INTRODUCTION. 25 



plants concerned, each of which has been sufficiently discussed to 

 indicate that they are counter to our purpose. 



In making a general study of the relation of climate to vegetation 

 for as large an area as the United States, it has been necessary for us 

 to disregard the small communities which are unlike the general vege- 

 tation of the surrounding region, such as the local prairies of Arkansas 

 and Mississippi and the bands of trees that border the rivers of the 

 western plains. These require special treatment, in which soil condi- 

 tions can be given a thorough investigation. It has also been neces- 

 sary for us to leave out of reckoning all of the minor plants in the 

 stratified communities, inasmuch as the conditions under which they 

 live are unlike those for the major plants. The climatic conditions 

 under which the major plants exist are modified by them in such a 

 manner as frequently to give the minor plants a very different environ- 

 mental complex. 



By means of these omissions we have done a great deal to simplify 

 our problem from the standpoint of the demarcation of vegetational 

 areas. It has been desirable, furthermore, to consider only the most 

 general features of the vegetation, because we have only very general 

 data as to the distribution of the climatic factors. The subsidiary 

 communities of every region are so largely controlled through the soil 

 conditions that it would have carried us beyond our investigation of 

 climatic controls to have entered upon a consideration of them. 



The statement that the occurrence and geographical range of all 

 plant communities is controlled by the physical, and rarely the chemi- 

 cal, characteristics of the environment is almost axiomatic. The 

 operation of these controls has been observed from time immemorial 

 by men of no technical training but of keen powers of observation, and 

 the knowledge of them has become more exact among the practical 

 pioAeers in the agriculture and forestry of many lands where the 

 natural vegetation has been used as an index of the cultural capabili- 

 ties of given situations. Any skepticism regarding the physical con- 

 trol of communities would be dissipated by an extended course of 

 travel in diversified regions, or equally well by a careful reading of 

 Schimper's Plant Geography, to say nothing of an examination of the 

 many scattered papers which give proofs in regard to particular 

 instances of such control. 



We can, in brief, put it down as a law of plant geography that the 

 existence, limits, and movements of plant communities are controlled 

 by physical conditions. The conditions that control the movements 

 of the community are those of the soil; the conditions that control the 

 broader geographical limits are almost solely those of the climate. The 

 existence of the community and the extent of the area occupied are, of 

 course, controlled by conditions of both soil and climate. 



