58 CHAS. C. ADAMS. 



we usually think of them as completely boreal forms and tending 

 southward in their dispersal. Turning now to a brief considera- 

 tion of the primary characteristics of these elements in the biota 

 and beginning with the one which invaded the glaciated region 

 first we have the following order of succession : 



First IVave. 



1. Tundral or Barren Ground Biota. This element of the 

 return movement exists to-day in the north beyond the tree limit 

 and as relicts farther south upon mountain summits. This is a 

 circumpolar type and has little peculiarly American. Its orig- 

 inal center of dispersal may have been farther south near the 

 centers of ice accumulation, or as Dixon ('95, p. 298) has sug- 

 gested in the elevated regions of the tropics. In either case it 

 has had a very nomadic existence. 



As there is reason to believe that the ice did not completely 

 cover all the northern land some of this type flourished there, in 

 all probability, even in Glacial times, as for example, in the Point 

 Barrow region of Alaska, where Nelson ('87, p. 27) has noted 

 the distinctly Siberian affinities of the biota. This factor sug- 

 gests that the life of this region is an overflow perhaps in pre- 

 glacial or Glacial times from unglaciated Arctic Siberia. 



Migration and Dispersal Routes. The first wave biota has 

 apparently reached its present location by a northward Glacial 

 and Postglacial migration, and has been supplemented by certain 

 Glacial relicts, from Alaska in particular, as has been suggested, 

 while in more recent times some additions have been received 

 from Asia and Greenland as has been shown by Stejneger for the 

 wheatear ('oi, a and /?). The migration route of the western 

 birds of this species points to India by way of Alaska, and the 

 eastern ones to Africa by way of Greenland. 



Probably the latest paths followed northward were along the 

 mountain chains, where occasionally colonies have lingered, in 

 favorable conditions, upon mountain peaks. 



Second Wave. 



2. Northeastern Biota. The second wave was of the biotic 

 type now represented by the northern transcontinental coniferous 



