THE STRUGGLE FOR EXISTENCE. 87 



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 individu- 

 als, 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 in- 

 variably see some species gradually getting rarer and rarer, 

 and finally disappearing ; and, the change of climate be- 

 ing 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 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 spe- 

 cies of all kinds, and therefore of competitors, decreases 

 northward ; 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 southward or in descending a mountain. 

 When we reach the Arctic regions, or snow-capped sum- 

 mits, or absolute deserts, the struggle for life is almost 

 exclusively with the elements. 



