DISPERSAL OF NORTH AMERICAN BIOTA. 69 



4. SUMMARY AND CONCLUSION. 



In summarizing we may note that recent advances made in 

 the study of the extratropical North American fauna and flora 

 have been primarily due to the rapid accumulation of data. In 

 the near future, rapid advances along the line of explanation and 

 interpretation of these facts may be expected. As the present 

 distribution is in part an effect it is therefore necessary to take 

 into account certain past conditions. Very important among 

 these factors have been Glacial and Postglacial influences upon 

 this region. These geological changes have had a great influ- 

 ence on the biota, not only on account of the wonderful changes 

 in the physical conditions of life, attending the decline of the Ice 

 Age, but also on account of the definiteness given to the dynamic 

 tendencies by this environment. When attempting to determine 

 the affinities and interrelations of the present biota too much 

 emphasis cannot be placed upon this definite dynamic tendency, 

 and to the sources and routes followed by the life on its return to 

 the glaciated region. This returning biota followed, in all proba- 

 bility, a definite successional relation and was composed of three 

 general belts or " waves," concentrically distributed south of the 

 ice margin. The first one was of the barren ground type, the 

 second was represented by distinct eastern and western conifer- 

 ous forest types, and the third by the biota of the southeastern 

 and southwestern states. The first wave was of a transcontinental 

 extent, the second while coniferous and transcontinental was com- 

 posed of two distinct types, the eastern, represented by the biota 

 of northeastern North America, and the western by that of the 

 Rocky Mountains and the Pacific coast. The northeastern biota 

 overflowed to the north, to the northwest into the Mackenzie basin 

 and even a few forms into the Yukon valley and to the Rocky 

 Mountains. The northwestern biota spread from the Rocky 

 Mountains and Pacific coast regions in the United States north 

 to British Columbia and Alaska. The third wave spread from 

 the southeastern center of dispersal northward to the conifers, and 

 west to the Great Plains. From the southwestern center the 

 life spread north on each side of the Rocky Mountains into 

 Canada, and only stragglers spread eastward into the humid 

 southeast. 



