64 NATURE OF THE CHECKS TO INCREASE. 



England, and, at the same time, if no vermin were de- 

 stroyed, 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 numbers of a species, and periodical seasons of ex- 

 treme cold or drought seem to be the most effective of all 

 checks. I estimated (chiefly from the 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 extraordinarially 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 indi- 

 viduals, 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 the 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 

 cliange of climate being conspicious, 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 suffering enormous destruc- 

 tion at some period of its life, from enemies or from com- 

 petitors for the same place and food; and if these enemies 

 or competitors be in the least degree favored by any slight 

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

 each area is already fully stocked with inhabitants, the 

 other species must decrease. When we travel southward 

 and see a species decreasing in numbers, we may feel sure 

 that the cause lies quite as much in other species being 

 favored, as in this one being hurt. So it is when we 

 travel northward, but in a somewhat lesser degree, for 

 the number of species of all kinds, and^ therefore of 



