The Mechanism of Adaptation 335 



will in the course of time bring about the diminution of un- 

 fitness and the general prevalence of fitness. Darwin showed 

 in masterly manner that the greater elimination of unfit 

 individuals in each generation and the more general preser- 

 vation of better fitted ones would gradually improve the 

 standard of fitness until finally such exquisite adaptations 

 as are found in the eye, for example, might be reached. This 

 seems to me to be the crowning glory of Darwin's great 

 theory; it is not so much its species-forming power which 

 impresses one as its ability to explain on simple and natural 

 principles very many of the wonderful adaptations of the 

 living world. 



3. Mutation Theory 



The mutation theory has to a certain extent changed our 

 point of view regarding adaptations as it has also regarding 

 species formation. Neither of these phases of evolution 

 can any longer be regarded as the result of minute varia- 

 tions which persist and replace ancestral forms, if they are 

 infinitesimally better adapted, but mutations may represent 

 relatively large changes both in form and usefulness. They 

 occur in many directions and are usually non-adaptive and 

 are frequently positively injurious. The latter are quickly 

 eliminated in a state of nature, but indifferent mutations 

 may persist, and it is no longer necessary to assume, as older 

 Darwinians did, that every structure of an organism is of 

 some benefit to its possessor; on the contrary, it appeared 

 without reference to its usefulness or its uselessness, and it 

 persists only if it is not injurious. 



Preadaptations. In applying the mutation theory to the 

 explanation of adaptations, Cuenot has proposed a modi- 

 fication of the Darwinian theory which he calls "preadapta- 

 tion." Mutations which are injurious or indifferent in the 



