LIFE IN THE PAST 177 



esting. The swim-bladder which we have just de- 

 scribed in other fishes is, with this lungfish, peculiarly 

 spongy in its walls, presenting a large surface full of 

 blood vessels which absorb the air on the inside of 

 the bladder. This air the fish changes with moderate 

 frequency, the result being that the swim-bladder 

 serves him exactly as the lung serves a higher animal. 

 To this fact he owes his name of lungfish. 



We sometimes gain much light concerning the past 

 history of any particular form of animal by studying 

 the development of that animal in the egg, or, in the 

 case of the mammals, before birth. It is an interest- 

 ing fact that when the lung begins to form in the 

 embryo it starts as a simple sac which is an offspring 

 from the gullet, and occupies the position of the swim- 

 bladder of the fish. This sac later divides into two, 

 and develops into the lungs of the animal. This 

 assures the zoologist that the origin of the lungs in 

 the higher animals is found in the swim bladder of 

 the so-called lungfish. In this Silurian time certain 

 of these lungfish were perhaps trapped in the basin in 

 the marsh by the uplifting of the border. The waters 

 becoming progressively shallower and more crowded, 

 these fishes took to the land, their fins developing into 

 awkward limbs which slowly became more perfect. 



To state the fact in this simple fashion is to make 

 it seem far less probable than is really the case. The 



