Chap. 111.] NATURE OF THE CHECKS TO INCREASE. 85 



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 at- 

 tribute 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 de- 

 struction 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 de- 

 crease. 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 north- 

 ward, 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 ele- 

 ments. 



That climate acts in main part indirectly by favour- 

 ing other species, we clearly see in the prodigious num- 



