| SLOTH, AN ANIMAL WHICH KEEPS TO TREES AND IS ALMOST HELPLESS ON THE GROUND |
The striking peculiarity of both these queer creatures, however, is the fact that they lay eggs. These are few—sometimes only one—and recall those of reptiles in their relatively large size, parchmentlike shells, and abundance of food-yolk. The duckbill deposits her eggs in her grass-lined burrow nest and covers them with her body until they quickly hatch. The blind and naked young then apply their lips to the nearest part of the mother's abdomen, and suck milk through the pores of the skin. In the echidna one sees a little advance on this extremely simple beginning of nursing; for here, instead of being laid in a burrow nest, and covered by the mother, the echidna's egg is placed by the mother within two parallel folds of skin which at that season form a deep groove in the abdomen inclosing the nursing area, and is held there until it hatches. When the young has attained a certain size the mother removes it from the "pouch," but takes it in from time to time to suckle it.
Such are the Prototheria—one of the grand divisions of Mammalia, set apart by reason of their laying the eggs from which the young will afterward be born, whereas in the other division or Eutheria ("proper mammals") the "embryos," or unborn young, escape from the eggs in a less or greater degree of development before their birth from the mother. This period between the conception of life in the egg and its emergence at birth is called the period of gestation, and is much longer in large animals than in small ones. Fundamental differences in method of birth divide the Eutheria into two groups, designated as Nonplacentals and Placentals.
THE MARSUPIALS
The word marsupial means "pouched," and refers to the most characteristic peculiarity of the nonplacental division (order Marsupialia), which is the possession of a more or less pocketlike fold in the skin of the abdomen of the females within which the extremely immature young are nourished.
The egg-laying mammals, also nonplacental, have the young inclosed in a protective shell that they keep warm, as do the birds, until the embryo is sufficiently matured to be safely born. In the marsupials nature meets the difficulty another way. The embryo is but little advanced when born, in fact it is utterly helpless and minute, being, even in the case of the largest kangaroos, hardly as big as a mouse. It would be fatal, of course, to turn it loose upon the world; and therefore the mother is provided with the pouch already described.
The instant an embryo is born the mother picks it up and places it within the pouch, where it crawls about until it touches and instinctively takes hold of one of the threadlike teats. As it gets stronger it leaves the pouch now and then, but returns to it for nursing, sleeping, and protection when alarmed, until finally it departs altogether.
This description applies to the most advanced families of the order. In the oldest and most generalized families of marsupials, such as the banded anteaters, there is virtually no pouch at all.
As almost the whole marsupial tribe are natives of Australasia, it is odd that the family with which we must begin a list of them—the true opossums—should be American, and quite unknown in Australia. This is explainable when it is known that this family (Didelphidæ) is the most archaic of this ancient tribe, and was well established in Cretaceous times, and then and later was widely distributed in Europe and on this continent; yet so little change has occurred in the race that teeth from the Laramie formations of Wyoming are hardly distinguishable from those in the jaws of our 'possum-up-a-gum-tree to-day. No wonder the quaint creature is hoary and wrinkled; he is a very Methuselah among mammals, and looks it! All opossums seem to have disappeared from Europe before the close of the Miocene, but continued to survive numerously in South America. They probably owe their long career, in competition with animals of so much higher grade, to their small size, forest life, nocturnal habits, ability to eat all sorts of food, and, most of all, to their great fecundity. Our common opossum is the most northern of its kind, and ranges over the whole country as far north as the latitude of Lake Erie; it appears never to have crossed the Hudson River until comparatively recent times, but is now frequently met with in New England and on Long Island. It is at home in all sorts of places, except, perhaps, on the dry plains, for it is primarily an arboreal animal, aided in climbing about trees by its naked, prehensile tail, by which it may hang to a branch while using its forefeet to rob a bird's nest or gather fruit. It will eat anything it can get hold of, and with its sharp teeth, which number fifty, will kill animals as large as itself; hence it is a destructive raider of henroosts and sitting birds as well as a seeker of mouse nests and insects.