86 DARWINISM STATED BY DARWIN HIMSELF. 



though fully grown plants ; thus out of twenty species 

 growing 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 

 number 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 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 AS A CHECK TO INCREASE. 



Climate plays an important part in deter- 

 mining the average numbers 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-'55 destroyed four fifths of the birds 

 in my own grounds ; and this is a tremendous destruc- 

 tion, when we remember that ten per cent is an extraordi- 

 narily severe mortality from epidemics with man. The 

 action of climate seems at first sight to be quite inde- 

 pendent of the struggle for existence ; but, in so far as 

 climate chiefly acts in reducing food, it brings on the 



