MIGRATION AND DISPERSAL OF INSECTS : FTNAT, CONSIDERATIONS. 263 



rardiii, and in North America by Anusia arc/u'pjHis and other species of 

 similar habits, which, when they spi'ead north, bring tlieir southern 

 continuously-brooded habits with them and attempt to do, under more 

 rigorous climatic conditions, what they do naturally in their more 

 southern homes, with the result that their progeny is almost entirely 

 exterminated during the autumn and winter following their immigra- 

 tion. On the other hand, should the locality reached prove favourable, 

 this again may become a new centre of dispersal. Such species, 

 continually seeking new grounds to which they may extend, may well 

 be termed dominant species, because they are least likely to undergo 

 absolute extinction owing to local causes, and also because, in the 

 process of extending their area, they are continuously being brought 

 under, and to be successful have to respond to, changed conditions 

 which must result in slow modifications of habit and possibly in 

 appearance, culminating in the development of new races, and possibly 

 ultimately of new species. The permanent settlement of such species 

 as Pi/ratiu'is canliti and Colias I'lliisa in a more northerly latitude than 

 that in which they winter easily now, must depend upon the ability of 

 the species to adopt a fixed hybernating habit which will carry them 

 through some months in such a climate, for example, as our own. 

 Colias ednsa attempts to do this in the larval, and Pi/rameis eardui in 

 the imaginal, state, and there is some little evidence that a few 

 examples may for a short time succeed, but these are met by the 

 next band of immigrants without the habit, and crossing rapidly 

 reduces or swamps altogether the hybernating power, and the 

 progeny has almost, as it were, to start again dc novo. Along 

 the southern shores of the Mediterranean, where the actual winter is 

 very short, some slight hybernation is succeesfully negotiated by both 

 these species ; if these individuals immigrated or spread into a 

 comparatively near country or district, the progeny of those of 

 the immigrants which had the hybernating tendency most strongly 

 developed would be able to tolerate a slightly severer climate, and 

 one can understand that, by easy stages, a race might be established 

 that could spread slowly into far distant countries, and show a 

 distinct definite hybernating habit differing greatly from that of 

 its early southern ancestors. There is little proof, however, that 

 the remarkable swarms of these species that more or less frequently 

 visit Britain come from neighbouring countries ; on the other 

 hand, they often occur in years when there is certainly no excess 

 of abundance anywhere in southern Europe, but such evidence as is 

 forthcoming, especially with regard to P. eardiii, suggests northern 

 Africa as the area of origin. Here the species is sometimes observed in 

 enormous numbers in March, the larva? stripping everything that they 

 can possibly eat in April, and producing imagines in May, many of 

 which are probably forced, by their local circumstances, to leave their 

 native homes and seek pastures new elsewhere. 



It has been suggested by Scudder {Psi/che, v., pp. 190 et .m/.) that 

 P. eardui was originally a native of the New World, and that its 

 present distribution — practically over the whole world (the Arctic 

 regions and part of South America excepted) — has been obtained by 

 the species spreading from America. He observes that the species 

 belongs to a subdivision of the genus Vmwssa, the members of which 

 (with the sole exception of this cosmopolitan species) are found 



