where they may be located it is possible, by means of the Mendelian theory of segregation of units in the germ cells and of chance combinations of these in fer- tilization to predict the number of genotypes and phenotypes which may be expected as the result of a given cross. 2. Modifications of the Principle of Dominance. Incomplete Dominance. — A large number of animal and plant hybrids show one contrasting character completely 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 Fx plants bear neither white nor red flowers but pink ones, and the F2 plants are white-flowered, red-flowered or pink-flowered. The whites and reds are always homozygous, the pinks heterozygous; pure white and pure red are produced only when their factors are duplex (WW}, (RR) ; when they are simplex (WR) pink is produced. In this case red is not com- pletely dominant over white, but the hybrid is more or less inter- mediate between the two parents (Fig. 28). It has long been known that the race of fowls called Blue An- dalusian 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 (homozygous), blue only when both black and white are present ( heterozygous). Again a cross of red and white cattle produces roan offspring, but the latter when interbred give rise to reds, roans and whites in the proportion of 1:2:1, showing that the roans are heterozy- gotes in which red is not completely dominant over white, while Phenomena of Inheritance 107 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 interme- diate between the two parents ; but when these hybrids were inter- bred they produced banded, faintly banded and uniformly col- ored snails in the ratio of 1:2:1, thus proving that Mendelian segregation takes place in the F2 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 develop- ment but becomes complete in adult stages. Davenport found that when white and black fowls are crossed the chicks, especially the females, are speckled white and black, but in the adult fowl domi- nance is complete and the plumage is white. Similar conditions of delayed dominance are well known in the color of hair and eyes of children, though dominance may become complete when they have reached adult life. Reversible Dominance. — In a few instances a character may be dominant at one time and recessive at another. Thus Davenport found that an extra toe in fowls is dominant under certain cir- cumstances and recessive under others. Tennent found that char- acters 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 environmental conditions, sometimes upon a particular combination of hereditary units. Dominance Not Fundamental. — 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 con- ditions of the germ or of the environment some characters may io8 Heredity and Environment develop in hybrids to the exclusion of their opposites whereas under other conditions these results may be reversed or the char- acters may be intermediate. The principle of dominance is not a fundamental part of Mendelian inheritance. Even when the characters of hybrids are intermediate between those of their parents, if the parental types reappear in the F2 generation we may be certain that we are dealing with cases of Mendelian inheritance. 3. The Principle of Segregation. — The individuality of inheri- tance units, and their segregation or separation in the sex cells and recombination in the zygote are fundamental principles of the Mendelian doctrine. Indeed the evidence for the individual- ity and continuity 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 segre- gate according 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 there was not a clear segregation of the par- ental types in the F2 generation or in which the ratio of dominants to recessives differed from the well kmfwn 3 to I ratio were sup- posed to be non-Mendelian. Unusual Ratios. — However further work has shown that most of these cases are really Mendelian. Sometimes offspring are in- termediate between their parents owing to incompleteness of domi- nance, rather than to incompleteness of segregation ; in such cases Phenomena of Inheritance 109 the parental types reappear in the F2 generation as in the cross between red and white "four o'clocks." Sometimes departures from the 3 to i ratio are caused by the fact that two or more fac- tors of the same sort are involved in the production of a single character. Nilsson-Ehle found that when oats with black glumes were crossed with varieties having white glumes the ratio of 3 white to i black was usually found in the second generation ; but one variety of black oats when crossed with white gave in the second generation approximately 15 blacks to i white which is the dihybrid ratio. From this and other evidence he concludes that in this variety of oats two hereditarily separable factors are involved in the production of black. In crosses between red- grained and white-grained wheat he usually got in the second gen- eration the monohybrid ratio of 3 red to i white, but three strains gave the dihybrid ratio of 15 to i and two gave the trihybrid ratio of 63 to i and in subsequent generations each of these strains continued to give the same ratios. 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 to three factors; notable departures from expected ratios may thus be explained. Other departures from regular Mendelian ratios are caused by the early death of certain gametes or zygotes due to lethal factors, as explained on page 105. Blending of Color in Mulatto. — Perhaps the most serious objec- tions which can be presented against the universality of the Men- delian doctrine are found in phenomena of "blending" inheritance. In some instances contrasting characters of parents appear to blend in offspring and even in the F2 and in subsequent generations the descendants remain more or less intermediate between the parents. One of the best known illustrations of this is found in the skin col- or of the mulatto which is intermediate between the white parent and the black one, and even in the F2 and in subsequent generations mulattoes do not usually produce pure white or pure black chil- dren, though the children of mulattoes show considerable variation no Heredity and Environment in color. Hence there seems to be a failure of the Mendelian principle of segregation. But white skin is not really white nor is black skin ever perfect- ly black. Davenport has shown that there is a mixture of black, yellow and red pigments 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 pigments 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 the way from 46 per cent to 6 per cent of black, the latter so far as skin color is concerned 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 chil- dren 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 F2 and in subsequent genera- tions 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. 33. Davenport, who has made an extensive study of this case, concludes that "there are two double factors (A A, BB) for black pigmentation in the full blooded negro of the west coast of Africa, and these are separably inheritable." These factor^ are lacking in white persons (this being indicated by the formula aa, bb). Since the germ cells carry only single factors and not double ones the cross between negro and white would have only one set of these fac- * In another family shown in Fig. 35 the father has 18 per cent black pigment, the mother 38 per cent and the children range from 17 per cent to 54 per cent. Phenomena of Inheritance ill AB Ab aB a b V AB Ab aB a b AB AB Ab AB aB AB ab AB AB Ab Ab Ab aB Ab ab Ab AB aB Ab aB aB 'aB ab aB AB a b Ab ab aB ab a b a b FIG. 34. CHECKERBOARD DIAGRAM SHOWING RESULTS OF CROSSING Two MULATTOES, each having color factors ABab. Types of male gerih cells are above the square, of female cells on the left and the possible combina- tions of these are shown in the 16 small squares. Homozygotes are found only along the diagonal. The color of the children varies all the way from black (upper left corner) to white (lower right corner). FIG. 35. MULLATTO HUSBAND AND WIFE AND THEIR SEVEN CHILDREN ranging in color from the one on left who "passes for white" to the youngest who is typically black. (From Davenport.) 112 Heredity and Environment tors for black color, as shown by the formula AB x ab = ABab ; hence the color of the Ft generation is intermediate between that of the two parents. In the F2 generation there should be a variety of colors ranging all the way from white to black (Fig 34), though pure white (ab ab) or pure black AB AB) would be expected in only i out of 16 of the offspring. 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. Blending of Size. — 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 intermediate length, and in all subsequent generations the ear length remained inter- mediate between that of the parents. He found the same thing true of length and breadth of the skull (Fig. 36) and of the size of other portions of the skeleton, and he concluded that such quantitative characters are not inherited in Mendelian fashion. More recently MacDowell, working on the inheritance of size in rabbits, concludes that this character as well as other quan- titative differences between parents which appear to blend in the offspring, such as Castle's case of ear length in rabbits, is not due to a single factor, 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 dihy- brids, trihybrids or polyhybrids. As a result offspring appear more or less intermediate in size between their parents. Thus it is possible to explain even "blending" inheritance as Phenomena of Inheritance due not to the real fusion or blending of inheritance factors but -to varying combinations of numerous or multiple factors, accord- ing to the Mendelian rules. The Mendelian principle of segrega- tion has been found to be of such general occurrence that there is a strong probability that it is universal,' and that all cases of "blending" inheritance are due to incomplete dominance or to multiple factors. Maternal Inheritance. — Another case which seems at first sight to be non-Mendelian is what may be called "maternal in- heritance" since certain characters are invariably derived from the mother and not from the father. Among these are the polarity, symmetry and pattern of the egg and of the adult animal which is derived from it (see p. 196). These characters are of such a general sort that they may not be recognized as phenomena of inheritance at all, and yet they form the background and frame- work for all the other characters. They do not come equally FIG. 36. INHERITANCE OF SIZE IN RABBITS. The skulls of two parents are shown in I and 3, of their intermediate offspring in 2. (From Castle.) Heredity and Environment from the egg and sperm, and they do not undergo segregation in the formation of the gametes, but are apparently derived from the egg cytoplasm. Among characters of this sort are the normal and inverse symmetry of snails, and of many other animals, including man, which are referred to on pages 197-205. Such characters are undoubtedly inherited, though they differ from other characters not only in 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 from a previous generation rather than inheritance factors. These egg characters probably appeared in the course of oogenesis under the influence of paternal as well as of maternal factors ; 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 Men- FIG. 37. X-RAY PICTURE OF RIGHT AND LEFT HANDS EACH WITH Six FINGERS (polydactyly) caused by splitting of the little fingers at an early stage. (From Journal of Heredity.) Phenomena of Inheritance 115 delian in the previous generation, this also being a case of pre- inheritance. It has been found by Correns, Baur, and Shull that the leaf colors of certain plants are not inherited in Mendelian fash- ion, but the chromoplasts, which produce 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 differentiation or development this may be a case of pre-inheritance ; if they are symbiotic organisms it is an instance of the inclusion of foreign bodies in the cytoplasm and not in- heritance at all. 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 particular chemical substances such as fat dyes through the egg but not through the sperm, and the transport of symbiotic or parasitic organisms, such as algae, bacteria, etc., through the female sex cell but not through the male cell. These substances or micro- itli iw % B A FIG. 38. X-RAY PICTURE, A OF A NORMAL, B OF A SHORT-FINGERED (brachydactyl) hand. (From Bateson.) u6 Heredity and Environment organisms are to be regarded as inclusions in the egg rather than as any permanent part of the germinal organization; consequently they are not inherited in the strict sense of that term. III. MENDELIAN INHERITANCE IN MAN The study of inheritance in man must always be less satisfac- tory and the results less secure than in the case of lower animals and for the following reasons : In the first place there are no "pure lines" but the most complicated intermixture of different lines. In the second place experiments are out of the question and one must rely upon observation and 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 with- in two months and Morgan almost as many generations of Dro- sophila within two years. Finally the number of offspring are so few in human families that it is impossible 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 char- acters is still uncertain. Davenport and Plate have catalogued more than sixty human traits which seem to be inherited in Mendelian fashion. About fifty of these represent pathological or teratological conditions while only a relatively small number are normal characters. This does not signify that the method of inheritance differs in the the case of normal and abnormal characters, but rather that ab- normal characters are more striking, more/ easily followed from generation to generation, and consequently statistics are more complete with regard to them than in the case of normal char- acters. In many cases statistics are not sufficiently complete to determine with certainty whether the character in question is dominant or recessive, and it must be understood that in some instances the classification in this respect is tentative. A par- tial list of these characters is given herewith: Phenomena of Inheritance 117 nS Heredity and Environment MENDELIAN INHERITANCE IN MAN NORMAL CHARACTERS Recessive Dominant Hair: Curly Straight Dark Light to red Eye Color; Brown Blue Skin Color: Dark Light Normal Pigmentation Albinism Countenance: Hapsburg Type (Thick lower Normal lip and prominent chin) Temperament: Nervous Phlegmatic Intellectual Capacity: Average Very great Average Very small TERATOLOGICAL AND PATHOLOGICAL CHARACTERS General Size; Achondroplasy (Dwarfs with Normal short stout limbs but with bodies and heads of normal size) Normal size True Dwarfs (With all parts of the body reduced in proportion) Hands and Feet: Brachydactyly (Short fingers Normal (Fig. 38) and toes) Syndactyly (Webbed fingers Normal and toes) Polydactyly (Supernumerary Normal t^Tg. 37) digits) Skin: Keratosis (Thickening of Epi- Normal dermis) Epidermolysis (Excessive for- Normal mation of blisters) Hypotrichosis (Hairlessness as- Normal sociated with lack of teeth) Phenomena of Inheritance 119 MENDELIAN INHERITANCE IN MAN (Continued) TERATOLOG1CAL AND Dominant Kidneys; Diabetes insipidus Diabetes mellitus Normal Nervous System: Normal Condition Nervous System: Normal Normal Normal Normal Normal Huntington's Chorea Muscular Atrophy Eyes: Hereditary Cataract Pigmentary Degeneration of Retina Glaucoma (Internal pressure and swelling of eyeball) Coloboma (Open suture in iris) Displaced Lens Ears.