Adaptations in Mammalia.—Of the thirty-five hundred species, most inhabit continents; a few species are found on different islands, and some, as the whale, inhabit the ocean. They vary in size from the whale and the elephant to tiny shrew mice and moles. Adaptations to different habitat and methods of life abound; the seal and whale have the limbs modified into flippers, the sloth and squirrel have limbs peculiarly adapted to climbing, while the bats have the fore limbs modeled for flight.
Lowest Mammals.—The lowest are the monotremes, animals which lay eggs like the birds, although they are provided with hairy covering like other mammals. Such are the Australian spiny anteater and the duck mole.
All other mammals bring forth their young developed to a form similar to their own. The kangaroo and opossum, however, are provided with a pouch on the under side of the body in which the very immature, blind, and helpless young are nourished until they are able to care for themselves. These pouched animals are called marsupials.
The other mammals may be briefly classified as follows:—
Classification of Higher Mammals
Order I. Edentata. Toothless or with very simple teeth. Examples: anteater, sloth, armadillo.
Order II. Rodentia. Incisor teeth chisel-shaped, usually two above and two below. Examples: beaver, rat, porcupine, rabbit, squirrel.
Order III. Cetacea. Adapted to marine life. Examples: whale, porpoise.
Order IV. Ungulata. Hoofs, teeth adapted for grinding. Examples: (a) odd-toed, horse, rhinoceros, tapir; (b) even-toed, ox, pig, sheep, deer.
Order V. Carnivora. Long canine teeth, sharp and long claws. Examples: dog, cat, lion, bear, seal, and sea lion.