1. The analogy from breeding experiments. Tame rats, for instance, are very docile; their offspring can be handled without a bit of trouble. The wild rat, on the other hand, is not at all docile.
W. E. Castle, of Harvard University, writes:[32] "We have repeatedly mated tame female rats with wild males, the mothers being removed to isolated cages before the birth of the young. These young which had never seen or been near their father were very wild in disposition in every case. The observations of Yerkes on such rats raised by us indicates that their wildness was not quite as extreme as that of the pure wild rat but closely approached it."
Who can suggest any plausible explanation of their conduct, save that they inherited a certain temperament from their sire? Yet the inheritance of temperament is one of the things which certain psychologists most "view with alarm." If it is proved in other animals, can it be considered wholly impossible in man?
2. The segregation of mental traits. When an insane, or epileptic, or feeble-minded person mates with a normal individual, in whose family no taint is found, the offspring (generally speaking) will be mentally sound, even though one parent is not. On the other hand, if two people from tainted stocks marry, although neither one may be personally defective, part of their offspring will be affected.
This production of sound children from an unsound parent, in the first case, and unsound children from two apparently sound parents in the second case, is exactly the opposite of what one would expect if the child gets his unsoundness merely by imitation or "contagion." The difference can not reasonably be explained by any difference in environment or external stimuli. Heredity offers a satisfactory explanation, for some forms of feeble-mindedness and epilepsy, and some of the diseases known as insanity, behave as recessives and segregate in just the way mentioned. There are abundant analogies in the inheritance of other traits in man, lower animals and plants, that behave in exactly the same manner.
If mental defects are inherited, then it is worth while investigating whether mental excellencies may not also be.
3. The persistence of like qualities regardless of difference in environment. Any parent with open eyes must see this in his own children—must see that they retained the inherited traits even when they left home and lived under entirely different surroundings. But the histories of twins furnish the most graphic evidence. Galton, who collected detailed histories of thirty-five pairs of twins who were closely alike at birth, and examined their history in after years, writes:[33] "In some cases the resemblance of body and mind had continued unaltered up to old age, notwithstanding very different conditions of life;" in other cases where some dissimilarity developed, it could be traced to the influence of an illness. Making due allowance for the influence of illness, yet "instances do exist of an apparently thorough similarity of nature, in which such differences of external circumstances as may be consistent with the ordinary conditions of the same social rank and country do not create dissimilarity. Positive evidence, such as this, can not be outweighed by any amount of negative evidence."
Frederick Adams Woods has brought forward[34] a piece of more exact evidence under this head. It is known from many quantitative studies that in physical heredity, the influence of the paternal grandparents and the influence of the maternal grandparents is equal; on the average one pair will contribute no more to the grandchildren than the other. If mental qualities are due rather to early surroundings than to actual inheritance, this equality of grandparental influence is incredible in the royal families where Dr. Woods got his material; for the grandchild has been brought up at the court of the paternal grandfather, where he ought to have gotten all his "acquirements," and has perhaps never even seen his maternal grandparents, who therefore could not be expected to impress their mental peculiarities on him by "contagion." When Dr. Woods actually measured the extent of resemblance to the two sets of grandparents, for mental and moral qualities, he found it to be the same in each case; as is inevitable if they are inherited, but as is incomprehensible if heredity is not largely responsible for one's mental make-up.
4. Persistence of unlike qualities regardless of sameness in the environment. This is the converse of the preceding proposition, but even more convincing. In the last paragraph but one, we mentioned Galton's study (cited at some length in our Chapter I) of "identical" twins, who are so much alike at birth for the very good reason that they have identical heredity. This heredity was found to be not modified, either in the body or the mind, by ordinary differences of training and environment. Some of Galton's histories[35] of ordinary, non-identical twins were also given in Chapter I; two more follow:
One parent says: "They have been treated exactly alike; both were brought up by hand; they have been under the same nurse and governess from their birth, and they are very fond of each other. Their increasing dissimilarity must be ascribed to a natural difference of mind and character, as there has been nothing in their treatment to account for it."