DISTRTBl i i 



OIH iii wet years, Mi>ture he* k- 1 he development of these and otl 

 j in ways ,i- yet ana M ertained; po^ibiy i' dire< ily ; 



lug tin- growth of fun . to which ii. jet L 



The absence of proper food i-> more ri'fcciivr than < limatc, as a direct 

 ( heck upon the spread of an animal; food itself In-in^, of course, de- 

 pendent ultimately upon climatal factors and soil. Many inset ts, being 

 (ontincd to a single food plant, can not exist lon.^ where 1 hi- plant does 

 not occur; hut they will follow the plant, as was just said, into n- 

 climates; thus Anosia plexippus is following the milkweed over the 

 world. Tin' butterfly Euphydryas phaeton is remarkably local in its 

 invnce, being limited to swamps where its chief food plant (Chelone 

 glabra) grows; and Epidemic c pi. van the is similarly restricted to cranberry 



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. 

 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 

 tlora of North America. "With the slow southward advance of the ice, 

 animals were crowded southward; with its recession they advanced 



tin 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 



