The next higher step in the ascent of Mammal life above the Monotreme is occupied by the Marsupials, or _milk-giving, pouched animals_, of which family the opossum and kangaroo are well known members. The characteristic feature of this family of creatures is the possession of an external pouch in the female, in which the young are kept and nourished until they can take care of themselves as the young of other animals are able to do. The young of the Marsupials are brought forth, or born, in an imperfect condition, and undeveloped in size and strength. There are fossil remains of Marsupials showing that in past ages creatures of this kind existed which were as large as elephants. In the more common form of Mammals the young are brought forth fully formed, they having received "nourishment, before birth, from the mother's body, through the _placenta_, the appendage which connects the fetus with the parent. The Placental Mammals were the best equipped of all the life-forms for survival and development, for the reason that the young were nourished during their critical period, and the care that the mammal must of necessity give to her young operated in the direction of affording a special protection far superior to that of the other forms. This and other causes acted to place the Placentals in the "Royal line" from which Man was evolved. The following families of Placental Mammals are recognized by Science, each having its own structural peculiarities: The _Edentata_, or Toothless creatures, among which are the sloths, ant-eaters, armadillos, etc. These animals seem to be closer to the Monotremes than they are to the Marsupials; The _Sirenia_, so called by reason of their fanciful resemblance to the sirens of mythology, among which are the sea-cows, manatees, dugongs, etc., which are fish-like in structure and appearance, the fore-limbs being shaped like paddles, or fins, and the hind-limbs being absent or rudimentary; The _Cetacea_, or Whale Family, including whales, Porpoises, dolphins, etc., which are quite fish-like in appearance and structure, their forms being adapted for life in the sea, although they are, of course, Mammals, bringing forth matured young which are suckled at the breast; The _Ungulata_, or Hoofed Animals, which comprise many varied forms, such as the horse, the tapir, the rhinoceros, the swine, the hippopotamus, the camel, the deer, the sheep, the cow, etc., etc.; The _Hyracoidea_, which is a small family, the principal member of which is the coney, or rock rabbit, which has teeth resembling those of the hoofed animals, in some ways, and those of the gnawing animals in the others. The _Proboscidea_, or Trunked Animals, which family is represented in this age only by the families of elephants, which have a peculiar appendage called a "trunk," which they use as an additional limb;