

DISTRIBUTION 325 



ous in wet years. Moisture checks the development of these and other 

 insects in ways as yet unascertained; possibly it acts indirectly by favor- 

 ing the growth of fungus diseases, to which insects are much subject. 



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, can not exist long where this plant does 

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

 climates; thus Anosia plexippus is following the milkweed over the 

 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. 



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 (Ends semidea on moun- 

 tain summits in New Hampshire and Colorado (particularly Mt. 

 Washington, 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 those 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 



