.■',«: msSatm siafiN ■'.;..; ;; ■77 Hi ffigg S3 ■ '•' ? ■' RSI ,, .!• *» Sfartljuiratrrtt ImurrBitg THE N. W. HARRIS LECTURES FOR 1914 <51j? J8L U. partis IGrrturrB were founded in igo6 through the generosity of Mr. Norma;i Wait Harris of Chicago, and are to he 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." iOrrturpa Alrraog fhtbltBirpft 1907 Personalism. Bordon P. Browne 1908 University Administration. Charles W. Eliot 1910 The Age of Mammals. Henry F. Osborn 191 1. Democracy and Poetry. Francis B. Gummere 1912 The Milk Question. Milton J. Rosenau 1913 The Constitution of Matter. Joseph S. Ames REVISED THIRD EDITION HEREDITY AND ENVIRONMENT IN THE DEVELOPMENT OF MEN BY EDWIN GRANT CONKLIN PROFESSOR OF BIOLOGY IN PRINCETON UNIVERSITY PRINCETON UNIVERSITY PRESS PRINCETON LONDON: HUMPHREY MILFORD OXFORD UNIVERSITY PRESS 1919 Qh i 19 n FIRST EDITION Copyright, 1915, by Princeton University Press Published February, 1915 Second Printing, June, 1915 revised second edition Copyright, 1946 Published May, 1916 Second Printing, August, 1917 Third Printing, March, 1918 REVISED THIRD EDITION Copyright, 1919 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. I 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- vi Preface lives, 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 subjects, 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 boped that the glossary at the end of the volume may rob these unfamiliar terms of many of their terrors. The first three chapters of this book appeared in Popular 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 i-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. \V. E. Castle ond Dr. ]. H. McGregor for the use of photographs which are reproduced in Figures Si, 82 and 96; and I wish especially to thank my as- si>tnnt, Marguerite Ruddiman, for her aid in preparing figures and manuscript for publication. Princeton, December, 101 / * These figure numbers apply to the first and second editions but not to the third. 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, 18, 19 have been added and besides minor corrections and addi- tions to all the chapters considerable changes have been made in Chapter 111, 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, i()]6. Publisher's Note. — In the second edition, second printing, August, T917, the text was corrected by the author so as to embody some of the results of the latest investigations in this subject. Vll 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 ampelophila. 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, Tpip. ix CONTENTS CHAPTER I. FACTS AND FACTORS OF DEVELOPMENT INTRODUCTION . A. PHENOMENA OF DEVELOPMENT I. Development of the Body i. The Germ Cells 2. Fertilization 3. Cleavage 4. Embryogeny 5. Organogeny 6. Oviparity and Viviparity 7. Development of Functions II. Development of the Mind 1. Sensitivity 2. Reflexes, Tropisms, Instincts \f. Memory 4. Intellect, Reason 5. Will 6. Consciousness 7. Parallel Development of Body and Mind B. FACTORS OF DEVELOPMENT 1. Preformation 2. Epigenesis ft. 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 1. Racial Characters xi xii Contents 2. Individual Characters a. Morphological Features b. Physiological Peculiarities c. Teratological and Pathological Peculiarities d. Psychological Characters II. Hereditary Differences i. New Combinations of Characters 2. New Characters or Mutations 3. Mutations and Fluctuations ^/q. 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 Con tc tits xiii CHAPTER III. CELLULAR BASIS OF HEREDITY AND DEVEL- OPMENT A. INTRODUCTORY i. Definitions 2. The Transmission Hypothesis 3. Germinal Continuity and Somatic Discontinuity •-.}. The Units of Living Matter . ^^ £ 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 Eertilization 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. Origin of Mutations B. CONTROL OF HUMAN HEREDITY I. Past Evolution of Man H. 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 ok Nature and ok Human Responsibility II. The Mechanistic Conception of Nature and oe 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. Th^InDIVIDUAL AND THE RACE /yi. The Conflict between the Freedom of the Individual and the Good of Society 2. Perpetuation and Improvement of the Race the Highest Ethical Obligation REFERENCES GLOSSARY INDEX 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, the 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 proper study of man is the lower organisms in which life processes are reduced to their simplest terms, and where alone they may be subjected to conditions of rigid 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 to apply consciously 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. PHENOMKNA 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 vivum 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 ami 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 nturofibrils and in a one nerve fiber, in b many processes, one of which (-}-) 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. (c) Reproduction, or the capacity of organisms to give rise to ] 'nets and /•'actors of Development 9 Memb ^> B Fig. 2. A nearly Ripe Human Ovum in the Living 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 (F) and a nucleus (Ar) embedded in a clear mass of prdto- plasm. Magnified 500 diameters (x 500). (From O. Hertwig.) B, two human spermatozoa drawn to about the same scale of magnification. (After G. Retzius.) 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. Germ Cells Alive. — Both the egg and the sperm are living cells io Heredity and Eninronment 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. The corresponding cells and adjacent parts of flowering plants are known as ovules and pollen. Col- lectively all kinds of sex cells are called gametes, and the indi- vidual 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 smaller than 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. The sperm, or male gamete, is among the smallest of all cells and is usually many thousands of times smaller than the egg. Ira facts and Factors of Development i i B ... c>» 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.) most animals, and in all vertebrates, it is an elongated, thread- like cell with an enlarged head which contains the nncleus, 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 \\\>< 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 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- 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, Astcrias 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 (cc) to meet it. (After Fol.) D, Mature spermatozoon of the sea-urchin, Toxopneustes, showing head (/i) ; middle-piece (hi) ; and tail (/)• E-H, Successive stages in the pene- Facts and Factors of Development 13 trosomc 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 — 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 Nczv 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 tration of the sperm nucleus (SN) and centrosome ( $ C) into the egg of Toxopncustcs. I-L, Stages in the approach of the sperm nucleus ($N) to the egg nucleus (9N), and in the division of the sperm centrosome ( SC) and the formation of the first cleavage spindle. (D-L after Wilson.) 14 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. B and C, Later stages showing the separation of the daughter chromosomes from one another and their movement toward the two poles of the spindle. D and 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.) I'm ts 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 starfish, 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 perceptible change is minute ; but the changes are innumerable and 16 Heredity and Environment 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 daughter chromosomes which move apart toward the two poles of the cell (Fig. 7, G, H) where all the daughter chromosomes come Fuels and Factors of Development \7 together to form the two daughter nuclei. The cell body then divides by a process of constriction into two daughter cells ( Fig. 7. r, /). 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- 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 i8 Heredity and Environment 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 individual chromosomes or chromosomal vesicles through the E 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 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 rise to a spindle (a) with astral radiations at its poles; /•', Middle stage of division in which the chromosomes lie in the equator of the spindle forming the equatorial plate (ep) ; G, II. 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; ;;, Nucleolus: /, 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.) :0 Heredity and Environment /f/jiwrn 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, nucleus at beginning of mitosis, having shrunk from dotted outline and showing chromosomal vesicles containing chromatin granules ; B, each chromosomal vesicle con- tains granules or chromomeres which are condensing to form a chromo- some ; C, amphiaster showing faint outlines of the chromosomal vesicles with their contained chromosomes ; D, amphiaster showing each chromo- Facts and Factors of Development 21 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. 9-1 1). 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 some beginning to split and the chromosomes dividing; /:, late phase of division showing daughter chromosomes at the poles of the spindle and each chromosome becoming vesicular; F, still later phase, each chromo- some a vesicle containing chromatin granules; (/, daughter nucleus slmu ing chromosomal vesicles containing scattered chromatin granules. (After Richards.) 22 Heredity and Environment * G Fig. 9. Successive Stages ln 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 ; A." 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; (/, dorsal; be, blas- toccel ; gc, gastroccel. Facts and Factors of Development 23 differ from one another. The differentiations of the cleavage cells appear much earlier in some forms than in others, but in all cases such differentiations appear during early or late- cleavage (Figs. 9-11). 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, n, //), and this in turn he- comes a gastrula (Figs. 9, 11, 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 he between the upper and lower poles (Fig. it, ;;/). 5. Organogeny. — 1 >y 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. 11, 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. ir, A", 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, li and /*"). The back-bone begins to appear as a delicate cellular rod ( Fig. 1 1 , c), which then in higher vertebrates becomes surrounded successively by a 24 Heredity and Emnronment 9« » Figs, io, ii. Diagrams of Frog's Eggs showing the Relations of the Axes and Substances of the Egg to the Axes and Principal Organs of the Embryo. All eggs viewed from right side, polar bodies above; A, anterior; P, posterior; D, dorsal; V, ventral; s, spermatozoon; $N, sperm nucleus, 9N, egg nucleus; m, mesodermal crescent where mesoderm will form; c and n, gray crescent, where chorda (c) and nervous system (») 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 anil luw tors of Development 25 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. 11. Sections in Median Plane of Embryos. G, 16-32-cell stage; H, blastula; /, early gastrula; /, late gastrula; A.', early embryo, /., 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 -7 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 Df.vki.opment ok the Human Oosperm. A, cleavage stage which has just. come into the uterus; />' and (", Mastodermic vesicles embedded in the mucous membrane of the uterus; D, 11 and F, longitudinal sections of later stages, the anterior and poster- ior poles being marked by the axis a />. In C" cavities have appeared in the ectoderm, entoderm and mesoderm. />, villi forming from the trophoblast (nutritive layer, tr) ; black indicates ectoderm (<•$77777ff772& 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 ; B, blastodermic 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 ap- peared, age ten to fourteen days; D, stage of fourteen somites showing enlargements 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 de- Facts and Factors of Development 29 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., (1) Construc- pression at the dorsal angle of the second cleft; G, embryo, of thirty-five somites showing eye, branchial arches and limb buds; //, embryo of thirty-six somites showing nasal pit, eye, branchial arches and clefts, limb buds and heart. (After Keibel.) 3Q 1 1 credit x and Environment live 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 0 Fjg. 14. A, human embryo of forty-two somites, age twenty-one days; /?, 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.) Facts and Factors of Development ,}i as development advances each of these functions becomes mine 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 arc 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. Certainly 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 crally 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 been branded as "materialism*' and promptly rejected without examination by those who are frightened by names. But the general spread of the scientific spirit is shown not only by the growing regard for evidence but also by the decreasing power of epithets. "Materialism," like many another ghost, fades away into thin air or at least loses many of 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 Xeo- 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 is the cause of the other, though each may modify or condition the other, but that they are two aspects of one common thing, viz., organization. In the same way I think that the body or brain is not the cause of mind, nor mind the cause of body or brain, but that both are inherent in one common 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. 1 / Germinal Bases oe 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 Anlagcn 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. 1. 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. 38 Heredity and Environment a B C J) E b JF 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.) 4, ec). It is an interesting fact that the same sort of response follows when a frog's egg is pricked hy 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, Facts and Factor's of Development 39 becomes differential sensitivity and special senses in the proa of embryonic differentiation. Such sensitivity is the basis of all psychic processes; sensations are the elements of the mind. 2. Reflexes, Tropisms, Instincts. — All the responses of germ a Fig. i6, a, b, c. Repulsion of Spirilla by Common Salt, a, condition immediately after adding crystals; b and c, later stages in the reaction. .r, 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 1 retreated before the distilled water. (From Jennings, after Massart.) 