74 MUTUAL CHECKS TO INCREASE. Chap. III. 



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 de- 

 stroy 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 concurring 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 the trees now 

 growing on the ancient Indian mounds, in the Southern 

 United States, display the same beautiful diversity and 

 proportion of kinds as in the surrounding virgin forests. 

 What a struggle between the several kinds of trees 



