STRUGGLE FOR EXISTENCE 1473 



greatly reduced numbers of nests in the spring) that 

 the winter of 1854-55 destroyed four-fifths of the 

 birds in my own grounds; and this is a tremendous 

 destruction, when we remember that ten per cent is 

 an extraordinarily severe mortality Trom 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 in- 

 dividuals, 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 individuals, 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 

 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 enor- 

 mous 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 spe- 

 cies 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 