40 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 a 19 ■ - - ' - . « - , 19* 19 26 ' 38' 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 190 ; 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 M toward or away from ;i 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- K Fig. 18. Phototropism of Seedling of White Mustard supported by a sheet of cork (AT, A') 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 Fig. 19. Geotropism of Seedling Oak. After starting to grow with the axis A-A in vertical position the seedling was gradually turned through 900 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. quently the case in animals, they are known as "instincts." Re- flexes and tropisms occur in the simplest organisms, such as bac- teria, protozoa, and 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, amnion and intestine are early manifestations of reflex motions. These appear chiefly in the involuntary muscles before nervous connections are Facts dud Factors of Development \$ formed, the protoplasm of the muscle cells probably responding directly to the ch< mical 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 arc present imme- diately after birth, such as the instinct of sucking or crying, though these are so simple when compared with some instincts which develop later that they might he classed as reflexes; it is doubtful whether any of the activities before birth could properly he designated as instincts. Reflexes, tropisms and instincts have had a phylogenetic as well as an ontogenetic origin, and conse- quently we might expect that they would in general make for the preservation of the species ; as a matter of fact we usually And 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 because 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- genetieally, 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 he 44 Heredity and Environment Fig. 20. Dionaca 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. very different from that to the first. Macfarlane found that if the sensitive hairs on the leaf of Dionaca, 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 Facts and Factors of Development 45 of some chemical substance which remains in the protoplasm for a certain time, during which time the effects of the stimulus are said to persist, or it may he 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 reason may be defined as the power of predicting future events and of reaching conclusions regarding unexperienced phenomena under the influ- ence of past experience. In the absence of individual experience young children have none of this power, but it comes gradually as a result of remembering past experiences and of' fitting such experiences into new conditions. Useful Responses. — Young infants and many lower animals lack the power of 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, 1, 2, 3) ; it then stands nearly still, merely rolling over and swerving toward the aboral side, and finally it goes ahead again, usually on a new course (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 in 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 Juniors of Development 17 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- mecia is heated and the other end is cooled by ice, the Par ante cia 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, Paramecium 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 unlit. 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 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.) 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. 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 Paramecium! 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 experienee 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 lowrer 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: (1.) ATo 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- diet 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, St en tor, 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 spouses. 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 he overcome to a certain extent. Here fear is at first a stronger stimulus than hunger and unless the stimulus of fear can he reduced the animal will starve to death in the presence of the most tempting food. (b) Compulsory Limitations. — Responses may also he 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 hack, 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 he- come fixed and constant through natural selection or other means of limitation may hecome 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 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 hut by feeling, that something is missing, and she leaves the nest after a 52 Heredity and Environment 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 scries 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 associations tracts may cause permanent loss of consciousness. Facts and Factors of Development 55 7. Parallel Development of Body ami Mind. The developn 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 (1 ) physii chemical events involved, (2) physiological processes, (3) mor- phological features, (4) ecological correlations and adaptations. (5) psychological phenomena, ((>) 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 ol 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 lie causally explained in terms of the preceding ones, and hence all he reducible to physics and chem- istry. But this is not now demonstrable and may not he 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: 56 Heredity and Environment DEVELOPMENT OF PSYCHICAL PROCESSES IN ONTOGENY AND PHYLOGENY All Living Things, Including Germ Cells and Embryos, Show: i. 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. Mature Forms of Higher Ani- mals Show : 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. 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 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. Facts and Factors of Development $7 1. Preformation. — 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 altogther. 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 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. Epigcncsis. — As opposed to this doctrine of preformation, which was founded largely on speculation, arose the theory of * 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, hut 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 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 undifferentiated, 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 ij^> and entitled "Theoria Generationis," marked the beginning of a great epoch in the stud)' 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. lie 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 zris 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. Endogcncsis and Epigenesis. — Modern students of develop- ment recognize that neither of these extreme views is true — adult parts are not present in the germ, nor is the latter homogeneous— but there are in germ cells many different structures and func- tions which are, however, very unlike those of the adult, and by the transformation and differentiation of this germinal organi- Facts and Factors of Development 59 zation the complicated organization of the adull arises. Develop ment is no! 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 presenl 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 hy 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 he the case; the) 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- 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 6o Heredity and Environment 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 extrinisic 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 hut 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 consti- tuent 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 < ialton 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 is trUe that inheritance is most clearly recognized in those characters in which offspring resemble their parents, even characters in which they differ from their parents may be inherited, as is plainly seen when, in any character, a child resembles a grandparent or a Phenomena of Inlicriltui. 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 I. Hereditary Resemblances i. 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 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 whi questions of the occurrence or distribution of particular phenomena are concerned; but the causes of heredity are individual and Fig. 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 groups of parents and the arrow heads indicate the average heights of their children. The offspring of undersized parents are taller and of oversized parents are shorter than their respective parents. (After Walter.) 8o Heredity and Environment physiological, and averages are of less value in finding the causes of such phenomena than is the intensive study of individual cases. By observation alone it is usually 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 and 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 (1) 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. The observational and statistical study of inheritance helped Phenomena of Inheritance 81 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, of the Konigskloster, an Augustinian monastery in Briinn, Moravia, published the results of his experiments on hybridization 82 Heredity and Environment 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 had been studied extensively before Mendel began hid 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 characters, which he followed through several generations of hybrids, viz., (1) Differences in the form of the ripe seeds, whether round or wrinkled. (2) Differences in the color of the food material within the seeds, Avhether pale yellow, orange or green. (3) Differences in the color of the seed coats (and in some cases of the flowers also), whether white, gray, gray brown, leather brown, with or without violet spots. (4) Differences in the form of the ripe pods, whether simply inflated or constricted between the seeds. Phenomena of Inheritance 8$ (5) Differences in the color of the unripe pods, whether light to dark green, or vividly yellow. (6) Differences in the positions of the flowers, whether axial, that is distributed along the stem, or terminal, that is bunched at the top of the stem. (7) Differences in the length of the stem, whether tall or short. 1. 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. 84 Heredity and Environment Fig. 26. Diagram Showing the Results of Crossing Yellow-seeded (Lighter Colored) and Green-seeded (Darker Colored) Peas. (From Morgan after Thompson.) 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- Phenomena of Inheritance 85 lion 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 recessives indefinitely. One-third of the dominants are also pure homozy- gotes, or "extracted" dominants, and when self-fertilized produce pure dominants indefinitely. On the other hand two-thirds of the dominants are heterozygotes and when self-fertilized give rise in the next generation to pure dominants, mixed domi- nant-recessives 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 mixed dominant- recessives, 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 mixed dominant- recessives, D (R). The parental generation is indicated by the letter P, and the successive filial generations by Parent Generation Ft a im rrj Homozygotes Heterozygotex IT I WrT ■ Fig. 27. Diagram Showing Results of Mendelian Splitting where the parents are pure dominants and pure recessives (homozygotes). All pure dominants are represented hy black circles, all pure recessives by white ones, while mixed dominant-recessives (heterozygotes) are repre- sented by circles half white and half black. Successive generations are marked F,, F2, F3, etc. 86 Heredity and Environment Fig. 28. Results of Crossing White-flowered and Red-flowered races of Mirabilis Jalapa ("four o'clocks") giving a pink hybrid in F^ which when inbred gives F2 1 white, 2 pink, 1 red. (From Morgan, after Correns.) Incomplete Dominance. — In the case of the peas studied by Mendel the hybrids of the F1 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 Fr generation had pink flowers and from these in the F2 generation there came white-flowered, pink-flow- ered and red-flowered forms in the proportion of 1 white : 2 pink: 1 red, as shown in Fig. 28. This is a better illustration of Mendel's principle of splitting than is offered by the peas, since Phenomena of hilieritan 87 in this case the mixed dominant-recessives P(P) arc 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 mixed domi- nant-recessives continue to split up in each successive generation into pure dominants, mixed dominant-recessives and pure reces sive in the proportion 1:2:1. The result of this is that with continued self-fertilization the relative number of dominants and recessives increases in successive generations, whereas the relative number of mixed dominant-recessives decreases, and in a few gen- erations a hybrid race will revert in large part to its parental types if continued hybridization is prevented. On the other hand there is no tendency for the relative number of dominants to in- crease and of recessives to decrease in successive generations ; an equal number of pure dominants and pure recessives is pro- duced in each generation (Fig. 27). "Purity" of Genu 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 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. 88 Heredity and Environment 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 mixed dominant- recessives in each generation. For if all germ cells are pure with respect to any particular character the hybrid offspring of any two parents with contrasting characters will produce in equal numbers two classes of 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. 2 germ cells D yR 1X1 $ germ cells D R Possible combinations 1 DD : 2 D(R) : 1 RR. Phenomena of Inheritance 89 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 ; b, Dominant-recessive X domi- nant-recessive = 1 pure dominant: 2 dominant-recessives: 1 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.- Other Mendelian Ratios. — When a pure dominant is crossed with a mixed dominant-recessive (Fig. 29 c) all the offspring show the dominant character, though one-half are pure dominants and the other half dominant-recessives. Thus if a pure round- seeded variety of pea is crossed with a hybrid between a round- seeded and a wrinkled-seeded one, all the progeny are round- seeded, though one-half of them carry the factor for wrinkled seed; this may be graphically represented as follows, R repre- senting the factor for round seed and IV that for wrinkled seed: 9 germ cells 7?* > R $ germ cells R W Possible combinations 2RR: 2R(JV) 90 Heredity and Environment 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(W) : I WW (Fig. 30). When a pure recessive is crossed with a mixed 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 1 : 1 This is due to the fact that the hybrid produces two kinds of germ cells, the pure-bred but one, and the possible combmations of these are as follows : 9 germ cells W^ W 1X1 $ germ cells R W Possible combinations 2 R{W) : 2 WW. This ratio of 2 : 2 or 1 : 1 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. 163). 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 F.2 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 character only, the offspring formed by their crossing are called monohybrids, when there are Phenomena of Inheritance 91 two contrasting characters in the parents the offspring arc dihy brids, when three, trihybrids, and when the parents differ in more than three characters the offspring are called poly hybrids. Th< 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. Dihybrids. — When two or more contrasting characters of the parents are followed to the F2 generation many permutations of these characters occur, thus giving rise to a larger number of R w p &^JB (o ojf o o To 0Y0 O) F, RlW) RlW) RlW) R(W) \o* B w^ V r^\ S*\ H ( ) ( ) w \™J /"wN /v\ w ) ) W w Fig. 30. Monohybrid Diagram Showing Results of Crossing Round- (R) seeded with Wrinkled- (IV) seeded Peas. Large circles represent zygotes, small ones, or single letters, gametes. In Ft all individuals are round but contain round and wrinkled gametes. In F, the S gametes are placed above the square, the 9 ones to the left, and the possible com- binations of $ and 9 gametes are shown in the small squares, the relative numbers of different types being 1 RR : 2 R(\V) : 1 WW, 92 Heredity and Environment types of individuals than when a single pair of characters is concerned. When there is only one pair of contrasting characters there are usually but two types of offspring apparent in the 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 four types of offspring in the F2 genera- YRCGWJ F* Fig. 31. Dihybrid Diagram Showing Results of Crossing Peas Hav- ing Yellow-round (YR) Seeds with Others Having Green-wrinkled (GIV) Ones. The hybrids of the first filial generation (F2) 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 (geno- types) of these $ and ? gametes are shown in the small squares. Since re- cessive characters do not appear when mated with dominant ones these 16 genotypes produce 4 phenotypes in the following relative numbers : 9 YR : 3 YW : 3 GR:i GW. There is 1 pure dominant (upper left corner), 1 pure recessive (lower right corner), 4 homozygotes in the diagonal line between these corners, and 12 heterozygotes. Phenomena of Inheritance 93 tion in the ratio of (3:i)2 = 9:3:3:1. Thus when Mendel crossed a variety of peas bearing round and yellow seeds with an- other variety having wrinkled and green seeds all the offspring of the Ft 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 (YIV), green and round (GR), and green and wrinkled (GIV) in the proportion of 9: 3 : 3 : 1 as shown in Fig. 31. In this case also this ratio may be explained by assuming that the germ cells (ovules and pollen) 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 