COSMOPOLITAN BUTTERFLIES. 1161 



Neopyrameis, tho members of which (witli tlio sole exception of this 

 cosmopolitan member) are all found exclusively in the New World ; while 

 the antithetical section. Pvraineis (with the single exception again of one 

 member found both in Europe and the Fnited States) is exclusively con- 

 fined to the Old World. Judging from this fact we may venture to 

 assert with considerable confidence that this cosmopolitan l)utterfly origi- 

 nated in America. Yet it is just on this continent that its distribu- 

 tion is the most limited ! It is known in only a comparatively small 

 portion of South America and occurs on none of the West India Islands, 

 witii the exception of Cuba where it is rare. The cause of this limitation 

 cannot be attributed to the food plant of the caterpillar ; for the thistles 

 upon which it lives are quite as abundant in these regions as in many 

 others which it has invaded, certainly sufficiently abundant for all its uses. 

 Nor can the heat of the tropics be placed as a difficulty in the way, since 

 there is no place where it flourishes more abundantly than in the tropics 

 and subtropics of the Old World, repeated invasions of Europe by hordes 

 from the south where they had outgrown their opportunities being already 

 on record. 



Assuming, then, America to have been its original home, it would seem 

 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 

 between 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 members 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. Pieris 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 represented in species, all feeding upon plants of the 

 same family ; now though there are among them species of the genera 

 Pontia and Pieris having intimate relationship with forms which have 

 more or less successfully contended with rapae in their own home, the inex- 

 perience of the American species with such a rude antagonist has made 

 them no match for it ; so that in the mere quarter of a century since its intro- 

 duction it has spread over half the territory of the United States, doing now 

 vastly more injury than all the others of its own tribe combined and con- 

 tending with them so successfidly that their scarcity where formerly 



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