April 1889.] 



PSYCHE. 



191 



as if we might fairly conclude that a 

 butterfly of a dominant type, after its 

 distribution in the region of its birth 

 had reached its limits (the balance be- 

 tween the competitors in the struggle 

 for existence being fairly struck), on 

 being introduced into a new world, 

 where it had to contend in the struggle 

 for supremacy with none of the mem- 

 bers of its own restricted group, which 

 had stood in its way in its native home, 

 would suddenly find that it had reached 

 a region ready for conquest and would 

 spread therein with such success as to 

 completely overrun that division of the 

 world. 



That this Is a probable picture of 

 events which actually transpired in this 

 instance, the result of which we see to- 

 day, is rendered more probable by other 

 events which have taken place under our 

 very eyes, which, though not strictly 

 parallel, seem to have a lesson. Pier is 

 rapae., originating in the Old World 

 among a circle of relatives far greater 

 than exists in North America, relatives 

 whose natural food plant is precisely its 

 own, has been suddenly transported to 

 America, where the group to which It 

 belongs is much more poorly repre- 

 sented in species, all feeding upon plants 

 of the same family ; and though there 

 are among them species of the genera 

 Pontia and Pieris having intimate rela- 

 tionship with forms which have more or 

 less successfully contended with rapae 

 in their own home, the inexperience of 

 the American species with such a rude 

 antagonist has made them no match for 

 it ; so that in the mere quarter of a cen- 

 tury since its introduction it has spread 



over half the territory of the United 

 States, doing now vastly more injury 

 than all the others of Its own tribe com-- 

 bined and contending with them so sue- 

 cessfullv that their scarcity where for- 

 merlv abundant is everywhere noticed. 

 In this latter instance commercial agen- 

 cies are amply sufficient to explain the 

 Introduction of this butterfly into our 

 countrv. It is, however, an Insect de- 

 pendant upon a group of food plants 

 which forbid its passage Into the tropics 

 and so will prevent its spread over more 

 than the north temperate zone. 



It is plain that no butterfly can become 

 cosmopolitan whose caterpillar does not 

 feed upon plants found in all quarters of 

 the globe. Yet this is plainly not a 

 sufficient cause for distribution. As a 

 proof of this it may be pointed out that 

 one of the most polyphagous of our 

 butterflies, Jasoniades glauais, which 

 has an unusually extended distribution In 

 North America, where it has several 

 allies, has never become cosmopolitan ; 

 while plants to which it might easily adapt 

 itself are found in every quarter of the 

 globe. Moreover, the alliances of the 

 genus are wholly with tropical American 

 forms and Its ancestors unquestionably 

 originated in that part of the world ; yet 

 the genus Is not found in the tropics. 

 Nor has It ever spread to the Old World ; 

 at the same time there are other genera 

 of the same tribe, not distantly related, 

 which do possess members In both the 

 New and Old Worlds, whose food Is of 

 a much more restricted range ; such are 

 the genera Iphiclides and Papilio. 



We have another instance of possible 

 cosmopolitanism wliich is perhaps more 



