The Black-and-white Warbler 



where this family was already highly differentiated, a varied cohort 

 invaded the timbered territory east of the Mississippi, found congenial 

 conditions, and spread rapidly northward. Encountering, presumably, 

 the ice barrier in the Laurentian Highlands, this eastern division found 

 relief in a northeasterly direction, flowed across the timbered stretches of 

 Ontario, Manitoba, and Saskatchewan, and reached at last the eastern 

 Rockies of British Columbia, and the limit of trees in Mackenzie, Yukon 

 Territory, and Alaska. 



A similar but smaller cohort, starting perhaps at the same time, 

 invaded the mountains of the West, proceeding slowly northward by way 

 of the Rocky Mountains, the Sierra-Cascades, and the continuous forests 

 of the Pacific slope. This western army of invasion has also reached the 

 limit of trees in Alaska, but it is represented there by only two forms, — 

 forms whose conspecific analogues of the eastern stream had long since 

 reached the limit of trees in Mackenzie. The eastern stream, also, has 

 three exclusive types already on the ground. 



In the early days an effective barrier existed between the eastern 

 and western divisions of the Mniotiltine army in the treeless plains west 

 of the Mississippi. This barrier still exists, but it is disregarded by such 

 eastern species as, having found a home in the British Northwest, now 

 cut "cross-lots" in going to and from their winter quarters, instead of 

 returning by the Mackenzie-Ontario-Atlantic route originally traversed. 

 It is worthy of remark, however, that the great majority of such northern 

 residents still return by the old route. In a similar way, we may suppose 

 that the western pioneers were originally separated by the Great Basin; 

 but having passed beyond it, and becoming now reunited in the North, 

 they have learned to traverse it unfearingly in their annual flights. 



Disregarding for the present the recent or the non-aggressive types, 

 eight in number, such as Vermivora hicice and Peucedramus olivaceus, which 

 are confined to our southern latitudes, there are in the West only eleven 

 species. Of these, five, presumably the oldest, that is, the first to enter 

 this territory, are peculiar to the West. But of these, in turn, one, and 

 that the dominant warbler of the West, D. anduboni, while specifically 

 distinct, is yet so closely related to the dominant bird of the North (east- 

 ern division), D. coronata, that it forms a connecting link with the sec- 

 ond grand ethnological division of western warblers, viz., those which 

 are subspecifically related to eastern forms. For instance, the Yellow 

 Warbler (D. cestiva subsp.), aggressive, dominant, has penetrated Alaska 

 both by way of the East and of the West; but in the days of separation, 

 slight differentiations have arisen within the species, not only between 

 the eastern and western birds, but in the West itself as between the more 

 aggressive northern birds and the more indolent south-keeping birds. 



439 



