The explanation of these remarkable phenomena is not easy. Whatever their causes may be, the change in form cannot be doubted. It might, however, be claimed that the changes are not due to deep physiological causes, but to the changes of certain external factors. The composition of the immigrant population might be such that the people who came here at different periods had distinct physical characteristics, and that these are now reflected in the descendants of the older generations when compared with the more recent immigrants. It can be shown, however, that the differences between the Hebrews who immigrated at different periods between 1860 and 1909 are so slight that they cannot account for the type of the descendants of immigrants. This important point can be elucidated more definitely by the application of a different method. For this purpose I have compared the cephalic index of all immigrants of a certain year with that of their descendants. It appears from these comparisons that the differences which are exhibited by the whole series exist also between the immigrants who arrived here in a certain year and their descendants. This purely statistical explanation of the phenomenon may therefore be dismissed. More difficult to investigate is the hypothesis that the mechanical treatment of infants may have a decided influence upon the form of the head, and that the changes in cradling and bedding which are made by some immigrants almost immediately after their arrival in America account for the changes of head-form. If this were true, the continued changes among the Hebrews might indicate merely that the American method of cradling is used the more frequently, the longer the family has resided in this country. A number of investigators have claimed that the position of the child on the back tends to produce short-headedness, and that the position on the side tends to produce long-headedness (Walcher). There is good evidence that a flattening of the occiput occurs when a very hard pillow is used and the child lies permanently on its back. This is the case, for instance, among many Indian tribes, and similar results might obtain if a swathed child were to lie permanently on its back. The prevalence of rachitis in New York would favor distortion due to pressure. While I cannot disprove the existence of such influences, I think weighty considerations are against their acceptance. If we assume that among the Hebrews the children born abroad have a lesser length of head than those born here because they are swathed and lie more permanently on their backs than the American-born children, who can move about freely, we must conclude that there is a certain compensatory decrease in the other diameters of the head of the American-born. Since this compensation is distributed in all directions, its amount in any one direction will be very small (Boas).