Checks on Increase 



lion, for example, was never so numerous as to 

 reach the limit of its food supply. Before the 

 white man obtained a foothold in Africa vast 

 herds of herbivores were to be seen in those 

 districts where lions were most plentiful. This 

 is a most important fact, for, if the numbers of 

 a species are not determined by those of the 

 animals that prey upon it, the particular colour of 

 an organism is probably not of any direct im- 

 portance to it. This cuts away the foundation 

 of some of the generally accepted theories of 

 animal colouration. 



" Climate," writes Darwin (p. 84), "plays 

 an important part in determining the average 

 numbers of a species, and periodical seasons of 

 extreme cold or drought seem to be the most 

 effective of all checks. I estimated (chiefly from 

 the greatly reduced numbers of nests in the 

 spring) that the winter of 1854-55 destroyed 

 four-fifths of the birds in my own grounds, 

 and this is a tremendous destruction when 

 we remember that 10 per cent, is an extra- 

 ordinarily severe mortality from epidemics with 



man." 



In our opinion, Darwin did not lay nearly 

 enough stress upon the importance of climate as 

 a check on the increase of species. We have 

 seen that he stated his belief that it is the most 

 effective of all checks. But even this is not a 

 sufficiently strong statement of the case. It 



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