THE PEAIRIE REGION 145 



pass from the treeless prairie to the forest region, which is 

 continued eastward as far as the Atlantic Ocean. In the 

 prairie region a struggle has been in progress for thousands 

 of years between the conditions favouring tree growth and 

 those adverse to them. The increase in the mean annual pre- 

 cipitation from west to east is the determinant factor in forest 

 production. The main cause, therefore, of the absence of trees 

 in the prairies lies, according to Professor Russell,* in the 

 climatic conditions, and principally in the lack of sufficient 

 rain during the long, hot summers. 



A thorough survey of the fauna of the prairie region haa 

 still to be made. Dr. Me-rriam f devotes only a short para- 

 graph to it. Most other writers have confined themselves to 

 a description of one or two typical prairie forms. The sole 

 attempt to give us a more lucid impression of the general 

 'features of the vertebrate life of the region was made by 

 Dr. Ruthven.f He noticed that the peculiar conditions of the 

 prairie region had an effect on the fauna in modifying the 

 species as they entered this region from the adjoining ones. 

 Yet he thinks that there is a great difference in the extent 

 to which the species of eastern North America push westward,- 

 or the plains -forms eastward, into the prairie region, before 

 becoming modified or checked. Dr. Ruthven's studies lead 

 him to the conclusion that the prairie region is an extensive 

 area of transition between the plains and eastern forest 

 regions, but he expresses the opinion that the conditions of 

 environment are either not intensive or not extensive enough 

 to mould the animals into a peculiar fauna. 



What was once the most characteristic animal of the prairie 

 region is now practically extinct in the United States in its 

 feral condition. I need no longer dwell on the history of the 

 extinction of the bison, the animal I am alluding to, for it 

 has been sufficiently described in the third chapter (pp. 

 65 67). When discussing the question of the bison's origin, 

 I suggested that its ancestors might have invaded North 



* Eussell, I. 0., "North America," pp. 89 96. 

 t Merriam, C. H., " Life in North America," p. 20. 

 I Euthven, A. G., "Faunal Affinities of Prairie Eegion," pp. 390 

 393. 



L.A. L 



