1 68 THE DARWINIAN THEORY 



the dry seasons are apt to become dried up. These 

 animals lie buried in the mud for months, and can 

 live for a long time out of water, owing to the fact 

 that the swimming bladder is used as a lung, a 

 slight change in the circulation causing aerated blood 

 to be returned from it to the heart. 



In Menobranchus, found in North America, both 

 lungs and gills are present throughout life, and it is 

 equally at home in water and on land. The lung 

 is better developed than in the Protopterus and 

 Ceratodus. 



InAmphiuma, found in North American swamps, 

 the lungs are still more perfect. The gills are lost 

 but the gill-slits remain. 



From this we reach the condition met with in the 

 ,newt and frog, which possess gills in the tadpole 

 / stage, but lose them in the adult or lung-breathing 

 state. (See Fig. 29.) 



We have thus a series of animals, all now living, 

 showing the actual transition from the swimming 

 • bladder to the lung, and from the gill-breathing 

 to the lung-breathing condition ; and we further see 

 that the frog actually repeats this history in its own 

 development. 



The explanation of the first commencement of the 

 bat's wing is more difficult, and there is still some 

 uncertainty about it. Let us consider another group 

 of Mammals, the Squirrels, in which the finest gra- 

 dation is known from animals with their tails slightly 

 flattened, the hind parts of their bodies wide, and the 

 skin of the flanks full, to the "flying squirrels," in 

 which the limbs and even the base of the tail are 



