72 THE PROBLEMS OF EVOLUTION 



every case forces outside of the body are involved, 

 but they act in different capacities. 



We have recognized the abiUty of individuals to 

 respond in various ways to environmental condi- 

 tions, and have concluded that everything belong- 

 ing to the individual belongs also to the species. 

 In a diflferent way entirely, individual responses 

 may become characteristic of the species if a 

 change in the environment is so extensive as to 

 affect all individuals; the resulting development of 

 individuals will then occur as uniformly as the 

 normally inherited complex of characters. En- 

 vironmental changes of this extent are likely to 

 occur only as the results of extensive and important 

 geological events, like the elevation of great land 

 masses. Such events do not come and go rapidly. 

 Once they occur, their effects are likely to be felt 

 for a long time. Generation after generation the 

 individuals constituting our hypothetical species 

 may respond to the same conditions by producing 

 the same expression of characters. The results will 

 be as certain of appearance while the environ- 

 mental conditions persist as will the characters 

 which inevitably develop during ontogeny. If the 

 climate of North America should suddenly become 

 subtropical we might confidently expect the paw- 

 paw butterfly, Papilio ajax, always to occur in its 

 summer form, while now its spring brood is recog- 

 nizably different. And if the rainfall of some trop- 



