68 
CHECKS TO INCKEASE. 
Chap. III. 
gradually kill tlie less vigorous, tliougli 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, wdiich 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 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 owui 
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 betw’^een the individuals, whether of the same 
or of distinct species, which subsist on the same kind 
of food. Even when climate, for instance extreme cold, 
