BOOK II 

 THE FACTORS OF DISTRIBUTION 



CHAPTER I 



THE CLIMATIC ENVIRONMENT AND THE RESPONSE TO IT 



IT is difficult to classify and arrange in logical sequence 

 the factors that determine or influence plant-distribu- 

 tion. 1 There can, however, be little doubt that climate 

 is the pre-eminent condition. Though climate has been 

 denned as the average of the weather, or of the physical 

 conditions of the atmosphere, and is often spoken of as 

 if it consisted solely in the temperature, moisture, and 

 movements of the air, meteorologists generally in practice 

 include the treatment of the intensity and other charac- 

 ters of the sun's light. It may be well to emphasise at 

 the outset the fact that in nature the various elements 

 that make up climate and other physical conditions, 

 such as aspect and the temperature or moisture of the 

 soil, are so intimately correlated that it is very difficult 

 to analyse their separate action. In estimating the 

 action of these agencies upon the life of plants, with 

 special reference, so far as we are concerned, to distribu- 

 tion, it has also to be borne in mind, not only that the 



1 " At least six main factors have contributed to the present 

 distribution of organisms, and none of these can be ignored. They 

 may be grouped in pairs: (a) the physical peculiarities of tne region 

 under discussion, and the constitutional peculiarities of the living 

 creatures; (b) the original headquarters of the stock (usually 

 uncertain) , and the means of dispersal in each case ; (c) the physical 

 changes of climate, earth-movements, etc., in the region; and the 

 changes brought about in the struggle for existence between the 

 various living tenants of the country. It may even be permissible 

 to use a mathematical expression and say that the distribution is 

 a function of six factors, some of which are variable dependently 

 and others independently " (Professor J. A. Thomson in The 

 International Geography, p. 86). 



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