126 THE EVOLUTION OF MAN. 



for brief intervals that they rise to the surface of the water 

 or creep out of the water on to the land ; and at these times 

 they breathe air through lungs. Some, however, of the 

 Tailed Amphibians, the Axolotl and the Salamander, live 

 exclusively in the water only when young, and afterwards 

 usually remain on land. In the adult state they breathe 

 only air through lungs. This is also the case with the most 

 highly developed Amphibians, the Frog-amphibia (Frogs and 

 Toads) ; some of the latter have even entirely lost the 

 gilled larval form.^^^ The same is true of a few small 

 snake-like Amphibia, the CsecilijE, which, like earth-worms, 

 live in the ground. 



The high degree of interest attached to the natural 

 history of the Amphibian class is especially due to the fact 

 that they hold a position exactly intermediate between the 

 higher and the lower Vertebrates. While the lower Am- 

 phibia are in their whole organization directly allied to the 

 Dipneusta and the Fishes, living mostly in the water and 

 respiring water through gills, the higher Amphibia are no 

 less directly related to the Amnion Animals, for, like the 

 latter, they live mostly on land, and breathe air through 

 lungs. But when young the higher forms resemble the lower, 

 and only attain their own higher degree of development 

 after undergoing complete modification. The individual 

 germ-history of most higher Amphibians still accurately 

 reproduces the tribal history of the whole class ; and the 

 various stages of modification which were necessitated in 

 certain low Vertebrates by the transition from aquatic to 

 terrestrial habits during the Devonian or Carboniferous 

 Period, are still to be seen every spring in each Frog as it 

 develops from the egg in our ditches and pools. 



