172 Review of Darwin’s Theory on the Origin of Species, — 
to be the most effective of all checks. I estimated that the winterof 
d 
struggle for existence; but in so far as climate chiefly acts in reducing 
, 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 
rectly, 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 se 
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 
f rget that each species, even where it most abounds, is constantly sufler 
decreases northwards; hence in going northward, or mm ascene” 
mountain, we far oftener meet with stunted forms, due to the — - 
jurious action of climate, than we do in proceeding south 
nding &@ mountain. When we reach the Arctic regions, or pa 
capped summits, or absolute deserts, the struggle for life is almost 
sively with the elements. i : 
“That climate acts in main part indirectly by favoring other spec 
we may clearly see in the prodigious number of plants in our become 
Which can perfectly well endure our climate, but which never par 
naturalized, for they cannot compete with our native plants, Bot 
destruction by our native animals.”—pp. 68, 69. 1y de 
After an instructive instance in which “cattle agers to 
termine the existence of the Scotch Fir,” we are Te 
eases in which insects determine the existence of cattle. . 
“Perhaps Paraguay offers the most curious instance of this; 
neither cattle nor horses nor dogs have ever run wild, though they om 
southward and northward in a feral state; and Azara and Reng pee 
shown that this is caused by the greater number in Paraguay ten firs 
ually checked by ge | 
nay vuceked by some means, probably by birds. Hence, pet 
— “ig ip Ss whose numbers are probably recsiaies ha — 
en entre), Were to increase in Paraguay, the flies would @ etsy 
