Chap. III. 
CHECKS TO INCKEASE. 
69 
acts directly, it will be tbe 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 conspi¬ 
cuous, 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 competi¬ 
tors 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 southward 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 
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 exclusively with 
the elements. 
That climate acts in main part indirectly by favoui’- 
ing 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. 