- •Normal Normal PATHOLOGICAL CHARACTERS Recessive Normal Normal Alkaptonuria (Urine dark after oxidation) General Neuropathy, e.g. Hereditary Epilepsy Hereditary Feeble-mindedness Hereditary Insanity Hereditary Alcoholism Hereditary Criminality Hereditary Hysteria Multiple Sclerosis (Diffuse de- generation of nerve tissue) Friedrich's Disease (Degenera- tion of upper part of spinal cord) Meniere's Disease (Dizziness and roaring in ears) Chorea (St. Vitus Dance) Thomsen's Disease (Lack of muscular tone) Normal Normal » Normal Normal Normal Normal Normal Deaf-mutism Otosclerosis (Rigidity of tym- panum, etc., with .hardness of hearing) I2O Heredity and Environment SEX-LINKED CHARACTERS* Recessive characters, appearing in male when simplex, in female when duplex. Normal Gower's Muscular Atrophy Normal Haemophilia (Slow clotting of blood) Normal Color Blindness (Daltonism; in- ability to distinguish red from green) Normal Night Blindness (Inability to see by faint light) Normal Neuritis Optica (Progressive atrophy of optic nerve) SUMMARY The principles of heredity established by Mendel are almost as important for biology as the atomic theory of Dalton is for chem- istry. By means of these principles particular dissociations and recombinations of characters can be made with almost the same certainty as particular dissociations and recombinations of atoms can be made in chemical reactions. By means of these principles the hereditary constitution of organisms can be analyzed and the real resemblances and differences of various organisms deter- mined. By means of these principles the once mysterious and apparently capricious phenomena of prepotency, atavism and reversion find a satisfactory explanation. Before the establishment of Mendel's principles, heredity was, as Balzac said, "a maze in which science loses itself." Much still remains to be discovered about inheritance, but the principles of Mendel have served as an Ariadne thread to guide science through this maze of apparent contradictions and exceptions in which it was formerly lost. * See page 187. CHAPTER III THE CELLULAR BASIS OF HEREDITY AND DEVELOPMENT CHAPTER III THE CELLULAR BASIS OF HEREDITY AND DEVELOPMENT A. INTRODUCTORY Heredity is to-day the central problem of biology. This prob- lem may be approached from many sides, that of the observer, the statistician, the practical breeder, the experimenter, the embry- ologist, the cytologist; but these different aspects of the subject may be reduced to three general methods of study, (i) the ob- servational and statistical, (2) the experimental, (3) the cyto- logical and embryological. We have dealt with the first and sec- ond of these in the preceding chapter and before taking up the third it is important that we should have clear definitions of the terms employed and a fairly accurate conception of the processes involved. i. Confused Ideas of Heredity. — Heredity originally meant the transmission of property from parents to children, and in the field of biology it has been defined erroneously as "the transmis- sion of qualities or characteristics, mental or physical, from par- ents 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 parents to son. And when it is said that a son inherits 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 development is temporarily forgotten. Of course * Century Dictionary. 123 124 Heredity and Environment there are no "qualities" or "characteristics" which are "trans- mitted" as such from one generation to the next. Such terms are not without fault when used merely as figures of speech, but when interpreted literally, as they frequently are; they are alto- gether misleading; they are the result of reasoning about names rather than facts, of getting far from phenomena and philosophiz- ing about them. The comparison of heredity to the transmission of property from parents to children has produced confusion in the scientific as well as in the popular mind. It is only necessary to recall the most elementary facts about development to recog- nize 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 genera- tion to the next is a very ancient one and was universally held until the most recent times. Before 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 produced blue-eyed germs, without attempt- ing 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 by the adult animal or plant and that the characteristics of the adult were in 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 parti- cles or "gemmules" were given off by/ every cell of the body, at every stage of development, and that these gemmules then col- lected in the germ cells which thus became storehouses of little germs from all parts of the body. Afterward, in the develop- ment of the embryo, the gemmules, or little germs, developed into cells and organs similar to those from which they originally came. The Cellular Basis 125 3. Germinal Continuity and Somatic Discontinuity. — Many ingenious hypotheses have been devised to explain things which are not real, and this is one of them. The doctrine that adult organ- isms 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 comes from a preceding cell by a process of division, and germ cells are formed, not by contributions from all parts of the body, but by division of preceding cells which are derived ultimately from the fertilized egg (Fig. 40). The hen does not produce the egg, but the egg produces the hen and also other eggs. Individual 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. Germ Cells and Body Cells. — There is a continuity of germinal substance, and usually of germinal cells, from one generation to the next. In some animals the germ cells are set apart at a very early stage of development, 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 general then germ cells do not come from differen- tiated body cells, but only from undifferentiated germinal cells, and if in a few doubtful cases differentiated cells may reverse the process of development and become embryonic cells and even germ cells it does not destroy this general principle of germinal continuity and somatic discontinuity of successive generations. Thus the problem which faces the student of heredity and de- velopment has been cut in two; he no longer inquires how the body produces the germ cells, for this does not happen, but merely how the latter produce the body and other germ cells. The germ is the undeveloped organism which forms the bond between suc- cessive generations; the body is the developed organism which arises from the germ under the influence of environmental con- 126 Heredity and Environment ancles <$ \\ FIG. 40. DIAGRAM SHOW- ING THE "CELL LINEAGE" OF THE BODY CELLS AND GERM CELLS IN A WORM OR MOL- 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 gam- etes, the genesis of the sper- matozoa being shown on the left and that of the ova on the right. Ogte*ll+ I , »«*J\ V Tid» • • * 4 Kb. Gametes ditions. The body develops and dies/in each generation; the germplasm is the continuous stream of living substance which connects all generations. The body nourishes and protects the germ; it is the carrier of the germplasm, the mortal trustee of an immortal substance. 4. Germplasm and Somato plasm. — This contrast between the germ and the body, between the undeveloped and the developed The Cellular Basis 127 FIG. 41 organism, is fundamental in all modern studies of heredity. It was especially emphasized by Weismann . in his germplasm theory and recently it has been made prominent by Johannsen under the terms "genotype" and "phenotype" ; the genotype is the funda- mental hereditary constitution of an organism, it is the germinal type; the phenotype is the developed organism with all of its visible characters, it is the somatic type. But important as this distinction is between germ and soma it has sometimes been overemphasized. This is one of the chief faults of Weismann's theory. The germ and the soma are generi- cally 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 undeveloped in the germ cells. But the germ cells do not remain 128 Heredity and Environment undifferentiated ; both egg and sperm are differentiated, the for- mer for receiving the sperm and for the nourishment of the em- bryo, the latter for locomotion and for penetration into the egg. But while the differentiations of tissue cells are usually irreversi- ble, so that they do not again become germinal cells, the differen- tiations of the sex cells are reversible, so that these cells, after their union, again become germinal cells. The ovum loses its power to form yolk and during the early development it gradually loses all the yolk which it had stored up ; the spermatozoon loses its highly differentiated tail or locomotor apparatus and its small compact nucleus absorbs substance from the cytoplasm of the egg and becomes a large germinal nucleus. Chromatin is Germplasm, Cytoplasm is Somatoplasm. — In many theories of heredity it is assumed that there is a specific "inheri- tance material," distinct from the general protoplasm, the func- tion of which is the "transmission" of hereditary properties from generation to generation, and the chief characteristics of which are independence of the general protoplasm, continuity from generation to generation and extreme stability in organization. This is the idioplasm of Nageli, the germplasm of Weismann. Such a substance is no mere fiction or logical abstraction, as many writers have affirmed, for there is in the nucleus of every cell a substance which fulfills all of these conditions, namely, the chromatin. It is relatively independent of the surrounding cytoplasm, it is self-propagating and consequently continuous from cell to cell, and from generation to generation and it is relatively stable in organization so that it is but little influenced by environmental conditions. There are many important rea- sons for believing that the chromatin is the germplasm, or at least that it contains the inheritance units, as we shall see later. It is present not only in germ cells but in every cell of the organ- ism, though in highly differentiated tissue cells it may undergo certain secondary modifications. On the other hand the cyto- plasm surrounding the nucleus, undergoes many marked differ- The Cellular Basis 129 entiations in the course of development and it constitutes in the main the body plasm or somatoplasm. Germplasm and somato- plasm are not, therefore, vague generalizations, but they are defin- ite cell substances which may be seen under the microscope. 5. The Units of Living Matter. — The entire cell, nucleus and cytoplasm, is the smallest unit of living matter which is capable of independent existence. Neither the nucleus nor the cytoplasm can for long live independently 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 capa- ble of responding to many kinds of stimuli. But while the parts of a cell are not capable of independent existence they may be dif- ferentiated to perform different functions. Panmerism. — Not only is the cell as a whole capable of assimi- lation, growth and division, but every visible 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 assimila- tion, growth and division occurs in manv parts of the nucleus and cytoplasm, such as the chromosomes, chromomeres, centro- sorries, etc. In all cases cells come from cells, nuclei from nuclei, chromosomes from chromosomes, centrosomes from centro- somes, etc. Indeed, the manner in which all living matter grows indicates that every minute particle of protoplasm has this power of taking in food substance and of dividing into two particles when it has grown to maximum size ; this is known as panmerism. Presum- ably 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 par- ticles are much larger than the largest molecules known to chem- istry. The smallest particle which can be seen with the most powerful microscope in ordinary light is about 250 ftp, (millionths of a millimeter) in diameter. The largest molecules are prob- 130 Heredity and Environment ably about 10 /U/A in diameter. Between these molecules and the just visible particles of protoplasm there may be other units of organization. These hypothetical particles of protoplasm have been supposed by many authors to be the ultimate units of assimi- lation, growth and division, and in so far as these units are sup- posed to be the differential causes of hereditary characters, they are known as inheritance units. Inheritance Units. — It is assumed in practically all theories of heredity that the "inheritance material," or the germinal proto- plasm, is composed of ultra-microscopical inheritance units which have the power of individual growth and division and which are capable of undergoing many combinations and dissociations dur- ing the course of development, by which combinations and dissociations they are transformed into the structures of the adult. Various names have been given to these units by different authors; they are the "physiological units" of Herbert Spencer, the "gemmules" of Darwin, the "plastidules" of Els- berg and Haeckel, the "pangenes" of de Vries, the "plasomes" of Wiesner, the "idioblasts" of Hertwig, the "biophores" and "de- terminants" of Weismann. With the publication of Weismann's work on the germplasm 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 existence of inheritance units of some kind once more became evident, and, without being able to define just what such units are or just how they behave, modern students of hered- ity assume 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 developed or- ganism, and which are in a measure independent of one another; though of course neither they nor any other parts of a cell are The Cellular Basis 131 really independent 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 combina- tion with other chemical elements in the form of molecules, and yet preserve their identity in many different combinations. It is certain that Mendelian factors are not to be regarded as gemmules or the germs of particular characters. There is not a separate factor for every character, and factors are not "repre- sentatives" or "carriers" of characters. They are the differen- tial causes of particular characters just as in the compounds H2SO4 and K2SO4 the hydrogen and potassium atoms are the differential causes of the properties manifested by these two substances. Location of Inheritance Units. — If there are inheritance units, such as determiners 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. There are many evidences that these genes are located in the chromatin of the nucleus, that they are arranged in a linear series when the chromatin takes the form of threads, or chromosomes, preparatory to cell division, that in the division of each chromosome every gene which it contains is also divided and that daughter chromosomes and daughter genes are distrib- uted equally to the daughter cells at every typical cell division (Figs. 6, 7, 8). For nearly fifty years this complex process of nuclear division, known as mitosis or karyokinesis, has been recog- nized as a mechanism for the equal distribution of the chromo- somes to the daughter cells, and for nearly that length of time it has been suggested that the inheritance material or germplasm was located in the chromosomes, but only within recent years has critical experimental evidience been obtained that inheritance units occupy definite positions in these chromosomes. With this ad- vance in our knowledge, which we owe chiefly to Morgan and his associates, it may be said that an important part, at least, of the "mechanism of heredity" has been discovered. 132 Heredity and Environment It must be said however that there are biologists who still re- fuse to believe that heredity is associated with any particular cell substance, while many others who would grant this are not yet ready to admit that there are particular units or genes which are concerned in the production of particular characters. However anyone who will examine at first hand the evidences in favor of this cannot fail to be impressed with its importance, and no one has proposed any other hypothesis that is at all satisfactory. 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 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 par- ticular 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 proto- plasm differentiated units which are combined into a system; in short, there is organization. 6. 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. Further- more, 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. 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 organization of the germ cells which differentiate in similar ways in successive generations if the extrinsic factors of develop- ment remain similar. The Cellular Basis 133 Definitions. — In short, heredity may be defined as the continuity from generation to generation of certain elements of germinal organization. Heritage is the sum of all those qualities whiah are determined or caused by this germinal organization. Develop- ment is progressive and coordinated differentiation of the oosperm, under the joint influence of heredity and environment, by which it is transformed into the adult organisation. Differ*- entiation is the formation and localisation 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 charac- ters but also the characters of every stage from the egg to the adult condition. For every inherited character, whether embryonic or adult, there is some germinal basis. 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 inheritance, but the actual stages and causes of inheritance, the real mechanisms of heredity, are to be found only in a study of the germ cells and their development. Although many phenomena of inheritance have been discovered in the absence of any definite knowledge 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 necessary to formu- late theories of heredity to account for the facts, but these theo- ries are only temporary scaffolding to bridge the gaps in our knowledge, and if we knew all that could be known about the 134 Heredity and Environment germ cells and their development we should have little need for theories. In the first chapter 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 func- tions. 