NATURE OF THE CHECKS TO INCREASE 83 



quently 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 probability, be less game than at present, al- 

 though 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 j-oung ele- 

 phant protected by its dam. 



Climate plays an important part in determining the aver- 

 age 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 1854-5 destroyed four- 

 fifths of the birds in my own grounds ; and this is a tremen- 

 dous destruction, when we remember that ten per cent, is 

 an extraordinarily severe mortality from epidemics with 

 man. The action of climate 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 cold, acts 

 directly, it will be the least vigorous individuals, or those 

 which have got least food through the advancing winter, 

 which will suffer most. When we travel from south to 

 north, or from a damp region to a dry, we invariably see 

 some species gradually getting rarer and rarer, and finally 

 disappearing; and the change of climate being conspicuous, 

 we are tempted to attribute the whole effect to its direct 

 action. But this is a false view; we forget that each species, 

 even where it most abounds, is constantly suft"ering enormous 

 destruction at some period of its life, from enemies or from 

 competitors for the same place and food; and if these ene- 

 mies or competitors be in the least degree favoured by any 

 slight change of climate, they will increase in numbers; and 



