40 DARWmiARA. 



« 



species can increase ; but very frequently it is not the obtaining 

 of food, but the serving as prey to other animals, which de- 

 termines the average numbers of species." — (p. 68.) 



" Climate plays an important part in determining the average 

 numbers of a species, and periodical seasons of extreme cold or 

 drought I believe to be the most effective of all checks. I 

 estimated that the winter of 1854-'55 destroyed four-fifths of 

 the birds in my own grounds; and this is a tremendous destruc- 

 tion, when we remember that ten per cent, is an extraordinarily 

 severe mortality from epidemics with man. The action of 

 climate seems at first sight to be quite independent of the 

 struggle for existence ; but, in so far as climate chiefly acts in 

 reducing food, it brings on the 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 in- 

 stance extreme cold, acts directly, it will be the least vigorous, 

 or those which have got least food through the advancing 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 at- 

 tribute the whole effect to its direct action. But this is a very 

 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 stocked with 

 inhabitants, the other species will 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 oi 

 species 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 



