|HUMAN EVOLUTION 359
into higher and lower races is fully justified . . . The white race appears to be the most progressive, as being the most retarded" (1929, pp. 25–27). "In his fetal development the negro [sic] passes through a stage that has already become the final stage for the white man" (1926a, p. 473). I have analyzed the racist argument in Chapter 5, and shall explore the subject no further here.
A different approach to our intraspecific differentiation has been little pursued but offers great promise: the comparison of intra- with inter-populational variation. Abbie (1958) calculated a significantly negative correlation between cephalic index and stature among male Australian aboriginals; similar data for intrapopulational variation of other peoples has been available since Boas' early study of 1899. Variation in adult size within a population follows the ontogenetic trend, with skulls becoming relatively longer as they depart from juvenile brachycephaly. But the correlation of mean cephalic index with mean stature for 50 different human groups is insignificantly positive (.211 for males, .132 for females, from Abbie, 1958). Differences between large and small peoples do not follow the ontogenetic trend of departure from juvenile proportions; the degree of brachycephaly of smaller peoples is retained or even increased among larger peoples. Abbie concludes: "With improvement in environment headform tends to adhere more to the fetal type: neither much longer nor much shorter than the mean fetal cranial index at about the middle of the human scale. It seems then, that paedomorphism is still an active factor in determining at least one human character" (1958, p. 203).
Bolk emphasized that his list of features reflected two different phenomena: the physiological retardation of development ("the retardation hypothesis of anthropogenesis"—1926a, p. 470) and the somatic retention of ancestral juvenile proportions ("the fetalization theory of anthropogeny"—1926a, p. 469). The two phenomena are joined in "the narrowest causal connection" ("in engstem kausalen Zusammenhang"—1926c, p. 12) because delayed development prolongs fetal growth rates and conserves fetal proportions: "The essential in his form is the result of a fetalization, that of his life's course is the result of retardation. These two facts are closely related, for, after all the fetalization is the necessary consequence of the retardation of morphogenesis" (1926a, pp. 470–471). If fetalized form is a consequence of retarded development, then the key to human evolution lies in the cause of this retardation: "There is no mammal that grows so slowly as man, and not one in which the full development is