The above diagram illustrates the explanation which has been advanced to account for the proportions obtained when individuals differing with respect to a single character are crossed, as in the case of the peas. The circles may be considered as smoothness and the squares as wrinkledness. The first three lines, I, II, and III, show the condition of reproduction in lines breeding true to type. A and B represent the two types, each of which can give off germ cells of its own type only (II, ab and cd). So long as the individuals of each line are bred separately they can produce only the combinations ah and cd. But when the two lines are crossed, every germ cell of A unites with a germ cell of B to produce an individual having determiners for both smoothness and wrinkledness (C). All the individuals of this first hybrid generation will have the same make-up and all will be alike in appearance. Although both characters are present in this hybrid generation, one is hidden by the other, as the square is enclosed by the circle, so that, in ex- ternal form, the individuals of type C are not to be distinguished from type A. When individuals of type C produce germ cells, they produce equal numbers containing the determiners for each of their characters, as is indicated in VI, a, b, c, d. When equal numbers of the two kinds of germ cells unite by chance, the result will be as indicated in VII. The possible combinations are ac, ad, be, bd, which produce one of type A, two of type C, and one of type B, the propor- tions of 3 to 1, which Mendel obtained with peas, since types A and C are not distinguishable from their external form. When two or more pairs of characters are involved, as greenness versus yellowness.
156 CONCERNING THE ORIGIN OF INSTINCTS roundness versus wrinkledness, each pair is found to be inherited, as in the above example, but the two pairs are found to be independ- ent of each other, so that they occur in all possible combinations: green-wrinkled, green-smooth, yellow-wrinkled, and yellow-smooth. In such a case the proportions obtained are 9 yellow-smooth, 3 yellow-wrinkled, 3 green-smooth, and 1 green-wrinkled. These are the proportions already mentioned in connection with the walnut combs of fowls and the proportions to be expected from the laws of chance.
Further details of the process, blending inheritance (as in the walnut comb), linkage of characters, and the struc- tural basis of the process of inheritance need not be en- tered into here. Certain important features may be em- phasized. First: the parts, organs, or characters of the plant or animal are to a great extent independent in in- heritance (hair color may be inherited from one parent and hair form from the other). Second: the characters are not altered in their passage from one generation to the next; they may blend together in the hybrid, as the rose and pea combs blend to form the walnut comb, but they will separate eventually into the component parts.
Some attempts have been made to apply these laws to behavior complexes, but as yet psychology has provided little foundation for such studies. The most thorough- going attempts have been made with human mental traits and some evidence has been collected here in favor of the view that differences in the instinctive behavior of individuals are inherited according to Men- delian ratios. But in the field of human psychology too little is known of the genesis of character, of the dis- tinction between native and acquired behavior, to provide a very firm foundation for the work of the geneticist. A few studies of the inheritance of some more readily deter- mined characters have been made upon animals. On p. 144 we presented the work of Yerkes on the direction of whirl- ing in the dancer and upon the inheritance of the in- stinctive traits of boldness, timidity, etc., in the rat. Certain studies have also been made upon the inheritance of wildness in ducks. Kammerer has found that certain variations in the brooding instincts of the obstetrical toad ORIGIN OF DIVERSITIES 157 are inherited in Mendelian ratios. None of the other studies has been carried far enough to show that the traits studied reappear in the offspring in Mendelian ratios.
II. Origin of Diversities in Organisms Introduction. — The laws of heredity thus far considered apply only to the transmission of characters already present in the organism. They say nothing as to the origin of these characters, yet the wide diversity of instincts with which the behaviorist is constantly confronted suggests, per- haps first of all, the question: how could such diverse forms of behavior have come into existence? This is a part of the problem of evolution, of which Bateson says, summing up the work bearing upon the question: " The many converging lines of evidence point so clearly to the central fact of the origin of the forms of life by an evolutionary process that we are compelled to accept this deduction, but as to almost all the essential features, whether of cause or mode, by which specific diversity has become what we perceive it to be, we have to confess an ignorance nearly total."