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 biologists, but nevertheless we have come far since that day in 1875 when Oscar Hertwig first saw the ap- 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. I. Fertilization, a. Stimulus to Development. — The development of the individual may be said to begin with the fertilization of the egg, though it is evident that both egg and sperm must have had a more remote beginning, and that they also have undergone 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 usually begin again until a spermatozoon has entered an ovum, or until the latter has been stimulated by some other outside means. Parthenogenesis. — In some animals and plants, eggs may de- velop regularly without fertilization, the stimulus to development being supplied by certain external or internal conditions ; in other cases, as Loeb discovered, eggs which would never develop if left to themselves may be experimentally stimulated by physical or chemical changes in the environment, so that they undergo regu- lar development. The development of an egg without previous fertilization is known as parthenogenesis or virgin reproduction; if it occurs in nature it is natural parthenogenesis, if in experi- The Cellular Basis 135 ments it is artificial parthenogenesis. Natural parthenogensis is relatively rare and in the vast majority of animals and plants the FIG. 42. DIAGRAMS OF THE MATURATION AND FERTILIZATION OF THE EGG OF A MOLLUSK (Crepidula}. A, B, First maturation division (ist Mat. Sp.). C, Second maturation division (2d Mat. Sp.) and first polar body (ist PB) resulting from first division. $N, Sperm nucleus. $C, Sperm centrosome. D, Approach of sperm nucleus ($N) and sphere ($S) to egg nucleus ( 9 A/") and sphere ( $ S) ; the second polar body (2d PB} has been formed and the first has divided {ist PB}. E, Meeting of egg and sperm nuclei and origin of cleavage centrosomes. F, First cleavage of egg showing direction of currents in the cell. 136 Heredity and Enmronment egg does not begin to develop until a spermatozoon has entered it. b. Union of Germplasms. — But the spermatozoon not only stim- ulates the egg to develop, as environmental conditions may also do, but it carries into the egg living substances which are of great significance in heredity. Usually only the head of the spermato- zoon enters the egg (Fig. 4) and this consists almost entirely of nuclear chromatin (Fig. 4 D-H, 42 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. 42 C). Both of these condensed chromatic nuclei then grow in size and become less chromatic by absorbing from the egg a substance which is not easily stained by dyes and hence is called achromatin (Figs. 4 I-L, 42 D-E). The chromatin then appears to become scattered through each nucleus in the form of granules or threads which are embedded in the achromatin; this is the condition of a typical "resting" nu- cleus. It is evident however that these chromatin granules are not scattered broadcast throughout the nucleus, since at the next mitosis they come together into particular chromosomes similar in every way to the chromosomes of the previous mitosis. Probably the chromosomes preserve their identity from one division to the next either in the form of chromosomal vesicles (Fig. 8, p. 20) or as strings of granules. The spermatozoon also brings into the egg a centrosome or division center, around which an aster ap- pears consisting of radiating lines in the protoplasm of the egg (Fig. 4F-7, Fig. 42, B-E). The moment that the spermatozoon touches the surface of the egg the latter throws out at the point 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. 42, D and E). In the movements of egg and sperm nuclei toward The Cellular Basis FIG. 43. FERTILIZATION OF THE EGG OF THE NEMATODE WORM Ascaris megalocephala. $ N, Egg nucleus. $ N, Sperm nucleus. Arch, Archiplasm. C, Centrosome. A, B, Approach of germ nuclei. C, D, Formation of two chromosomes in each germ nucleus. E. F, Stages in the division of the chromosomes which are split in E and are separating in F; only three of the four chromosome pairs are shown in F. (From Wilson after Boveri.) 138 Heredity and Environment 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 cytoplasm which moves according to its pre- established organization. 2. Cleavage and Differentiation. — When the sperm nucleus has come close to the egg nucleus the sperm centrosome usually divides into two minute granules, the daughter centrosomes, which move apart forming a spindle with the centrosomes at its poles and with astral radiations running out from these into the cyto- plasm (Figs. 4, 42 F, 43 B-E). Egg and Sperm Chromosomes. — At the same time the chro- matin granules and threads in the egg and sperm nuclei take the form of chromosomes, and at this stage it is sometimes possible to see that each chromosome is composed of a series of granules, like beads on a string; these granules are the chromomeres (Fig. 4 L). The number of chromosomes is constant for every species and race, though the number may vary in different spe- cies. In the thread worm, Ascaris megalocephala, there are usually two chromosomes in the egg nucleus and two in the sperm nucleus (Fig. 43 D). In the gastropod, Crepidula (Fig. 45), there are about thirty chromosomes in each germ nucleus and sixty in the two. Distribution of Chromosomes. — Then the spindle and asters grow larger and the nuclear membrane grows thinner and finally disappears altogether, leaving the chromosomes in the equator of the spindle (Figs. 5 A, 6 F, 42 F, and 43 F). Each of the chromosomes then splits lengthwise into two equal parts, and in the splitting of the chromosomes 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 constriction which pinches the cell in two in the plane which passes through the equator of the spindle (Figs. 5, 7, 43 F, 45 B). Finally the chromosomes The Cellular Basis 139 grow in size by the absorption of achromatin from the cell body forming the chromosomal vesicles in which the chromatin takes the form of threads and granules, the chromosomal vesicles unite to form the daughter nuclei and these nuclei come back to a "rest- 9 * * H FIG. 44. MATURATION AND FERTILIZATION OF THE EGG OF THE MOUSE. A, First polar body and second maturation spindle. 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, for- mation of chromosomes in each germ nucleus. /, First cleavage spindle showing chromosomes from egg and sperm on opposite sides of spindle. (After Sobotta.) 140 H'eredity and Environment ing" stage similar to that with which the division began, thus completing the "division cycle" of the cell (Fig. 8). Identity of Chromosomes. — During the whole division cycle it is possible in a few instances to distinguish the chromosomes 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 daughter 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 are sometimes clearly double, one-half of each having come from the egg chromosomes and the other half from the sperm chromo- somes (Fig. 45). At every subsequent cleavage of the egg the chromosomes di- vide in exactly the same way as has been described for the first cleavage. Every cell of the developing animal receives 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 sometimes the case when dif- ferent races or species are crossed, these two groups of chromo- somes may still be distinguished at advanced stages of develop- ment. Where the egg and sperm chromosomes are not thus dis- tinguishable 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 an advanced stage of the cleavage (Fig. 45). Distribution of Cytoplasm. — At the same time that the mater- nal and paternal chromosomes are being distributed with such precise equality to all the cells of the developing organism the different substances in the cell body outside of the nucleus may be distributed very unequally to the cleavage cells. The move- ments of the cytoplasm of the egg, which began with the flowing of the surface layer to the point of entrance of the sperm, and The Cellular Basis 141 which continue during every cleavage of the egg, lead to the segre- gation of different kinds of plasms in different parts of the egg FIG. 45. SUCCESSIVE STAGES IN THE CLEAVAGE OF THE EGG OF A MOLLUSK (Crepidula), showing the separateness of the male and female chromo- somes ($ch, 9-c/O and of the male and female halves of each nucleus I42 Heredity and Environment and to the unequal distribution of these substances to different cells (Figs. 10,46,47). 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 differ in color, so that their distribution to different regions of the egg and to different cleavage cells may be easily followed, and even photographed, while in the living condition. The periph- eral layer of protoplasm is yellow and it gathers at the lower pole of the egg, where the sperm enters, forming a yellow cap (Fig. 46, i, pi.). This yellow substance then moves, 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. 46, 2-4). On the an- terior side of the egg a gray crescent is formed in a somewhat similar 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 protoplasm. The yellow crescent goes into cleavage cells which become muscles and mesoderm, the gray crescent into cells which become nervous system and notochord, the slate blue substance into endoderm cells and the colorless substance into ectoderm cells. (Figs. 47 and 48; see also Figs. 10 and n.) Localization of Substances. — Thus within a few minutes after the fertilization of the egg, and before or immediately after the first cleavage, the anterior and posterior, dorsal and ventral, right and left poles are clearly distinguishable, and the substances which will give rise to ectoderm, endoderm, mesoderm, muscles, noto- chord and nervous system are plainly visible in their character- istic positions. At the first cleavage of the egg each of these substances is di- vided into right and left halves (Fig. 46, 5). The second cleavage cuts off two anterior cells containing the gray crescent from two posterior ones containing the yellow crescent (Fig. 46, 6 and Fig. 47, i). The third cleavage separates the colorless protoplasm in the upper hemisphere from the slate blue in the lower (Fig. 47, The Cellular Basis 143 1P.S. 6 FIG. 46. SECTIONS OF THE EGG OF Styela, showing maturation, fertiliza- tion and early cleavage, i P.S., First polar spindle, p.b., Polar bodies; $ N, sperm nucleus, $ N, egg nucleus, p. I., peripheral layer of yellow pro- toplasm. Cr., Crescent of yellow protoplasm. As, Aa, Anterior cells, B3, Ba Posterior cells of the 4-cell stage. In I the sperm nucleus and cen- trosome are at the lower pole near the point of 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. 144 Heredity and Environment 2). And at every successive cleavage the cytoplasmic substances are segregated and isolated in particular cells, and in this way the cytoplasm of the different cells comes to be unlike (Figs. 47 and 48). When once partition walls have been formed between cells the substances in the different cells are permanently sepa- rated 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. 9, 10, n), though the segregation of substances and the differ- entiation of cells are not so evident in the last named animals be- cause these substances are not so strikingly colored. Indeed the segregation and isolation of different 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, indeed 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 chro- mosomes is equal and non-differential. When once the various tissues have been differentiated the further divisions in these tissue cells are usually non-differential even in the case of the cell bodies. Significance of Cleavage. — There can be no doubt that this re- markably complicated process of cell division has some deep significance; why should a nucleus divide in this peculiarly in- direct manner instead of merely pinching in two, as was once supposed to be the rule ? What is the relation of cell division to embryonic differentiation ? In this process of mitosis, or indirect cell division, two important things take place: (i) Each chro- mosome, chromomere and centrosome is divided exactly into two equal parts so that each daughter structure is at the time of its The Cellular Basis 145 formation quantitatively and qualitatively precisely like its mother structure. (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 localization of dif- ferent parts of the cytoplasm in definite regions of the cell, and this cytoplasmic 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. Cytoplasm Differentiates, Nuclei Do Not. — 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 suggests at once that the nucleus contains non-differen- tiating material, viz., the idioplasm or germplasm, which is char- acteristic of the race and is carried on from cell to cell and from generation to generation ; whereas the cell body contains the dif- ferentiating 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 division of the chromo- somes during development was of a differential character, the daughter chromosomes differing from each other at every dif- ferential division in some constant and characteristic way, and that these differentiations of the chromosomes produced the charac- teristic differentiations of the cytoplasm which occur during development. But there is not a particle of evidence that the ordi- nary division of chromosomes is differential; on the contrary, there is the most complete evidence that their division is remark- ably 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 cytoplasm upon the nuclei, as is true for example, in the differentiation of the chn>- mosomes in the somatic cells as contrasted with the germ cells of 146 Heredity and Environment Ascaris (Fig. 49). In this case Boveri has shown that the nuclei and chromosomes of germ cells and of somatic cells are at first alike whereas the cytoplasm in these cells is unlike; later the nuclei and chromosomes of these two kinds of cells become unlike, FIG. 47. CLEAVAGE OF THE EGG OF Styela, showing distribution 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. The Cellular Basis 147 owing probably to the peculiarities of the cytoplasm of these cells. These nuclear differentiations are caused by differential divisions of the cytoplasm and not of the nucleus. But while the chromo- FIG. 48. GASTRULA AND LARVA OF Styela, showing the cell lineage of vari- ous organs, and the distribution of the different kinds of protoplasm to these organs. Muscle cells are shaded by vertical lines, mesenchyme by horizontal lines, nervous system and chorda by stipples. 148 Heredity and Environment somes themselves divide equally, other portions of the nucleus may not do so. Nuclear achromatin and oxychromatin, like the cytoplasm, may divide unequally and differentially, and this is probably a prime factor in development. On the other hand, the differential division of the cytoplasm is a regular and characteristic feature of ontogeny ; indeed, the seg- regation and isolation of different kinds of cytoplasm in differ- ent cells is one of the most important functions of cell division during development. Thus we find in the division apparatus of the cell a mechanism for the preservation in unaltered form of the species plasm or germplasm of the nucleus, and for the progres- sive differentiation of the personal plasm or somatoplasm of the cell body. 3. The Origin of the Sex Cells. — The sex cells are among the latest of all cells of a developing animal to reach maturity, and yet they may be among the earliest to make their appearance. Every sex cell, like every other type of cell, is a lineal descendant of the fertilized egg (Fig. 41), 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 stage of development 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 differentiation such as fibres, secretions, etc., 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 result of this difference in the rate of growth and division the spermatogonia become much smaller and immensely more numerous than the oogonia. This The Cellular Basis 149 period in the genesis of the sex cells is known as the division period (Fig. 41). FIG. 49. DIFFERENTIATION OF GERM CELLS AND SOMATIC CELLS IN THE EGG OF Ascaris. A and B, Second cleavage division showing that the chro- mosomes 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 contain all of their 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.) Heredity and Environment (b) The Growth Period. Oocytes and Spermatocytes. — This period of rapid cell division is followed by a period of growth without division during which the developing sex cells are called primary oocytes or spermatocytes. 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 reproduc- tive period ; during 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 FIG. 50. DIFFERENT STAGES IN THE DEVELOPMENT OF THE EGG OF THE RABBITT. 