54 THE ORIGIN OF SPECIES 



years in 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 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 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 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 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 suffering enor- 

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

 competitors 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 competitors, 

 decreases northward, or in ascending a mountain, we far oftener 

 meet with stunted forms, due to the directly injurious action of 

 climate, than we do in proceeding southward or in descending 

 a mountain. When we reach the arctic regions, or snow-capped 