combinations RW and YG. Accordingly there are four possible combinations of these characters in the pollen and four in the ovules as follows : Y G I X I i.e. YR, YIV, GR, GIV. R IV Each of these four kinds of pollen may fertilize any of the four kinds of ovules, thus giving rise to sixteen combinations, 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. Accordingly the sixteen possible combinations yield seeds of four different appearances and in the following proportions: 9 YR : 3 GR:$ YIV: 1 GIV. Only one individual in each of these four classes is pure (homo- zygous) 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 other individuals are heterozygous and show Mendelian splitting in the next generation. Trihybrids. — When parents differ in three contrasting char- acters there are eight types of offspring in the F2 generation in 94 Heredity and Environment the proportion of (3:i)3 — 27:9:9:9:3:3:3:1. Thus if a pea with round (R) and yellow (Y) 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 Fx generation have round and yellow seeds and tall stem, R, Y, and T being dominant over IV, G, and D. In the F2 generation there are sixty-four possible combinations (genotypes) of these six char- acters (Fig. 32) ; but since a recessive character does not develop if its contrasting dominant character is present there are only eight types which come to expression (phenotypes) and in the following numbers : 27 RYT : 9 RYD : .9 RGT : 9 WYT'.i RGD: 3 WYD : 3 WGT: 1 WGD. Of these sixty-four genotypes 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 (d) ; inflated (E), or constricted 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 inheritance formula of a plant having all seven of the dominant characters is ABCDEFG ; of one having all of the recessive characters abedefg. When two such plants are crossed the inheritance formula of the hybrid is AaBbCcDdEeFfGg, and since the dominant and recessive characters (or rather determin- Phenomena of Inheritance 95 RYT WGD RYT RYD RGT ROD WYT WYD WGT WGD ?z 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 F, hybrid, as shown in the $ gametes above the square and the 9 ones to the left of it, and the possible combinations (genotypes) of these $ and 9 gametes are shown in the 64 small square of which only 1 is pure dominant (upper left corner), I pure recessive (lower right corner) and 8 homozygotes (in diagonal line between these corners), The relative numbers of the different phenotypes are 27 RYT: 9 RYD:g RGT: 9 WYT: 3 RGD : 3 WYD : 3 WGT: 1 WGD. 96 Heredity and Environment 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)1 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 (128)2 or 16,384. Every one of these more than sixteen thousand genotypes may be represented by various combinations of the letters ABCDEFG and abedefg. 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 other writers regard these contrasting charac- ters as positive and negative expression of a single character, and consequently they speak of the presence or absence of single characters : thus round seeds are due to the presence of a factor for roundness {A) while wrinkled seeds are characterized by the absence of that factor (a). Round seeds are wrinkled seeds plus the factor for roundness. Most of the phenomena of Men- delian inheritance are more simply stated in terms of presence or absence of single characters than in terms of contrasting charac- ters. But it is practically certain that recessive characters are hot due to the absence of factors for dominant characters ; there are Phenomena of Inheritance 97 many genetical and philosophical objections to such a view, which leads logically to some strange conclusions, such as Bateson's speculations on evolution (p. 268). When both gametes carry similar positive factors the zygote has a "double dose" of such factors and is said to be duplex: when only one of the gametes carries such a factor the zygote has a "single dose" and is simplex, when neither gamete carries a positive factor or factors, the zygote receives only negative fac- tors and is said to be nulliplex. Thus the union of gametes AB ( 2 ) and AB (S) yields zygote AABB, which is duplex in constitution; gametes Ab (9) and aB (S) yield zygote AaBb, which is simplex ; gametes ab ( 2 ) 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, that 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 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 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 98 Heredity and Environment 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 having been de- rived from the male and the other half from the female sex cell. This double being, or zygote, again becomes single in the forma- tion of the germ cells only once more to become double when the germ cells unite in fertilization. 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 Phenomena of Inheritance 99 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, so that at present it is practically certain that there is no other kind of inheritance than Mendelian. 1. 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, 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 that in 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. ioo Heredity and Environment 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 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 every inherited difference in adult organization must have had some differential cause or factor in germinal organization. Mendel did not speculate about the nature of hereditary units though he evidently 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 Phenomena of Inheritance 101 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 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 formulas 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 Ft 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 102 Heredity and Environment 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. t 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. 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- Phenomena of Inheritance 103 F Fig. 33. Results of Crossing Two Different Races (A and B) of White Sw-eet Peas; all the Fa hybrids (C) are purple with blue wings like the wild ancestral stock; in F2 six colored varieties are formed rang- ing from purple with hlue wings (D) to tinged white (I) and several kinds (genotypes) of white varieties (AT). (After Punnett). 104 Heredity and Environment 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 genetics 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 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. Phenomena of Inheritance 105 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 F1 plants bear neither white nor red flowers but pink ones, and the F2 plants bear white, red and pink flowers. The whites and reds are always homozygous, the pinks heterozy- gous ; pure white and pure red are produced only when their fac- tors are duplex (WW), (RR) ; when they are simplex (WR) pink is produced. In this case red is not completely dominant ovef white, but the hybrid is more or less intermediate 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 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. 106 Heredity and Environment Sometimes dominance is incomplete in early stages of develop- ment but becomes complete in adult stages. Davenport found that when pure white and pure black Leghorn fowls are crossed the chicks are speckled white and black, but in the adult fowl domi- nance is complete and the plumage is black. 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 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 F„ 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 Phenomena of Inheritance 107 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 Fz generation or in which the ratio of dominants to recessives differed from the well known 3 to 1 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 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 1 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 1 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 1 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- io8 Heredity and Environment grained and white-grained wheat he usually got in the second gen- eration the monohybrid ratio of 3 red to 1 white, but three strains gave the dihybrid ratio of 15 to 1 and two gave the trihybrid ratio • of 63 to 1. Consequently he concludes that while the red color of wheat grains is usually due to one factor for red, it may in some cases be due to two or even 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 104. 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 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, 3; 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 Phenomena of Inheritance 109 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 factors are lacking in white persons (this being indicated by the formula aa, bb). Since the AB c?AB Ab aB ab Ab aB ab AB, Ab AB aB AB ab AB Ab Ab Abs Ab £b Ab AB aB Ab aB \B all ab aB AB ab Ab ab aB ab \b Fig. 34. Checkerboard Diagram Showing Results of Crossing Two Mulattoes, each having color factors ABab. Types of male germ 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 dotted diagonal. The color of the children varies all the way from black (upper left corner) to white (lower right corner). * In another family shown in Fig. 36 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. no Heredity and Environment germ cells carry only single factors and not double ones the cross between negro and white would have only one set of these fac- tors for black color, as shown by the formula AB x ab = ABab ; hence the color of the F1 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 1 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. 35) 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, hut some of these factors go into certain cells and others into others, as in the case of dihy- Phenomena of Inheritance 1 II Fig. 35. Blending Inheritance of Size in Rabbits. The skulls of two parents are shown in I and 3, of their intermediate offspring in 2. (From Castle.) 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 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. H2 Heredity and Environment symmetry and pattern of the egg and of the adult animal which is derived from it (see p. 190). 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 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 191-195. 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- delian in the previous generation, this also being a case of pre- inheritance. Somewhat similarly it has been found by Correns, Baur, and Shull that the leaf colors of certain plants are not inherited in Mendelian fashion, but the chromoplasts, which produce the chro- matophores (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. Phenomena of Inheritance "3 ( Mlier forms of transmission arc known in which substances are carried over from one generation to the nexl through the <\^.ur, but they are probably not casts 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- 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 Fig. 36. Mulatto 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.) «-*-! 03 o C/> V r-] "H. o E o ,*5 o C/D bo rt c ^_, c/} & 2 ^^ o • »— . p^ .G ^ •ts t/i £ "a ^ rt <5j 2 E 5 2 •—• o E H-, X O »»— i c O' .5 w N^^X •O 1^ £ rt 5 o -*-» ^ co f Q — ' •— * & £ -4-J <; 5 u O « o JH. w XI rj w ^ r- > Q « C/j E w -t-» o H 1-* L> o c c 0) fc > u nj 9 - S u i-i Phenomena of Inheritance "5 "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 difficult to determine what all the hereditary possibilities .of a family may be. Bearing in mind these serious handicaps to an exact study of inheritance it is not surprising that the method of inheritance of many human 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- Fig. 38. 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,) n6 Heredity and Environment B A Fig. 39. X-ray Picture, A of a Normal, B of a Short-fingered (brachydactyl) hand. (From Bateson.) 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 : Mendelian Inheritance in Man normal characters Dominant Recessive Hair: Curly- Dark Eye Color: Brown Skin Color: Dark Normal Pigmentation Straight Light to red Blue Light Albinism (Fig. 37) Phenomena of Inheritance 117 Mi miki.iax Inheritance in Man (Continued) Dominant Recessive Countenance; Hapsburg Type (Thick lower lip and prominent chin) Temperament; Nervous Intellectual Capacity; Average Average TERATOLOGICAL AND PATHOLOGICAL CHARACTERS General Sice; Achondroplasy (Dwarfs with short stout limbs but with bodies and heads of normal size) Normal size Normal Phlegmatic Very great Very small Normal (Short (Webbed fingers fingers (Supernumerary I funds and Feet Brachydactyly and toes) Syndactyly and toes) Polydactyly digits) Skin; Keratosis (Thickening of Epi- dermis) Epidermolysis (Excessive for- mation of blisters) Hypotrichosis (Hairlessness as- sociated with lack of teeth) Kidneys; Diabetes insipidus Diabetes mellitus Normal Nervous System; Normal Condition True Dwarfs (With all parts of the body reduced in proportion) Normal (Fig. 39) Normal Normal (Fig. 38) Normal Normal Normal Normal Normal Alkaptonuria (Urine dark after oxidation) General Neuropathy, e.g. Hereditary Epilepsy Hereditary Feeble-mindcdness Hereditary Insanity Hereditary Alcoholism Hereditary Criminality Hereditary Hysteria n8 Heredity and Environment Mendelian Inheritance in Man {Continued) TERATOLOGICAL AND PATHOLOGICAL CHARACTERS Dominant 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 Ecus; Normal Normal Recessive 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) 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) * See page 180. bo £ c a s <=> o ^ c E as w u 3 O £ < 3 c a E O T3 ►J S.I £ 3 (U W Q E w • = w W .2 S * fe O en vo 120 Heredity and Environment 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. 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. 1. 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 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 Wave 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 he erroneous. Neither germ cells nor any other kind of cells are formed by the body as a whole, hut 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. 41). 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 Gametti O \V *\t Zygote ABCD Fig. 41. 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. Off** l*Mal / \. Cyies 11m m Gametes ditions. The body develops and dies in each generation ; the germ-plasm is the continuous stream of living substance which connects all generations. The body nourishes and protects the germ ; it is the carrier of the germ-plasm, the mortal trustee of an immortal substance. 4. Genu plasm and Somatoplasm. — This contrast between the germ and the body, between the undeveloped and the developed The Cellular Basis 1-7 organism, is fundamental in all modern studies of heredity. It was especially emphasized by Weismann in his germ-plasm 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 visihle 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 Weismamrs 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 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. 128 Heredity and Environment This is the idioplasm of Nageli, the germ-plasm 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 germ-plasm, 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- 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 many parts of the nucleus The Cellular Basis [29 and cytoplasm, such as the chromosomes, chromomeres, centre somes, etc. In all cases cells come from cells, nuclei from nuclei, chromosomes from chromosomes, centrosomes from ccntro- 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 /1/1 (millionths of a millimeter) in diameter. The largest molecules are prob- ably about 10 fifi 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. 130 Heredity and Environment With the publication of Weismann's work on the germ-plasm, in 1892 speculation with regard to these ultra-microscopic units of life and of heredity reached a climax and began to decline, owing to the highly speculative character of the evidence as to the existence, nature and activities of such units. But with the rediscovery of Mendel's principles of heredity the necessity of assuming the 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 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 represent the dif- ferential causes of particular characters just as in the compounds H2S04 and K„S04 the hydrogen and potassium atoms represent 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 The Cellular Basis 131 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 forty 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 germplasni was located in the chromosomes, but only within recent years has critical experimental evidence 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. 