DISPERSAL OF NORTH AMERICAN BIOTA. 59 



forest belt. But this belt was not homogeneous from east to 

 west and the eastern element in this wave will be considered 

 first. On account of its wanderings, this wave as in the case of 

 the first cannot be geographically defined in the east as a center 

 of dispersal, without danger of confusion. Although this biota 

 reaches its best development at the present time, in the northeast, 

 yet it is only a relatively late arrival in that region. For eastern 

 North America this was the "second wave" (Adams ,'02, b, p. 

 309) to pass north after the retreating arctic climate attending 

 the decline of the Wisconsin ice sheet. The region now occu- 

 pied by this biota is an area with abundant lakes, peat bogs and 

 a region of poor drainage. The area is covered by coniferous 

 forests but it is of a very different type from that found in the 

 Rocky Mountain region as has been shown by Rydberg ('oo, 

 pp. 871-873). Here the very characteristic bog plant society 

 reaches its best development as shown by Transeau ('04). This 

 is the region of fur-bearing animals, and there are very few 

 reptiles and amphibians. On the west this biota swings north 

 of the Great Plains in Canada to the Rocky Mountains and then 

 north into the Mackenzie basin. 



Migration and Dispersal Routes. The northeastern type of 

 biota has moved from about the latitude of the Ohio valley north 

 to its present position. Certain elements have apparently pushed 

 far northwest to the Rocky Mountains, to the Mackenzie basin, 

 and even overflowed into the Yukon valley --the reverse route, 

 in all probability, was followed by certain Asiatic forms into 

 America. This westward and northwestward dispersal has 

 tended perhaps to overemphasize the transcontinental distribu- 

 tion of these northern types and shows how the determination of 

 faunal areas based primarily upon the present conditions tends to 

 obscure the compositeness and diversity of origin of their con- 

 stituent elements. This biota reaches its greatest southward ex- 

 tension along the Appalachian Mountains. Laggards of this 

 and the barren ground type form the " boreal " islands, when sur- 

 rounded by the life of the following wave. These occur not only 

 upon mountain tops, in bogs and on sand dunes but also in cer- 

 tain deep lakes, where the "abyssal " fauna shows very decided 

 northern affinities, and clearly suggests them as Glacial relicts. 



