64 ON THE ORIGIN OF SPECIES 



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, 

 or those which have got least food through the advanc- 

 ing 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 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 will decrease. 

 When we travel southward and see a species decreas- 

 ing 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 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 southwards or in 

 descending a mountain. When we reach the Arctic 

 regions, or snow-capped summits, or absolute desert-?, 

 the struggle for life is almost exclusively with the 

 elements. 



That climate acts in main part indirectly by favour- 

 ing other species, we may clearly see in the prodigious 

 number of plants in our gardens which can perfectly 

 well endure our climate, but which never become 

 naturalised, for they cannot compete with our native 

 plants nor resist destruction by our native animals. 



When a species, owing to highly favourable circum- 



