Next above the Bird family comes the highest form of all—the Mammals. But before we begin our consideration of these high forms, let us take a hasty glance at the "connecting-links" between the Birds and the Mammals. The lowest forms of the Mammals resemble Birds in many ways. Some of them are toothless, and many of them have the same primitive intestinal arrangements possessed by the birds, from which arises their name, Monotremes. These Monotremes may be called half-bird and half-mammal. One of the most characteristic of their family is the Ornithorhynchus, or Duck-bill, which the early naturalists first thought was a fraud of the taxidermists, or bird-stuffers, and then, when finally convinced, deemed it a "freak-of-nature." But it is not a freak creature, but a "connecting-link" between the two great families of creatures. This animal presents a startling appearance to the observer who witnesses it for the first time. It resembles a beaver, having a soft furry coat, but also has a horny, flat bill like a duck, its feet being webbed, but also furnished with claws projecting over the edge of the web-foot. It lays eggs in an underground nest—two eggs at a time, which are like the eggs of birds, inasmuch as they contain not only the protoplasm from which the embryo is formed, but also the "yolk." on which the embryo feeds until hatched. After the young Duck-bill is hatched, it feeds from teatless glands in the mother's body, the milk being furnished by the mother by a peculiar process. Consider this miracle—an animal which lays eggs and then when her young are hatched nourishes them with milk. The milk-glands in the mother are elementary "breasts."
The above-mentioned animal is found in Australia, the land of many strange forms and "connecting-links," which have survived there while in other parts of the globe they have vanished gradually from existence, crowded out by the more perfectly evolved forms. Darwin has called these surviving forms "living fossils." In that same land is also found the Echidna or spiny ant-eater, which lays an egg and then hatches it in her pouch, after which she nourishes it on milk, in a manner similar to that of the Duck-bill. This animal, like the Duck-bill, is a Monotreme.
Scientists are divided in theories as to whether the Monotremes are actually descended directly from the Reptiles or Birds, or whether there was a common ancestor from which Reptiles and Birds and Mammals branched off. But this is not important, for the relationship between Reptiles, Birds and Mammals is clearly proven. And the Monotremes are certainly one of the surviving forms of the intermediate stages.
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.;