134 THE PROBLEMS OF EVOLUTION 



may be cited. The curved claws of the ant-eater 

 are used primarily to tear open the nests of ter- 

 mites, but in case of need they are effective weap- 

 ons of defense. Hairy vestiture in plants protects 

 them against undue loss of moisture and charac- 

 terizes many xerophytes, but it serves also as a 

 protection against cold. And in the past evolution 

 of animals such structures as the fins of the Cros- 

 sopterygii were evidently developed in one environ- 

 ment for one purpose, only to become exceedingly 

 important in another environment for another 

 purpose. Preadaptation adds another concept for 

 the explanation of the adjustment of organisms to 

 the environment without explaining the origin of 

 the characters involved. 



The attitude which forces itself inevitably upon 

 the student of evolutionary processes is that the 

 theories which are based upon the belief that the 

 organism alone is the source of change are limited 

 in possibility. Certainly an organism may accom- 

 plish nothing for which it lacks hereditary capacity, 

 but all of the selection theories, all ideas of isola- 

 tion, all of our knowledge of mutations, serve only 

 to show that the characters which make up a species 

 may be reassorted, redistributed, preserved in part 

 and destroyed in part, or modified in some degree. 

 Only when we have recourse to a vitalistic prin- 

 ciple or to an environmental control do these factors 

 align themselves with the evolution which has 



