80 THE NEXT GENERATION 



the wishes of those who suffer from it or of those who are 

 blessed by it. And this points the difference between a 

 mutation and an acquired character. 



An acquired character cannot be inherited ; a mutation is 

 inherited. The difference between the two is striking. And 

 this explains why mutations have done so much for past gen- 

 erations of living creatures, for if a mutation gave its owner 

 any advantage in the struggle for existence, — if it made the 

 creature in the least degree more fit to survive, — through 

 the law of inheritance it was sure to play an important part 

 in changing succeeding generations of descendants. 



It is evident, then, that not only such variations as are 

 striking enough to be called "sports," but all variations, 

 wherever they appear, belong to one or the other of two 

 great classes. They are either heritable or nonheritable. 



No mutations have taught plainer lessons than do those 

 which Dr. Tower brought about among his American potato 

 bugs. These are discussed in the thirteenth and fourteenth 

 chapters. Just now, however, we turn to the law of isolation 

 as it is illustrated by snail shells on the Hawaiian Islands. 