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 arc combined into a system ; in short, there is organization. 132 Heredity and Environment 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. Definitions. — In short, heredity may be defined as the particu- lar germinal organisation which is transmitted from parents to -offspring. Heritage is the sum of all those qualities which are determined or caused by this germinal organisation. Develop- ment is progressive and coordinated differentiation of this ger- minal organisation, by which it is transformed into the adult organisation. Differentiation is the formation and localisation of many different kinds of substances out of the germinal substance, of many different structures amd 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. We receive from our parents germ cells of a particular kind and constitution, and under given conditions of environment these cells undergo regular transforma- tions and differentiations in the course of development, which differentiations lead to particular adult characteristics. In the The Cellular Basis 133 last analysis the causes of heredity and development arc problems of cell structures and functions, problems of the formation of particular kinds of germ cells, of the fusion of these cells in fer- tilization, 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 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. 1. Fertilization, a. Stimulus to Development. — The development 134 Heredity and Environment 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- ments it is artificial parthenogenesis. Natural parthenogensis is relatively rare and in the vast majority of animals and plants the egg does not begin to develop until a spermatozoon has entered it. 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 The Cellular Basis *35 dN^"' -2d PB Fig. 42. Diagrams of the Maturation and Fertilization of the Egg of a Mollusk (Crcpidula). A, B, First maturation division (1st Mat. 136 Heredity and Environment 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-I, 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 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 accordingly 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 Sp.). C, Second maturation division (2d Mat. Sp.) and first polar body (1st PB) resulting from first division. $N, Sperm nucleus. $ C, Sperm centrosome. D, Approach of sperm nucleus ( $ N) and sphere (SS) to egg nucleus ( ? N) and sphere ( 2S) ; the second polar body (2d PB) has been formed and the first has divided (1st 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. The Cellular Basis *37 Fig. 43. Fertilization of the Egg of the Nematode Worm Ascaris megalocephala. 9 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 J\ (From Wilson after Boveri.) 138 Heredity and Environment 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 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- 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 The ( ellular Basis [39 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 **;* » * 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 Heredity and Environment 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 which continue during every cleavage of the egg, lead to the segre- gation of different kinds of plasms in different parts of the egg 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, The Cellular Basis 141 and even photographed, while in the living condition. The peri- pheral layer of protoplasm is yellow and it gathers at the lower pole of the egg, where the sperm enters, forming a yellow cap Fig. 45. Successive Stages in the Cleavage of the Egg of a Mollusk (Crcpidula), showing the separatcness of the male and female chromo- somes ($ch, V ch) and of the male and female halves of each nucleus ($N, 9N). 142 Heredity and Environment (Fig. 46, 1, 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. (Fig. 47; see also Figs. 10 and 11). 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, 1). The third cleavage separates the colorless protoplasm in the upper hemisphere from the slate blue in the lower (Fig. 47, 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 separ- ated so that they can no longer commingle. What is true of Stycla in this regard is equally true of many other ascidians, as well as of Amphioxus and of the frog (Figs. 9, 10, 11), though the segregation of substances and the differ- The Cellular Basis 143 IPS. Fig. 46. Sections of the Egg of Stycla, showing maturation, fertiliza- tion and early cleavage. / PS., First polar spindle, p.b., Polar bodies ; $N, sperm nucleus, $>iV, egg nucleus. />./., peripheral layer of yellow pro- toplasm. Cr., Crescent of yellow protoplasm. A.-, ./, Anterior cells, B3, B3 Posterior cells of the 4-cell stage. In 1 the sperm nucleus and cen- 144 Heredity and Environment 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 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- 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. The ( ellular Basis 145 fercnt 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 germ-plasm, 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 di- vision of chromosomes is ever differential ; on the contrary, there is the most complete evidence that their division is always 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 chro- mosomes in the somatic cells as contrasted with the germ cells of 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, 146 Heredity and Environment 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- somes invariably divide equally, other portions of the nucleus A« 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 i and of Fig. 62, and this suggests that the differential factors for these charac- ters 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. Morgan does not admit the validity of their conclusions, since these apply only to sex- limited and not to sex-linked characters. However these experi- ments may be interpreted, there is good evidence that the factors for the determination of sex and of sex-linked characters are dis- tributed in the same way as the "sex chromosomes" are, and it would be a surprising thing if these two phenomena should be found not to be related causally. Haemophilia. — Another case of sex-linked inheritance is found in an abnormal condition in man known as haemophilia, which is 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- 1 84 Heredity and Environment Eyes n 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 Faetors. — 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 Eyes Chromosomes XX Parents x 0. ? X M XX Fi Gametes ¥2 X Fig. 64. Digaram 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.) i86 Heredity and Environment 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 1 male. Other lethal factors have occurred in other chromosomes of 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 linked together in hered- ity without being associated with sex. Up to 191 6 Morgan had found and studied about one hundred mutations of the fruit fly, Drosophila, which are inherited in four groups, all the charac- ters of each group usually going together. At that date there had been found in the first group 47 different characters, in the sec- ond 27, in the third 22 and in the fourth 2. Corresponding with the number and size of these groups there are four pairs of chro- mosomes in Drosophila, three of which are large and one is very small (Fig. 65). The sex chromosomes {XX in the male, XY in the female) constitute one of the large pairs and the genes of reiuu ""« IV I r Fig. 65. Chromosomes (Diploid) of D. ampelophikt. The sex chromo- somes are the lower ones in each case, XX in the female and XY in the male, Y being J-shaped. There are three other pairs of chromosomes. (From Morgan.) The Cellular Basis \Xj the characters which are sex-linked are probably located in these chromosomes; the genes of the second and third groups of char- acters are presumably in the other large chromosomes, while the fourth group of only two characters probably have their gem :s in the very small chromosomes (Fig. by). If this interpretation is correct, linkage is due to the grouping together of certain genes in certain chromosomes, there are as many groups of characters as there are pairs of chromosomes and as long as the chromosomes preserve their individuality the linkage of genes in the chromosomes and of characters in the developed organism 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 inherited in the same 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 all the sons are yellow and have white eyes while all the daughters are gray and have red eyes, but when these latter are crossed about 99 per cent of the offspring show the same linkage of the colors yellow-white, gray-red, but in 1 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 inter- change 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 expla- nation of the interchange or "cross-over" of genes and conse- quently 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 same way. If a pair of chromosomes are twisted round each other i88 Heredity and Environment A B ai A B C D E a A N // a A b b \\ JjB b c OWL ) ) c C d f fl D d E{{ fi a A B b C D d E f 9. a B D F Fig. 66. Diagram Showing the Possible Chromosomal Mechanism by which "Crossing Over" is Caused. Pairs of chromosomes, one from the .father, the other from the mother, are shown in synapsis. On the left they lie parallel to each other and when they separate in the reduction division they remain as they were before union ; in the second column they are shown crossing each other one or more times and in the remaining figures are shown the results of the chromosomes breaking at the points of crossing and the interchange of sections of the two chromosomes. Let- ters indicate loci of allelomorphic factors in the chromosomes. (After Wilson.) , I * at more than one place and are then broken at these points we get double or multiple crossing over and a correspondrng re-grouping of genes and of characters (Fig. 66). 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. On this basis Mor- gan has constructed a "map" of each chromosome of Drosophila indicating the positions of those genes which have been deter- The Cellular Basis 189 «* YELLOW. SPOT. O.T LETnAL I. « » 111 TE. EOSIN. CnEBBY. • o ABNORMAL. M BIFID. !•-• SHIFTED. M.« LETHAL 111. •T.» TAN. »».o VER1HLION. HI MINIATCRE. «1T LETHAL V. *3-0 SABLE. «.» LETHAL IV. «SJ RID1MENTART. ••.» LETHAL 8. - •* STREAK. ■ - • dacus. *o.« PVJRPLE. BO VESTIGIAL. ».ori!. egg and sperm nuclei ; do, yolk ; k, peripheral layer of protoplasm; dh, vitelline membrane of egg; ch, chorion. (After Kor- schelt and Heider.) I92 Heredity and Environment 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) Localisation 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, 11, 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, 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 The Cellular Basis 193 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 First and Second Cleavages. 194 Heredity and Environment Fig. 71. Normal and Inverse Symmetry of the 3D, 4th, 5TH and 6th Cleavages. The cells la-id, 2a-2d and 2>a-2,d give rise to all the ectoderm; 6,d or M gives rise to mesoderm ; A, B, C, D to endoderm. 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 The Cellular Basis 195 Fig. 72. Normal and Inverse Symmetry in Late Embryos and Adult Stages. In 1, 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. 196 Heredity and Environment CTENOPHORE eel. TURBELLARIAN mes. ECHINODERM ASCIDIAN1 mes. ASCIDtAN II ect. c'9w* rLiD / proto. | 1 0 1 i . ) end. ~— =" V ji^ ms. 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 (cct). 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; eh., chorda; e.g., cerehral ganglion; v.g., ventral ganglion; proto., prototroch. The Cellular Basis 197 egg and of the developed animal arc 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 contained within their nuclei, just as in the differentiation of any tissue cell ; but in the case of the spermatozoon these cytoplasmic differentiations are lost when it enters the egg, 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 fertilization the hereditary potencies 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, white 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. There is evidence that the chromosomes of the egg and sperm are the seat of the differential factors or determiners for Mendelian characters, but the general polarity, symmetry and pattern of the embryo are egg characters which were determined before fertilization. But these egg characters, like any other character of the female, were probably determined by chromo- somes derived from her father and mother; if so they are Men- delian characters inherited by the mother through her chromo- somes and carried over to the first filial generation, not as factors but as developed characters. Such cases may be called "maternal inheritance" since the characters come only from the egg, or "pre- inheritance" since these egg characters are developed before fer- tilization. (See pp. in, 112.) 198 Heredity and Environment 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 are characters of the same kind in the egg and in the adult, and the correspondence is compara- tively 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 (1) 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- 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. 1. 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. The Cellular Basis 199 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 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- 200 Heredity and Environment ■ 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. (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 The Cellular Basis 201 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 patts 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 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. 