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, Later stages showing pairs of chromosomes due to conjugation. F, Later stage in which the distinctness of the chromo- somes is temporarily lost. (After Winiwarter.) The Cellular Basis 151 cells. The growth -period of a spermatocyte lasts for a briefer time than does that of an oocyte so that the former remains relatively small (Fig. 41). Sy nap sis. — All of the cell divisions which take place during the division period are of the usual kind, in which every chromo- some splits lengthwise into two and the two halves then separate and move to opposite poles of the spindle where they swell up into chromosomal vesicles and form the daughter nuclei, as is shown in Figs. 7, and 43. But during the growth period of the oocytes and spenmatocytes the chromosomes form a closely wound coil of long chromatin threads (Fig. 50 A and B), and when these threads uncoil later it is seen that the chromosomes have united in pairs (Figs. 50 D and E, 50 a, 51 B, 52 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 a temporary one and is not so close that they lose their identity. By this union of the chromosomes into pairs the number of separate chromosomes is reduced to half the normal number ; if there are usually 4 chromosomes, as in As- caris, they are reduced to 2 pairs ; if 48 chromosomes, as in man, there are 24 of these pairs. Conjugation of Homologous Chromosomes. — In the conjuga- tion of the chromosomes it is plain that, generally speaking, those chromosomes unite which are similar in shape and size; big chromosomes unite with big ones, little ones with little ones, and those of peculiar shape with others of similar shape (Figs. 50 a, 51 B, 52 B, 54, 58). It is probable that the two members of a pair of conjugating chromosomes are homologous not merely in shape and size but also in function, though this homology does not amount to identity. These homologous chromosomes may be com- pared to the fingers of the two hands ; each digit differs from every other one but the thumb, index finger and other fingers of the right hand are homologous but not identical with the correspond- 152 Heredity and Environment FIG. 50 a. SYN APSIS (CONJUGATION) OF CHROMOSOMES in the grasshopper Phrynotettix. A. At the left, Telophase of Spermatogonium showing chromosomes in nucleus, among them X and a pair B. At the right, are 12 pairs of B chromosomes in synapsis, each pair from a different animal, and one member of each pair from the father, the other from the mother. Homologous chromomeres (granules I, 2, 3, 4, 5) are shown in each chromosome. B similar stages in chromosome pair A. Some of the chromosomes in the middle show a "secondary" longitudinal split and a "crossing over" (?) of the halves. C. Tetrads (conjugated chromosomes) of pair B formed by shortening and thickening of the chromosome pairs and by the appearance of the "secondary" split. (After Wenrich.) ing digits of the left hand, and the conjugation of homologous chromosomes may be compared to the placing together of the two hands* so that homologous digits come together. In some instances it can be proved that one member of each conjugating pair of chromosomes comes from one parent and the other from the other parent, and it is probable that this is always The Cellular Basis 153 true. In every cell of every individual which has developed from a fertilized egg there are two full sets of chromosomes, one of which came from the sperm and the other from the egg; but when this individual in its turn produces germ cells homologous chromosomes of each set unite in pairs, side by side, during the growth period. This again may be compared to the union of the two hands, the right digits, for example, representing the paternal and the left the maternal chromosomes which come to- gether in homologous pairs, corresponding joints lying opposite each other, as corresponding genes in the chromosomes lie oppo- site each other. These synaptic pairs are the bivalent chromosomes, and in ad- dition to showing the line of junction by which they are united they frequently 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 chromosomes are known as tetrads (Figs. 51 B, 52 B, C). (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 maturation divisions and they are the last divisions which take place in the formation of the egg and sperm. Reduction Division. — In one or the other of these two matura- tion divisions the pairs of chromosomes 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. 51 C, D, 54) ; 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- 154 Heredity and Environment A. ^-rrr>^ -B FIG. 51. SPERMATOGENESIS OF A NEMATODE WORM (Ancyracanthus} . A, Chromosomes of sperm mother cell, n in number, before their union into pairs. B, early stage of first maturation division; 10 of the chromo- somes have united into 5 pairs and each of these has split lengthwise; I chromosome remains unpaired. C, First maturation division after the 5 pairs of chromosomes 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 re- sulting from the division of the two cells like D, in which every chromo- some has split into two so that two of the cells contain 5 and two contain 6 chromosomes. F , two of these cells changing into spermatozoa, one con- taining 5 and the other 6 chromosomes. (After Mulsow.) The Cellular Basis A B 155 FIG. 52. OOGENESIS OF A NEMATODB WORM (Ancyracanthiis) . 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. E and F, Eggs after fertilization; the egg nucleus is above and contains 6 chromosomes, the sperm nucleus is below and contains 5 chromosomes in one case and 6 in the other; in the former case the egg becomes a male with n chromosomes, in the latter a female with 12 chromosomes. (After Mulsow.) 156 Heredity and Environment 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 resemblances but indi- vidual differences, ,antf consequently 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 (Fig. 58). Again this is comparable to the separation of the digits of the two hands after these have been placed together in homologous pairs, except that all the right digits must go with the right hand, all the left ones with the left hand since they are permanently at- tached to the hands. But the homologous chromosomes are free and each chromosome of a pair may separate to the right or to the left as the digits might do if they were severed from the hands. Then each digit of a pair could separate either to the right or to the left and it would rarely happen that all the right hand ones would go in one direction and all the left hand ones in the other, but since the members of each pair could separate in two directions and since there are five pairs, the possible number of different combinations after separation would be (2)5 or 32, and yet in each of these combinations the full set of digits from the thumb to the little finger would be present. Finally the fer- tilization of the egg may be likened to the game of "bean por- ridge" in which different hands (gametes) each with its full set of digits (chromosomes) are .struck together thus making new c6mbinations of digits (chromosomes). 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 fertilization the whole number is again restored. The double set of chromosomes is known as the diploid number, the single set as the haploid 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 The Cellular Basis 157 two maturation divisions (Fig. 51 C, D), and that the second of these divisions is like an ordinary mitosis in that each chromo- some splits into two and the halves move apart, such a division being known as an equation division (Fig. 51 E), but it is pos- sible that some chromosome pairs undergo an equation division in the first maturation mitosis and a reduction division in the sec- ond, while other chromosome pairs may reverse this order. It is an interesting fact that long before the reduction of chro- mosomes had been actually seen Weismann maintained on theoret- 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 matura- tion divisions; this hypothesis of Weismann's is now an estab- lished fact. Mature Egg and Sperm. — As the result of these two matura- tion divisions four cells are formed from each cell (spermato- cyte or oocyte) of the growth period. In the spermatogenesis each of these four cells is transformed into a functional sperma- tozoon (Figs. 41, 51 F) by the condensation of the nucleus into the sperm head and the outgrowth of the centrosome and cyto- plasm to form the tail. In the oogenesis only tfne of these four cells becomes a functional egg while the other three are small rudimentary eggs which are called polar bodies and which take no further part in development (Figs. 41, 42 C-F). The fertili- zation of the egg usually takes place coincidently with the for- mation of the polar bodies, and so we come back once more to the stage from which we started, thus completing the life cycle. C. SEX DETERMINATION In the formation of the sex cells one can distinguish at an early stage differences between the larger oogonia and the smaller and more numerous 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 158 Heredity and Environment fifth week, and it is evident that the real causes of this differ- ence 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. Hundreds of hypotheses have been advanced to explain this perennially interesting phenomenon. 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 determined. Unfortunately these hypotheses and rules are gen- erally founded upon a few observations 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 forgets the failures of a rule and remembers the times when it holds good it is possi- ble to believe 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. i. Chromosomal Determination: XO Type. — This was the condition regarding the causes of sex determination which pre- vailed up to the year 1902. Immediately preceding that year it had been found that two kinds of spermatozoa were formed in equal numbers in certain insects; one of these kinds contained a peculiar "accessary" or "odd" chromosome, and the other lacked it. The manner in which these two types of spermatozoa were formed had been carefully worked out by several investigators without any suspicion of the real significance of the facts. It was shown that an uneven number of chromosomes might be present in the spermatogonia of certain insects and that when maternal and paternal chromosomes united in pairs in synapsis one "odd" chromosome was left without a mate (Fig. 51 B). Later, in the reduction division, when the synaptic pairs sep- arated, the odd chromosome werit entire into one of the daughter cells, and the spermatozoa formed from this cell contained one The Cellular Basis 159 chromosome more than those formed from the other daughter cell (Fig. 51 C-F). Chiefly because these two kinds of spermatozoa 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 accessory chromosome, there was only one type of egg, since every egg con- tained the accessory chromosome, and he pointed out that if an egg were fertilized by a sperm containing an accessory, two ac- cessories 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 Pratenor Type Oocyto Bmrmatooyte Kedvcti6n Sp/armaAids FIG. 53. DIAGRAMS OF SEX DETERMINATION IN THE BUG, Protenor. The oocyte contains 6 chromosomes and the spermatocyte 5 chromosomes which are not yet united into synaptic pairs; the "sex" chromosomes are shown in black, two are present in the oocyte, but only one in the sperma- tocyte. In the reduction division the synaptic pairs separate, giving rise to two types of spermatids, one of which has the sex chromosome and the other lacks it ; all ova are alike in this regard. If an egg is fertilized by a sperm without the sex chromosome a male results; if fertilized by a sperm containing the sex chromosome a female results. (After Wilson with modifications.) i6o Heredity and Environment derived from the egg (Fig. 52 E and F, Fig. 53). This "acces- sory" chromosome was therefore called the "sex determining" or merely the "sex" chromosome and was designated by the letter X; consequently its double occurrence in the female was indicated by XX; its single occurrence in the male by XO, the O standing for zero or no chrompsome. XY Type. — In other cases Miss Stevens as well as Wilson dis- covered that two accessory chromosomes, differing in size, might be present in the male whereas in the female they are of equal size (Fig. 54). In such cases two types of spermatozoa are pro- duced in equal numbers, one containing a large and the other a small accessory chromosome, whereas every egg contains one large accessory chromosome. If such an egg is fertilized by a sperm containing a large accessory (the X chromosome) it gives rise to a female with the formula XX, if by a sperm containing Oocyte Fertilized FIG. 54. DIAGRAMS OF SEX DETERMINATION 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 sper- matocyte 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.) The Cellular Basis 161 a small accessory (the Y chromosome) it gives rise to a male with the formula XY (Fig. 54). In other animals one may not be able to distinguish separate X or Y chromosomes and yet such structures may be joined to one or two ordinary chromosomes. This is the case in the thread worm, Ascaris (Fig. 55), 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 pres- ent. Here also two classes of spermatozoa are found one with and the other without the accessory element, 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. 55). Recurring to the comparison of digits and chromosomes, the chromosomal theory of sex determination may be illustrated by assuming that all males have lost a particular digit, say the Atcaris Type Jterf action Division Mature Egg and Polar U W ®r -^5 FIG. 55. DIAGRAMS OF SEX DETERMINATION IN THE THREAD WORM, Ascaris. The X chromosomes (black) are here joined to ordinary chro- mosomes, 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 one type of sperm produce females, those fertilized by the other type produce males. (From Wilson.) 1 62 Heredity and Environment thumb, from one hand while all females have the full number on both hands. When the hands (gametes) are struck together as in the game of "bean porridge" there will be an equal number of cases in which a hand with five digits meets one with five (female) and one with four meets one with five (male). In the latter case there will be an "odd" thumb (chromosome) which has no mate. Sex- Determination in Man. — Even in man sex is determined in the same manner, according to several recent investigators. Wini- warter concluded that there are in the spermatogonia of man 47 chromosomes, one of which is the X or accessory chromosome. These unite in synapsis into 23 pairs, leaving the X chromosome unpaired; in the reduction division the pairs separate, while the X chromosome goes entire into one of the daughter cells, which consequently contains 23 -{- X chromosomes, whereas the other daughter cell contains 23 chromosomes. In the female there are probably 48 chromosomes, there being two X chromosomes, one from each parent, and after the reduction divisions every egg contains 24 chromosomes. Winiwarter held that if an egg is fertilized by a sperm containing 24 chromosomes an individual with 48 chromosomes, or a female, is produced; if fertilized by a sperm with 23 chromosomes an individual with 47 chromosomes, or a male, results. It is a curious fact that it has been very difficult to determine the exact number of chromosomes in man. This is probably due to the difficulty of preserving in an unaltered condition the chro- mosomes of mammals in general, as McClung and his pupils have shown, and also to the peculiar difficulty of obtaining human tis- sues in a perfectly fresh and normal condition. Thus Guyer and Montgomery found not 47 but about 22 chromosomes in the spermatogonia of man. Since both the latter investigators worked on negroes whereas Winiwarter worked on white men it was suggested by Morgan and Guyer that there may be twice as many chromosomes in the white race as in the black. A The Cellular Basis similar condition in which one race has twice as many chro- mosomes as another race of the same species is found in two races of the thread worm, Ascaris megalocephala. However, more recent work has shown that the number of chromosmes in white men and in negroes is the same. Wieman found 24 chromosomes in the spermatogonia ( ?) of both races and he inferred that in the male there is an XY pair of sex chromosomes, in the female an XX pair. On the other hand, Evans* found in hundreds of counts that there were constantly 48 chromosomes in the spermatogonia of a white man, thus indicating the presence of an XY pair of chromosomes in the male and an XX pair in the female. The most recent work on the number of chromosomes in man is by Painter,* who has studied normal testes of both whites and Mr* FIG. 56. SPERMATOGNIAL CHROMOSOMES OF NE*GRO. A. Spermatogonium showing 48 chromosomes, the smallest of which is the sex chromosome Y; the other sex chromosome X, is a small rod-shaped one. As reproduced the figure is magnified about 3000 diameters. From a preparation and drawing by Painter. B. The same chromosomes spread out and arranged, according to size and shape, in 24 synaptic pairs. * See Babcock and Clausen, p. 538. 