ON THE QUICK GROWING WALNUT 



endeavoring to gain a clear notion as to just what 

 are the underlying principles 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 with- 

 out the aid of the illustrative cases that have been 

 presented. 



NATURAL SELECTION 



In the preceding chapter we briefly reviewed 

 the story of the vicissitudes to which plant life has 

 been subjected in the course of recent geological 

 eras. We were concerned there with the elimina- 

 tion of unadaptable species rather than with the 

 evolution of adaptable ones. 



But it should of course be understood that the 

 same 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 re- 

 gion 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 con- 

 stitute the flora of the temperate zone in our own 

 day. 



[203] 



