NATURE OF THE CHECKS TO INCfcEASE. 61 



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 the 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 destruc- 

 tion at some period of its life, from enemies or from com- 

 petitors 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 species of all kinds, and therefore of 

 competitors, decreases northward, or in ascending a moun- 

 tain, we far oftener meet with stunted forms, due to the 

 directly injurious action of climate, than we do in proceed- 

 ing southward or in descending a mountain. When we 

 reach the arctic regions, or snow-capped summits, or abso- 

 lute deserts, the struggle for life is almost exclusively with 

 the elements. 



That climate acts in main part indirectly by favoring 

 other species, we clearly see in the prodigious number of 

 plants which in our gardens can perfectly well endure our 

 climate, but which never become naturalized, for they 

 cannot compete with our native plants nor resist destruction 

 by our native animals. 



When a species, owing to highly favorable circumstances, 

 increases inordinately in numbers in a small tract, epidem- 

 ics — at least, this seems generally to occur with our game 

 animals — often ensue; and here we have a limiting check 

 independent of the struggle for life. But even some of 

 these so-called epidemics appear to be due to parasitic 

 worms, which have from some cause, possibly in part 

 through facility of diffusion among the crowded animals, 

 been disproportionally favored : -and here comes in a sort of 

 Struggle between the parasite and its prey. 



