DISPERSAL OF NORTH AMERICAN BIOTA. 6 1 



require much more study before its true position can be deter- 

 mined. In marked contrast with the northeastern biota this one 

 has long been bounded on the south and east by an arid climate. 

 Migration and Dispersal Routes. With the exception per- 

 haps of Glacial relict colonies in favorable spots along the Pacific 

 coast, and in the unglaciated parts of Alaska, the biota of the 

 Pacific coast and Canadian Rockies must have pushed into this 

 region primarily from two directions in Postglacial times. To a 

 limited extent there was an overflow of the northeastern biota 

 but the great bulk of the population came from the Rocky Moun- 

 tains and Pacific coast region south of the Canadian border. 

 Dispersal must have been carried on under great disadvantages, 

 on account of the topographic difficulties. But this biota, on 

 account of its proximity, and early invasion of the region, had 

 manifest advantages over the later arrivals. That a great wave 

 of life moved north along the mountains from this western center 

 is very apparent from the present affinities of the life of the region 

 extending from southern British Columbia to Alaska. The pri- 

 mary highways were probably the mountains themselves, and a 

 narrow coastal strip, now largely submerged. These lines of 

 dispersal are today migration routes for birds. Bishop ('oo, p. 

 50) has shown that a large part of the Yukon valley birds winter 

 in western United States, and this clearly suggests their western 



origin. 



The extensive distribution of certain forms in northwest North 

 America, and their occurrence as well in the northeast, has 

 suggested the northwestern origin of such forms. From the 

 present standpoint it seems more likely that most of these 

 northwestern forms have been derived from the western center 

 of dispersal from which they spread north or later overflowed 

 into the northeast. It also seems that the northern biota in 

 general has had a northern rather than a southern trend to 

 its dispersal. 



The Alaskan region, in addition to its Glacial relicts, was ap- 

 parently repopulated in part by a northward invasion from the 

 western center of dispersal along the mountains, by a double 

 invasion from Asia (Stejneger) north and south of the Stanovoj 

 Mountains, and by contributions from the northeastern biota. 



