THE PRAIRIE 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 has 
still to be made. Dr. Merriam 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.^; 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 tire 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 
* Russell, I. C., “North America,” pp. 89—96. 
f Merriam, C. H., “ Life in North America,” p. 20. 
\ Ruthven, A. G., “Faunal Affinities of Prairie Region,” pp. 390— 
393. 
L.A. 
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