CATALOG OF HUMAN CRANIA — HRDLICKA 171 



or more separate parts of even the same tribe, that do not present 

 some physical differences. Yet all these tribes are basically closely 

 related, and all belong plainly to one and the same stem of humanity. 

 The differences are manifested, though never collectively, in most of 

 the physical characters of both the living and the skeleton. The 

 most marked ones are in stature, shape of the head, and robustness 

 of the parts. 



These differences parallel those within the other two main stems of 

 mankind, the 'VMiite and the Black, and their explanation is not yet 

 possible, but it may be approached. It is clear that all these differ- 

 ences could not have existed from the beginnings of the species, for 

 none of the human varieties of present times are of such antiquity; 

 many in fact must be rather recent. Therefore they must have 

 arisen in the course of man's biological history and can have been 

 due only to internal or external contemporaneous agencies. In an 

 extended sense therefore they were not inherent but were acquired. 

 Just what the reasons were that underlay these organic acquisitions 

 it is not possible to fathom clearly, but we may be sure that the causes, 

 multiple and elusive as they may be, are all natural, and as such all 

 subject to eventual definitive determination. They may legitimately 

 be called the causes of "raciogeny," and their study will constitute 

 perhaps the most attractive and important task of future anthropology. 

 For the present it may suffice to view all these human subtypes, types, 

 or varieties, Amxcrican or other, as so many more or less fixed results 

 of the reactions between a plastic class of organisms and various 

 sufficiently potent internal and external agencies. 



