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We have seen that among the individuals composing a population considerable differences in the rate and ultimate result of development are found and that these depend largely on external conditions under which growth and development take place. The question arises whether it is possible to separate the variability caused by environment from hereditary conditions.

In an attempt to answer this question we have to investigate the relations between the bodily forms of parents and children and those between members of fraternities. The method by which these relations can be investigated may be explained by the example of the head index.

If in a given population fathers with unusually rounded heads are compared with their children, it is found that their children have also the tendency to have rounded heads, but less so than the selected group of fathers. For instance, if the average head index of a population is 80% and a sufficiently large number of fathers with an index of 86% is picked out, the average head index of the children of this group will be 82%, about one-third^[58] of the deviation of the fathers from the norm of the selected group of fathers. In this way all the body parts may be investigated and a relation similar to the one just mentioned may be established. In populations which have lived for a long time in the same place and whose families have intermarried the “similarity” of fathers and children, as here defined, is about one-third. The same value has been found for the similarity of mothers and their children. The similarity among brothers and sisters is much greater. In a population like the one just referred to a group of men may be selected whose headform is characterized by the index 86%. Then the average index of their brothers will be 83%, in other words the difference between the norm and the average of the brothers of the selected group is about one-half of the difference between the norm and the value of the selected group. We may say that the similarity of the brothers is about one-half.

Observations regarding the similarities between parents and children and those between brothers and sisters are similar to those mentioned here: about one-third and one-half. Often the values are a little different. Much larger values have been found for twins, particularly for identical twins—that is, those developed from a single ovum. For the present the values given here may be considered as the norms for European populations.

In parts of the body which in the course of growth undergo marked individual changes, such similarities may be considered in the following way: Besides the hereditary similarities which are determined by the values just given each individual develops independently of all the others, with varying tempo and intensity, according to more or less favorable conditions of life. If these causes are of such a character that they are expressed in the bodily form of the adult, there must be, besides the hereditary causes, other accidental ones which are different for the different individuals. In this case a weakening of the similarities may be expected. Therefore, if the observed similarities are so arranged that the similarities of those forms which are permanently established shortly after birth are compared with those which take their permanent forms later in life, it may be expected that the latter group will show lower degrees of similarity.

I have carried through this inquiry for the statures of East European Jews measured in New York. The results show a considerable decrease for the degree of similarity as compared to that of the measurements of the head. The similarity between parents and children for length and width of head, which are established early in life, obtained from more than 2,300 observations is about .36, while that for stature referring to parents and their adult children is only .21. The similarity of siblings (brothers and sisters) in regard to stature is .33 as compared to .56, the similarity for head measures. We may conclude from this that the reduction of variability is essentially due to external differences under which the individuals develop.^[59]

If it is assumed that the hereditary part of the similarity is the same for all parts of the body—an assumption that needs proof—it is possible to determine the part of the variability to be ascribed to heredity and the other to be ascribed to environment.

I have assumed that every individual is advanced or held back in his development entirely independently of all the others. This hardly corresponds to actual conditions. Comparing poor and well-to-do families, all members of each family are obviously more or less exposed to similar conditions. The poor are less favorably developed than the rich, so that a secondary family resemblance develops not dependent upon heredity but upon sameness of environment. The same may be said of various parts of a population forming social strata or inhabiting different geographical localities.

At present the real amount of the environmental influences to which families are subjected cannot be determined.

We must furthermore remember that even those forms that reach their final form at an early age are exposed to outer influences, and that the values obtained by the assumption that the values observed for early completed forms are maximal values express only a minimum for the effects of environmental determinants. This is indicated by the increased similarity of heterozygous twins which must be explained as due to the sameness of their prenatal life.

It follows from these considerations that we may expect not only differences between individuals due to exogene causes, but that there will also be differences between populations due to the conditions under which they live. We may also conclude from available data that among individuals of the same descent such variations in stature may be measured by a standard variability of ± 3.5 cm., so that in the same population according to outer conditions differences in stature of several centimeters may be expected. This is quite in accord with the observation made both in Europe and America showing that stature has increased by about 3 cm. during the last fifty years.

I repeat that the maximal value for similarity obtained from head measurements must not be considered as an expression of hereditary influences only, but that a certain amount of external influences is contained in it. The same considerations that we made before may, therefore, be applied, and, as we found that the stature in populations must be variable according to environmental conditions although the hereditary character of the population remains constant, so we may say in general that modifications of type may result from environmental conditions without any fundamental, hereditary changes in the hereditary character of populations. It seems to me that this explanation of the changes observed in the United States is quite adequate and that it is, for the present at least, unnecessary to look for hereditary changes.

It should be emphasized again that I have assumed here that the degree of hereditary similarity for all characters is equal—an assumption that may not be taken for granted without further investigation. Thus it seems probable that in a mixed population, descendants of one population with high and another with low head index, while the hereditary element of stature is the same in either, the degree of similarity for head index and for stature would be quite different. Nevertheless the material used in this discussion seems to indicate that the method can be used in fairly homogeneous populations. Extended investigations on similarity are needed in order to show whether the suggested approach is admissible.

[57]	Zeitschrift für Ethnologie, vol. 45 (1913), pp. 622 et seq.

[58]

I have used here a value lower than the one found by Karl Pearson (Biometrika, vol. 2 [1902-3], pp. 378, 379; see also E. Schuster, Ibid., vol. 4 [1905-6], p. 478), since the following calculations are based on material which gives the value here adopted (Franz Boas, Changes in Bodily Form, p. 156). The low value of the regression is due to the character of the series which consists of Russian Jews, a population more homogeneous than Pearson’s English material. See in regard to this matter Franz Boas, The Mind of Primitive Man, 1938, pp. 60 et seq.

[59]	Here follows an attempt to evaluate the relative contributions of hereditary and environmental determinants of variability which is here omitted.