The smaller mammals live everywhere, from the tropical end of Florida
to the uttermost lands of the frozen North, and from the seashore to
the limit of vegetation on the high mountains. The heaviest forests,
open meadows, rugged mountain slopes, arctic barrens, and sun-scorched
desert plains all have their small four-footed habitants. Many
modifications of parts and organs of the various species have been
necessary to adapt the small mammals to specialized modes of life.
This is strikingly illustrated in the case of those true rodents, the
pocket-gophers, which apparently found competition on the surface of
the ground so acute that they took the unoccupied territory below the
surface, where they live as miners and tunnel from place to place in
search of edible roots, with an occasional stealthy excursion above
ground to seize some of the food available there.
Another excellent illustration is furnished by the moles, which,
leaving the numerous closely related species--the shrews--to feed upon
insects above ground, have descended and, like the pocket-gophers,
live in tunnels which they make in the pursuit of earthworms and
insects below the surface; like the gophers, they, too, make occasional
excursions above ground in search of food.
The mink and the muskrat, representing the carnivores and rodents, have
rivals for their food supply on land and have become amphibious, being
as much at home in the water as on shore, one feeding on fish and flesh
and the other on aquatic vegetation. Certain forms of the squirrel
tribe are heavy-bodied and live in underground burrows, while other
more slender and graceful species make their homes in the tree-tops.