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 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 favored
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 northward; 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
southward 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 favoring 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 naturalized, for
they cannot compete with our native plants, nor resist destruction by our
native animals."--(pp. 68, 69.)


After an instructive instance in which "cattle absolutely determine the
existence of the Scotch fir," we are referred to cases in which insects
determine the existence of cattle:


"Perhaps Paraguay offers the most curious instance of this; for here
neither cattle, nor horses, nor dogs, have ever run wild, though they swarm
southward and northward in a feral state; and Azara and Rengger have shown