384 HEREDITY AND ENVIRONMENT 



but there is no evidence that such changes in 

 hereditary constitution are reversible and that 

 the race reverts to its former state when the 

 old environment is restored. Such reversible 

 changes undoubtedly occur in somatic char- 

 acters but they are not inherited; they are 

 modifications of development, not of heredity ; 

 they are personal fluctuations, not racial 

 mutations. 



II. Artificial Selection 



Since the beginning of historic times, and 

 probably through long prehistoric ages, breed- 

 ers have followed the method of selecting de- 

 sirable individuals for propagating their stock. 

 There can be no doubt that almost all that man 

 has done in the production of domestic ani- 

 mals and cultivated plants has been accom- 

 plished by this process of selection. How has 

 selection acted? 



Until very recently it was generally 

 believed that continued selection of individuals 

 which showed desirable characters gradually 

 led to the improvement of those characters and 

 thus to the production of new races; it was 



