54 NATURE OF THE CHECKS TO INCREASE. [Chap. Ill 



destroyed by beasts of prey ; for even the tiger in India most rarely 

 dares to attack a young elephant protected by its dam. 



Climate plays an important part in determining the average 

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

 drought seem to be the most effective of all checks. I estimated 

 (chiefly from the greatly reduced numbers of nests in the spring) 

 that the winter of 1854—5 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 

 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 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 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 favoured by any slight change of climate, tbey will increase 

 in numbers ; and as each area is already fully stocked with inhabi- 

 tants, the other species must decrease. When we travel south- 

 ward and see a species decreasing in numbers, we may feel sure 

 that the cause lies quite as much in other species being favoured, 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 northwards ; hence in going 

 northwards, 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 southwards 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 favouring 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 naturalised, for they cannot compete with our native 

 plants nor resist destruction by our native animals. 