202 Heredity and Environment 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. The only living bond between one generation and the next is found in the sex cells and all inheritance must take place through these cells. Inherited traits are not transmitted from parents to offspring but the germinal factors or causes are transmitted, and under proper conditions of environment these give rise to developed characters. Every oosperm as well as every developed organism differs more or less from every other one and this remarkable condition is brought about by extremely numerous permutations in the distribution of the chromosomes of the sex cells in maturation and fertilization. Sex is an inherited character dependent, primarily, upon an alter- native distribution of certain chrmosomes 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 associations and dissociations of germinal units which lead to the formation of new materials, and in part by the segregation and localization of these in definite cells. Germ cells and probably all other kinds of cells are almost incredibly complex. We know that former students of the cell greatly 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 tWv„ 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 grcar 206 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 of food, that sex is determined by food or tem- perature, mentality by education, and that in general individual peculiarities are due to environmental differences. Many philosophers of the seventeenth and eighteenth centur- ies taught that man was the product of environment and educa- tion and that all men were born equal and later became unequal through unequal opportunities. Decartes begins his famous "Dis- course on Method" with these words: "Good sense is, of all things among men, the most equally dis- tributed . . . The diversity of our opinions does not arise from some being endowed with a larger share of Reason than others, but solely from this, that we conduct our thoughts along different ways, and do not fix our attention on the same objects." The Declaration of Independence merely reflected the spirit of the age in which it was written when it held this truth to be self evident, "that all men are created equal." The equality of man has always been one of the foundation stones of democracy. Upon this belief in the natural equality of all men were founded systems of theology, education and government which hold the field to this day. Upon the belief that men are made by their en- vironment and training rather than by heredity are founded most of our social institutions with their commands and prohibitions, their rewards and punishments, their charities and corrections, their care for the education and environment of the individual and their disregard of the inheritance of the race. To a large extent civilization itself means good environmental conditions, and the advance of civilization means improvement of environment. Influence of Environment 207 2. Present Emphasis on Heredity. — On the other hand modern studies in genetics are emphasizing the immense, the overwhelm- ing importance of heredity, in both phytogeny and ontogeny. No one now takes seriously the assertion that life can be experi- mentally produced at the present time from non-living matter. It is evident that the artificial production of life is a much more difficult problem than was once supposed, and it may be an in- soluble problem. The first flush of enthusiasm over experimental methods in biology led to the expectation that we would soon be making species by the process of experimental evolution, but the results of one or two decades of such experimental work have been somewhat disappointing. Inherited variations do appear, incipient species arise, but there is very little evidence to show that they appear in response to environmental changes and at present we have no means of controlling such variations. Belief in the omnipotence of environment in the evolution of species has steadily waned in recent years, while a belief in the intrinsic causes of evolution, such as the mutation theory and ortho- genesis, has increased. In ontogeny also the environmental or extrinsic factors of de- velopment have been relegated to a subordinate place, while the intrinsic or hereditary factors appear more important than ever. The old view that men are chiefly the product of environment and training is completely reversed by recent studies of heredity. The modifications which may be produced by environment and educa- tion are small and temporary as compared with those which are determined by heredity. 3. Both Indispensable. — These conclusions are, in the main, well founded. The evidence of the tremendous importance of heredity is so complete that we may rest assured that thinking men will never again return to the position which prevailed until a few years ago regarding the all-importance of environment. And yet there is danger of going too far in the opposite direction. Neither environment nor heredity is all-important, but both are necessary to development. The germ cells with all their inherent 208 Heredity and Environment possibilities would forever remain germ cells were it not for environmental stimuli. The realization of germinal possibilities is dependent upon the responses of the germ to environmental stimuli, and although heredity is a relatively constant factor while environment is a more variable one, nevertheless the two are in- dispensable to development. Only by experiment can the relative importance of heredity and environment in development be de- termined. Extensive experiments have been made within recent years on developing animals and plants in order to discover the factors involved in development, and the modifications which may thus be produced are very striking. B. EXPERIMENTAL MODIFICATION OF DEVELOPMENT The study of development under experimental conditions has given rise to a new branch of biology, viz., experimental embry- ology or the physiology of development. By changes in environ- mental conditions notable modifications may be produced in adult organisms, but these modifications are much greater when the changed environment acts on the organism during the period of its development. I. Developmental Stimuli It is by no means easy to define such general terms as "environ- ment," "stimulus," and "response." In its common use "environ- ment" means all that lies outside the individual, if it is defined from the standpoint of the entire organism. But from the stand- point of an organ or cell it is the surrounding organs, cells or fluids of the body ; the latter may be defined as "internal envi- ronment." If developmental stimuli arise outside the organism they are plainly extrinsic or environmental, but if they arise within the organism they are said to be intrinsic though they may be due to changes in the "internal environment." Stimuli are chiefly energy changes of a physical or chemical nature. A stimulus to which an adult organism responds by Influence of Environment 209 movements or other activities may call forth or inhibit develop- mental responses when applied to germ cells or embryos. These developmental stimuli may be classed as: 1. Physical stimuli including the following, (a) mechanical, (b) thermal, (c) electrical, (d) radiant, (e) light, (£) density of medium, (g) gravity and centrifugal force, etc. 2. Chemical stimuli include the action of (a) substances found in normal development, such as oxygen, carbonic acid, water, food, secretions of ductless glands, etc. and (b) substances not found in normal development, such as various salts, acids, alkalis, alcohol, ether, tobacco, etc. 3. General vs. Specific Stimuli. — In general the action of these stimuli during development does not call forth a perfectly specific and definite response of the organism; various stimuli may pro- duce the same result. Thus artificial parthenogenesis has been produced by almost every stimulus named, and weakened or re- tarded development is produced by many different stimuli. By the elimination of certain of these stimuli which are normally present or by introducing stimuli which are not usually present very important and even profound changes in development may be produced. In this way animals have been formed which are turned 'inside out, or side for side, or in which heads or nervous systems or muscles or backbones are lacking, or in which the various organs are not found in normal positions. In this way dwarfs and giants and one-eyed monsters as well as all sorts of double and partial embryos have been formed. In general monstrous and defective forms of development are due to altera- tions of the normal environment rather than to defective heredity. II. Developmental Responses The character of developmental responses to stimuli depends primarily upon (a) the nature of the organism and (b) the stage of development at which the stimulus acts. Modifications are more easily produced and are more profound during cell division than during intervening periods and at early stages of develop- 210 Heredity and Environment ment than at later ones ; indeed conditions which have no serious effect on an adult organism may greatly modify the development of an embryo or germ cell. i. Modifications of Germ Cells before Fertilization. — It has been found by many investigators that development may be pro- foundly changed by influences acting upon the germ cells before fertilization. In general environmental changes acting during the growth of eggs or spermatozoa and especially during their matura- tion may produce marked changes in development though rarely if ever in heredity. Tower has found that unusual conditions of temperature and humidity during the later stages of oogenesis and spermatogenesis may lead to the production of new races in the case of the potato beetle (Fig. 99) and MacDougall's experi- ments on plants point to the conclusion that chemical substances may influence the ovules so as to change the hereditary character of the plant. Other workers have failed to confirm these results and it is doubtful whether these changes in hereditary constitu- tion were caused by the changed environment. Bardeen and the Hertwigs have shown that great monstrosities may be produced if X-rays, radium or various chemical substances are allowed to act on spermatozoa before fertilization, but there is no evidence that these changes are inherited. Effects of Alcohol. — Stockard subjected adult male and fe- male guinea-pigs to the fumes of alocohol for some time before breeding them and then studied the effects of this drug on their offspring- He finds that the influence of alcohol on the sperma- tozoa is as deleterious as when acting on the ova and that it pro- duces sterility, or greatly reduced fertility, a great excess of still- births, and weak and sickly offspring (Fig. 74). lie and Papani- colaou have studied the offspring of alcoholized parents to the fourth filial generation and while the deleterious effects ultimately disappear they attribute this to the elimination of those germ cells, embryos and developed individuals that were most injured and to the introduction of normal germplasm by crossing with un- treated animals ; consequently the final survivors may be stronger Influence of Environment jm and more vigorous than the controls in which the weak arc pi served along with the strong. Pearl found that the offspring of alcoholized chickens were on the whole stronger than (hose from normal animals and he attributes this to the elimination of weaker germ cells and em- bryos, so that only the most sturdy survive. The work of both Stockard ami Pearl leaves no grounds for doubting thai alcohol injures many germ cells and Stockard has demonstrated that such injured cells may give rise to defective individuals and that this injury may persist through two or three generations. The facts that spermatozoa are affected even more than ova and that the injury persists to the third filial generation show that the chro- matin of these cells is injured. Undoubtedly chromatin as well as cytoplasm may be injured by various unfavorable conditions, and if the injury is not too great it may persist through several generations and may cause defective development; hut this is probably a different thing from the "inheritance of an acquired character"; its effects are seen not in particular characters hut in a general weakening of development; not in mutative changes in genes, but in their temporary injury. In venturing to apply Stockard's discoveries to human beings it should not be forgotten that his guinea pigs were alcoholized to a degree far greater than ever occurs in man. Some of them that were five years old had been kept intoxicated for more than four years. It is probable that the use of alcoholic beverages never produces such serious effects on germ cells as in the case of these guinea pigs which were compelled to inhale the fumes of strong alcohol throughout the greater part of life. Elderton and Pear- son made a mathematical study of children, approximately nine years old, from temperate and from intemperate parents and they concluded that parental alcoholism had practically no effect upon them. However, the more serious the injury to germ cells the sooner they die and it may he that children that survive for nine years come from germ cells that were least injured, while those that were more seriously injured produce individuals that died before or shortly after birth. 212 Heredity and Environment Hoppe believes that a single drunken debauch may so injure the germ cells of man as to produce abnormal and defective off- spring, though this is by no means proved ; while Hertwig con- cludes that the great prevalence of the drug habit may seriously affect the germ cells and their subsequent development. Forel has for many years maintained that one of the most serious causes of human malformations and degenerations is to be found in the effect of alcohol on the germ cells, especially at the time of con- ception. ^ri • Lto, ^B BLr '"i^l ^B Jt Rfe- 4 i B ^1 Fig. 74. Dwarfed Guinea-pigs on the Left and Normal Ones on the Right. All are of approximately the same age though the normal ones are nearly twice the weight of the dwarfs. The normals came from nor- mal parents, the dwarfs from a normal mother and an alcoholic father; the dwarfing has therefore been produced by the influence of alcohol on the spermatozoa. (From Stockard.) Influence of Environment 213 2. Modifications During Fertilisation Stages. — Environmental changes acting during fertilization may cause more than one sper- matozoon to enter the egg or may injure the egg or sperm; in either case the resulting development is abnormal. Where two or more spermatozoa enter the egg the nuclear divisions are usually abnormal, as Boveri has shown in the case of the sea urchin ; the distribution of chromosomes to different cleavage cells is unequal and such cells do not undergo typical development, while the embryo or larva produced is not capable of continued life. In cases where an egg is fertilized by a spermatozoon belonging to a different phylum or class (heterogeneous fertilization) the foreign sperm, after stimulating the egg to begin development, may itself die or remain inactive, in which case the hereditary traits which develop are those of the mother only. In many ani- mals unfertilized eggs may be stimulated to begin development by a great variety of changes in the medium, all such cases being known as "artificial parthenogenesis." 3. Modifications of Development after Fertilization. — Envi- ronmental changes, acting upon the oosperm after fertilization, or upon the embryo, may produce an almost infinite variety of ab- normal types of development, but so far as known none of these modifications becomes hereditary. It seems probable that changes in hereditary constitution take place in the main before fertiliza- tion and especially during the maturation divisions. Isolation of Cleavage Cells.— If the cleavage cells are separated from one another in the 2-cell or 4-cell stage each of them may give rise to an entire animal (Fig. 75) ; in this way two com- plete animals may be derived from a single egg of a star-fish or sea-urchin, of an amphioxus, and of several other animal types. If the frog's egg is turned upside down in the 2-cell stage, double- headed or double-bodied embryos may result (Fig. 76). In such cases each cleavage cell is said to be totipotent, that is, it is ca- pable of giving rise to an entire animal. On the other hand in certain animal phyla such as the cteno- phores, mollusks, annelids and ascidians isolated cleavage cells 214 Heredity and Environment Fjg. 75. Dwarf and Double Embryos of Amphioxus. A, isolated blas- tomere of the 2-cell stage segmenting like an entire egg. B, twin gastrnlse from a single egg. C, double cleavage resulting from partial separation of the first two cleavage cells. D, E, F, double gastrulae arising from such forms as C. (From Wilson.) give rise only to parts of an animal; in this way one may get a right or left half of an animal (Fig. jj) from right or left cleav- age cells; an anterior half (Fig. 78), or a posterior half (Fig. 79) from anterior or posterior cleavage cells; or any one of the cells of the 4-cell stage may produce the corresponding quarter of an entire animal. Such cases are known as "mosaic devel- opment." Influence of Environment 215 Fig. 76. Double Embryos of Frog Developed from Eggs Inverted when in the 2-cell Stage. A, twins with heads turned in opposite directions. B, twins united back to back. C, twins united by their ventral sides. D, double headed tadpole. (From Wilson after O. Schultze.) There has heen much discussion among biologists as to the meaning of these results. On the one hand it has been said that the totipotence of any one of the first four cleavage cells proves that all of these cells are alike and that they have not yet begun to differentiate. On the other hand it is said that a part of an egg may give rise to a whole animal for the same reason that parts of certain adult animals may do the same thing, viz., because they have the power of regeneration. However there are many animals which are incapable of regenerating lost parts of their 2l6 Heredity and Environment Fig. 77. Half and Three-quarter Embryos of Styela. np, nerve plate ; tit, nerve tube; E, eye; »hc7i, mesenchyme; ms, muscle; ch, notochord. A, right half-blastula which developed after the left half of the egg, A3B3, had been killed. B, left half larva from the two left cells of the 4-cell stage, the right cells, A3B3, having been killed. The muscle cells (stippled) occur only on one side of the notochord. D, three-quarter larva, the left an- terior cells having been killed. E, F, three-quarter larvae, the right pos- terior cell B3 having been killed. Influence of Environment 217 Fig. 78. Antkrior Half-embryos of Stycla, the posterior cells having been killed in the 4-cell stage. Neural plate, eye-spots and chorda cells present but no muscle cells or tail. are 2l8 Heredity and Environment olm'ch. Fig. 79. Posterior Half -embryos of Styela, the anterior cells having heen killed in the 4-cell stage. Muscle cells and intestinal cells are present hut no portion of neural plate or chorda. bodies, and similarly there are cases in which part of an egg cannot give rise to a whole animal. The evidence available at present favors the view that in cases where one of the cleavage Influence of Environment 219 cells is capable of giving rise to a whole animal there is a greater capacity of regeneration or regulation, and possibly also a lower degree of initial differentiation, than in those cases in which part of an egg is capable of producing only part of an animal. Effects of Centrifugal Force. — If the fertilized egg is whirled rapidly on a centrifugal machine it may be subjected to a pressure several thousand times that of gravity. Under such conditions the heavier particles are thrown to one side of the egg and the entire substance of the egg becomes stratified into layers or zones. In the ascidian egg, where the different kinds of protoplasm give rise to different tissues and organs, this rearrangement of the egg suhstances may lead to a marked dislocation of organs; the animal may be turned inside out, having the endoderm on the outside and its ectoderm or skin on the inside, etc. (fig. No). < )n the other hand in some mollusks and echinoderms the devel- opment of centrifuged eggs is practically normal. In the for- mer case the formative substances were dislocated ; in the latter they were probably not. Double Monsters and Identical Twins. — If the cleavage cells are only partially separated they may produce animals which are partially separated, such as Siamese twins, two-headed forms, Fig. 80. Two Larvae ok Styela which were centrifuged in the 4-cell stage thereby changing the position of various organ-forming substances. Nervous system (11s), eyes (E), notochord (ch) and muscles iius) have been displaced, and the larva has been turned inside out, the endoderm (end) being outside and the ectoderm (ret) inside. 