74 
MUTUAL CHECKS TO INCKEASE. 
Chap. 111 . 
whole genus of humble-bees became extinct or very 
rare in England, the heartsease and red clover would 
become very rare, or wholly disappear. The number 
of humble-bees in any district depends in a great 
degree on the number of field-mice, which destroy 
their combs and nests; and Mr. H. Newman, who has 
long attended to the habits of humble-bees, believes 
that more than two-thirds of them are thus destroyed 
all over England.” Now the number of mice is largely 
dependent, as every one knows, on the number of cats ; 
and Mr. Newman says, ^^Near villages and small towns 
I have found the nests of humble-bees more numerous 
than elsewhere, which I attribute to the number of cats 
that destroy the mice.” Hence it is quite credible that 
the presence of a feline animal in large numbers in a 
district might determine, through the intervention first 
of mice and then of bees, the frequency of certain 
flowers in that district! 
In the case of every species, many different checks, 
acting at different periods of life, and during different 
seasons or years, probably come into play; some one 
check or some few being generally the most potent, but 
all concur in determining the average number or even 
the existence of the species. In some cases it can be 
shown that widely-different checks act on the same 
species in different districts. When we look at the 
plants and bushes clothing an entangled bank, we are 
tempted to attribute their proportional numbers and 
kinds to what we call chance. But how false a view 
is this ! Every one has heard that when an American 
forest is cut down, a very different vegetation springs 
up; but it has been observed that ancient Indian ruins 
in the Southern United States, which must formerly have 
been cleared of trees, now display the same beautiful 
diversity and proportion of kinds as in the surrounding 
