94 THE NEXT GENERATION 



4. The region covered by a single species of Hawaiian 

 snail is often not over two or three square miles. The region 

 covered by a single species of potato bug is as wide and as 

 long as the continent of North America. 



These facts, put together in this way, show that the power 

 to migrate and to live in different kinds of environment has 

 much to do with the number of species in this region or that. 

 If potato bugs had been kept in narrow sections of the coun- 

 try, if they had been able to live on but one kind of food, 



LEPTINOTARSA 



The dark one is bright red ; the light one is bright yellow. Both are decorated 



with black spots, both help destroy the crops, and both will be changed into flying 



beetles. (From W. L. Tower) 



if colonies had gone out from them rarely and at long inter- 

 vals, if there had been no chance for different colonies to mix 

 with each other, the potato bug would have ended by being 

 divided into many species, as are the snails of Hawaii. 



Instead, this potato bug lives in any environment. Neither 

 soil nor climate daunts him. Storms and blizzards simply 

 drive him on his way and improve his chances. In spite of 

 all this, however, there are certain kinds of beetle environ- 

 ment which make all the difference in the world with his 

 descendants of succeeding generations. The next chapter 

 takes up this part of the subject. 



