In such ways, by thousands of adaptations and modifications of the
typical four-footed mammal, are they fitted to their varied modes of
life, each so far as possible in some special place of its own.
The effect of the pressure of environment and competition upon the
various species of mammals in any region could not be better shown
than by the kangaroos of Australia. That continent is occupied by many
species of these peculiar mammals, some of which inhabit the open
plains like our jack-rabbits in the West; others have learned to climb
and live arboreal lives in the tree-tops; and still other members of
this group have become burrowers and live in dens underground like some
of our native rats and mice.
From the instances mentioned above it is evident that the mammalian
organism is very plastic and has been molded by the environment
to which it has been subjected during the ages. The larger effects
evidenced by profound modifications in the anatomy are the result of
continued pressure extending far back in time. The far more numerous,
modern, and superficial changes known to naturalists as geographic
variations are everywhere in evidence.
[Illustration:
© F. J. Haynes
“BABES IN THE WOOD”
The American black bear, of which the brown bear is a color phase, is
not aggressive and will attack man only when wounded or in defense of
its young. The hungry twins were born in mid-winter and came into the
world entirely devoid of fur overcoats. Their coats soon developed,
however; in a month their eyes were open, and in two months they were
following their mother about the great forests of the Yellowstone.]
By the collection of great series of specimens in North America and
elsewhere in the world it has been proved that it is common for a
single species of mammal to occupy a great area, including such diverse
climatic conditions as humid forested districts near the sea-level,
sections of arid desert plains in the interior, and high rugged
mountain slopes. In each area of differing conditions it is ordinarily
found that representatives of a species, under certain conditions, vary
from those in other areas mainly in shades of color and in proportions.