108 THE ORIGIN OF SPECIES 



north, or from u 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 con- 

 stantly 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 cli- 

 mate, 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 north- 

 ward, but in a somewhat lesser degree, for the number 

 of species of all kinds, and therefore of competitors, de- 

 creases northward; hence in going northward, or in as- 

 cending 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 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, 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 circum- 



