PROGRESS OF THE BACKBONED FAMILY. 743 



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 hin^ 

 and it is gradually sucked in, growing shorter and shorter, till it dis- 

 appears. Thus our backboned animal has succeeded in getting out of 

 the water on to the land. 



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Pig. 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. 