164 Heredity and Environment negroes, removed in castration and fixed immediately by the most approved methods. He finds that in both whites and blacks, there are 48 chromosomes in the spermatogonia, or 24 synaptic pairs in the first maturation division; one of these is plainly the XY pair, the X and Y being unequal in size. I am indebted to Painter for the privilege of using one of his unpublished drawings (Fig. 56A) and for permission to quote his conclusions. Guyer has also informed me personally that in new and better material he now finds 48 chromosomes in human spermaftogonia, one of these being an X and another a Y. These discoveries appear to settle once for all this vexing question and to establish the fact that in man, as well as in many other animals, sex is determined by the chromo- somes, the sex chromosomes being XX in the female, and XY in the male. Sex a Mendelian Character — Correlations Between chromo- somes 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 Lepidoptera and birds, the evi- dence while not entirely convincing seems to point to the fact that two types of ova are produced and but one type of sperma- tozoa; but the general principle that sex is determined by the chance union of male-producing or female-producing gametes is not changed by such cases. Sex, therefore, appears to be inherited, that is, its factors are present in the germ cells but probably not as particular genes oc- cupying definite loci in a chromosome but rather as a relation of whole chromosomes, such as XX, XY or XO (see p. 172) ; it is a Mendelian character in which the female is usually homozygous for sex while the male is heterozygous. Consequently in the formation of the gametes every egg cell receives one sex de- terminer, while only one-half of the spermatozoa receive such a determiner, the other half of them being without it. If then an * Painter, T. S. — The Y Chromosome in Mammals, Science, May 27, 1921. Also letter dated November 3, 1921. The Cellular Basis egg is fertilized by a sperm with one of these determiners a fe- male is produced, if by a sperm without the sex determiner a male results. This is graphically illustrated in Fig. 57 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 female germ cells yields females (XX) and males (XO) in equal num- bers. Of course there is no such thing as a "sex-producing" chromosome, sex being a developed character which is the result of many intrinsic and extrinsic causes. The A" chromosome is only one factor in the development of sex but if it is a factor which differs in the case of the two sexes, it is a "sex-differential." 2. Environmental Influence. Alteration of Sex Ratios. — On the other hand there are many observations which seem to indi- Garnetes FIG. 57. 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 2:2 or i: I ratio of females to males. 166 Heredity and Environment cate that the sex ratio may be changed by environmental condi- tions acting before or after , fertilization and therefore it has been concluded that sex is determined by extrinsic rather than by intrinsic causes. Many of these observations, as already remarked, are now known to be erroneous or misleading, since they do not prove what they were once supposed to demonstrate. But there remain a few cases which cannot at present be explained away in this manner. Perhaps the best attested of these are the observa- tions of R. Hertwig and some of his pupils on the effect of the time of fertilization on the determination 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 pre- vented for two or three days from laying the eggs after they have entered the oviducts, the proportion of males to females is enor- mously increased. A wholly similar result has been observed by Pearl and Parshley in the case of cattle, where delayed fertiliza- tion of the egg leads to a great preponderance of males. Hertwig attempts to explain his extremely interesting and important ob- servations 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 with- drawing water from them by placing them in solutions of salts, acids, sugar, etc., but the manner in which drying increases the proportion of females is wholly unknown. All of these experi- ments on sex determination in frogs and toads are somewhat complicated by the difficulty of determining the sex of tadpoles and young animals and the evidence is by no means conclusive that in these cases sex is determined by extrinsic causes. Quite recently Whitney- has shown that in several species of rotifers a scanty diet produces in the second filial generation only female offspring while a copious diet produces as high as 95 per The Cellular Basis 167 cent of male offspring. Many earlier investigators had found that food influences sex, though usually it was held that scanty food led to the production of males and abundant food to females, but this older work, unlike Whitney's, was generally uncritical. A. F. Shull finds that "the irrevocable event leading to the determination of the sex of any given parthenogenetically produced individual (rotifer) occurs in the maturation of the egg from which that in- dividual's mother develops .... Probably a definite chemical change in the proteins of the chromosomes occurs at the time of maturation." The diet may thus affect the chromosomes and through these the sex. Extensive statistics show that in many animals including man more males are born than females, whereas according to the chro- mosome theory of sex-determination as many female-producing spermatozoa are formed as male-producing ones. It is possible to explain such departure from the I : i ratio of males to fe- males in conformity with the chromosome theory if one class of spermatozoa are more active 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, but if before fertilization the activity or vitality of male- producing spermatozoa is greater than that 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 fertilized eggs, and in one of these cases, viz., Phylloxera, Morgan has discovered that this 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, Whitney and others have brought about may be explained as due to a differential action of the modified egg cells or of the environment upon the two types of sperma- tozoa. In the Drosophila work many lethal mutations have ap- 1 68 Heredity and Environment peared which cause the early death of those zygotes in which the lethal gene is not balanced by a normal allelomorph. Some of these lethals are sex-linked and alterations of the normal sex ratio in certain cases may be explained as the result of these lethal factors. Finally the chromosomal theory of sex determination is so well supported in so many instances where at first it seemed impossible of application, that it ought not to be rejected until unmistakable evidences can be adduced against it. 3. Hermaphrodites and Intersexes. — Finally a number of cases have been brought to light which indicate the necessity of dis- tinguishing between the hereditary determination of sex and its ontogenetic development. It has been known for a long time that in bees and ants the workers are imperfect females, while the queens are perfect females, and that the kind or amount of food which is fed to the larvae determines whether they will be workers or queens (see p. 233). Again in many animals the development of male or female characters is dependent upon internal secretions or hormones from the sex glands or other organs (see pp. 234-237). In these cases it is evident that sex was determined at an early stage, probably at fertilization, but the development of these male or female characters, which occurs later, is influenced also by the external or internal environment (p. 218). Breeders of cattle are familiar with the fact that when twin calves are of opposite sex, the male is sexually perfect, but the female usually has many male characters and grows into a steer-like animal which is sterile and is known as a "free martin." In a recent paper Lillie has shown that in all such cases the twins are connected by blood vessels at an early stage in utero and that there is a more or less complete circulation of blood from one foetus to the other, and he concludes that "sex hormones," which are probably formed earlier in the male than in the female, are carried from the male to the female twin, thus causing the de- velopment of male organs in an animal which would otherwise have been a female. Therefore the chromosomal or "zygotic The Cellular Basis 169 determination of sex is not irreversible predestination but a quan- titative overbalance in the direction of one sex or the other" which may later be changed. Somewhat similar conclusions had previously been reached by Whitman and by Riddle regarding the sex of pigeons, by Shull in the case of Lychnis, and especially by Goldschmidt for the gypsy-moth. Goldschmidt supposes that sex is determined by certain enzymes which he calls "andrase" and "gynase" ; an excess of the former leads to the development of males, an excess of the latter to females, and varying mixtures of the two to varying intergrades or "intersexes." He assumes that these enzymes are present in the "sex determining" chro- mosomes at fertilization as well as in later stages and thus he attempts to identify all sex determining factors with these "sex enzymes." The difference between determination by chromo- somes and by internal secretions, that is, between heredity and development, is found chiefly in the time at which these enzymes act. Morgan has shown that "gynandromorphs" or "sex mosaics" are due to the irregular distribution or loss of certain "sex chromosomes" owing to abnormalities in fertilization or cleavage. In such cases one portion of the body shows male characters, an- other portion female ones and a study of the chromosomes in these regions shows that in the former the male combination of chromosomes is present, in the latter the female combination. This comes as near to a demonstration of the truth of the chro- mosomal theory of sex determination as is possible. Finally, the most notable recent work on this subject is by Bridges.* In his studies of the pomace fly, Drosophila melano- gaster, he found intersexes whose genetical behavior was such as to suggest that they had more or less than the usual number of chromosomes. By means of breeding experiments as well as by microscopical study O'f their germ cells he has demonstrated that *Bridges, C. B. — Triploid Intersexes in Drosophila Melanog aster. Science, Sept 16, 1921. 170 Heredity and Environment this is true. In the normal fly of this species there are four pairs of chromosomes (Fig. 65) ; the first pair (Chromosomes I) are the sex chromosomes which are XX in the female and XY in the male; the second and third pairs (Chromosomes II and III) are large and V-shaped; the fourth pair (Chromosomes IV) are very small and round. Through the failure of chromosome pairs to separate in the maturation divisions an egg or sperm may come to contain an abnormal number of any or all of these chromo- somes. As opposed to the sex chromosomes, all others are known col- lectively as "autosomes." A normal female has two X chromo- somes and two of each of the autosomes ; a normal male has one X and two of each of the others ; but when two X's occur with three of each of the others, or with three of some of them, inter- sexes result, and these may grade all the way from perfect females to perfect males depending upon the ratio of X chromosomes to autosomes. Sex is therefore determined by a quantitative rela- tion of the X chromosomes to the autosomes, and if one assumes that there are sex enzymes, such as Goldschmidt postulated, it is probable that the X chromosomes produce mainly "gynase" and the autosomes "andrase." Such an explanation harmonizes well not only with all that is known regarding the chromosomal determination of sex and of intersexes but also with much that is known concerning the possibility of modifying the development of sex by enzymes or hormones from glands of internal secretion. In either sex many secondary sexual characters of the other sex are present during development and traces of these may per- sist in the adult; but one set of these characters develops fully in the male and another set in the female, so that they may be called sex-limited. The development of the secondary sex char- acters is usually determined by internal secretions from the ova- ries or testes, though in some cases they may develop after these organs have been removed, but in the last analysis both primary and secondary sex characters are dependent upon the sex deter- The Cellular Basis 171 miners in the germ cells. Sex and sex-limited inheritance are only special cases of Mendelian inheritance and the full develop- ment of the male or female condition is dependent upon the predominance of male-determining, or of female-determining factors, both hereditary and environmental; while the condition of "intersexes" is the result of the lack of such predominance. On one point there is general agreement, namely every organ- ism is at the beginning of ontogeny so evenly balanced between maleness and femaleness that very slight changes in heredity or environment may cause it to go one way or the other ; every or- ganism is potentially both male and female, and even in the fully developed state each sex carries the vestiges of suppressed organs of the other sex. D. 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 fertiliza- tion. We have briefly traced the origin, maturation and union of male and female sex cells in a number of animals, and in these phenomena we have the mechanism of the hereditary continuity between successive generations. But in addition to these specific facts there are certain general considerations which need to be emphasized. I. THE SPECIFICITY OF GERM CELLS The conclusion is inevitable that the germ cells of different species and even those of different individuals are not all alike. Every individual 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 environmental stimuli, i.e., they may be intrinsic or extrinsic, but as a matter of fact experi- ence has shown that they are generally intrinsic in the germ. In 172 Heredity and Environment the same environment one egg becomes a chicken and another a duck; one becomes a frog and another a fish and another 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, etc. Since these differences may occur in the same environment they must be due to differences in the germ cells concerned. Environment Non-specific. — On the other hand different en- vironmental conditions may be associated with similar develop- mental results. Loeb and others have found that artificial par- thenogenesis 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 nothing specific in these different stimuli. Similarly Stockard has discovered that cyclopia, or one-eyed monsters, may be produced by magnesium salts, alcohol, chlore- tone, chloroform, and ether, and to this list McClendon has added various other salts and anaesthetics. In all such cases it is evident that the specific results 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- 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 another to a sperm? If these dif- ferences 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. Protoplasm Specific. — Many differences in the material sub- stances of cells are visible, and many more are invisible though still demonstrable. These differences may not be detectable by chemical or physical tests, and yet they may be demonstrated The Cellular Basis 173 i physiologically and developmentally. The most delicate of all tests are physiological, as is shown by the Weidal test in typhoid fever, the Wassermann reaction in syphilis, the reactions of im- munized animals to different toxins, etc. Lillie has recently shown that egg cells give off a substance which he calls "fertil- izing which can be detected only by the way in which spermato- zoa react to it. No chemical or physical test can distinguish be- tween the different eggs or spermatozoa produced by the same individual, but the reactions of these cells in development prove that they are different. Undoubtedly chemical and physical dif- ferences are here present but no chemical methods at present available are sufficiently delicate to detect them. It is one of the marvelous facts of biology that practically every sexually produced individual is unique, the first and last of its identical kind, and although some of these individual dif- ferences are due to varying 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? Mie- scher 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 many kinds of albumin and other proteins, some with probably more than 700 carbon atoms- In such a complex sub- stance as protoplasm 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 different kinds of individuals in the world. Permutation's of Chromosomes. — Even with regard to morpho- logical elements which may be seen with the microscope it can be shown that an enormous number of permutations is possible (Fig. 58). It seems probable, as Boveri has shown, that different chromosomes of the fertilized egg differ in hereditary potencies, 174 Heredity and Environment and where the number of chromosomes is fairly large the number of possible combinations of these chromosomes in the germ cells becomes very great. In man, where there are probably 48 chro- mosomes and, after synapsis, 24 pairs of maternal and paternal ones, the possible number of permutations in the distribution of these chromosomes to the different germ cells would be 