THE BIRDS OF THE PEIBILOF ISLANDS. 367 



life. This became greatly intensified when the glacial influence reached a maximum. 

 Those PalaearctiG types which under the former mild, cooling conditions had effected a 

 foothold on the American continent were then forced southward and readily displaced 

 the previous Neotropical stock. This must have been a comparatively easy task, for 

 the cooling climatic conditions were in their favor* Thus a number of forms of various 

 genera and families of Palaearctic types were cut off from the parent body and subse- 

 quently became differentiated into what should be considered as a branch or subregion 

 of the Triarctic (or Solarctic) region^the Nearctic subregion. The retreat of the ice 

 permitted not only the reoccupation of the more northern portion of the continent by 

 these now fixed Nearctic forms, but also a northern extension of the remains of the 

 more northern Neotropical forms. These movements, the result of the retreat of the 

 ice sheet, plus the altering topographical conditions resulting and in connection there- 

 with, or following, have effected the present mixed condition of the Alaskan avifauna 

 and of North America generally. The northwestern trend of the western outline of 

 the ice sheet and the lacustrine conditions thereby induced as the glacial influence 

 diminished, plus the influence of the Missouri and Yukon drainage systems, assisted 

 in the extension, even to the mouths of the Yukon, of the common migratory forms of 

 the Mississippi Valley. The same result evidently occurred on the Pacific watershed, 

 but modified by the western trend of the Eocky Mountains and the barrier of the 

 Mount St. Elias region. The probably extensive island condition then of Bering Sea, 

 and perhaps of the North Pacific, assisted in permitting the return of Asiatic forms 

 to again reside in summer in Alaska. The extreme cold of the glacial periods and the 

 volcanic activities of the North Pacific region have undoubtedly effected the extermi- 

 nation of some prior forms and the dispersal of others. The habits of the sea species, 

 especially the AMdae, have favored their preservation. 



The common migrating forms of Limosa and Gharadrius have a peculiar status, 

 which may be mentioned here as illustrating one phase of Alaskan bird life. The first, 

 is closely related to the European L. lapponica, but is paler and larger and does not 

 occur in North America except in western Alaska. The second is closely related to 

 the American G. dominicus, though smaller, but also occurs on the Pacific coast of 

 Asia as a migrant. Both occur in their greatest abundance in winter on the islands 

 of the Pacific and rarely if ever on the mainlands except in summer in Alaska and 

 Siberia, and both are good subspecies. The winter distribution of the Limosa is 

 more southerly than the other. Of Arenaria, another extensive Pacific islands' 

 migrant, rarely found on the northern mainland in winter, I have treated fully under 

 that genus. 



It would appear then that we should be right in considering that the Palaearctic 

 ancestors of our present Alaskan avifauna were originally breeding residents on the 

 island land areas immediately about the North Pole and became diffused as that 

 region became covered with ice. As this refrigeration began at a center — whether 

 that center was at the Pole or elsewhere is immaterial — it would have caused, more 

 markedly than before, the individuals of each species to migrate southward over 

 ocean island routes when that suited their needs and over land routes when that was 

 more to their taste. This separation of the main. body of breeding birds and their 

 segregation into several noncommunicating parts began early in the first glacial 

 period and continued and increased as time went on. Then there occurred, naturally, 



