Chap. III. CHECKS TO INCREASE. 69 



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 attribute 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 com- 

 petitors be in the least degree favoured by any slight 

 change of climate, they will increase in numbers, and, 

 as each area is already fully stocked with inhabitants, 

 the other species will 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 spe- 

 cies 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 there- 

 fore of competitors, decreases northwards ; 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 

 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 exclu- 

 sively with the elements. 



That climate acts in main part indirectly by favouring 

 other species, we may clearly see in the prodigious 

 number of plants in our gardens which can perfectly 

 well endure our climate, but which never become natu- 

 ralised, for they cannot compete with our native plants, 

 nor resist destruction by our native animals. 