While the origin of a new race or type of animals has been observed in but few cases (and some writers are in- clined to believe that even these are not truly new, but merely the reappearance of something already present but hidden), the indirect evidence obtained from comparative anatomy, embryology, and historical geology is so complete as to leave no room for doubt that new forms are con- stantly arising from the old, by some process the details of which are not yet clear.
In the present incomplete stage of the problem it will be impossible to reach any final conclusion as to the man- ner in which evolution has taken place. At best we can but point out the paths of investigation which promise to yield the solution of the problem, and summarize (very incompletely) the evidence which has been collected in support of the various theories as to how new characters arise. The first students of evolution sought to find the method of origin of new species, of groups of animals dif- fering rather markedly from other groups, usually with 158 CONCERNING THE ORIGIN OF INSTINCTS respect to several characters. The results of Mendelian experiments have changed this point of view in great measure. The student of evolution is now concerned chiefly with the origin of single new characters, of " unit characters," such as the wrinkledness of the pea or the single comb of the fowl. Davenport says of this: " The fact that most characteristics are not necessarily associated — that they may occur in various combinations — certainly accounts for the multiplicity of 'varieties' in domesticated species; and for much of the variation in feral species. Moreover, it probably ac- counts for the presence of many ' species ' in a genus."
In order to recognize the origin of a new character, it is necessary that the experimenter be thoroughly familiar with the normal conditions in a small group of related organisms. Not only must he know what characters are present visibly in the bodies or soma of his material, but he must know also what characters are potentially present in their germ cells, as recessives in the Mendelian sense. Students of variations have accomplished this in investiga- tions of many morphological- characters, such as hair color in mice; but students of behavior have been lax in analyz- ing and recording the many individual differences in in- stinctive behavior which have come to their attention. The few studies of variation in instinct which have thus far been completed have given results of considerable importance for theories of evolution. Practically all of the work has been done by students of the inheritance of acquired char- acters and will be considered with the evidence bearing upon this problem. Owing to the lack of material in the field of behavior, it will be necessary to base our discus- sion of the method of the origin of variations upon morpho- logical studies of extra-neural structure, disregarding, for the moment, the difference between structure and behavior; or rather, assuming that there is no fundamental difference between the two, since, ultimately, all behavior must be looked upon as the functioning of structure.
Darwin's conception of variations. — Recent studies of variation, with advancing knowledge of the mechanism of heredity, have shown clearly that not all diversities (varia- ORIGIN OF DIVERSITIES 159 tions) are of the same nature either with respect to causa- tion or heritability. This fact was perceived vaguely by the early investigators, but they did not understand its full significance. Darwin distinguished at least two kinds of variations: the so-called continuous ones, constantly ex- hibited by all organisms, slight differences of size, pro- portion, depth of pigmentation, etc., in which continuous series may be recognized; and discontinuous variations, sports or monsters which appear suddenly, differ rather widely from the racial type, and show no intermediate stages. He considered this latter class as of little sig- nificance in evolution, as mere accidents of nature, of too rare occurrence or of too extreme a form to affect the course of development of species. The former class, he believed, furnished the differences, which, preserved and fixed by natural selection, formed the basis for new varie- ties. Concerning the cause of these variations he w^as uncertain, but inclined to ascribe them to some funda- mental property of living matter, of equal rank with irri- tability or with the power of reproduction itself.
Continuous variation due to the direct action of en- vironment upon the developing organism. — It is now gen- erally admitted that continuous variation is due to the di- rect action of the environment upon the body or soma of the developing organisms. If large numbers of individuals of any pure race of animals or plants are considered, it will be found that the greatest number of them are nearly alike, forming the racial average, while the variants in the two directions from the racial average grow proportionately fewer, with an increasing degree of variation. This con- dition may be illustrated by a group of measurements of the size of bean seeds descended from a single individual (a pure line) — ^taken from the measurements of Johannsen.
