THE GENEALOGY OF ANIMALS 83 



to rise and fall in the water, is probably the 

 degenerated lung of the lung fishes. 



From the lung fishes or allied forms developed 

 the amphibians, the well-known fish quadrupeds 

 of our bogs and brooks. The amphibians are 

 genuine connectives — living links between the life 

 of the sea and the life of the land. In early life 

 they are fishes, with gills and two-chambered 

 hearts. In later life they are air-breathing quad- 

 rupeds, with legs and lungs and three-chambered 

 hearts. Here is evolution, plenty of it, and of the 

 most tangible character. And it takes place right 

 before the eyes. The transformation from the fish 

 to the frog is, however, no more wonderful than 

 the embryonic transformations of other vertebrates. 

 It is simply more apparent, because it can be 

 seen. The lungs of amphibians and the lower 

 reptiles are simple sacks opening by a very short 

 passage into the mouth. Some amphibians, as the 

 axolotl of Mexican lakes, ordinarily retain their 

 gills through life, but may be induced to develop 

 lungs and adapt themselves to terrestrial life by 

 being kept out of the water. Others, as the newts, 

 which ordinarily develop lungs, may be compelled 

 to retain their gills through life by being forced to 

 remain uninterruptedly in the water. The black 

 salamander, inhabiting droughty regions of the 

 Alps, brings forth its young bearing lungs, and 

 only a pair at a time. But if the young are pre- 

 maturely removed from the body of the mother 

 and placed in the water, they develop gills in the 

 ordinary way. These are remarkable instances oi 



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