EVOLUTION 173 



entirely external to animal or plant. It was a natural 

 first explanation, but it was too superficial, and en- 

 vironment as a direct cause of evolution soon passed 

 into the historical background. 



In 1801, Lamarck, in a series of lectures, announced 

 his explanation, calling it the theory of "appetency." 

 This was really the first explanation with a body of 

 doctrine, and hence Lamarck has often been called 

 the "founder of organic evolution." The term "appe- 

 tency," however, has been abandoned, and its real mean- 

 ing expressed by the "effect of use and disuse." With 

 Lamarck, environment is not the direct cause of the 

 change, but the occasion for the change. The cause is 

 the striving, the effort to do something that had become 

 necessary. Thus organs would become developed as a 

 consequence of some change in environment calling; 

 them into use; and conversely, organs would become 

 gradually aborted as a consequence of some change in 

 environment that eliminated their use. This explana- 

 tion rests absolutely upon the inheritance of acquired 

 characters, meaning characters not inherited by the 

 possessor, but acquired during the lifetime of the in- 

 dividual. 



In 1858 the epoch-making explanation by Charles 

 Darwin was announced, an explanation which was dom- 

 minant for about fifty years. It is too familiar to need 

 explanation, but I wish to call attention to the steps by 

 which it developed in Darwin's mind. These steps have 

 been spoken of as five facts and one inference, and the in- 

 ference appeared so natural that there was no escaping 

 it. The five facts in their logical sequence are the ratio 



