260 FISHES 



powerful swimmers, both feeding on other fishes, both ranging 

 through the surface waters of the open ocean. We cannot 

 perceive that the bony fish was derived from the shark-like fish 

 by adaptation to a different mode of life in the sea, as the 

 Amphibian was derived from the fish by adaptation to a life 

 on dry land in its adult condition. What then is the explana- 

 tion offered by the doctrine of evolution for the existence of 

 these two types ? 



When we turn to the study of fossil fishes, we find that in 

 the most ancient stratified rocks both types were represented, 

 but the fossil forms of the bony type, belonged exclusively to 

 certain groups of which at the present time very few species 

 exist, and these only in fresh water, namely the Fringe-finned 

 Ganoids, represented now by the African Polypterus, and the 

 Lung-fishes which occur now in tropical rivers or swamps. 

 Both these kinds of fishes have lungs, or open air-bladders 

 actually used for breathing air. The Lung-fishes evidently re- 

 semble the ancestors of the Amphibia, while from the Fringe- 

 finned Ganoids we have a series of diverging forms leading to 

 the great variety of existing bony fishes. The conclusion is 

 that the bony fishes of the sea at the present day are descended 

 from fresh-water fishes of ancient times, which were adapted to 

 breathe air in order to supplement the original respiration by 

 gills. In this way we can understand the origin of the air- 

 bladder, which is wanting in the shark type. The air- or gas- 

 bladder is evolved from lungs, not lungs from air-bladder. At 

 the same time the skeletal structures both within the body and 

 in the skin underwent a change to the bony type with the 

 change of habitat from the sea to fresh water. From the 

 Fringe-finned Ganoids descended numerous diverging forms 

 which populated the various fresh waters of the globe, and ulti- 

 mately reached the sea again and established themselves there 

 along with the fishes of the shark type. The first important 

 change in the transition from the fringe-finned type to the 

 modern fishes was the reduction of the internal skeleton of the 

 paired fins, so that the basal fleshy lobe disappeared and the fin 

 became fan-like. It is difficult to say with certainty, which of 

 the extinct fan-finned Ganoids were first to reach the sea: it is 

 probable enough that many of the groups lived along the sea- 

 coast, but we know that, excepting the Sturgeons, the only sur- 



