There is another hypothesis which might account for the observed changes of type. If it were assumed that among the descendants of immigrants born in America there are an appreciable number who are in reality children of American fathers, not of their reputed fathers, a general assimilation by the American type would occur. Socially this condition is not at all plausible; but, on account of the importance of the phenomenon that we are discussing, it should be considered. I do not think that any of the observations that have been made are in favor of this theory. The changes that occur in the Bohemians who arrive here as young children, the different directions of the changes in distinct types, particularly the shortening of the head of Bohemians and of Italians, do not favor the assumption. Furthermore, if the modifications were due to race-mixture, the similarity between fathers and American-born children should be less than the similarity between fathers and foreign-born children, but there is no indication that this is the case.
This hypothesis is also shown to be untenable by the comparisons of fathers and mothers with their own foreign-born children. These comparisons show that the differences are the same in the case of fathers and children, and of mothers and children; so that obviously the same conditions must control the relations between fathers and their children, and mothers and their children. In other words, the fathers must be considered as the true fathers of their children.
Earnest advocates of the theory of selection might claim that all these changes are due to the effects of changes in death-rate among foreign-born and American-born; that either abroad or here individuals of certain types are more liable to die, and that thus these changes are gradually brought about. On the whole, it seems to my mind, the burden of proof would be entirely on those who claim such a correlation between head-index, width of face, etc., and death-rate,—a correlation which I think is highly improbable, and which could be proposed only to sustain the theory of selection, not on account of any available facts. I grant the desirability of settling the question by actual observations; but, until these are available, we may point out that the very suddenness of the changes after immigration, and the absence of changes due to selection by mortality among the adult foreign-born, would require such a complicated adjustment of cause and effect in regard to the correlation of mortality and bodily form, that the theory would become improbable on account of its complexity.
It would be saying too much to claim that all the distinct European types become the same in America, without mixture, solely by the action of the new environment. First of all, I have investigated only the effect of one environment, and there is every reason to believe that a number of distinct types are developing in America; but we will set aside this point, and discuss only our New York observations. Although the long-headed Sicilian becomes more round-headed in New York, the round-headed Bohemian and Hebrew more long-headed, the approach to a uniform general type cannot be established, because we do not know yet how long the changes continue, and whether they would all lead to the same result. I confess, I do not consider such a result as likely, because the proof of the plasticity of types does not imply that the plasticity is unlimited. The history of the British types in America, of the Dutch in the East Indies, of the Spaniards in South America, favors the assumption of a strictly limited plasticity. Certainly our discussion should be based on this more conservative basis until an unexpectedly wide range of variability of types can be proved. It is one of the most important problems that arise out of this investigation, to determine how far the instability or plasticity of types may extend.
Whatever the extent of these bodily changes may be, if we grant the correctness of our inferences in regard to the plasticity of human types, we are necessarily led to grant also a great plasticity of the mental make-up of human types. We have observed that features of the body which have almost obtained their final form at the time of birth show modifications of great importance in new surroundings. We have seen that others which increase during the whole period of growth, and are therefore subject to the continued effect of the new environment, are modified even among individuals who arrived here during their childhood. From these facts we must conclude that the fundamental traits of the mind, which are closely correlated with the physical condition of the body, and whose development continues over many years after physical growth has ceased, are the more subject to far-reaching changes. It is true that this is a conclusion by inference; but if we have succeeded in proving changes in the form of the body, the burden of proof will rest on those who, notwithstanding those changes, continue to claim the absolute permanence of other forms and functions of the body.
In order to gain a correct understanding of the importance of changes in the frame of the human body, it seems desirable to view the type of modern man from a somewhat different standpoint.
It is quite a number of years since Fritsch, in his studies of the anthropology of South Africa, pointed out that a peculiar difference exists in the form of the body of the Bushman and the Hottentot as compared to that of Europeans, in that the former exhibit slenderer forms of the bones, that the bone is very solid in its structure; while in the European the skeleton appears heavier, but of more open structure. Similar differences may be observed in a comparison between the skeletons of wild animals and those of domesticated animals; and this observation has led to the conclusion that the Bushmen are in their physical habitus to a certain extent like wild animals, while the Europeans resemble in their structure domesticated animals.
This point of view—namely, that the human race in its civilized forms must be compared, not with the forms of wild animals, but rather with those of domesticated animals—seems to me a very important one; and a somewhat detailed study of the conditions in which various races are found suggests that at the present time, even among the most primitive types of man, changes incident to domestication have taken place almost all over the world.
There are three different types of changes due to domestication which must be clearly distinguished. On the one hand, the bodies of domesticated animals undergo considerable transformations, owing to the change in nutrition and use of the body. On the other hand, selection and crossing have played an important part in the development of races of domesticated animals.
Some changes of the former class are due to the more regular and more ample nutrition; other changes are due to modifications of the kinds of food which the domesticated animal uses when compared with the wild animal of the same species; still others are due to the different manner in which the muscular and the nervous systems are put into use. These changes are not quite the same among carnivorous and among herbivorous animals. The dog and the cat, for instance, are fairly regularly fed when they are found in domestication; but the food which is given to them is of a quite different character from the food which the wild dog and cat eat. Even among people whose diet consists almost entirely of meat, dogs are generally fed with boiled meat, or rather with the boiled, less nutritious parts of animals; while, among other tribes which utilize to a great extent vegetable food, dogs are often fed with mush and other vegetable material. The same is true of our cats, whose diet is not by any means entirely a meat diet. The exertions which wild carnivorous animals undergo to obtain food are incomparably greater than those of domesticated carnivorous animals; and it is obvious that for this reason the muscular system and the central nervous system may undergo considerable changes.