220 Heredity and Environment etc. (Figs. 75, 76). Or these double monsters may be produced by division or budding of the embryo at a later stage of develop- ment. In the human species, no less than in other animals, all sorts of double monsters may be formed in this way by the par- tial division of a single egg or embryo (Fig. 81). If the division is slight the developed individual may show only the beginnings of a division into two, as in two-headed forms ; if the division of the egg or embryo is complete two separate and perfect individ- uals may be formed from an originally single oosperm. When two individuals are formed from a single egg they have exactly the same heredity and accordingly they are always of the same sex and are so similar in appearance that they are known as "identical" or "duplicate" twins (Fig. 81, right end). On the other hand twins which develop from different eggs do not have the same heredity and may differ in sex as well as in other features ; they are known as "fraternal" twins. Mfftff *' A" A* ** *» *» »„ tu la ^Y^ ^^ Fig. 81. Diagram Showing the Different Types of Union of Double Human Monsters, each being produced by a partial division of a single egg or embryo. If the division is a complete one, duplicate twins are formed, as shown by the figures at the right end of each line. (From Wilder.) Influence of Environment _>_>i Other Monstrous Forms. — If the temperature or density of the surrounding medium is altered during the gastrula stages the endoderm may he caused to turn out instead of in (exogastrula >, thus producing an animal which is turned inside out (Fig. 82). In other cases (vertebrates) the gastrula mouth may fail to close, thus producing animals in which the spinal cord and vertebral ma Fig. 82. Exogastrula of Crcpidula. The endoderm {End) has been turned out instead of in, thus leaving the digestive layer of cells on the outside of the body; Shg, shell gland; V, velum. column are split in two (spina bifida) ; or the brain may be forced outside of the head or may be lacking altogether (anencephaly). In some cases eyes are wholly lacking, in others the two eyes fuse together into a single one as in the fabled Cyclops (Fig. 83). Practically all such cases of monstrous development are due to abnormal environmental conditions, in early stages of ontogeny. Effects of Food. — In addition to such monsters, which are in- capable of long life, many peculiar if not abnormal types of ani- mals are produced by the action of unusual environmental stimuli during later stages of development. Gudernatsch found that if tadpoles of the frog were fed on the thyroid gland they trans- formed into minute frogs, scarcely larger than flics, but if fed on thymus gland they grew to be large, dark-colored tadpoles 222 Heredity and Environment «. ■. *■•■■/ Fig. 33. Young Fish. On the right a normal individual with two eyes ; on the left cyclopean monsters with one eye ; produced by treatment with magnesium solutions. (From Stockard.) but did not change into frogs; if fed on the adrenal gland they produced extremely light-colored forms. If canary birds are fed on sweet red pepper they become red in color. If the larva? of bees are fed on "royal jelly," which is a bee food rich in fats, Fig. 84. The Three Castes of the Honey Bee. A, worker or imper- fect female ; B, queen or perfect female ; C, drone or male. The differ- ences betwen workers and queens are produced by the type of food sup- plied to the larvae. Influence of Environment 22$ they become fertile females or queens; if fed on ordinary "bee bread" they become iniYrtile females or workers ( Fig. 84 1. There are marked structural differences between the workers and die queens but the differences in their habits and instincts arc even more striking; all of these differences whether in bodily structure or in instincts are determined by the character of the food and not by heredity. Innumerable cases of a similar sort could be named which show tin- greal effect of environmental stimuli on development but not upon heredity. C. FUNCTIONAL ACTIVITY AS A FACTOR OF DEVELOPMENT Another factor of development which is partly intrinsic and part- ly extrinsic is functional activity or use. Functional activity is re- sponse to stimuli which may be external or internal in origin. The entire process of development may lie regarded as a series of such responses on the part of the organism, whether germ cell, embryo or adult. The nature of the response is determined by the nature and state of the organism and by the character of the stimulus. By the normal, or usual, series of stimuli certain parts are kept active while other parts are kept inactive or are inhibited. Developmental Movements. — Normal development is depend- ent on the correlated activity of many parts of the organism. If in any part stimuli and responses are lacking the development of that part is arrested or inhibited. For example in the cleav- age stages different substances are sorted and localized by pro- toplasmic movements within cells and these substances are then isolated by cell divisions and by the formation of partition walls between cells; these protoplasmic movements occur in response to stimuli and if these movements are stopped cleavage and dif- ferentiation are arrested. In later stages the infolding of the gastrula, or neural tube, or alimentary canal, and the foldings of layers in general, which play so important a part in development, are due to the movements of substances within cells and to the movements of cells in the layers in which they lie, and if these movements are inhibited normal development is prevented. 224 Heredity and Environment Nutrition and Development. — Another type of functional ac- tivity which is a potent factor in development is found in the trophic or nutritive relations which exist between different parts of the organism. Organs long unused undergo regressive changes and may become rudimentary, for example the muscles of a limb, which has been paralyzed or placed in a cast, shrivel ; on the other hand use increases the size and strength of any organ. In- activity or atrophy of one part usually leads to imperfect nourish- ment and development of related parts ; for example, the optic nerve atrophies when the eye is lost, and muscles atrophy when the nerves leading to them are destroyed or paralyzed. In gen- eral the normal development of any part is dependent upon its proper nutrition and this is dependent upon the functional activity of this and other related parts. Internal Secretions; Hormones. — Still another phase of func- tional activity is found in the effects of certain secretions and chemical substances which are formed by different glands. In many cases the secondary sexual characters which distinguish the male or the female are due to chemical substances from the testes or the ovary, which stimulate or inhibit the formation of these characters. If the ovary is removed from a young hen she develops the larger size, the more brilliant plumage and the pecu- liar comb, wattles and spurs of the cock. These secondary sexual characters of the male are potential in the female but are kept from developing or are inhibited by the activity of the ovary. On the other hand the castration of the young cock does not prevent the development of most of the secondary sexual char- acters of the male. In the case of mammals removal of the ovar- ies of a young female or of the testes of a young male does not lead to the development of the secondary sexual characters of the other sex, but both sexes remain in a sexually undeveloped or infantile condition, that is, the presence of ovaries or testes serves as stimulus to call forth the development of the secondary sexual characters in mammals, and not as inhibitors to prevent the de- velopment of the secondary sexual characters of the opposite sex, Influence of Environment _>_■- as in the female fowl. If bits of the ovary of a guinea-pig are inserted under the skin of a young male which has been pre- viously castrated, the latter develops mammary glands similar to those of a normal female; in short he is "feminized" by the stim- ulus of substances from the ovary. Another gland whose secretions exercise a profound influence on development is the thyroid, which is found in the neck near the "Adam's apple." If this gland becomes enlarged it gives rise to goitre, protruding eyeballs, rapid heart beat; on the other hand if the thyroid is deficient in a young child it causes the peculiar type of idiotic dwarf known as "cretin." If the gland which lies between the roof of the mouth and the base of the brain and which is known as the hypophysis is deficient the child, or young animal, remains infantile; if the hypophysis is too large the individual's hands, feet and face become enlarged and he may grow to be a deformed giant, but with weak body and mind. Correlative-Differ en tiation and Self-D iffc rent ia t ion . — Many cases are known in which the development of a part is dependent upon the presence of another part; this is technically known as "correlative differentiation." Thus it has been found that the lens of the eye will develop from any portion of the ectoderm, or outer layer of the skin, if only the primitive retina, or optic cup, is brought near to this layer; if the optic cup is transplanted from the head to the thorax or abdomen a lens will form wherever the cup comes in contact with the ectoderm. If an embryonic limb is transplanted from its normal position to the middle of the back or belly, it will develop, and nerves and blood vessels will grow into it which would have had very different positions and distributions if the limb had not been there. If one of the first four cleavage cells is separated from the others it may develop into an entire ani- mal though it would have formed only a quarter of an animal if it had remained in contact with the other three-quarters of the egg. All such cases are known as "correlative differentiation," implying that differentiation is dependent upon the stimuli which come from surrounding parts. On the other hand if the differen- 226 Heredity and Environment tiation has already begun before the relation of a part to surround- ing parts has been changed, it may continue to differentiate as if no change of position or relation had taken place. Thus if a right limb is transplanted to the left side of the body after it has begun to differentiate it remains a right limb and is not modified by its new relations (Harrison) ; if the cleavage cells are already dif- ferentiated in the four-celled stage, each cell when separated from the others will give rise to only one-quarter of an animal. In short the organ or cell is already set, or fixed, or differentiated to such an extent that it can not change its fate even though its environment should change. Such cases are known as "self- differentiation." Many students of the physiology of development have been led to the view that the fundamental causes of development are to be found not in the egg cell itself but in environmental stimuli and in the interaction of the various parts. Driesch in particular regards the egg, or any cleavage cell, as an "harmonic equipoten- tial system," that is, any part is capable of any fate and its actual fate is determined by its relation to other parts; in the striking phrase of Driesch, "The fate of a part is a function of its posi- tion." We now know that this expresses only a fraction of the truth. The fate of a part is primarily a function of its organiza- tion and only secondarily a function of its position. These are only a few illustrations of the many kinds of abnor- mal development which may be caused by changed environment or by unusual functional activities. At all stages of ontogeny the course of development may be altered by extrinsic stimuli but earlier stages may be more profoundly influenced than later ones. D. INHERITANCE OR NON-INHERITANCE OF ACQUIRED CHARACTERS Few questions in biology have been discussed so fully and so fruitlessly as this. It is a problem of the greatest interest not only to students of biology but also to sociologists, educators and philanthropists and yet it is still to a certain extent an unsolved problem. Influence of Environment _>_>- Opinions of Lamarck and Darwin. — It is well known thai La- marck taught that characters due to desire or need, use or disuse, and to changed environment or conditions of life were inherited and thus brought about progressive evolution. Long ago desire or need was repudiated as a factor of evolution. Lowell satir- ized it in his Biglow Papers in these words : "Some filosifers think that a fakkilty's granted The minnit it's felt to be thoroughly wanted, * * * * * That the fears of a monkey whose holt chanced to fail Drawed the vertibry out to a prehensile tail." Darwin wrote to Hooker, "Heaven forfend me from Lamarck's nonsense of adaptation from the slow willing of animals" ; but although he repudiated this feature of Lamarckism he held that characters due to use or disuse and to changed conditions of life might be inherited and he proposed his hypothesis of pan- genesis in order to explain the process of the transmission of such characters to the germ cells. Weismann's Theories. — Weismann introduced a new era in biology by denying the inheritance of all kinds of acquired char- acters, and by challenging the world to produce evidence that would stand a rigorous analysis. But Weismann's greatest ser- vice lay in his constructive theories rather than in destructive criticism ; he forever disposed of theories of pangenesis and the like by showing that the germ cells are not built up by contribu- tions from the body and that characters are not transmitted from generation to generation ; but on the other hand that there is transmitted a germ plasm which is relatively independent of the body and which is relatively very stable in organization. This epoch-making theory of Weismann's has naturally undergone some changes, as the result of new discoveries. It is no longer believed that the germ plasm is really independent of the body, nor that it is absolutely stable, as Weismann at one time held. There is no doubt that the germ cells and the germ plasm are 228 Heredity and Environment physiologically related to