264 THE entomologist's record. 



exclusively in the New World, whilst the antithetical section (with the 

 single exception again of one member found both in Europe and the 

 United States) is exclusively confined to the Old World, and yet it is 

 just in America that its distribution is most limited : " It has a wide 

 distribution in North America, is, however, known only in a compara- 

 tively small portion of South America, and occurs in none of the West 

 Indian Islands except Cuba, where it is rare. The cause of this limita- 

 tion cannot be attriljuted to the want of food-plants, which are quite as 

 abundant in the districts where the species does not occur as where it 

 does ; nor can the heat of the tropics be placed as a difficulty in the 

 way, since there is no phxce, he says, where it flourishes more abundantly 

 than in the tropics and subtropics of the Old World. Scudder's explana- 

 tion of its success in the Old World appears sound, for it is clear that, in 

 the New World, its distribution is limited by its competition with its 

 nearest relatives, whilst "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, it 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." The force of this 

 reasoning is excellently illustrated by the facts connected with the spread 

 of the European Pieris rapac in America. This species, first noticed in 

 America (Quebec) in 18G0, had spread to Cacouna, to the Eastern Town- 

 ships, and to the state of Maine by 1866, in 1867 it was noted at Montreal, 

 whilst in 1868 there was a fresh importation by way of New York (a 

 German naturalist having obtained chrysalides from Europe allowed 

 the imagines to escape). From the two centres the spread continued, till 

 in 1876 the species had covered the whole of Western Ontario, and in 1881 

 had invaded the whole country from the Atlantic Ocean to Texas, Kansas, 

 Nebraska and Lake Superior; in 1884, specimens were met with on the 

 shores of Hudson's Bay and at the foot of the Rocky Mountains, whilst 

 at the present time the butterfly has practically covered the whole 

 of Temperate America. Coming from a district (Europe) in 

 which it has several near relations which compete closely with 

 it for food, and possessed, as it is, of a strong migrating tendency 

 which leads it to push itself even against these relations, it found 

 itself in a country where the group of butterflies to which it 

 belongs was poorly represented, and where its food-plant was 

 excessively abundant. The consequence is that it has spread at 

 an amazing rate, and in a quarter of a century has covered Canada 

 and more than half of the territory of the United States, extending 

 itself all over the temperate area, and elbowing out its native rehitions 

 which are unable to compete with this hardier and more capable type. 

 In fact, the scarcity of one of them, l^icris proUxlici', formerly a 

 common insect, in the districts which the European species has 

 invaded, is most distinctly marked. Here we see how a stranger can 

 become dominant in a short time and crowd out its less pushing 

 neighbours. But it is only in temperate regions that this species can 

 spread, for its food-plants (the cabbage tribe) are scarce or entii'ely 

 absent beyond the temperate zone. On the other hand, Pi/raiiieis 

 can/M?" sutlers no such restrictions, its food being practically everywhere. 

 There has been, therefore, everything in favour of its becoming a 

 cosmopolitan species, and if there be one really deserving this term in 

 the world, this species is probably it. 



