Chap. III.] STRUGaLE FOR EXISTENCE. . 91 



ber. of mice is largely dependent, as every one knows, 

 on the number of eats; and Col. Newman says, "Near 

 villages and small towns I have found the nests of 

 humble-bees more numerous than elsewhere, which I 

 attrihute 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 deter- 

 mine, 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 will concur in determining the average number 

 or even the existence of the species. In some cases it 

 can be shown that widely-different cheeks 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 beauti- 

 ful diversity and proportion of kinds as in the surround- 

 ing virgin forest. What a struggle must have gone 

 on during long centuries between the several kinds 

 of trees each annually scattering its seeds by the thou- 

 sand; what war between insect and insect— between 

 insects, snails, and other animals with birds and beasts 

 of prey — all striving to increase, all feeding on each 



