CONSIDERED AS A DEGENERATE LUNG. 201 



may call them, which spent much of their life on shore. And 

 their habits of land life would naturally be attended by a gradual 

 change of the fins into better walking organs, from which by a long 

 continued process of evolution, may have arisen the leg and foot 

 of the primordial batrachian. For this purpose, to become fully 

 developed, however, the development of an internal bony skeleton 

 was necessary, and with the completion of this step of evolution 

 the lung-breathing fish probably directly unfolded into the true 

 amphibian. Were we left to imagine what such a fish would be 

 like, could we draw any form which would differ much from the 

 existing Perennibranchiates ? 



But with the existence on land of vertebrates with bony 

 skeletons, the dominion of the fish must have gradually decreased. 

 The formidable reptiles which for a time had the mastery of the 

 world, would make terra firma less and less of a desirable resort. 

 The fin, too, could not compete with the leg and foot as an organ of 

 land motion, and those Dipnoi which had remained mainly fish-like 

 were probably driven back to the water. As a result of this 

 change of conditions, a retrogressive change took place in the air- 

 breathing organ. Some fishes never cease to use it as a lung, as 

 modern Dipnoi. Yet, with the Ganoids, as a rule, it has been used 

 only temporarily for breathing purposes. But with their successors, 

 the Teleosteans, it has lost all air-breathing capabilities, and has 

 passed through every stage of degeneration, from a condition 

 closely resembling that of the Ganoids to complete extinction. 

 This theory is not vitiated by the fact that the organ has developed 

 secondary uses. Innumerable instances exist of secondary adapta- 

 tions of organs which embryology shows had once far different uses, 

 as in the transformations which the gill arches have undergone 

 in the higher vertebrates, and the still more extraordinary modifi- 

 cations of functions in the primitive kidneys. One remarkable 

 fact may be mentioned as showing that the air-bladder has 

 an important function in the aeration of the blood in deep-sea 

 fishes. In fresh-water forms the organ contains nearly pure 

 nitrogen, whilst in deep-swimming sea-fish oxygen forms its main 

 contents (Dr. Gunther states that the proportion is sometimes as 

 high as 87 per cent, of oxygen). This difference is not due to any 

 differences in the gases contained in water at various depths, for 



