EVOLUTION OF PHYSICAL CHAKACTEES 381 



usually genetically favourable ; there will be the advantage of 

 hybrid vigour, though this is always temporary, and there may 

 very possibly be the advantage of valuable character re-com- 

 binations.^ 



Migration is important not only because it is connected with 

 war and racial crossing but also because of the selection which 

 follows upon transfer to new climatic conditions. To the nature 

 of these changes allusion has already been made ; it has been 

 pointed out that races adapted to maintain themselves in one 

 environment cannot as a general rule maintain themselves in a 

 very different environment. To new surroundings, which only 

 differ shghtly from the old, they may become adapted by selection, 

 and much selection on these lines must have taken place as a 

 result of racial movements in Europe. It is probably now taking 

 place among Europeans who have migrated to America. Dublin 

 and Baker, for instance, have shown that the death-rate varies 

 considerably among the different racial elements who have 

 immigrated, some elements being probably better adapted to 

 the new environment than others.^ Similarly selection may be 

 going on within modern races owing to the rapid urbanization of 

 industrial countries. Urban conditions may be more favourable 

 to some types than to others. But it has not yet been shown in 

 what direction these changes, if they are in progress, are taking. 



Summing up our conclusions we may say that the great changes 

 in human bodily form were accomplished in the intermediate 

 period when the splitting up into varieties may also have begun. 

 This splitting was continued in the first period and the modern 

 types were formed before the end of that period. The chief 

 characteristic, however, of the first and second periods was the 

 maintenance of the types evolved, though this was interfered with 

 by migration which brought about elimination of certain types, 

 racial mixture, and further climatic adaptation. Finally, in 

 modern times there has been a lessening of the stringency of 

 selection, which previously tended to maintain the existing 



1 Recent research bearing on this problem has been summed up by East and 

 Jones in their book Inbreeding and Outbreeding. 



^ Dublin and Baker, Amer. Stat. Soc, vol. xvii, 1920. Nevertheless, as shown 

 by an important memoir by Hrdlicka, a new variety is not being produced, at 

 least with any rapidity, under the influence of the American environment. Hrdlicka 

 studied representatives of the old white American stock, whose ancestors had been 

 for four generations in the United States, and concluded that no new sub-type 

 has yet emerged. In fact the faithful preservation of the traits of tlie original 

 immigrants is the outstanding result of the study. 



