Chap, in.] NATURE OF THE CHECKS TO INCREASE. 85 



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 con- 

 spicuous, 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 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 inhabi- 

 tants, 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 north- 

 wards; 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 by favouring 

 other species, we clearly see in the prodigious number 



