THEORY OF EVOLUTION 147 



If we measure, or weigh, or classify any 

 character shown by the individuals of a popu- 

 lation, we find differences. We recognize that 

 some of the differences are due to the varied 

 experiences that the individuals have encoun- 



FIG. 71. Series of leaves of a tree arranged according to 

 size. (After de Vries.) 



tered in the course of their lives, i.e. to their en- 

 vironment, but we also recognize that some of 

 the differences may be due to individuals hav- 

 ing different inheritances different germ 

 plasms. Some familiar examples will help 

 to bring home this relation. 



If the leaves of a tree are arranged accord- 

 ing to size (fig. 71), we find a continuous ser- 

 ies, but there are more leaves of medium size 

 than extremes. If a lot of beans be sorted out 



