142 EVOLUTION OF THE EARTH 



is, after all, a limited environment and the descent to the ter- 

 restrial habitat was as necessary to further evolution manwards 

 as was that older emergence upon land on the part of our 

 piscine ancestry. In the former case as in the latter, the actual 

 attainment of the terrestrial habitat Is supposed to have been 

 forced by geologic change of a very similar character. The 

 presumption Is that central Asia was the evolutionary center 

 of the anthropoids and so far as our records go this is borne 

 out by the fossils, the oldest of which are found In the Slwalik 

 Hills of northern India In rocks of Lower Pliocene age. From 

 this center of radiation these primates took their departure, 

 the gorilla and chimpanzee southwestwardly toward the Dark 

 Continent, while the gibbons and orang went toward the south- 

 east. The prehuman, on the other hand, remained in central 

 Asia nearest the dispersal center, which, we have seen, is 

 generally true of the latest and most highly specialized of a 

 race. The forms which migrated southward felt the enervat- 

 ing languor of the tropics and remained static, if they did not 

 actually retrogress, whereas the prehuman In the more invig- 

 orating and variable climate progressed In his evolution toward 

 a higher type. Central Asia has proved to be the theater 

 wherein the highest mammalian evolution could be attained, 

 for here the forms which man has chosen for his companions, 

 the domestic animals, almost without exception attained their 

 evolutionary completion. 



The actual descent from the trees seems to have been due 

 to a chain of events which In many ways parallel those which 

 Impelled the emergence.^ The Miocene upljft, with the con- 

 sequent aridity, has been mentioned. As a direct result, the 

 stupendous barrier of the Himalayas began to arise, cutting 

 off the forests of central Asia from their old-time continuity 

 with those in India, and thereby severing the lines of com- 



^Barrell, J., "Probable Relations of Climatic Change to the Origin of the 

 Tertiary Ape-man." Scientific Monthly, January, 1917, pp. 16-26. 