224, or 16,777,216, and the possible number of different combinations of fertilized eggs or oosperms which could be produced by a single pair of parents would be (16,777,21 6) 2, or approximately three hundred thousand billions.f But probably other things than chro- mosomes differ in different germ cells, and it is by no means cer- tain that individual chromosomes are always composed of the same chromomeres, or units of the next smaller order, and in view of these possibilities it may well be that every human germ cell differs morphologically and physiologically from every other one, in short that every oosperm and every individual which de- velops from it is absolutely unique. Significance of Sexual Reproduction. — Indeed the production of unique individuals seems to be the chief purpose and result of sexual reproduction. In asexual reproduction the individual var- iations which occur are chiefly if not entirely due to environment, but in sexual reproduction they are also due to new combina- tions of hereditary elements. The particular germinal organiza- tion transmitted from one generation to the next depends upon (a) the organization inherited from ancestors, (b) the particular character of the cell divisions by which the germ cells are formed, (c) the particular kinds of egg and sperm cells which combine in fertilization. The inherited organization determines all the gen- eral characteristics of race, species, genus, order, phylum. It de- termines the possibilities and limitations of individual variations. Given a certain inherited organization, the individual peculiarities t Excluding duplications there would be 324 different genotypes and 224 different phenotypes, assuming that chromosomes always preserve their identity and that dominance is always complete. The Cellular Basis 175 1 9 GAMETES | ZYGOTES | 6 GAMETES I FIG. 58. DIAGRAM SHOWING SOME OF THE POSSIBLE DISTRIBUTIONS OF CHROMOSOMES OF GRANDPARENTS TO GRANDCHILDREN. Chromosomes of maternal grandfather shaded black, of maternal grandmother with 'cross lines; of paternal grandmother stippled, of paternal grandfather un- shaded. "Sex-chromosomes" are /-shaped, a pair being present in the female, a single one in the male. In the maturation of oocyte and of sper- matocyte homologous chromosomes unite in pairs and then separate into the gametes. With 6 pairs of chromosomes, each free to separate in 2 directions the largest possible number of combinations in the gametes is (2)6 or 64 and since any sperm may unite with any egg, the number of combinations possible in the zygotes is (64) 2 or 4096 and, excluding all duplications, there are (3)6 or 729 genotypes only 4 of which are shown in the diagram. 176 Heredity and Environment of the germ cells are determined by the particular character of cell division by which the germ cells are formed, and the peculiari- ties of the individuals or persons which develop from these cells are determined in large part by the particular kinds of germ cells which unite in fertilization. Comparison of Cards and Chromosomes. — The behavior of chromosomes in maturation and fertilization is like the shuffle and deal of cards in a game, and apparently with the same object, namely, never to deal the same hand twice. To make this compari- son 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 ; suppose that in the shuffling of these cards corresponding cards of the red and 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 denomina- tion in each hand, but if the cards are dealt indiscriminately each hand may contain two black cards of any denomination, two red cards or one black and one red- This description parallels what takes place in the maturation of the human germ cells, except that there is no evidence that there are more than two suits of chromo- somes, one of which is maternal and the other paternal. If now we complete this comparison by extending 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 denomination 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 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 mixture of chromosomes from dis- The Cellular Basis 177 tinct 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 oosperm and every individual which develops from it is different from every other one. Germ Cells as Specific as Persons. — This conception of the spe- cificity of every germ cell, as well as of every developed individ- ual, 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 (see p. 57) 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 transformation of the specific characters of the germ into those of the adult. From beginning to end de- velopment is a series of morphological and physiological changes but not of new formations or creations except in so far as new structures or functions appear as a result of "creative synthesis." It is only the incompleteness 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 become visible at certain stages, but their real beginnings are indefinitely remote. 178 Heredity and Environment 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 developed 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 correlation of organization between the germ and the adult. What correlations are known to exist between the oosperm and the developed ani- mal? Inheritance Factors and Developed Characters. — We have con- sidered in the preceding chapter (pp. 100-105) the evidence that there are specific inheritance factors or genes which are correlated with the development of specific adult characters. These factors are not the characters in miniature nor are they the "representa- tives" or "carriers" of characters, but they are the differential causes of characters. Every inherited character must have a differential cause in the germ cells, but this cause may not be a peculiar vital corpuscle nor even a peculiar atom or molecule but it must at least be some particular combination of atoms or mole- cules. It is practically certain that this differential cause of each inherited character is associated with some material substance which occupies some place in the germ cells. Location of Inheritance Factors. — If it is asked whether there are particular structures in germ cells which correspond to par- ticular inheritance factors it must be admitted 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 in the development of every character, and on the other hand that the differential cause of any character may be located in some differentiated 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 The Cellular Basis 179 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 biologi- cal analysis and when the contents of cells were supposed to be "perfectly 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 suppose that organization ceases with the powers of our present microscopes. With every improvement of the microscope and of microscopical technique, structures have been found in cells which were undreamed of before, and it is not probable that the end has been reached in this regard. We know that cells contain nuclei and chromosomes and chromo meres, centrosomes and plastosomes and microsomes, and we know that some of these parts differ in function as well as struc- ture. 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. 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 character, is a familiar part of Mendelian inheritance (Fig. 59). Even if these units be regarded as physiological processes they must be associated with particular structures, since function and structure are inseparable in life processes. What are the units in terms of cell structures and where are they located in the cell? i. CHROMOSOMAL INHERITANCE THEORY. — We have in .the chromosomes, as Wilson especially has emphasized, an apparatus which fulfils all the requirements of carriers of Mendelian fac- tors (Fig. 60). 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 Figs. 59 and 60; and so far as is known the chro- i8o Heredity and Environment mosomes are the only portions of the germ cells which ful- fil these conditions. The association, segregation and distribu- tion of Mendelian factors and of maternal and paternal chromo- somes are exactly parallel and it is not reasonable to suppose that this remarkable coincidence is without significance. Furthermore there is much additional evidence that the chro- mosomes are important factors in heredity and development: 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. McClung, Stevens, Wilson and others have dis- covered that the determination of sex is associated with the pres- ence or absence of a particular chromosome, the X or Y chromo- some, in the spermatozoon which fertilizes the egg. If an egg FIG. 59. DIAGRAM SHOWING UNION OF FACTORS IN FERTILIZATION AND THEIR SEGREGATION IN THE FORMATION OF GERM CELLS. With 4 pairs of fac- tors (Aa, Bb, Cc, Dd) 16 types of gametes are possible as shown in the two series of small circles at the right. (After Wilson.) The Cellular Basis 181 is fertilized by a sperm which lacks the X chromosome a male is produced, if fertilized by the other type a female results. This correlation between the presence or absence of a whole chromo- some and of a developed character such as sex, was the first case of the kind that was known and more than anything else it served to prove that the chromosomes contain the Mendelian factors. The absence of a whole chromosome is plainly visible under the microscope, whereas the absence of a single factor or gene from a chromosome would never have been seen. These fortunate cases in which the male lacks a whole chromosome seem almost to have been intended to give ocular proof of the chromo- somal theory of heredity; they are to biology what the rings of Saturn are to astronomy, — a visible confirmation of a great theory. Bynaprii AbeD *•*««••• Oenn Cell. Dii-iiion Simple* Group, Somatic Division* Duple* Group* FIG. 60. DIAGRAM OF GERM CELLS CORRESPONDING TO FIG. 59, show- ing the union of maternal chromosomes (ABCD) and paternal ones (abed) in fertilization, their distribution in cleavage, 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.) 182 Heredity and Environment We have in these facts a remarkable correlation between the distribution of the chromosomes and the occurrence of certain characters of the adult animal. The association of maternal and paternal chromosomes in fertilization 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 of chromosomes in cleavage is ab- normal the larva shows abnormal characters (Boveri) ; sex de- termination is associated with the distribution of a particular chromosome to one-half of the spermatozoa, and the fertilization of the egg by one or the other type of spermatozoa. (Wilson). There are many parts of a germ cell, all of which may be con- cerned in heredity and development, but the chromosomes appear to be the seat of the differential factors for Mendelian characters. On the other hand it has been objected by certain investigators, notably Child, Foot and Strobell, et al, 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 activi- ties." This objection seems to confuse the idea of natural cause with that of final cause. Science knows nothing 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 subse- quent results. Undoubtedly the chromosomes are the results of antecedent processes, and yet they may also be the causes of sub- sequent results. No thoughtful person has ever maintained that chromosomes or any other things in nature are autonomous, abso- lute, uncaused causes. Abnormal Distribution of Chromosomes and Factors. — Experi- mental evidence that the chromosomes are the seat of inheritance factors is found in the correlation between the abnormal distribu- tion of chromosomes and the development of abnormal characters in the embryo or adult. An abnormal distribution of chromo- somes to the cleavage cells may be caused in a variety of ways but one of the least injurious methods of accomplishing this is by The Cellular Basis 183 causing two spermatozoa instead of one to enter an egg. In such doubly fertilized eggs Boveri discovered that different cleav- age cells receive a different number of chromosomes and in gen- eral those cells which receive the largest number develop most typically, while those which receive a small number develop atypi- cally. By a skillful analysis Boveri proved that normal develop- PRIMARY BOM -DISJUNCT ION IN MALE PRIMARY HOB-DISJUNCTION JH FEMALE .SYGOTES FIG. 60 a. DIAGRAM OF NON-DISJUNCTION OF THE SEX CHROMOSOMES in the Maturation of the Egg and Sperm of Drosophila melanogaster, with the resulting types of Zygotes (According to Bridges). Primary non- disjunction in the sperm leads to the production of XXY females and XO males; primary non-disjunction in the egg leads to four combina- tions, two of which (XXX and YO) are non-viable; secondary non- disjunction occurs in the maturation of an XXY egg. In Drosophila Y is an "empty chromosome," i.e., it appears to contain no genes, yet it has some influence since a male without it (XO) is sterile. Zygotes are matro- clinous or patroclinous depending upon whether they get a larger number of X chromosomes from the egg or from the sperm. 184 Heredity and Environment ment depends not so much upon the absolute number of chromo- somes in a given cell as upon a complete set of all the different kinds of chromosomes, and when a complete set was not present certain characters failed to develop. By this means he showed that different chromosomes of a set differ in hereditary value, as, for example, the fingers of a hand differ from one another, and that two chromosomes of one kind could not make up for the lack of one of another kind, any more than two thumbs could make up for the loss of a little finger. A still more detailed correlation between the presence or absence of a particular chromosome and the presence or absence of parti- cular characters in the developed organism has been described by Bridges (1916). In his study of the pomace fly Drosophila melanogaster, he found that the occasional appearance (i in 1700) of a matroclinous daughter or patroclinous son was due to the fact that the members of the XX pair of chromosomes of the oocyte fail to separate in the reduction division so that both XX's are included in the egg (Fig. 65, C) or both are extruded in the polar body, the eggs being accordingly XX or O; or the two chromo- somes of the XY pair in the spermatocyte fail to separate in the reduction division so that one sperm may have both X and Y, and another lack both of these chromosomes (Fig. 60 a). This phenomenon he calls "non-disjunction" and it results in the pro- duction of matroclinous daughters or patroclinous sons, and in many other irregularities of inheritance which follow precisely the abnormal distribution of these chromosomes. A patroclinous son is the result of the fertilization of an O egg by an X sperm ; such an XO son is sterile whereas the normal XY son is not, thus showing that the chromosome Y has some function though in Drosophila it does not contain any of the genes ; fertilization of an O egg by a Y sperm produces a combination which is non-viable. Fertilization of an XX egg by a Y sperm produces a matroclinous daughter (XXY), whereas fertilization of an XX egg by an X sperm produces a combination (XXX) which is non- viable. The Cellular Basis 185 These relations are shown schematically in the accompanying diagrams (Fig. 60 a). 