6O CHAS. C. ADAMS. 



3. Western Center of Dispersal. In the West we recognize 

 a second center of northward migration. It is represented by 

 the biota of the Rocky Mountains and the Pacific coast region. 

 Its great extent, even in Glacial times, south of the ice margin and 

 its present occupation of the field, allows this biota to be geograph- 

 ically defined as the Western Center of Dispersal. In contrast 

 with the region dominated by the eastern part of this wave the 

 western branch occupied a high mountain country. It was a 

 coniferous forest belt, but as has been mentioned, was of a very 

 different type from that of the northeast. The present flora of 

 the Rocky Mountains and the coast region is of the same gen- 

 eral type, as shown by Coville ('93, pp. 29-31) and Rydberg 

 ('oo, p. 871), although the climatic conditions are quite different 

 in several respects. It should also be recalled that much of the 

 recent botanical work has been done in the Rocky Mountains 

 near the Canadian bolder so that later studies in the southern 

 Rockies may, to some degree, lessen this apparent uniformity. 

 These facts do not favor the idea of transcontinental unity of the 

 coniferous forests but show that the direction of geographic origin, 

 the adaptations of the biota to mountain conditions, and prox- 

 imity, are factors which must be reckoned with in understanding 

 the Postglacial repopulation of the Northwest. The same fac- 

 tors also suggest that the usually accepted transcontinental dis- 

 tribution of the fauna may be overestimated. At least it is 

 very evident that many of the characteristic animals of the 

 western mountains are lacking in the relatively low eastern Appa- 

 lachians. Such a relation may have been closer in the past than 

 it is at present, as is suggested by the occurrence of the pica 

 (Ochotona) in the Pleistocene of Pennsylvania, although now, 

 in North America, it occurs only in the western mountains. 

 There is also in the West a great increase of Asiatic types, in 

 addition to certain native elements. The mountain goai(0ream- 

 mts] and big horn (Ovis} are representative mountain forms of the 

 west but lacking, even as fossils, in the east. Among some in- 

 vertebrates this is also true, as for example, the butterfly genus 

 Parnassius, the crawfish Potainobius, the west coast Unionida; 

 and certain Arionta-\fa& land shells, are quite distinct from eastern 

 types. The composition and affinities of this western biota 



