THE EVOLUTION OF MAN 201 



account of the discovery in his book on The Pedigree of the Human 

 Race. This species has been named Australopithecus ofricanus. 



A single molar tooth discovered in Nebraska which bears the 

 name Hesperopithecus haroldcooki has caused much comment. 

 This tooth is said to indicate a stage intermediate between Pithe- 

 canthropus and modern man, but its standing is in dispute. Such 

 a stage of human development is not to be expected in North 

 America and so it demands very accurate analysis. 



With these limited remains of prehuman species we cannot, of 

 course, establish a continuity of descent comparable to the phylo- 

 geny of the horse. They are significant, however, for they verify 

 many points in the theory of derivation from arboreal ancestors 

 by the physical "finishing touches" of erect posture and the de- 

 velopment of intelligence. All show greater brain capacity than 

 the apes. All show the reduction of the jaws and teeth. Finally, 

 within limits, all indicate increasing erectness. Such checks, how- 

 ever incomplete, can only strengthen our belief in evolution. 



Climatic Factors. Not only are the structural features of 

 these remains of prehuman species in accordance with the theory 

 of descent, but also the climatic conditions under which they must 

 have lived. Geologists tell us that the late Pliocene witnessed the 

 first glaciation. Over the northern hemisphere the extension of 

 ice sheets from the north in some regions accompanied a general 

 lowering of temperature which marked the culmination of a proc- 

 ess long in operation. The gradual cooling of the climate brought 

 about a change of flora. Tropical forests could no longer flourish, 

 and the trees which existed were more like those of the north tem- 

 perate zone of the present, finally to be replaced by conifers in 

 regions far south of their present range. 



These changes tended first of all to destroy the arboreal habitat 

 of the great anthropoids. They were faced with two alternatives 

 as the tropical forests disappeared and the climate became more 

 severe, namely, to migrate southward with the retreat of condi- 

 tions favorable for their continued arboreal existence, or to remain 

 where they were and meet the changed conditions with changed 

 habits. As in the case of other animals which we have considered, 

 there is every reason to believe that they could do either; very 

 probably they did both. Those which migrated had no reason to 

 change, but those which remained in the north must have been 

 subjected to exactly those stimuli which we have considered. 



