•J 6 NATURE OF THE CHECKS TO INCPwEASE. Chap. III. 



and finally disappearing- ; and the change of climate being con- 

 spicuous, we are tempted to attribute the whole effect to its 

 direct action. But tliis 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 aheady fully stocked with inhabitants, the other 

 species will decrease. "\\Tien we travel southward and see a 

 species decreasing in numbei"s, Ave may feel sure that the cause 

 lies quite as much in other species being favored, as in tliis 

 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; hence, in 

 going northward, or in ascending a mountain, we far oftener 

 meet with stvuited forms, due to the directly injurious action 

 of climate, than we do in proceeding southward or in descend- 

 ing 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 favoring other 

 species, Ave may clearly see in the prodigious number of plants 

 in our gardens which can perfectly Avell endure our chmate, 

 l)ut which never become naturalized, for they cannot compete 

 with om- native plants, nor resist destmction by our native 

 animals. 



When a species, owing to highly favorable circumstances, 

 increases inordinately in numbers in a small tract, epidemics — 

 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 jiart through facihty of diffusion 

 among the crowded animals, l)ecn disproportionally favored ; 

 and here comes in a sort of sti-uggle between the parasite and 

 its prey. 



On the other hand, in many cases, a large stock of individ- 

 uals of the same species, relatively to the numbers of its ene- 

 mies, is absolutely necessary for its preservation. Thus we can 

 easily raise plenty of corn and rape-seed, etc., in our fields, be- 

 cause the seeds are in great excess compared with the mnnber 

 of birds which feed on them ; nor can the birds, though liavinq 



