Ii8 CHAPTERS ON EVOLUTION. 



true history of the air-bladder. Equally ancient with the sharks and 

 their allies, the ganoids, from their habits and ways of life, became 

 provided with an air-bladder, which, as time passed, became still 

 better specialised through the effects of use aided by " natural 

 selection " as the propagating principles of a structure useful and 

 advantageous to the race. As offshoots from a more ancient type 

 of fishes, the first representatives of our common fishes probably 

 developed an air bladder, which once again, owing probably to 

 variations in habit, has become well developed in some (such as the 

 carps, herrings, perch, and the like), but obliterated in others 

 (;uch as the flounders and their neighbours), most probably from 

 disuse. 



The ganoid race has declined in numbers since the days of 

 Devonian oceans, but its living members represent within their select 

 circle the stages in the modification of the swim-bladder. In the 

 sturgeons, the type of the organ is of primitive kind. In the 

 Polypteri (Fig. 53, a) the air-bladder has become double ; but in the 



FIG. 55. DEVELOPMENT OF FROG. 

 A, B, and C, Tadpoles ; D, Head of Tadpole (magnified). 



bony pike (Lepidosteus) it is not merely double, but exhibits a cellular 

 or lung-like structure internally; and it is equally lung-like in Amia, 

 another well-known type of ganoid fishes. Still more lung-like does 

 the organ become in the Ceratodtis or Barramunda (Fig. 50), where it 

 is placed in relation with the blood-system. When, however, we reach 

 the mud-fishes or Lepidosirens (Fig. 54), we pass the definite boundary 

 which separates the swim-bladder from the lung, and discover an 

 organ, not merely lung-like in structure, but which performs all the 



