68 CHECKS TO INCREASE. Chap. III. 



the more vigorous plants gradually kill the less vigorous, 

 though fully grown, plants : thus out of twenty species 

 growing on a little plot of 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 destruction 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 proba- 

 bility, be less game than at present, although hundreds 

 of thousands of game animals are now T annually killed. 

 On the other hand, in some cases, as with the elephant 

 and rhinoceros, none are destroyed by beasts of prey : 

 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 numbers of a species, and periodical seasons of 

 extreme cold or drought, I believe to be the most 

 effective of all checks. I estimated 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 ten per cent, is an extraordinarily severe 

 mortality from epidemics with man. The action of cli- 

 mate seems at first sight to be quite independent 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 same kind 

 of food. Even when climate, for instance extreme 



