50 NATURE OF THE CHECKS TO INCREASE. [CHAP, IIL 



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 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 

 jchiefly 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 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 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 favoured 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 favoured, 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 northwards ; hence in going northwards, 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 

 southwards or in descending a mountain. When we reach the 

 Arctic regions, or snow-capped summits, or absolute deserts, the 

 struggle for life is almost exclusively with the elements. 



That climate acts in main part indirectly Jjy favouring other 

 species, we clearly see in the prodigious number of plants which 