other cells and to other plasms, and similarly there is no doubt that the germ plasm although very stable can and does change its constitution under some rare conditions. But in the main the germ plasm theory is accepted by the great majority of biologists to-day, and recent work in genetics and cytology has brought many confirmations of this theory. Distinction between Hereditary and Acquired Characters. — As long as it was believed that the developed characters of an or- ganism could be transmitted as such to its -descendants it was cus- tomary to speak of developed characters as hereditary or ac- quired and to talk of the inheritance or non-inheritance of acquired characters. This distinction is not a logical one for all developed characters are invariably the result of the responses of the ger- minal organization to environmental stimuli ; and of course no developed character can be purely hereditary or purely environ- mental. But when a given character arises in many individuals of the same genotype under different environmental conditions it is probable that heredity, which is the constant factor in this case, is also the determining factor for that character. On the other hand if a character develops in response to peculiar stimuli and does not appear in other individuals of the same genotype in which such stimuli are lacking it is said to be an environmental or acquired character. In fine, inherited characters are those whose distinctive or differential causes are in the germ cells, while acquired characters are those whose differential causes are environmental. Statement of Problem. — Briefly stated the question of the in- heritance of acquired characters is this : Can the differential cause of a character be shifted from the environment to the germ plasm? Can peculiarities of the environment which influence the development of somatic characters so affect the germ cells that they will produce these somatic characters in the absence of the peculiar environment? Can the characteristics of a developed organism enter into its germ cells and be born again in the next Influence of Environment 229 generation? Considering the fact that germ cells arc cells and contain no adult characteristics, it seems very improbable that any peculiarity of environment whether of nutrition, use, disuse or injury, which brings about certain peculiarities of developed char- acters in the adult, could so change the structure of the germ cells as to cause them to produce this same character in subsequent generations in the absence of its extrinsic cause. How, for ex- ample, could defective nutrition, which leads to the production of rickets, affect the germ cells, which contain no bones, so as to produce rickets in subsequent generations, although well nour- ished? Or how can over-exertion, leading to hypertrophy of the heart, so affect the germ cells that they, in turn, would produce hypertrophied hearts in the absence of over-exertion, seeing that germ cells have no hearts? Or how could the loss or injury of eyes or teeth or legs lea'd to the absence or weakened development of these organs in future generations, seeing that inheritance must be through germ cells which possess none of these structures ? Lack of Evidence for Inheritance of Acquired Cliaracters. — But, apart from these general objections to the doctrine of the in- heritance of acquired characters, there are many special difficul- ties. There is no conclusive and satisfactory evidence in favor of such inheritance. Almost all the evidence adduced serves to show only that characters are acquired, not that they are inherited. It is a matter of common observation that mutilations are not inherited; wooden legs do not run in families, although wooden heads do. The evidence for the inheritance of peculiarities due to use or disuse is wholly inconclusive; for example, did the giraffe get his long neck because he browsed on trees, or does hi' browse on trees because he has by inheritance a long neck? Did attempts to fly lead to the development of wings in birds, or d<> birds ily because heredity has given them wings? Did life in caves make cave animals blind, or did blind animals resort to caves because the struggle for existence there was less severe for them? The evidence is in favor of the second of each of these alternatives rather than of the first. 230 Heredity and Environment There still remains the question of the inheritance of certain characters due to environment, though here also the most clear- cut evidence is against this proposition. That unusual conditions of food, temperature, moisture, etc., may affect the germ cells so as to produce general and indefinite variations in offspring is probable, but this is a very different thing from the inheritance of acquired characters. The germ cells being a part of the paren- tal organism may be modified by such changes in the environment as affect the body as a whole, they may be well nourished or starved, they may be modified by changed conditions of gravity, salinity, pressure, temperature, etc., and these modifications of the germ cells probably lead to certain general modifications of the adult, which may be larger or smaller, stronger or weaker, accord- ing as the germ is well or poorly nourished, but it is incredible that the environment which produces rickets, or hypertrophied heart, or loss of sight in one generation should modify the germ cells in such a peculiar and definite way that they should give rise in the next generation to these particular peculiarities, in the absence of the extrinsic cause which first produced them. The inheri- /tance of acquired characters is incredible, because the egg is a 1 cell and not an adult organism ; and in this case there is no suffi- cient evidence that the thing which is incredible really does hap- pen. No Inherited Influence of Stock on Graft. — If specific changes of environment produced specific changes in heredity we should expect to find that where different plants or animals are grafted together each would modify more or less the hereditary consti- tution of the other. But this does not occur. Everybody knows that when a branch of a particular kind of fruit tree is grafted upon a tree of a different variety the quality of the fruit borne by that branch is not altered by its close union with the new stock. The same is true of all forms of animal grafts. Harrison cut in two young tadpoles of two species of frog, Rama sylvatica and Rana palustris, and spliced the anterior half of one to the posterior half of the other. These frogs and their tadpoles differ in color Influence of Environment 231 as well as in other respects, R. sylvatica being more deeply pig- mented than R. fialustris. In the grafted tadpoles each half preserved its own peculiarities even up to the adult condition (Fig. 85). A still more striking case of the persistence of heredity in spite of environmental changes is found in experiments in which the ovaries are removed from one variety of animal and transplanted to another variety. Guthrie made such transplantations in the case of fowls and concluded that there was some influence of the 1 ter mother upon the transplanted ovary, but Davenport, who re- peated his experiments, was unable to confirm his results. Fi- nally Castle and Phillips furnished the most conclusive demon- Fig. 85. 'Grafted Frog Embryos, anterior part, Rami sylvatica, posterior part, R. palustris. In later stages, and even in the adult condition, the two parts preserve their peculiarities. (From Harrison.) 232 Heredity and Environment stration that the hereditary characteristics of the transplanted ova are in no wise changed by the foster mother. They removed the ovary from a pure black guinea-pig and put it in the place of the Fig. 86. Effect of Transplanting Ovaries in Guinea-pigs. Above, young black female; in the middle, mature white female; below, mature white male. The white female's ovary was removed and in its place was put the ovary from the black female. The white female (with "black" ovary) was then bred to the white male. (From Castle.) Influence of Environment 233 ovary of a pure white animal. After recovery from the operation this white female with the "black" ovary was hred to a pure white male (Fig. 86). Three litters of offspring from these parents were all pure black as shown in Figure 87. Although both parents Fig. 87. Results of Cross Described in the Preceding Figure. All the offspring are black, though both parents are white, because the white female contains only "black" eggs and black is dominant over white. (From Castle.) 234 Heredity and Environment were pure white all the offspring of the F1 generation were black because they came from "black" eggs and black is dominant over white. The fact that these "black" eggs developed in the body of a white female did not in the least change their hereditary con- stitution. Dominants and Rcccssivcs Remain Pure. — A still more inti- mate union takes place when the dominant and recessive char- acters come together in any zygote. These characters, or rather the factors which determine them, may be intimately associated in every cell of the organism throughout an entire generation and yet we may get a clean separation of these characters in the next generation ; in many cases neither the dominant nor the re- cessive character has been at all modified by its most intimate association with the other. Climatic Effects Not Inherited. — A striking instance of the purely temporary effect of the environment and of the long persistence of hereditary constitution amidst new environmental conditions, which have greatly changed the appearance of the developed organisms, is found in the case of alpine plants. Nageli says that such plants, which have preserved the characters of high mountain plants since the ice age, lose these characters perfectly during their first summer in the lowlands. Summary. — If acquired characters were really inherited we should expect to find many positive evidences of this instead of a few sporadic and doubtful cases. In particular why do we not find in plant or animal grafting that the influence of the stock changes the hereditary potencies of the graft? Why do we not find that transplanted ovaries show the influence of the foster mother as Guthrie supposed — a thing which has been disproved by Castle (Figs. 86 and 87) ? Why do dominant and recessive characters remain pure, even after their intimate union in a hybrid, so that pure dominants and pure recessives may be ob- tained in subsequent generations from this mixture? Why does every child have to learn anew what his parents learned so la- Influence of Environment j\^ boriously before him? Even the strongest defenders of the inheritance of acquired characters are constrained to admit that it occurs only sporadically and exceptionally. Neo-Lamarckism. — Many modifications of the Lamarckian hy- pothesis of the inheritance of acquired characters have been pro- posed in recent years. Foremost among these are the "mneme" theory of Semon and the "centro-epigenesis" theory of Rignano. To Semon as to many other biologists the apparent resemblance between memory and heredity has seemed significant, and tluN furnishes the basis of his theory. Semon holds that every condi- tion of life, every functional activity of an organism leaves a permanent record of itself in what he calls an "engramme.'' If these conditions or activities are long continued their engrammes are heaped up and affect heredity. Semon does not ask if "ac- quired characters" are inherited, but rather "Are the hereditary potencies of the germ cells altered by stimuli acting on the paren- tal body?" This is a very different thing from the inheritance of a particular acquired character, and there is some evidence that such stimuli may in rare instances produce changes in the heredi- tary constitution of the germ plasm though these evidences are by no means conclusive. Temporary Effects of Environment; "Induction." — On the other hand certain changes may be produced in germ cells or embryos which last for only a generation or two and then dis- appear. It is well known that plants grown in poor soil are smaller and produce smaller seeds than those grown in good soil, and deVries, Bauer and Harris find that such seeds produce smaller plants having smaller seeds than do seeds of normal size. This is an after effect of poor nutrition which changes the amount of food material in the seeds and through this the size of the plant which develops from the seed, but it does not change the heredi- tary constitution. Woltereck found that in Daphnia there is an after effect of cold lasting for one or two generations, and this he calls "induction" when the effect lasts for one generation, or "pre-induction" when it lasts for two or three generations. Whit- 236 Heredity and Environment ney found that rotifers poisoned with alcohol were weaker in resistance to copper salts and were less fertile than others, and when brought back to normal conditions the first generation was weak but the second was normal. On the other hand Stockard finds that the injurious effects of alcohol on guinea pigs persist through two or more generations. In man alcohol may have an "induction" effect on offspring, but fortunately it does not seem to alter hereditary constitution. Probably of a similar character are Sumner's results ; he found that mice raised in the cold have shorter tails than those raised at higher temperatures and this modified character appears in the next generation. If this is an after effect or "induction'' it should disappear in the following generations. Kammerer found that salamanders with black and yellow spots when reared on yellow soil gradually lose their black color becom- ing more yellow, and their young continue to grow more yellow until finally almost all black may disappear. The offspring of such salamanders are said to be more yellow than normal; but this work has been called in question and needs confirmation. Even if confirmed the result may be an after effect or "induction" which would soon disappear under usual conditions, and there is no evidence that it is really inherited. Such cases are not instances of true inheritance ; they do not signify a change in the hereditary constitution but an influence on the germ cells of a nutritive or chemical sort comparable with what takes place when fat stains are fed to animals ; the eggs of such animals are stained and the young which develop from such eggs are also stained, though the germinal constitution re- mains unchanged. The very fact that the changed condition is reversible and that it disappears within a short time is evidence that it is not really inherited. In conclusion : ( 1 ) Developed characters, whether "acquired" or not, are never transmitted by heredity, and the hereditary con- stitution of the germ is not changed by changes in such charac- ters. (2) Possibly environmental stimuli acting upon germ cells Influence of Environment 237 at an early stage in their development may rarely cause changes in their hereditary constitution, but changes produced in somatic cells do not cause corresponding changes in the hereditary con- stitution of the germ cells. (3) Germ cells like somatic cells may undergo modifications which are not hereditary; if starved they may produce stunted individuals and this effect may last for two or three generations; they may he stained with fat stains and the generation to which they give rise he similarly stained; they may be poisoned with alcohol or modified by temperature and such influence be carried over to the next generation without becoming hereditary. All such cases are known as "induction" and many instances of the supposed inheritance of acquired char- acters come under this category. (4) Environment may pro- foundly modify individual development but it does not generally modify heredity. E. APPLICATIONS TO HUMAN DEVELOPMENT: EUTHENICS Man's Larger Environment. — Man's environment is more ex- tensive than that of any other animal, and its influence on his de- velopment is correspondingly greater. In addition to chemical and physical stimuli which are potent factors of development in the case of all organisms, man lives in a world of psychical, so- cial and moral stimuli which exert a profound influence on him. He is stimulated not merely by present environment but also by memories of past experiences and anticipations of future ones. Through intelligence and social cooperation he is able to control environment for particular ends, in a manner quite impossible to other organisms. On the other hand heredity is no more