EVOLUTION IN NATURE 159 



which encounter a permanent change of environ- 

 ment may very well develop new characters which 

 will appear generation after generation in spite of 

 fluctuations in, or the occasional failure of, the 

 inciting stimulus. 



The possibility of these characters becoming so 

 definitely established that they must appear in 

 every generation involves the question of the 

 initial external environmental stimulus being sup- 

 planted by an internal condition which would 

 normally produce the character independently of 

 external conditions. Can such a change occur? 

 So far it has not been proved possible. Logically 

 the possibility exists, but the lack of significant 

 evidence makes it desirable to consider the matter 

 elsewhere. 



Once the heritage of the hypothetical stock has 

 acquired diversity, whatever the cause, all of the 

 processes of selection may be operative in some 

 degree to break it up into different groups, but as 

 we have already noted, these processes account 

 only for the manipulation of existing things and 

 not for the origin of new characters. 



Preadaptation no doubt plays a considerable 

 part in the adjustment of organisms to new condi- 

 tions, either in their original area or in newly in- 

 vaded territory. Many structures are capable of 

 serving an organism in several ways and it is in- 

 conceivable that they should fail to serve it in any 



