86 BUTTERFLIES 



troduction it had become a serious pest over practically 

 all the United States and Canada. 



The introduction and spread of such a pest is of inter- 

 est in itself, but in this case there is to be noted the addi- 

 tional fact that the presence of this foreigner has prac- 

 tically led to the extinction of two native species of but- 

 terflies, both closely related to each other and to the in- 

 vader and both feeding upon the same plants. An almost 

 pure white butterfly — the Gray- veined White — was form- 

 erly exceedingly abundant in many of the Northern states, 

 while farther south there was another species, the Check- 

 ered White, which was also abundant. Both of these 

 have now so completely disappeared that in some localities 

 they are almost never seen, while their imported relative 

 has become perhaps the most abundant of all American 

 butterflies. 



The Gray-veined White 



Pieris napi 



One would naturally suppose that when a butterfly 

 was reduced to the greatest possible simplicity in its color- 

 ing there would be little chance for the development of 

 geographical or seasonal varieties. But he would only 

 have to study a large collection of specimens of this species* 

 taken at different seasons and in different regions, to 

 find his supposition at fault. Here is a butterfly which 

 is essentially a slender black-bodied creature with four 

 white wings scarcely touched with color, and yet we are 

 told that there are eleven varieties in the United States so 

 distinct that they have received scientific names, not to 