pow- erful as a factor of development in the case of man than in any other organism. Consequently the relative importance of hered- ity and environment is not the same in the development of an intelligent and social being, like man of the present age, as it is in lower organisms. For man and for every other living crea- ture heredity fixes the possibilities of development, it "sets bounds about us which we cannot pass" ; but the more complex those 238 Heredity and Environment possibilities become the more complex must be the environment which calls them forth and the more varied become the results of development under altered conditions of life. Capacity for Training and Education. — Functional activity also plays a larger part in man's development than in that of any other animal, owing to the longer period of his development and to the more extensive and varied training which he is capable of under- going. It is a notable fact that the period of immaturity in man is longer than in any other animal, and it is during this formative period that environment and education have their greatest influ- ence. Other animals develop much more rapidly than man but that development sooner comes to an end. The children of lower races of mankind develop more rapidly than those of higher races but in such cases they also cease to develop at an earlier age. The prolongation of the period of infancy and of immaturity in the human race greatly increases the importance of environment and training as factors of development. The possible training of human faculties is also more varied and extensive than in other animals, not only because those facul- ties are more numerous but also because they are more plastic and are capable of higher development, that is, are more edu- cable. Human faculties are functions and methods of reaction, which are dependent in part upon the bodily mechanism and in part upon environment and training. Habits are the usual meth- ods of responding to stimuli, and they may be classified as in- herent or acquired. The latter are in a sense forced upon organ- isms by environmental conditions. All education is habit for- mation, and good education like good environment is such exper- ience as leads to the formation of good bodily, intellectual, social and moral habits ; it consists in placing the individual in such an environment and bringing such stimuli to bear upon him as to call forth desirable responses and to suppress undesirable ones. Good and Bad Environment. — Only that environment and training are good which lead to the development of good habits and traits and to the suppression of bad ones. What we com- Influence of Environment 239 monly call "good environment" is frequently the worst possible, what is often called a bad environment may be the best possible. We are all strangely blind with regard to these matters. We know of many cases in which men began their careers on a farm, in the backwoods, on a flat-boat, amidst hardships and discom- forts of every sort and yet who achieved great distinction. And we speak of such men as winning in spite of disadvantages, for- getting that often these very disadvantages, hardships, discom- forts, have been stimuli which have given them sturdy bodies, good judgments, good morals, and have called forth all their best qualities. On the other hand under different circumstances or with different men such conditions may prove to be too hard, too severe, and the result be disastrous. But environment may be too good as well as too hard. Food may be too rich and too abundant for good health, life may be too easy and luxurious for the development of character. Luxury, easy lives, refined sur- roundings have less of educational value than we commonly sup- pose and they may be a positive menace. Any environment is bad, however cultured, refined or pleasant it may be, which leads to the development of bad traits of body or mind. In general the best environment is one which avoids extremes, one which is neither too easy nor too hard, one which calls for sustained effort and produces maximum efficiency of body and of mind. In education also we are strangely blind as to proper aims and methods. Any education is bad which leads to the formation of habits of idleness, carelessness and failure, instead of habits of industry, thoroughness and success. Any religious or social in- stitution is bad which leads to habits of pious make-believe, insin- cerity, slavish regard for authority and disregard for evidence, instead of habits of sincerity, open-mindedness and independence. Frequently the training of the human being, like the training of a star-fish, consists in limiting his activities to particular lines. Some physical defect which prevented a child from engaging in ihe usual activities of children has often turned his attention to scholarship. Galton says that great divines have usually had very 240 Heredity and Environment poor health. Genius is frequently associated with physical de- fects. Great specialization is associated with corresponding limi- tations in other directions. Society needs the genius and the specialist, but for the general good of mankind the generalized type is needed even more than the specialized. No given environment or training can be good for every in- dividual, nor for the same individual at every stage of develop- ment. Every individual is unique and if the best results are to be had he must have unique environment and training, which must be supplied by omniscient intelligence. Such an ideal may not be practicable but the impossibility of securing the absolutely best conditions of development need not prevent society from securing better conditions than those which now prevail. Relative Importance of Heredity, Environment, Education. — It is plain that environment and education play a greater part in the development of man than in that of other animals, whereas hered- ity plays the same part; but it is difficult if not impossible to de- termine the relative importance of these three factors. In the field of intellect and morals most persons are inclined to place greater weight upon the extrinsic than upon the intrinsic factors, but this opinion is not based upon demonstrable evidence. So far as organisms below man are concerned there is general agreement that heredity is the most important factor, and this opinion is held also for man by those who have made a thorough study of hered- ity. Galton has made the best scientific study of this subject in the case of identical twins, in which as we know heredity is the same in the two, both individuals having come from the same oosperm (Fig. 81). In bodily and mental characters such twins are remarkable alike; the differences which exist are slight and may usually be traced to different environmental and educational influences, and particularly to different illnesses. Galton sums up his study with these words: "There is no escape from the conclusion that nature prevails enormously over nurture when the differences of nurture do not exceed what is commonly to be. found among persons of the same rank of society and in the same country." Influence of Environment 241 The part played by these different factors of development may be graphically illustrated by the accompanying diagram (Fig. 88), in which the base line represents heredity and the other lines rep- resent the extrinsic factors of environment and education. For each individual heredity is a constant factor but environment and 2/ \, 13 c ^ 03 rt r- rt 1 -t-T Ih u < O 6 £ en o Sh p J3 fe O v~' •z t/1 < U o O -4-» Ih U IS u u. £ >, o r-! .Q 02 to *d & 3 O O ti »- 1 |h 3 < bfl O u a S3 o 2 ""H «> Q <—• > « £ +-» u V U .fi E Ih Mh • o o o «*-!. £ o o ja Control of Heredity: Eugenics 259 tin 10 On O OS W Q X, o w /. o H ^, W ^ o u w K H pq a u o £> Q O as Ph c/j w a. >< H M W K H O 260 Heredity and Environment of the phenomena in question. It is possible that the divisions of the cell body in these protozoans is not always into qxactly equivalent halves in which case variations might take place in the descendants, which might then be heaped up by selection ; or perhaps there are multiple or modifying factors in this case also so that selection has acted as in Castle's rats. Value of Selection. — In conclusion, the evidence which is most clear-cut and abundant indicates that selection by itself is unable to change inheritance factors or unit characters. Nevertheless selection is of great service in separating good lines or races from poor ones, and this is the chief significance of the artificial selec- tion practiced by breeders. The elimination of certain races by natural selection is an im- portant factor in evolution though it has nothing to do with the formation of new characters or new races but serves merely as a sieve, as deVries has expressed it, to sort the individuals which are supplied to it. Although selection has no power to make or change characters, it preserves certain lines and elim- inates others and thus fixes the type of a species. Finally the elimination of the unfit by natural selection is still the only natural explanation of fitness, or adaptation, in organisms. III. Methods of Modern Genetics i. Mendclian Association and Dissociation of Characters. — Breeders have long known that it is possible to get certain desir- able characters of an organism from one race and others from another race. But since the discovery of the Mendelian princi- ples of heredity such new combinations of old characters have been made repeatedly, and with almost the same certainty of results as when the chemist makes combinations of elements or compounds. In Fig. 95, A and B, are shown two guinea-pigs, one having long (L), rough and tumbled (T), white (W) hair, and the other having short (S), smooth (Sm), red (R) hair. When such ani- mals are' crossed one should get in the F2 generation 64 genotypes Control of Heredity: Eugenics 261 and 8 phenotypes, one of each of the latter being homozygous and breeding true, as is shown in Fig. 32 for trihybrid peas. These 8 phenotypes of this cross are STR, STIV, SSmR, SSmW , LTR, LTW, LSmR, LSmW. In Fig. 95, C and D, and Fig. 96, A, B, C, are shown 5 of these 8 phenotypes which were ob- tained by Castle from this cross. These figures well illustrate the new combinations of Mendelian characters which may be obtained by cross breeding. Hybridisation. — This is the chief method employed by Bur- bank in producing his really wonderful "new creations in plant life." By extensive hybridization he brings about many new com- binations of old characters, a few of which may be commercially valuable, and sometimes actually new characters or mutations appear, possibly as a result of the interaction of old characters, or rather of their factors. Lotsy, for example, maintains that the sole source of variation is crossing, and Bateson says that the new breeds of domestic animals made in recent times are the carefully selected products of the recombination of pre-existing breeds, and that most of the new varieties of cultivated plants are the outcome of deliberate crossing. One of the striking results of modern work in plant-breeding has been the discovery of the greatly increased vigor of certain hybrids as compared with either pure-bred parent. In general it is not possible to tell without previous experience what the char- acter of the hybrid of two races or "lines" will be; sometimes it is more and sometimes less vigorous than either parent, but not infrequently it is more vigorous. East and Shull have shown that hybrids between two races of corn may be very much larger and more fertile than either parent. In some instances the yield of corn per acre has been increased from 20-30 bushels to 80-90 1 bushels, and in one case to more than 250 bushels per acre (Figs. 97, 98). Unfortunately such hybrid races of corn do not con- tinue to breed true and the crossing must be made anew in each generation if maximum results are to be had. Nevertheless this method of hybridization or "heterozygosis," as it has been called, 262 Heredity and Environment offers an extremely important means of quickly producing very vigorous and fruitful individuals, but not lines or races which breed true. 2. Origin of Mutations. — Mendelian association and dissocia- tion of characters produces new forms of adult animals and plants but not new hereditary characters. Permutations of Men- delian characters we may have almost without number, of new combinations of these there may be no end, but no new unit char- acters are formed by such temporary combinations, no new in- heritance factors are created or evolved. New combinations of factors may be compared to new combinations of chemical ele- ments ; you can always get out of the combination what went into it, whereas new factors are comparable to the changes which take place in certain atoms, for example radium, by which the element itself is changed in an irreversible manner. The discoveries of Mendel show us how to follow old characters through many com- binations and through many generations, but they do not show us how new characters arise. These discoveries have given us an invaluable method of sorting and combining hereditary qualities, but Mendelian inheritance as such does not furnish the materials for evolution. In 1901 Hugo deVries startled the scientific world by the pub- lication of his great work on the "Mutation Theory" of evolu- tion in which he proved that the evening primrose, Oenothera lamarckiana occasionally produced "sports" or "mutations" which differed so much from the parent form that they deserved to be called new species (Fig. 100). He discovered and studied a large number of these mutations in Oenothera as well as in some other plants and concluded that evolution takes place by steps or jumps rather than by "creeping on from point to point" as Darwin believed. Several genetecists have expressed doubt as to whether there are any such things as mutations in the sense of deVries, maintaining that all his results may be explained by assum- ing that Oenothera is a hybrid and that the various "mutations" Control of Heredity: Eugenics 263 which he has described arc due to segregation and recombi- nation of old factors rather than to the appearance of new ones. Indeed Davis has made up an Oenothera by hybridiza- tion that is very similar to O. lamarckiana. There are many things which seem to indicate that this species and probably other species of Oenothera are not genetically pure and it is probable that some of deVries' results may be due to this fact. But it is certain that mutations do take place in species where there is no evidence of genetic impurity, as for example in Drosophila am- pelophila, and it is an extraordinary circumstance that some at least of the mutations of Oenothera upon which deVries founded his great theory are probably not mutations at all but are, as Muller has said, "merely the emergence into a state of homozy- gosis, through crossing over, of recessive factors constantly pres- ent in the heterozygous stock." DeVries himself suggested this explanation for his double reciprocal crosses and as Muller says, "it probably lies at the root of nearly all the unusual genetic phe- nomena of this genus." That there are lethel factors also in Oenothera which produce their effects upon the gametes rather than the zygotes is indicated by the partial or complete failure to form fertile pollen in certain forms of this genus. It is probable that many natural or Linnean species, other than O. lamarckiana, are not pure and homozygous; within every such species there are usually found many "elementary species" and by intercrossing of these a mixture of many lines or strains re- sults from which new forms may occasionally arise by segre- gation. Lotsy maintains that all mutations arise in this way. But such an explanation does not account for the existence of the original "elementary species" and if they be referred to still earlier crossings it is evident that we only put off the explanation to a more remote period. On the other hand the exact and ex- haustive work of Morgan and his associates on Drosophila has proved that the mutations in this species are not due to such segregation and it plainly indicates that they are caused by sud- den transformations in the Mendelian factors themselves, com- parable to changes in chemical composition. 264 Heredity and Environment rt O % -*-» . - t^ o> O to $5 H \ 55 £ <( 0 w ^ 1/3 a . \ in W O M ■ « E * H 0 V-i ' k 0 to j H -— ' OS <: +j j js i 5 M s • — * M 1-1 "> c > g J5 H ~ m a U, « 0 f £ fc *! 0 -° . U ■ U to 5 ° O OS *J~ CO "« % § . c/) O C to '5 Ih -4-»