156 HEEEDIiy AND SELECTION IN SOCIOLOGY 



fixed to be capable of modification except after many genera- 

 tions ; and in the meantime the enemy multiplies and gains 

 more and more grounnd. This failure to readapt itself must, as 

 we have said, have proved fatal to many a species. But other 

 causes, such as direct physical inferiority, or a change of cUmatic 

 conditions, of food, of the flora and fauna of a region, may also 

 operate ; or else many, or all, of these causes may combine. 

 In some cases we may be able to point, almost with certainty, 

 to the predominance of a particular cause ; in other cases it is 

 almost impossible to distinguish the exact part played by the 

 various factors. But we may say with confidence that there is 

 no reason for referring the extinction of species to any cause 

 beyond the changes produced in the environing conditions, in 

 climate, food, etc., and to the failure to efiect with sufficient 

 rapidity the changes necessary to readaptation. The extinction 

 of species does not warrant a behef in the theory of a vital force 

 any more than their origin does.^ 



^ The case in whioli a species is destroyed by being directly inferior to 

 a new species (for instance, tbe extermination of the giraffe by man) may 

 bejincluded in the category of tbe changes produced in tbe environing 

 conditions. The contact of the giraffe with man effects a fatal change in 

 the giraffe's conditions of life. 



