DISTRIBUTION 303 



The absence of proper food is more effective than climate, as a direct 

 check upon the spread of an animal; food itself being, of course, de- 

 pendent ultimately upon climatal factors and soil. , Many insects, being 

 confined to a single food plant, cannot exist long where this plant does 

 not occur; but they will follow the plant, as was just said, into new cli- 

 mates; thus Anosia plexippus is following the milkweed over trie world. 

 The butterfly Euphydryas phaeton is remarkably local in its occurrence, 

 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 distribution 

 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 prevailed throughout what 

 is now the Arctic region. 



The extraordinary isolation of the butterfly (Eneis semidea on moun- 

 tain summits in New Hampshire and Colorado (particularly Mt. Wash- 

 ington, N. H., and Pikes Peak, Col.) is explained by glacial geology. 

 The ancestors of this species, it is thought, were driven southward be- 

 fore an advancing ice-sheet and then followed it back as it retreated 

 northward, 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 lowlands, followed the retreating glacier 

 into the far north; and at present there occurs throughout Labrador a 

 species of (Eneis 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 southward 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 in- 

 habitants or with such limitations to the integrity of the fauna as this 

 interruption of local life may have caused." (Scudder.) Probably 

 many species were exterminated and many others became greatly modi- 

 fied, though little is known as to the relationship of the present fauna to 



