After the Duck-bills came the marsupial mammals—mammals whose young, born not yet fully developed, are carried for a time in a pouch attached to the body of the mother. The kangaroo (Fig. 20) belongs to this class. Here the advance is from an egg-laying mammal to one whose young is partly formed in the body of the mother. I say partly formed, for, although the kangaroo is as large as a man, its young, when born, though it is no larger than the little finger, is still a fœtus, so imperfectly formed that it must be carried for months in the mother’s pouch, so that it may complete its development as a babe. Meanwhile, being unable to feed itself, the mother, by an exercise of her muscles, forces milk down its throat. Once more Nature, in her forward march, is blazing a new trail. Life, by employing crude makeshifts and adaptations, is fashioning for itself a higher mould. [Illustration: FIG. 18.—THE ARCHAEOPTERYX. The lower picture shows the joints of the tail with the tail feathers.] The marsupial mammals were followed by the placental mammals, animals whose young are nourished before birth by a disc-like organ, called the placenta—the after-birth. The Pariasaurus Baini (Fig. 21) shows Nature, the apprentice, trying to make a quadruped. I say trying, for see what a crude, raw specimen this monster was. The best thing that can be said of this fellow is that he had his day of fighting for a place in the sun and was then supplanted by higher creatures. Many ages of progress, during which life assumed a rich variety of forms, including the early stages of most of the hoofed animals, brought the process of evolution to the lemurs, the monkey-like creatures that make their home in trees (Fig. 22). The lemurs differ from monkeys in that the milk glands of the female are on the abdomen instead of the breast, while the index finger of each hand, and the second toe of each foot, are furnished with claws, all the other fingers and toes having flat nails. Here again is a link between the lower and the higher life. Nature is stumbling and bungling, but getting there. [Illustration: FIG. 19.—THE DUCK-BILL, KNOWN IN AUSTRALIA AS THE PLATYPUS.] After the lemurs came the Slow Loris (Fig. 23). This species of the Loris has no tail, and its front foot bears a strong resemblance to the human hand. From these creatures, or possibly from similar creatures, were evolved the true apes (Fig. 24), and from these came the anthropoid or man-like apes.