From the reptiles came the birds. The first birds had teeth, claws on their wings, and bony tails of many joints (Fig. 18). The fossil remains of two of these reptile-birds—the earliest birds known—were found, some years ago, in the Jurassic limestone strata of Bavaria. These creatures had thirty-two teeth, three clawed fingers on each wing, and a lizard-like tail of twenty joints, with two long feathers growing out of each vertebra. Occupying the ground midway between the reptile and the bird, having the characteristics of both—the link between the four-legged animal and the feathered songster of the air—the Archaeopteryx, as this ancient bird is called, was about the size of a crow. Another line of development led from the reptiles to the mammals—the hair-clothed creatures that suckle their young. This was the most promising line of Nature’s advance, for at the end of this line, man was destined to appear. [Illustration: FIG. 17.—THE CERATOSAURUS. A giant reptile of the Jurassic Period.] An amazingly curious link, which connects the reptile with the bird on the one hand and with the mammal on the other, is the Duck-bill (Fig. 19). This creature, whose home is in Australia, is covered with dense fur and suckles its young, like a mammal; but, on the other hand, it lays eggs like the reptile and the bird. The eggs have large yolks, like those of birds; are hatched by the warmth of the mother’s body; and when the young is born, it lives on milk drawn from its mother’s breast. Observe, too, that the mother Duck-bill has no nipples, but mere depressions in the breast, from which the milk oozes out among the fur, to be sucked up by the young. Think of a fur-covered, five-toed, web-footed, duck-billed, flesh-eating, swimming animal, housing itself in a burrow in the bank of a stream, being born from an egg, like a bird; formed in part like a reptile, and deriving its early sustenance by sucking the milk-ooze from its mother’s teatless breast! This link between reptile, bird and mammal, this crude combination of three forms of life, shows finished forms in the making. It is the living proof of the manner in which Nature has accomplished her work—of the steps by which evolution has advanced. It is what Darwin called “a living fossil.”