Class distribu- ) centigramms. ) Number of seeds of.
seeds. ^ 160 CONCERNING THE ORIGIN OF INSTINCTS As was first shown by Quetelet, the distribution of such variations around the racial average follows the law of chance distribution. The effect of environment upon the production of variation in the soma can be seen quite clearly in the following investigations of Stockard, Fuld, and Cunningham. Stockard has shown that while the eggs of the fish {Fundulus) under ordinary conditions produce normal two-eyed fish, the addition of magnesium salts to the water causes them to develop into C3^clopean mon- sters. Some of the changes are apparently of such a na- ture as to make the animal better fitted to meet the demands of its environment. Thus, Fuld found that dogs which had their forelegs removed during the first year of life later showed changes in the proportion of the femur and tibia of the hind-legs which mimicked the normal con- ditions in leaping animals of the type of the hare. Cun- ningham illuminated the ventral surface of young floun- ders, which are normallj^ unpigmented there, and induced the formation of pigment.
The non-inheritance of continuous variation. — The be- lief that continuous variations of the type considered are inherited was shared by all investigators up to recent times. The whole Darwinian theory of evolution is based upon this concept. In 1903 the heritability of continuous variations was put to a crucial test by W. Johannsen. He studied a population of beans and found that it was made up of a number of races which differed from each other in the average size of their seeds and in other characters. The descendants of one plant gave seeds of small size, varying, it is true, from small to medium, yet maintaining for generation after generation an average size consider- ably lower than that of the population. The variations within this race were not inherited at all; the smallest and largest seeds, if only they had the same ancestry, gave progeny of the same average size. The descendants of other plants taken from the mixed population had larger seeds and the difference between the races persisted for as many generations as they were studied. By selecting large and small individuals from the mixed population it was MUTATIONS 161 MUTATIONS 161 possible to isolate large or small races, but after tire race was once isolated a selection of variations occurring within it did not lead to any change in the average size of the race.
Since the publication of this result the experiments of Jennings, De Vries, Pearl, Shull, Woltereck, and many others have confirmed it and extended it to many other classes of organisms. The results of this experimental work dealing with the constancy of heritable characters and the effects of selection of slight differences in these characters prove conclusively that the vast majority of the variations of organisms are not inherited. Just what is the cause of all such variations is not yet certain, but it is probable that all are the responses of the organism to changes in its environment and that these changes affect only the body of the individual and not the germ cells. ^ Fig. 35 (3) indicates this condition. The body of the or- ganism is changed as an effect of environmental action, but its germ cells (represented in lighter color in the abdomen of the diagrammatic moth) and hence the characters of the progeny remain unchanged.
Discrete variation or mutations. — We have considered the lack of influence of continuous variations (fluctuating variations of Darwin) upon heredity. Darwin himself, as has already been noted, had observed another type of varia- tion which he failed to consider because he believed that its occurrence was too rare to influence the course of evolution. He called such variations ' ' sports ' ' or monsters. The first investigator to lay great stress upon the importance of dis- continuous variation of this type was De Vries, influenced by his work on the evening primrose ((Enothera lamarchi- ana). For over twenty years he bred this plant under con- ditions of scientiflc accuracy. His first culture gave a wholly new sport or mutation and in the continued culti- vation of the plant many other mutants were obtained, some ^ There is, however, accumulating a considerable body of evidence that indicates that some somatic variations, perhaps of a special kind, are inherited. This will be considered in the discussion of the inheritance of acquired characters.
162 CONCERNING THE ORIGIN OP INSTINCTS Fig. 35. Possible Modes of Inheritance of New Characters 1. Somatic induction, the transmission of the effects of the environ- ment upon the soma to the germ cells. 2. The direct action of the environment of the organism upon the germ cells without effect upon the body (ordinary mutations), 3. Somatic variation, changes in the body which do not affect the germ cells. 4. Parallel induction, the same changes produced by the independent action of the environment upon the body and the germ cells. From Kammerer, " Die Abstam- mungslehre," Gustav Fischer, Jena, 1911 (after Ziegler).
of whieh, such as the giant and dwarf varieties, are very striking. Some fifteen types of mutants in the evening primrose have been described, which for the most part breed MUTATIONS 163 true, i.e., retain their individuality in later generations when hybridized or inbred. De Vries obtained similar re- sults with other plants and assumed that such discontinu- ous mutations furnish the differences which make the basis for evolution.
