I04 The Third and Fourth Generation 



strains that ripen at earlier and earlier times, 

 and the resultant very early ripening variety 

 would come about by the elimination of all 

 those plants that did not have in their germ 

 cells the early ripening trait. 



All through the Central West we have one 

 species of chipmunk. In the Rocky Mountain 

 region, however, we have a great many dif- 

 ferent species. Wherever there is an isolated 

 vaUey, hemmed in by impassable mountain 

 barriers, there you are very prone to find a 

 species peculiar to that vaUey. It looks as if, 

 when the mountains were upheaving, some of 

 the chipmunks that originally roamed the 

 entire region were isolated, and had gradually 

 changed their character as the environmental 

 influences of this particular locality had time 

 to operate upon them. Now these new species 

 breed true, and again it looks as if we had the 

 transmission of an acquired modification. But 

 we may interpret the phenomenon in a different 

 way. It may be that the hereditary germ mate- 

 rial is subject to wide variation, and that only 

 those varieties have survived whose germinal 



