Page:Popular Science Monthly Volume 22.djvu/763

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PROGRESS OF THE BACKBONED FAMILY.

throat, which afterward divides into two, forming a pair of lungs, which he uses when out of the water, though still using his gills when below. Little by little the blood-vessels going to the gills grow smaller, and those going to the lungs grow larger; while the fish's two-chambered heart is dividing into three chambers—one to receive the blood from the body, another to receive it from the lungs, and one to drive this blood back again through the whole animal. Now that he can leap and swim with his legs, his tail is no longer of use to him and it is gradually sucked in, growing shorter and shorter, till it disappears. Thus our backboned animal has succeeded in getting out of the water on to the land.

PSM V22 D763 Early reptiles.jpg
Fig. 4.

If we glance back to the far-off time when the ancient fishes were wandering round the shores and in the streams of the coal-forests, we find that the amphibia were not then the small, scattered groups they are now, but huge and powerful creatures, which sported in the water or wandered over the land with sprawling limbs, long tails, and bones on which gills grew, while their heads were covered with hard, bony plates, and their teeth were large, with folds of hard enamel on the surface.