The rediscovery of Mendel's laws of segregation and heredity have made possible a different interpretation of much of De Vries' material. In the light of these dis- coveries it has been shown that his original strain of 0. lamarckiana was a poly-hybrid, and that most of the mutants from it, if not all, were, in reality, but the reap- pearance of combinations of characters already present, although hidden, in the original stock. But although the greater part of the original mutations do not seem to have produced anything new in the race, the impetus given to the study of mutations by De Vries' work has led to the discovery of other " sports " which seem really to have produced something new. The number of such mutations so far discovered in laboratory experiments which may be truly said to add something not present in the parent stock, i.e., to be progressive, is small. Many recorded mutants seem rather to be due to the loss of some character. Thus albinism, which has appeared more than once in a pure strain, is due apparently, not to the addition of any factor which will cause albinism, but rather to the loss of what- ever substances in the germ cells cause the production of pigment. In some cases progressive changes have ap- peared. Many such are recorded as occurring suddenly among wild organisms, but the true nature of these is usually uncertain. Several mutations have been experi- mentally produced by Standfuss, Fischer, Tower, Morgan, and McDougall. Tower subjected various species of the potato beetle at the time of the ripening of the germ cells to extremes of temperature and moisture. The result was the formation of many new types of beetles, differing most conspicuously in pigmentation. These new beetles showed no tendency to revert to the parent form but 'bred true for generation after generation. Thus races having lighter color, darker color, differences in the arrangement of the 164 CONCERNING THE OEIGIN OP INSTINCTS markings on the wing covers, etc., were produced, races which were in all respects '' good " species. Furthermore, it is asserted that these races differed in their instinctive equipment. Some, e.g., were far more rapacious than others.
Frequency of mutations. — Thus the sudden appearance of new, discontinuous, and heritable diversities in organisms may be looked upon as established. But there is little agreement as to the extent and frequency of such muta- tions. There seems to be a growing tendency among biolo- gists to hold that they may be, in many cases, of very slight extent and of very great number, so as to form what is almost a continuous series. Davenport brings out this point of view clearly in the following: " Sometimes a prominent character is represented by a single determiner like (perhaps) roseness of the comb of the fowl; but in most cases there is a multiplicity of factors, as in human hair and skin pigments, in the yellow of mice, in shank feathering of fowls, and in seed coats of oats. In consequence of the fact of this multi- plicity of factors and of the fact tliat a variable number may be present in different cases, the adult character appears in numerous grades of development.
Indeed, the gradation of characters is, in these cases, such that one has to recognize that discontinuous variation passes over into continuous variation, in the sense that 40, 41, 42 form a continuous series, if not in the sense that X, X -)- dX, X -\- 2dX, etc., do. If a desire for uniformity leads us to conclude that all variations in the germ-plasm are discontinuous, at least we see in many of these variations sufficient justification for the continuity hj'pothesis of the old-fashioned selectionist."
Cause of mutation. — "When w^e come to ask the cause of mutation we find very little certain evidence. Various theories have been advanced and must be considered in some detail. A possibility w^hich has received great em- phasis in the past, chiefly in the works of Weismann, is to be found in the union of diverse elements in the two germ cells at the time of fertilization. AVe have seen that such union does sometimes produce new forms by combination of Mendelian unit characters, as in the production of the walnut comb of the fowl. But in such eases the new character does not seem to be permanent, but breaks up in later generations into its component parts. Whether new CAUSE OF MUTATIONS 165 and fixed forms may be produced in this Avay is not yet decided. The results of Morgan upon linkage of characters suggest that this is quite possible if a rearrangement of the constituents of the chromosomes takes place in such a way that characters which originally segregated freely can no longer do so. The greater amount of evidence points to the direct action of the environment upon the germ cells as the cause of mutation. McDougall injected various chemicals into the immature ovaries of CEnothera hiennis with the result that new heritable forms were produced. Similar results were obtained with other plants. Standfuss and Fischer have produced new forms of the butterflies Vanessa and Arctia by subjecting the pupas to low tem- peratures, and similar results have been obtained with other lepidopterous insects. Tower, as we saw on p. 163, has ob- tained mutations by subjecting the pupse of the potato beetle (Leptinotarsa) to abnormal conditions of tempera- ture and moisture during the period when the germ cells are ripening. This type of evidence, of which a consider- able amount has been collected, goes far towards proving the dependence of mutations upon changes in the germ cells caused by direct environmental action. Whether or not this is the only cause of progressive mutation cannot be de- cided from the evidence at hand. Whatever be the cause, it is certain that many differences in organisms do arise suddenly as '' sports " and are transmitted unchanged to the descendants of the individuals first showing them.
