160 THE PROBLEMS OF EVOLUTION 



way favored by the environment. The preserving 

 and destroying activities of natural selection and 

 other selective processes are important in accom- 

 plishing slight degrees of differentiation within the 

 species; they are, in fact, adaptive processes in 

 their relation to species, whereby the adaptive 

 processes of individual life are supplemented for 

 the provision of the best available equipment at 

 birth to the offspring of each surviving individual. 

 But they do not provide for absolute departure 

 from the existing state of development. Neither 

 does preadaptation, to be sure, but it has an ad- 

 vantage in that it brings new stimuli to bear on 

 old structures. 



Assuming that our hypothetical species spreads 

 in all directions, we may trace its development 

 according to the several possibilities. To the north 

 it may encounter low temperatures and as a result 

 may develop smaller appendages, like Sumner's 

 mice. To the west it may be forced to diverge in 

 order to find favorable surroundings on opposite 

 sides of a high mountain range. Here isolation 

 may play a part in establishing northern and 

 southern races if the divergent lines receive ini- 

 tially different heritages, without any other factor 

 intervening to change the species. And to the 

 south the species might well work its way gradually 

 into warmer and warmer regions. The abundance 

 of competing forms might cause it to seek an easier 



