278 U. S. NATIONAL MUSEUM BULLETIN 217 



isolation of the breeding populations which has brought about the 

 differentiation. For the differentiation into races to have affected so 

 many species implies that the isolation of the northwestern breeding 

 populations has been in operation for some time. This view leads 

 to an inquiry as to the nature of the special conditions which are re- 

 flected in the distinction of the avifauna of northern Alaska and 

 Yukon. 



The geography of the country was discussed in chapter 1 

 (see p. 6). A. E. Porsild (1951) has remarked that the flora of 

 arctic Alaska and Yukon is very old and rich in isolated and endemic 

 species. At the Mackenzie River a sharp boundary separates the 

 Yukon flora from that of Arctic America in the east. In contrast 

 there appears an affinity of relation to Asiatic plants in one third of 604 

 species of Alaska and Yukon. The North American isothermal lines 

 move rapidly northward in spring west of the Mackenzie River, al- 

 though in winter only the western coast is markedly warmer than the 

 interior. Spring advances rapidly northward in the west, or more 

 correctly, warm Pacific air moves inland over the Rocky Mountains 

 and over the Alaska Range to the Yukon Valley and extends about as 

 far eastward as the Mackenzie Valley. Many species of migratory 

 birds reach arctic Alaska and Yukon by May 15, or at about the date 

 when they come to northern Ontario in the east. At the time of spring 

 migration the isothermal lines above freezing extend nearly 20 de- 

 grees of latitude farther north in western than in eastern America. 



The distinctive flora and fauna of northern Alaska and Yukon 

 about correspond with the area in which the lowlands were not cov- 

 ered with ice during the last stage of Wisconsin glaciation (see figs. 

 3 and 9) . In thinking about the effect of this area as a refuge in which 

 life might survive while the rest of the northern part of the continent 

 was covered with ice it must be recalled that some climatic conditions 

 which left the country free from continuous ice in Wisconsin times 

 still exist. The general formation of the mountains has not varied, 

 and the resulting climate and low preciptation may well have been a 

 persistent distinction of the extreme northwest. Until we can discover 

 their remains and assign them to dates in glacial history, it is pure 

 speculation to view the wintering birds now present as derived from 

 stock which survived in an unglaciated arctic Alaskan refuge. It 

 seems more plausible to suspect that the warmth of the Pacific Ocean 

 early melted the coastal ice and made the coasts habitable for maritime 

 birds and a fit path for coastwise migration before the interior was 

 cleared. But even now the glaciers around the St. Elias Mountains 

 leave only a narrow path for land birds along the coast. Although 

 the majority of the taxonomically differentiated migratory races in 

 the Alaskan breeding avifauna are western American birds, the fact 



