In the early forms of life it is difficult to distinguish between the animal and the plant forms, in fact the early forms partake of the qualities of both. But as we advance in the scale a little there is seen a decided "branching out," and one large branch is formed of the evolving plant forms, and the other of the evolving animal forms. The plant-branch begins with the sea-weeds, and passes on to the fungi, lichens, mosses, ferns, pines and palm-ferns, grasses, etc., then to the trees, shrubs and herbs. The animal-branch begins with the _monera_, or single-cell forms, which are little more than a drop of sticky, glue-like protoplasm. Then it passes on to the _amoebae_, which begins to show a slight difference in its parts. Then on the _foraminifera_, which secretes a shell of lime from the water. Then on a step higher to the _polycystina_, which secretes a shell, or skeleton of flint-like material from the water. Then come the sponges. Then the coral-animals, anemones and jelly-fish. Then come the sea-lilies, star-fish, etc. Then the various families of worms. Then the crabs, spiders, centipedes, insects. Then come the mollusca, which include the oysters, clams and other shell-fish; snails, cuttle-fish, sea-squirts, etc. All of the above families of animal-forms are what are known as "invertebrates," that is, without a backbone. Then we come to the "vertebrates," or animals having a backbone. First we see the fish family with its thousands of forms. Then come the amphibia, which include the toads, frogs, etc. Then come the reptiles, which include the serpents, lizards, crocodiles, turtles, etc. Then come the great family of birds, with its wonderful variety of forms, sizes, and characteristics. Then come the mammals, the name of which comes from the Latin word meaning "the breast," the characteristic of which group comes from the fact that they nourish their young by milk, or similar fluid, secreted by the mother. The mammals are the highest form of the vertebrates. First among the mammals we find the aplacentals, or those which bring forth immature young, which are grouped into two divisions, _i.e_., (1) the _monotremes_, or one-vented animals, in which group belong the duck-bills, spiny ant-eaters, etc.; and (2) the _marsupials_, or pouched animals, in which group belong the kangaroo, opossum, etc.