THEORIES IN PRACTICE 355 



are through. Here it seems worth while, in con- 

 nection with the story of the hybrid walnuts, to 

 attempt a more comprehensive view of the entire 

 field of heredity, endeavoring to gain a clear 

 notion as to just what are the underlying prin- 

 ciples that determine whether or not a certain 

 heritable character or pair of characters shall 

 Mendelize ; and in so doing we may correlate our 

 earlier studies and secure a clearer notion of the 

 underlying principles of evolution, and of the 

 origin and development of species, than could 

 perhaps have been gained without the aid of the 

 illustrative cases that have been presented. 



NATURAL SELECTION 



The principle of natural selection applies to the 

 preservation and to the weeding out of species. 



In the case under consideration, it was the 

 changed climatic conditions, through which the 

 Northern Hemisphere was transformed from a 

 region of tropical heat to one of arctic cold, that 

 resulted in the destruction of countless species, 

 leaving only a tithe of the original number to 

 constitute the flora of the temperate zone in our 

 own day. 



It is easy to see how the altered conditions of 

 temperature made the struggle for existence 

 unduly hard for many species, because there is a 



