CHAPTER XXIII HEREDITY AND ENVIRONMENT Two Problems of Environment. As heredity treats of all the inborn traits which form the raw material of the individual, so the environment includes all the external forces which may influence this raw material. The environmental factor gives rise to two distinct problems: first, that of its possible power to influence heredity by modifying the inborn characters themselves; and, second, that of its relative importance in determining the development of the individual after birth. Both of these problems involve an inquiry into the extent of the environmental influence and also into the method of its action. Problem of the Inheritance of Acquired Characters. The first problem, ordinarily known as that of the inheritance of acquired characters, is the point of contention between two distinct schools of evolutionary theory. The first school, known as the Lamarckian, maintains that the environment directs the course of evolution by modifying the characteristics of the individual in the process of adaptation, and that these individual modifications are transmitted to future generations by inheritance. The other school, influenced chiefly by the work of Weismann, maintains that the environment can influence the direction of evolution only in determining which individuals shall survive, and not at all through any direct action upon surviving individuals; and that environmental modifications of the individual never become a part of the equipment of the species, because they are not inherited. In support of his theory Weismann contended that the germ plasm was not derived immediately from the somatic, or body cells, but came directly from the germ cells of the parents, so that the germ plasm was continuous from generation to generation and was therefore unaffected by somatic changes. This theory, supported by convincing experiments, and further confirmed by enlightening researches into the mechanism of inheritance, won over the majority of biologists. The Weismann school insists that no evidence exists of the inheritance of acquired characters, and claims moreover that the assumption that they are inherited is not essential to complete the theory of evolution, as all the facts can be accounted for by processes which are well established. Within recent years, however, new experiments have added weight to the Lamarckian viewpoint, and on the whole biologists are inclined to admit that the question is still open, and they acknowledge that many facts could be more easily explained by the hypothesis of the inheritance of acquired characters than by any other theory. In the course of experiment and discussion the two schools have come nearer together and are fairly well agreed on certain points, as for instance the non-inheritance of mutilations. Ever since Weismann cut off the tails of twenty-two generations of mice, and failed to get a tailless species for his pains, it has been generally agreed that mutilations are not inherited. And it is fortunate that they are not, for if they were the human race would present a melancholy appearance by this time. The Lamarckians themselves agree that this is not the sort of influence which would impress itself upon the germ plasm. Quite a different problem concerns the effect of the use of disuse of parts, for it is conceivable that, though a mutilation imposed from without might have no effect on future generations, a change in the organism due to individual effort might have an influence. However, biologists generally agree that there is no evidence of the inheritance of the effects of the use or disuse of parts, such as muscles; though there are scientists who prefer to interpret the existence of vestigial organs as inherited results of disuse, and who explain the blindness of cave animals also as resulting from the disuse of the organs of sight. The last case, on the other hand, is explained by their opponents as a result of the discontinuance of selection, or perhaps rather the reversal of selection. Animals with defective eyesight probably had a positive advantage in dark places over animals which relied upon sight, and therefore survived; while animals with normal vision either perished or found their way to the light. The question of the inheritance of the use or disuse of parts is admittedly difficult of solution, for its most ardent advocates now admit that any immediate heritable effect would be only slight, though it could be cumulative generation after generation. In the case of organs dependent upon individual exercise for development, it would be almost impossible to prove that exercise by the parent had any effect upon the offspring. As has already been suggested, inheritance of the use or disuse of parts is usually denied on negative grounds; that is, because of the general lack of any evidence to indicate inherited effects of somatic changes. The question will probably have to be settled by experiments of a different nature. If it should ever be positively proved that acquired characters of any kind were inherited, there would be more willingness to admit that the use and disuse of parts gradually affected future generations. The most notable experiments tending to prove the inheritance of acquired characters have been those of Kammerer, who has made extended investigations over a period of several years. According to reports, he altered the breeding habits of toads by changing temperature and found that these habits were inherited as a Mendelian dominant character; and he also altered the