That the amphibian has been evolved from the fish may be seen in the evolution of the frog (Fig. 15). Number 1 shows the newly-hatched tadpoles; 2 and 2a show the branching, external gills; 3 to 8 illustrate further steps in the evolutionary process. The fish-like tail, so prominent in the early stages, is finally absorbed and we have the finished frog. In the evolution of the frog we have a most suggestive illustration of the transformation of a creature during a single lifetime. The fish becomes an amphibian; the gilled, water-breathing creature becomes a lunged, air-breather; a water animal leaves its habitat for a home on land; a vegetable diet is abandoned for one of flesh. Truly a striking summary instance of the power of evolution! [Illustration: FIG. 15.—THE EVOLUTION OF THE FROG.] All that man has become, all the wealth and worth of the civilization he has achieved, has been due to the fact that he has possessed a hand which could obey the command of his brain. Without a hand, without fingers, man would still be a wild beast of the forest. It was in the amphibian that Nature first produced the five divisions of the foot, which, inherited by the reptiles and then by the mammals, in the end became specialized into the human hand. The beginning of the hand is seen in the foot of the frog. In the Carboniferous period, when the coal measures were laid down, appeared the wedge-headed amphibian, shown in Fig. 16 (above) and later, in the Permian period, the roof-headed amphibian (Fig. 16, below) was born into the world. This roof-headed amphibian is all the more interesting, for from some of these creatures were born the reptiles, from which, in turn, arose the mammals. From the amphibians were developed the true reptiles, and these branched out into many forms. Some lived in the water, some roamed on the land, some flew in the air. In a warm climate, and where food abounded, some of these creatures, like the Ceratosaurus (Fig. 17), the Atlantosaurus and the Diplodocus, grew to a prodigious size. Some were fifty, some a hundred, some a hundred and fifty feet long; some had a hundred teeth, and eyes fifteen inches across; some weighed ninety tons, and made footprints a yard square. It was in the Mesozoic times, millions of ages ago, when these ungainly monsters were the monarchs of the earth. Happily, they have long since been extinct, and to-day their colossal, though harmless, skeletons may be studied in the museums of the world. [Illustration: FIG. 16.—THE HEAD OF THE FAMOUS ARCHAEGOSAURUS. The wedge-headed Amphibian (above); the Branchiosaurus—the roof-headed Amphibian (below).]