220 THE PROBLEMS OF EVOLUTION 



experiments have dealt with organic responses of 

 these kinds. In every case some hereditary capacity 

 has been involved, and the degree of its expression 

 has been controlled by artificially controlling the 

 environment. The procedure followed has in- 

 volved the subjection of the organisms to a given 

 stimulus through several generations in all of the 

 experiments known to me, and the return of the 

 stock, finally, to the normal ancestral environ- 

 ment. In no case have I found a record of an 

 attempt to determine the effect of a gradual in- 

 crease of a stimulus over a succession of genera- 

 tions, and in no case has the experiment dealt with 

 the usefulness of a character under other condi- 

 tions than those which determined its production. 

 The various conditions suggested here for the 

 possible modification of living things through indi- 

 vidual response to environment present very differ- 

 ent possibilities for experimental verification. The 

 time element is always important in such studies, 

 but probably to the greatest degree in those experi- 

 ments which involve the modification of a simple 

 character in response to a simple environmental 

 condition. Such a response may be adaptive or it 

 may be incidental, but in any case we can scarcely 

 expect it to carry the development of a character 

 quickly beyond its former latitude of response, if 

 at all. It is conceivable that an increasing or de- 

 creasing^ stimulus may ultimately accomplish this 



