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 Io 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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