84 NATURE OF THE CHECKS TO INCEBASB. [Chap. IIL 



on a little plot of mown turf (three feet by four) nine 

 species perished, from the other species being allowed to 

 grow up freely. 



The amount of food for each species of course gives 

 the extreme limit to which each can increase; but 

 very frequently it is not the obtaining food, but the 

 serving as prey to other animals, which determines the 

 average numbers of a species. Thus, there seems to be 

 little doubt that the stock of partridges, grouse, and 

 hares on any large estate depends chiefly on the de- 

 struction of vermin. If not one head of game were 

 shot during the next twenty years in England, and, 

 at the same time, if no vermin were destroyed, there 

 would, in all probability, be less game than at present, 

 although hundreds of thousands of game animals are 

 now annually shot. On the other hand, in some cases, 

 as with the elephant, none are destroyed by beasts of 

 prey; for even the tiger in India most rarely dares to 

 attack a young elephant protected by its dam. 



Climate plays an important part in determining the 

 average number 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 1854-5 destroyed four-fifths of the birds in my own 

 grounds; and this is a tremendous destruction, when 

 we remember that ten per cent, is an extraordinarily 

 severe mortality from epidemics with man. The ac- 

 tion of climate seems at first sight to be quite indepen- 

 dent of the struggle for existence; but in so far as 

 climate chiefly acts in reducing food, it brings on the 

 most severe struggle between the individuals, whether 

 of the same or of distinct species, which subsist on the 