Mutations not swamped by cross-breeding. — In so far as the transmission of the new characters produced by muta- tion has been studied it appears to follow the laws of Men- delian heredity (p. 151). The new characters retain their individuality and are not swamped by cross-breeding with the race from which the mutant sprang. This fact is of extreme importance for evolution, since new characters produced by mutation are thus fixed immediately and give rise to new races which are in no danger of being swamped by interbreeding with the great mass of the population.
Frequent repetition of the same mutation. — Another fact of importance is that when a group of organisms has 166 CONCERNING THE ORIGIN OF INSTINCTS once produced a mutation, the probability is that the same mutation will occur again and again. De Vries was the jSrst to point out this condition. He obtained the same mutant many times from his cultures of CEnotliera. The same condition has" been observed in other organisms. This interesting fact may be explained either upon the assumption that the environmental action was the same in the various cases, or that some factor within the plant determined the form of the mutation, without regard to the nature of the environmental stimulus. The latter seems to be the true one. MacDougall obtained the same type of mutants from plants whose ovaries had been injected with different chemicals and treated with radium. He also ob- tained different mutants from different plants injected with the same chemical.
Such results suggest that the type of the mutation is determined by something already present in the organism and that the unusual conditions in the environment serve to start the change. The abnormal environment sets off the mutation, but it appears that the character of the latter is determined by the nature of the organism in which it occurs. This fact leads to another important conclusion, that there is no adaptive relation between the mutation and the environment. So far as can be determined from the evidence at hand, the application of excessive heat to growing rats might, conceivably, produce either extra toes or sparse hair in the next generation.
The Darwinian conception of natural selection. — The fact last brought out, viz., that mutations need not be adaptive and often have no conceivable relationship to the demands made by the environment upon the animal, leads to a consideration of the influence of selection upon muta- tions. On the Darwinian hypothesis natural selection is looked upon as an actual causative agent. As has been brought out, he supposed that, given fluctuating varia- tions, natural selection would shape the direction of evolu- tion in such a way that, given time, the organism would become perfectly adapted to its habitat. On such an hy- pothesis every instinctive act observable in an organism DARWIN'S CONCEPTION OF SELECTION 167 must be looked upon as having adaptive value. Two quo- tations from Darwin seem clearly to bring this out.
" But the mere existence of individual variability and of some well- marked varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in nature. How have all those exquisite adaptations of one part of the organism to another part, and to the conditions of life and of one organic being to another being, been perfected? We see these beautiful co-adaptations most plainly in the woodpecker and in the mistletoe; and only a little less plainly in the humblest parasite which clings to the hairs of the quadruped or the feathers of the bird; in the structure of the beetle which dives through the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world."
" Again it may be asked how is it that varieties which I call incipient species become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species? How do those groups of species which constitute what are called distinct genera and which differ from each other more than do species of the same genus, arise? All these results...follow from the struggle for life. Owing to this struggle, variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals and will generally be in- herited by the offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term natural selection, in order to mark its relation to man's power of selection."
Changes in the concept o£ natural selection. — With the discovery, on the one hand, that Darwinian fluctuations are not inherited, and, on the other, that new characters appear suddenly, due possibly to the direct action of the environment upon the germ cells, there has come about necessarily a marked change in our conception of the function of natural selection. In the first place, the muta- tion hypothesis has relieved the investigator of the burden of attempting to find adaptive value in the various activ- ities of animals, and has allowed him to examine such behavior without preconceived notions. It is now admitted that adaptation, the fitness of the organism for the condi- tions of its life, is by no means so perfect as has been sup- 168 CONCERNING THE ORIGIN OF INSTINCTS