376 HEREDITY AND ENVIRONMENT 



vironment does sometimes act in this way, 

 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. 



1. The methods of selection have varied con- 

 siderably with different breeders; sometimes 

 they have selected: 



(a) The best individuals grown in the best 

 environment. 