2. LINKAGE OF CHARACTERS AND CHROMOSOMAL LOCALIZATION. — Finally the study of characters which are linked together in heredity, joined with the study of the chromosomes and their dis- tribution in the maturation and fertilization of the germ cells, has not only confirmed the chromosomal theory of heredity but has also shown that certain chromosomes carry the genes for certain characters and has even indicated the relative positions of dif- ferent genes in the chromosomes. Thanks to the work of Bateson, Morgan, and many others, it is now known that many characters are linked together in inheri- tance. Darwin had long ago noted that male albino cats with blue eyes are usually deaf and many other cases of the association of peculiar characters had been reported by earlier observers. In 1906, Bateson and Punnett found that sweet peas with purple flowers usually have elongated pollen grains, whereas those with red flowers have round pollen. Since 1910 Morgan and his pu- pils have discovered about four hundred new characters, or mutations in the pomace fly, Drosophila melanogaster, which are usually linked together in four groups. a. Sex-linked Inheritance. — The first cases of such linkage studied by Morgan were in characters which are usually asso- ciated with one or the other sex, but which may have nothing to do with reproduction and may affect any part of the body. Such characters are not necessarily limited to one sex or the other, as are many 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" in- heritance) 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 chromosomes and has named them sex-linked characters. In the pomace fly, Drosophila, he has discovered a large number of such characters which are 1 86 Heredity and Environment FIG. 61. SEX -LINKED INHERITANCE OF WHITE AND RED EYES IN Dro- sophila. Parents, red-eyed female and white-eyed male (females are larger than males; F,, red-eyed males andefemales; F2, red-eyed females and equal numbers of red-eyed and white-eyed males. The distribution of sex chromosomes is shown through the middle of the figure, the straight rods being X-chromosomes and the hooked rods Y-chromosomes ; the X-chromosomes that carry the factor for red eye are black, those that carry the factor for white eye are unshaded ; the Y-chromosomes carry no factors for eye color. (From Morgan). The Cellular Basis 187 FIG. 62. RECIPROCAL OF CROSS SHOWN IN FIG. 61. Parents, white-eyed female and red-eyed male ; F , red-eyed females and white-eyed males ("criss-cross inheritance") ; F«, equal numbers of red-eyed females and males. The distribution of the sex-chromosomes is shown as in Fig. 61. (From Morgan.) i88 Heredity and Environment linked with sex, such as the color of the eyes and of the body, the length of the wings, etc. A typical case is shown in Figs. 61 and 62. The eye color of this fly is normally red, but mutations have arisen in which the eye is white. Such a mutation first ap- pears 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 F2 have red eyes while half of the males have red eyes and the other half have white eyes (Fig. 61). On the other hand if one of the F± females of this cross is bred with a white- eyed male (Fig. 62, Fx), half of the females of F2 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 FIG. 63. DIAGRAM OF INHERITANCE OF COLOR BLINDNESS THROUGH THE MALE. A color blind male (here black) transmits his defect to his grand- sons only. The corresponding distribution of the sex chromosomes is shown on the right, the one carrying the factor for color blindness being black. Recent work shows that the six chrosomes of the human male are XY and not XO, as shown in this diagram. The Cellular Basis 189 male (Fig. 62, P) all the females of the Ft generation are red- eyed and all the males white-eyed ("criss-cross" inheritance) and if these are interbred there are produced in the F2 generation equal numbers of red-eyed and white-eyed males and females (Fig. 62). The distribution of the maternal and paternal sex chromo- somes exactly parallels this distribution of this sex-linked char- acter, as is shown in Figs. 61 and 62, and this proves that the differential factors for these characters are carried in these sex chromosomes. By a series of ingenious experiments Foot and Strobell have shown that the differential factors for certain sex-limited char- acters in insects, that is, characters which are limited to one sex, are not contained in the "sex chromosomes," and they argue that the differential factors for sex and for sex-linked characters can- not be located in these chromosomes. Their conclusions apply only to sex-limited and not to sex-linked characters. There is Chromosomes XX Parents 9 ^M Gametes Gametes FIG. 64. DIAGRAM OF INHERITANCE OF COLOR BLINDNESS THROUGH THE FEMALE. A color blind female transmits her defects to all her sons, to half of her granddaughters and to half of her grandsons. Corresponding distribution of" sex chromosomes on right. (After Morgan.) 19° Heredity and Environment no doubt that the factors for the determination of sex and sex- linked characters are distributed in the same way as the "sex chromosomes" are, and this proves that these factors are located in the "sex chromosomes." Haemophilia. — Another case of sex-linked inheritance is found in an abnormal condition in man known as haemophilia, which i? characterized by a deficiency in the clotting power of the blood and consequently by excessive bleeding after injury. "Bleed- ers" are almost always males, though the defect is always trans- mitted to a son from his mother who does not usually show the defect because it appears in females only when both parents were affected. The manner of inheritance of this character is exactly similar to the inheritance of white eyes in Drosophila and is in all probability due to similar causes. Daltonism. — One of the most striking cases of sex-linked in- heritance is that form of color blindness known as Daltonism, in which the affected person is unable to distinguish between red and green. It is known that males are more frequently affected than females, and that color blindness is in some way associated with sex. It requires two determiners for color blindness, one from the father, the other from the mother, to produce a color blind female, whereas only a single determiner is necessary to produce a color blind male, just as is true of sex. The accom- panying diagrams (Figs. 63, 64) illustrate the method of inheri- tance of color blindness. J[ represents the normal X-chromo- some, Q its absence (or rather the F-chromosome since the sex chromosomes of the human male are XY and not XOQ and X the X-chromosome 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 daugh- ters and not to his sons the sex-determiner which carries the factor for color blindness. But since color blindness does not develop in females unless it is duplex (i.e. comes from both father and mother) whereas it develops in males if it is simplex The Cellular Basis 191 (i.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 nor- mal because they carry no determiner for color blindness. But these daughters transmit to one-half of their children the single determiner for color blindness and if any of those receiving this determiner are males they will be color blind. Consequently we have the curious phenomenon of simplex color blindness appear- ing only in males and being transmitted to them only through ap- parently normal females. On the other hand if a female is color blind she has inherited it from both father and mother, i.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 daughters will be normal, though carrying the simplex deter- miner for color blindness. Sex-linked Lethal Factors. — One of the most interesting cases of linkage is found where early death is linked with sex. In Drosophila a considerable number of lethal mutant factors have been demonstrated in the X chromosome and all individuals in which such a factor is not balanced by a normal allelomorph die early. All males that receive such a lethal die, since there is only one X chromosome in the male ; all homozygous females that have the factor in both X chromosomes die, while only those sur- vive that are heterozygous for this factor. Such a heterozy- gous female produces in equal numbers eggs with and without the lethal factor and if she is bred to a normal male all of the daughters are viable though half of them carry the lethal factor in one of the X chromosomes, but all of the males that receive the lethal factor are non-viable since the male has only one X chromosome, while all the males that survive lack this factor altogether. Thus the sex ratio in this case is 2 females to i male. Other lethal factors have occurred in other chromosomes of /92 Heredity and Environment Drosophila but they were first studied and are most easily demon- strated in the X chromosome. b. Other Cases of Linkage. — In addition to characters which are sex-linked other characters may be bound together in hered- ity without being linked with sex. Morgan and his associates have found and studied about four hundred mutations of Drosophila (see Figs. 101-103), which are inherited in four groups, all the characters of each group usually going together. There have been found in the first group 140 different mutant characters, in the second 125, in the third 120 and in the fourth 3. Or eliminating lethal and modifying factors and those of more doubtful location, there remain 188 mutant genes, 50 of which are in the first group, 70 in the second, 65 in the third, and 3 in the fourth. Correspond- ing with the number and size of these groups there are four pairs of chromosomes in Drosophila, three of which are large and one is very small (Fig. 65). The sex chromosomes (XX in the female, XY in the male) constitute one of the large pairs and the genes of the characters which are sex-linked are located in these chro- mosomes ; the genes of the second and third groups of characters are in the other large chromosomes, while the fourth group of only three characters have their genes in the very small chromosomes (Fig. 65). If this interpretation is correct, linkage is due to the FIG. 65. CHROMOSOMES (DiPLOio NUMBER) OF Drosophila melanogaster. A. Female with 2 X chromosomes; B. Male with i X and I Y; C. Ma- troclinoii'S female (XXY) resulting from non-disjunction of the 2 X chromosomes of the egg. (After Morgan.) The Cellular Basis 193 grouping together of certain genes in certain chromosomes, there are as many groups of characters as there are pairs of chro- mosomes and as long as the chromosomes preserve their identity the linkage of genes in the chromosomes and of characters in the developed organisms will persist. c. "Cross-Overs." — But linkage of inherited characters is not quite so simple as this statement would indicate for an extensive study of this phenomenon in Drosophila has shown that while characters are usually linked in four constant groups this is not al- ways true. For example Morgan has found that when a female fly with white eyes and yellow wings is crossed to a male with red eyes and gray wings, the genes for these characters being linked together in the X chromosomes, all the sons are yellow and have white eyes while all the daughters are gray and have red eyes, gray wings and red eyes being dominant over yellow and white; but when these F^s are crossed about 99 per cent of the offspring show the same linkage of the colors yellow-white, gray- red, but in i per cent the linkage is yellow-red, gray-white. This interchange of characters in the two groups, or "cross-over" as Morgan calls it, may be explained by assuming that there has been an interchange of genes between the two sex chromosomes of the female.* When the paired chromosomes lie side by side in synapsis it is known that they sometimes twist around each other and if in their subsequent separation each chromosome should break at the point where the two cross and a portion of one chromosome should be joined to the other one we would have a relatively simple explanation of the interchange or "cross-over" of genes and consequently of the breaking up of the old group of characters and the establishment of a new group (Fig. 66). Similar interchange of characters takes place in each of the other three groups of Drosophila, and can be explained in the * Such "cross-overs" occur only in the female of Drosophila, though they may occur in the males of other species. They occur when the synaptic pairs of chromosomes are long, slender threads. 194. Heredity and Environment same way. If a pair of chromosomes are twisted round each other at more than one place and are then broken at these points we get double or multiple crossing over and a corresponding re-grouping of genes and of characters. Unless chromosomes of a pair are very tightly twisted two cross-overs will not occur near together and in general the farther apart points are in a chromo- some the more likely is a cross-over to occur. If one per cent of "crossing over" occurs the genes are assumed to be one unit of distance apart; if ten per cent, ten units, etc. On this basis Mor- gan and his associates have constructed a "map" of each chromo- some of Drosophila indicating the positions of those genes which d FIG. 66. DIAGRAM SHOWING THE PROBABLE CHROMOSOMAL MECHANISM BY WHICH "CROSSING OVER" is CAUSED. Pairs of chromosomes, one from the father, the other from the mother, are shown in synapsis (a, b, c) and in the reduction division (rf). Homologous chromomeres (or allelomorphic genes) are represented by the black and white circles at the same level. In a and b the chromosomes are shown "crossing over"; in c they have broken at the point of crossing and half of each chromosome is joined to half of the other one; in d the "crossed over" chromosomes are separating in the reduction division (from Morgan). CHROMOSOME I The Cellular Basis CHROMOSOME II CHROMOSOME III 195 CHROMOSOME IV 0.0 o.& 1.5 YELLOW'SCUTE' J3ROAD* WHITE* -2.0, 0.0 TELEGRAPH STAR* 0.0 RDUGHOID* ! 5.5 7.5 ECHINUS' RUBY* 4.0 9.0 EXPANDED TRUNCATE* 10.0 STAR-IHTENSIFIER 14.0 CROSSV'NL'3" 14.0 STREAK 15.0 SMUDGE 20.0 CUT* 2E.O CRKAU-B 20.0 DWARFOID 27.5 •TAN* 29.0 DACH3 i:§ SEPIA* HAIRY* 33.0 36.0 VERMILION* MINIATURE* 33.0 SKI-II* 32.0 34.0 DIVER(2NT CREAM-III 38.5 DICHAETB* 44.5 SARNEi* 46,5 BLACK* 42.0 45. C SCARLET* PtNK* m FORKED* BAR* m CINNABAR* •PURPLE* fti 59.Q SPINELESS* BITHORAX (aASS 65.0 70.0 CLEFT BOBBED 65.6 70. Q 73.5: VESTIGIAL* LOBE* CURVED* 63.5 65.5 67.5 72,0 DELTA HAIRLESS* EBONY* WHITE.-OCBLLI* 85.0 88.0 ttINUTE-2 HUMFX 8615 90.0 «6UGH* POINTED WING ' 95.5 •CLARET* 98.5 103.0 105.0! PLEXUS* BROWN* SPECK* 101. a MINUTE-23* FIG. 67. MAPS OF THE FOUR CHROMOSOMES OF Drosophila melanogaster showing positions of a few of the nearly 400 mutant genes ; those that have been most accurately located are marked with a star. (From information furnished by Morgan and Bridges.) Figures of several of these Droso- phila Mutuals will be found on pp. 286-288. 196 Heredity and Environment have been determined most accurately (Fig. 67). Thus not only do they locate particular genes in particular chromosomes but they are able to locate the relative positions of these genes in each chromosome. This is in all respects the most remarkable work which has ever been done in this field; for the first time it gives us a detailed picture of what Weismann called the "architecture of the germplasm," — for the first time it assigns to different genes "a local habitation and a name." 3. CYTOPLASMIC INHERITANCE. — The most direct and the earli- est recognized correlations between the oosperm and the devel- oped animal are found in the polarity and symmetry of the egg cytoplasm and of the animal to which it gives rise. (a) Polarity. — In all eggs there is polar differentiation, one pole, at which the maturation divisions take place, being 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 surrounding 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 embryo and in every great group of animals it bears a constant relationship to the chief axis of the adult animal. The polarity of the developed animal is thus di- rectly connected with the polarity of the egg from which it came (Figs. 42,45, 46,47, 68, 69). (b) Symmetry. — In many cases the symmetry of the developed animal is foreshadowed in the cytoplasm of the egg. The eggs of cephalopods (Fig. 68) and of insects (Fig. 69) are bilaterally symmetrical while they are still in the ovary ; in other cases, such as ascidians, Amphioxus and the frog, bilateral symmetry ap- pears immediately after fertilization (Figs. 9, 10, 46, 47), 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 The Cellular Basis 197 now know whether it is present in earlier stages or not ; but wher- ever it can be recognized in the earlier stages it is probable that the bilateral symmetry of the egg becomes the bilateral symmetry of the developed animal. (c) Inverse Symmetry. — In most animals bilateral symmetry is not perfect, certain organs being found on one side of the mid- line and not on the other, or being larger or differently located on FIG. 68 FIG. 69 FIG. 68. OUTLINES OF THE UNFERTILIZED EGG OF A SQUID, Loligo, show* ing the polarity and symmetry of the egg with reference to the axes of the developed animal; d, dorsal; v, ventral; /, left; r, right; a, anterior; p, posterior. (After Watase.) FIG. 69. MEDIAN SECTION THROUGH EGG OF A FLY, Musca, just after fer- tilization, 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, micro- pyle 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 Kor- schelt and Heider.) 