color of salamanders by changing the backgrounds and found the new color inherited; and he made experiments on flounders, showing the effect of light on color and its tendency to become fixed. Another experimenter, Mr. W. F. Guyer, obtained young rabbits with defective eyesight by injecting a serum into the mother, and found that the defect was transmitted to as many as eight generations with a tendency to an increase rather than a diminution in the effect. When crossed with normal rabbits the defect seemed to behave as a Mendelian recessive. While these experiments have not been generally accepted as proving the case, they have aroused much interest. Bateman and Huxley particularly are skeptical of Kammerer's results; but McBride believes that the inheritance of acquired characters bas now been demonstrated and explains his position as follows: “ The exercise of function tends to enlarge the organs and if con 1 Eugenics Review, 8: 329. 1 tinued through generations slowly modifies the structure of the stock till eventually the resulting change can show itself in the young before the exercise has been begun. Conversely persistent disuse of function tends to atrophy the organ till eventually it is developed only in a reduced form.” The problem of the effect of the use and disuse of parts has appeared also among the psychologists, who have argued concerning the probable effects of nerve reactions on offspring. Thorndike believes that the strengthening of a nerve response in an individual would be unlikely to modify the germ cells to produce stronger response in the children. But H. W. Chase argues, from what is known of changes in secretions through the exercise of the emotions, that there would be good reason to assume an effect on offspring, provided the reflexes were attended by intense integration of the organism. And Castle admits that the inheritance of instinct is very difficult to explain unless the inheritance of acquired characters is admitted. The doctrine of the inheritance of acquired characters has been objected to on the ground that we know of no mechanism by which it could be accomplished, - an objection which would seem to indicate a good deal of scientific complacency. The advocates of the theory have suggested that the stimulation of secretions, or chemical changes in the blood, might furnish the mechanism. It is incontestable that the germ plasm represents ancestry rather than the immediate individual. And it is certainly difficult to see how changes in the individual could affect the inheritance by acting on the germ cells; but it is also difficult to understand how complex nervous reactions could develop by chance variations if the experience of the individual had no effect on the offspring. Experiments already made will doubtless have to be verified and extended before much reliance can be placed upon them. And even if the principle in question were established, it is possible that its usefulness might lie chiefly in helping to explain obscure processes of evolution rather than in any practical effects to be derived from its use. The Lamarckian theory has already been partially accepted, for it is generally admitted that 1 American Journal of Psychology, 28: 175. the environment may affect simultaneously the somatic cells and the germ cells, though it is not generally admitted that the germ cells are affected through the body cells. The environment then may have a direct influence in shaping the course of evolution despite the fact that modifications may not be inherited. Even though the inheritance of acquired characters were further corroborated, the sociologist should apply the theory with great caution, because the effects of inheritance would evidently be so exceptional and so slight as to be practically negligible in a single generation. It would be safer for the sociologist to assume that the effects of the life and habits of the parents, whether for good or ill, were not, technically speaking, inherited. True Meaning of Heredity. The general conclusion concerning the non-inheritance of acquired characters may be accepted, however, only with a clear understanding of two points: first, the true meaning of heredity, and, second, the ways in which the germ cells may be affected by external influences. Inheritance means that the chromosomes in the germ cells of the offspring so resemble those of the ancestor that the offspring reproduces the same trait. In this sense therefore a disease could not be inherited. Most disease germs are larger than the sperm cells, and it is entirely inconsistent with the known mechanism of heredity that germ diseases should be hereditarily transmitted. Even though the ovum, which is larger than the sperm, could be attacked by a disease germ it would still be a case of infection and not a variation in the carriers of inheritance. However, defective organs may be transmitted by inheritance just the same as perfect organs are, and they may cause disease. A few germ diseases like syphilis, though not inherited, may be congenital; that is, infection takes place during the period of prenatal development. Such a distinction between inherited and congenital characters may of course be of no practical importance to the victim himself, but it is of great importance to the theory. Other diseases which appear late in life, like tuberculosis, not in themselves either inherited or congenital; but it is supposed that power of resistance to disease germs is inherited in varying degrees. Whenever a disease germ is so prevalent that are |