37 ENTOMOLOGY 



butterfly Euphydryas phceton is remarkably local in its occur- 

 rence, being limited to swamps where its chief food plant 

 (Chelone glabra] grows; and Epidemia epixanthe is similarly 

 restricted to cranberry bogs, though its food-habits are as yet 

 unknown. 



Former Highways of Distribution. Many facts of dis- 

 tribution which are inexplicable under the present conditions 

 of topography and climate become intelligible in the light of 

 geological history. The marked similarity between the fauna 

 of Europe and that of North America means community of 

 origin ; and though the Arctic zone now interposes as a barrier, 

 there was once an opportunity for free dispersion when, in the 

 early Pleistocene or late Pliocene, a land connection existed 

 between Asia and North America and a warm climate pre- 

 vailed throughout what is now the Arctic region. 



The extraordinary isolation of the butterfly CEncis se- 

 midea on mountain summits in New Hampshire and Colo- 

 rado (particularly Mt. Washington, N. H., and Pike's Peak, 

 Col.) is explained by glacial geology. The ancestors of this 

 species, it is thought, were driven southward before an advan- 

 cing ice-sheet and then followed it back as it retreated north- 

 ward, adapted as they were to a rigorously cold climate. 

 Some of these ancestors presumably followed the melting ice 

 up the mountain sides, until they found themselves stranded 

 on the summits. Other individuals, undiverted from the low- 

 lands, followed the retreating glacier into the far north ; and 

 at present there occurs throughout Labrador a species of 

 CEneis which differs but slightly from its lonely ally of the 

 mountain tops. 



Glaciation undoubtedly had a profound effect upon the 

 fauna and flora of North America. " With the slow south- 

 ward advance of the ice, animals were crowded southward; 

 with its recession they advanced again northward to reoccupy 

 the desolated region, until now it has long been repopulated, 

 either with the direct descendants of its former inhabitants or 

 with such limitations to the integrity of the fauna as this inter- 



