316 Distribution of Moths of the Sub-family Bistonince. 



the Californian, British Columbian and intervening regions ; 

 such beings, then possess but few representatives in the west. 



This, no doubt, is possible but that the dwellers there 

 to-day, save these very few relict forms such as the Welling- 

 tonias (Sequoice), form a complex of very recent aggregation 

 indeed. In this, complex careful disentanglement reveals a 

 structure composed of five elements : — 



(i) The not inconsiderable endemic group certainly, in 

 the first place, evolved from intruders from the south. 



(2) Forms palpably of southern origin. 



(3) Forms just as decidedly from the north-west and there 

 fore from Asia. 



(4) The very few eastern forms that have crossed the 

 mountains. 



(5) The Tertiary relict forms probably of diverse origin. 

 Of these five divisions only the third concerns us here. 



After Pacific America emerged from the sea, and prior to 

 the Glacial Period, a gradual infiltration of Asiatic forms took 

 place via the land now overwhelmed by the Behring Straits ; 

 nor did the occurrence of the Glacial epoch in Europe interrupt 

 this entirely, for the advent of the Ice Age was long postponed 

 in Asia as well as in Western America. Clearly, all such 

 migrating species would be of temperate proclivities. 



Concerning the fate of these intruders but little doubt can 

 be felt but that the local glaciation experienced in the Cali- 

 fornian area was not sufficient to exterminate them. More- 

 over, there never has been a protracted (relatively) period 

 without its uplift rendering the passage of these types possible, 

 although it was interrupted at the climax of the Ice Age. 



Even if this survival cannot be conceded, one must admit 

 that, throughout the Northern Hemisphere, the Glacial Period 

 was succeeded by a climate so genial that the average annual 

 temperature was far higher than that of our era. 



Preceding this and persisting through it, a slight uplift 

 of the sea bottom brought about the reunion of America and 

 Eastern Asia. Aided by the conjunction of a milder climate 

 in northern latitudes with the reopened causeway, a flood of 

 forms of all groups, 'ranging from buttercups to larches, and 

 from butterflies to the Rocky Mountain Sheep (Ovis montana) 

 migrated from Asia to America. Unless such forms were 

 Arctic or montane, they found in the Rockies an insurmount- 

 able barrier, and were forced to restrict their habitats to the 

 tract between California and Alaska. As any such invasion, 

 granting the maximum possible increment in average annual 

 temperature, was only permissible for forms of temperate 

 tendencies all such immigrant species are bound to be char- 

 acteristic of temperate climates. If not contemporaneously 

 then not far behind or before, Europe was re-opened for 



Naturalist, 