198 Heredity and Environment one side as compared with the other ; among all such animals var- iations occasionally occur which show a complete reversal of these asymmetrical organs, i.e., 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 instead of the right, etc. Among certain snails this inversion of symmetry may occur regularly in certain species and not in others, the inverse form being known as sinistral and the ordinary form as dextral (Fig. 72). In these sinistral snails, and probably in all animals show- ing inverse symmetry, the embryo is inversely symmetrical and every cleavage of the egg from the first to the last is the inverse of that which occurs in dextral snails (Figs. 70-72). There is good reason to believe that in such cases the unsegmented egg is also inversely symmetrical as compared with the more usual type (Fig. 70). In all of these cases there is a direct correspondence between the polarity and symmetry of the oosperm and the polar- ity and symmetry of the developed animal (Figs. 68-72). (d) Localization Pattern. — In many animals the ectoderm and mesoderm may be traced back to areas of peculiar protoplasm in the oosperm, but, in addition to this, one can recognize in the as- cidian 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 posi- tions and proportions which they will have in the embryo and larva (Figs. 10, n, 46-48). Indeed there are types of localization of these cytoplasmic materials in the egg which are characteristic of certain phyla ; thus there are the ctenophore, the flat-worm, the echinoderm, the an- nelid-mollusk and the chordate types of cytoplasmic localization (Fig. 73). The polarity, symmetry and pattern of a jellyfish, starfish, worm, mollusk, insect or vertebrate are foreshadowed by the characteristic polarity, symmetry and pattern of the cyto- plasm of the egg either before or immediately after fertilization. In all of these phyla, eggs may develop without fertilization, The Cellular Basis 199 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 latter had been fertilized. The conclusion seems to be justified that these earliest and most fundamental differentiations which distinguished the eggs of various phyla are not dependent upon the entering sperma- tozoon. Share of Egg and Sperm in Heredity. — All of these corre- spondences between the polarity, symmetry and pattern of the egg and of the developed animal are found in the cytoplasm. No doubt the differentiations of the 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 chromosomes as well as of the environment, 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, whereas those of the egg persist. In short on- togeny begins in the egg before fertilization 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 fertilisation the poten- cies of the two germ cells are not equal, the polarity, sym- metry, type of cleavage, and the pattern, or relative positions and proportions of future organs, being foreshadowed in the cyto- plasm of the egg cell, while only the differentiations of later de- velopment are influenced by the sperm. In short the egg cyto- plasm determines the early development and the sperm and egg nuclei control only later differentiations. 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. The chromosomes of the egg and sperm are the seat of the differential factors or determiners for Mendelian characters, 200 Heredity and Environment FIGS. 70, 71, 72. THE CAUSE OF INVERSE SYMMETRY IN SNAILS. In each case the right-hand column represents dextral forms, the left-hand column sinistral ones. FIG. 70. NORMAL AND INVERSE SYMMETRY IN THE UNSEGMENTED EGG AND IN THE FlRST AND SECOND CLEAVAGES. The Cellular Basis 201 FIG. 71. NORMAL AND INVERSE SYMMETRY OF THE 30, 4TH, STH AND 6TH CLEAVAGES. The cells la-id, 2a-2d and sa-sd give rise to all the ectoderm ; 4d or M gives rise to mesoderm ; A, B, C, D to endoderm. 202 Heredity and Environment FIG. 72. NORMAL AND INVERSE SYMMETRY IN LATE EMBRYOS AND ADULT STAGES. In i, cross-hatched area is blastopore ; cells shaded by lines, meso- derm; other cells, endoderm; the spiral twist of the snail begins in oppo- site directions in the two embryos. In 2, the adult organization is shown with all organs inversely symmetrical; os, olfactory organ, a, anus; L, lung; V, ventricle; K, kidney. In 3, sinistral and dextral shells of adult snails are shown. The Cellular Basis 203 CTENOPHORE TURBELLARIAN eel. end. EOHINODERM end ASCIDIAN I end. mcs. ASCI WAN ii end. FIG. 73. TYPES OF EGG ORGANIZATION IN DIFFERENT PHYLA; cross- hatched area, mesoderm or mesenchyme (mes) ; horizontal lines, endo- derm (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 cleav- age; 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; n.p., neural plate; ch., chorda; e.g., cerebral ganglion; v.g., ventral ganglion; proto., prototroch. 2O4 Heredity and Environment but the general polarity, symmetry and before fertilization. But these egg characters, like any other character of the female, were probably determined by chromosomes derived from her father and mother; if so they are Mendelian characters inherited by the mother through her chromosomes and carried over to the first filial generation, not as factors but as developed characters. Such cases may be called "maternal inheritance" since the charac- ters come only from the egg, or "preinheritance" since these egg characters are developed before fertilization. (See pp. 113, 114.) Share of Chromosomes and Cytoplasm in Heredity and Devel- opment.— It will be observed that the correlation between chromo- somes and adult characters is different in kind from that between the cytoplasm of the egg and adult characters; in the latter case polarity, symmetry and pattern of localization are characters 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 organization is more fundamental than that of the cytoplasm; the chromo- somes contain the germplasm, the cytoplasm is the somatoplasm ; the chromosomes are chiefly concerned in heredity, the cytoplasm in development. E. 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 accomplished ? Many different processes are concerned in embryonic differ- entiation. From the standpoint of the cell the most important of these are (i) the formation of different kinds of substances in cells, (2) the localization and isolation of these substances, (3) the transformation of these substances into various struc- The Cellular Basis 205 tures which are characteristic of the different kinds of tissue cells. We shall here describe only the first and second of these pro- cesses which are of more general interest than the last. i. The Formation of Different Substances in Cells. — Embry- onic differentiation consists primarily in the formation of dif- ferent kinds of protoplasm out of the protoplasm of the germ cells. It is plain that different kinds of protoplasm 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 interaction with one another of substances or parts originally present and by their reactions to external stimuli new substances and parts appear which had no previous existence just as new substances result from chemical reactions. This is "creative synthesis" in general science, epigenesis in development. Differentiations appear chiefly in the cytoplasm but only as the result of interaction be- tween cytoplasm and nucleus. Similarly, it may be argued, smal- ler units of organization such as chromosomes or chromomeres do not in themselves give rise to any adult parts, but only as they interact upon other units are new parts formed. In many cases the first formation of such new substances ap- pears in the immediate vicinity of the nucleus and, like assimila- tion itself, this is evidently brought about by the interaction of nu- cleus 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 chromosomes of the previous cell division, it is probable that the chromosomes are a factor in this process. Weismann maintained that the chromosomes and the inheri- tance units contained in them undergo differentiation by a pro- cess of disintegration and that these disintegrated units escape into the cell body and there produce different kinds of cytoplasm 2o6 Heredity and Environment in different cells. A somewhat similar view was advanced by deVries in his theory of "intra-cellular pangenesis." However, as we have seen already, there is good evidence that the chromosomes do not undergo progressive differentiation in the course of devel- opment; they always divide with exact equality, and even in highly differentiated tissue cells their number and form usually remain as in embryonic cells. On the other hand the cytoplasm undergoes progressive dif- ferentiation, and when by pressure or centrifugal force it is brought into relations with other nuclei the differentiations of the cytoplasm are not altered thereby, thus showing that the dif- ferent nuclei are essentially alike and that differentiations are mainly limited 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. Never- theless differentiations do not take place in the absence of nu- clear material, and it seems probable that the interaction of nucleus and cytoplasm is necessary to the formation of the new cytoplas- mic substances which appear in the course of development. 2. Segregation and Isolation of Different Substances m Cells. — But differentiation consists not only in the formation of differ- ent kinds of substances in cells but also in the separation of these substances from one another. This separation is brought about to a great extent by flowing movements within cells which are associated especially with cell division. In all these processes of heredity and development cell divi- sion plays a particularly important part. If cell divisions were always exactly alike there could be no initial difference between the daughter cells, and unless acted upon by different stimuli all cells would remain exactly alike. But there is much evidence that daughter cells are often unlike from the time of their for- mation, and that different stimuli act upon them to increase still further this initial difference. The Cellular Basis 207 (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 divisions of all the smallest elements of the cell are of this sort; there is no good evidence that the plastosomes, the chromomeres, or the chromo- somes ever divide into unlike halves, though in the maturation divisions the separation of whole chromosomes leads to the ap- pearance of a differential division of the chromosomes. But while all of the cell elements may be supposed 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 chro- matin is equally distributed to the daughter cells, except in the case of the maturation divisions, the achromatin and the oxy- chromatin 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 frequently differential and such divisions are known to play a great part in embryonic differ- entiation. (b) Isolation of Cytoplasmic Substances by Division Walls. — 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 di- vision 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 dif- ferent substances within the cell body may freely commingle, es- pecially during nuclear division when the cytoplasmic movements are especially active; in such cases differentiation may be ar- rested even though nuclear division continues. In the develop- ing eggs of most animals partition walls between daughter cells are necessary to prevent the commingling of different kinds of substances, which are sorted by the movements within the cell and 208 Heredity and Environment are isolated by the partition walls. In some cases, as for example in certain protozoa, the commingling of different kinds of pro- toplasm within a cell may be prevented by the viscosity of por- tions of the protoplasm, or by the formation of intracellular mem- branes, or by a reduction to a minimum of the mitotic movements within the cell by the persistence' of the nuclear membrane dur- ing division. In general the degree of differentiation may be measured by the degree of unlikeness between different cells, and by the completeness with which the protoplasm of different cells is kept from intermingling. (3) The Chromosome Theory of Heredity Applied to Embry- onic Differentiation. — According to the chromosome theory of heredity the inheritance factors are located in the chromosomes, and the cytological evidence shows that chromosomes always di- vide equally and presumably every cell of an individual contains the same kinds of chromosomes and the same kinds of inheritance factors. How then is it possible to explain embryonic differen- tiation ? How can identical factors give rise to different products in different cells? This is evidently due to the fact that while the division of chromosomes is non-differential, that of the cell body is often differential and the same 'chromosomes and genes acting upon different kinds of cytoplasm will produce different results. But differential cell-division is the result of definite movements in the cytoplasm, of definite orientations of spindles and cleavage planes, and ultimately of a definite polarity and symmetry of the cyto- plasm. There is abundant evidence that these cytoplasmic orien- tations are not the immediate results of chromosomal activity and even if some of them may be the remote results of such activity it is logically impossible to place all the differential factors of de- velopment in non-differentiating genes. On the other hand if embryonic differentiations are produced by the interaction of chromatin and cytoplasm, and if the chro- matin does not undergo differentiation, it follows that some of the The Cellular Basis 209 differential factors of heredity and development must be located in the cytoplasm. Such factors would probably not be genes and would not be transmitted in Mendelian fashion, -but they would need to be present in the cytoplasm from the very beginnings of ontogeny. They need not be numerous — in fact they are prob- ably few in number — but they are absolutely indispensable to development. If a few orientating differentiations such as polarity and symmetry are present in the cytoplasm at the be- ginning of ontogeny all other differentiations of development can be explained as due to the interaction of non-differentiating genes on different parts of this cytoplasm, but there is no mecharr- ism by which embryonic differentiations could come from the action of non-differentiating genes on a homogeneous cytoplasm. The genes or Mendelian factors are undoubtedly located in the chromosomes and they are sometimes regarded as the only dif- ferential factors of development, but if this w"ere true these genes would of necessity have to undergo differential division and dis- tribution to the cleavage cells; since this is not true it must be that some of the differential factors of development lie outside of the nucleus and if they are inherited, as most of these early orientations are, they must lie in the cytoplasm. SUMMARY All the phenomena of life, including heredity 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. In sexual reproduction 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 off- spring but the germinal factors or causes are transmitted, and under proper conditions of environment these give rise to developed char- acters. Every oosperm as well as every developed organism differs more or less from every other one and this remarkable condition is 2io Heredity and Environment brought about by extremely numerous permutations in the distri- bution of the chromosomes of the sex cells in maturation and ferti- lization. Sex is an inherited character dependent, primarily, upon an alternative distribution of certain chromosomes to the germ cells. There is much evidence that the factors for all sorts of Mendelian characters are associated with the chromosomes. The differen- tiation of the oosperm into the developed organism is accom- plished in part by the interaction of chromosomes and cytoplasm which leads to the formation of new materials, and in part by the segregation and localization of these materials 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 underestimated this complexity and there is no reason to suppose that we have fully comprehended it. What Darwin said of the entire organism may now be said of every cell : It "is a mi- crocosm— a little universe, formed of a host of self -propagating organisms, inconceivably minute and numerous as the stars in heaven." CHAPTER IV INFLUENCE OF ENVIRONMENT CHAPTER IV INFLUENCE OF ENVIRONMENT The development of an individual or the evolution of a race is dependent upon the interaction of two sets of factors or causes, the intrinsic and the extrinsic. The former are represented by the organization of the germinal protoplasm, the latter by all other conditions ; the former are known as heredity or constitu- tion, the latter as environment or education; or in the words of Galton, these two sets of factors may be called "nature" and "nur- ture." The great problem of development is the unraveling of these two factors, the assignment of its true value to each, and the ultimate control of development so far as this may be possible through the knowledge thus gained. A. RELATIVE IMPORTANCE OF HEREDITY AND ENVIRONMENT The distinction between these two factors of development is generally recognized and the question of the relative importance of the two has been discussed for ages. Which is the more im- portant, constitution or environment? What characteristics are due to nature and what to nurture? To what extent is man the creature of heredity, to what extent the product of education? The old question "Which of you by taking thought can add one cubit to his stature," is a vital question to-day. To what extent may nature be modified by nurture ? To what extent may educa- tion make up for deficiencies of birth? i. Former Emphasis on Environment. — Formerly very great 213 214 Heredity and Environment emphasis was placed upon influences of environment in phylogeny and ontogeny. From the earliest times it has been believed that species might be transmuted by environmental changes and that even life itself might arise from lifeless matter through the in- fluence of favorable extrinsic conditions. If environment could exert so great an influence on the origin of species or even of life itself much more could it affect the process of development of the individual. It is still popularly supposed that complexion is dependent upon the intensity of light, and stature upon the quan- tity and quality