1892.] The Origin of Lungs. 985 
the Teleostean or bony fishes, as it doubtless was in the loss 
of its primitive function by, and the subsequent changes of, 
the air-bladder. 
Such of the Ganoids and Dipnoi as survived in their old 
condition had to contend with adverse circumstances. Most 
of them in time vanished, while the Ganoids which still exist 
have lost in great measure their air-breathing powers, and the 
Dipnoi, in which the development of the lung had gone too 
far for reversal, have degenerated into eel-like, mud-haunting 
creatures, in which the organs of locomotion, which perhaps 
once served them efficiently for land travel, have become con- 
verted into the feeble paddle-like limb of Ceratodus and the 
filamentary appendages of the other species. 
As regards the presence of a large quantity of oxygen in 
the bladders of deep-swimming marine fishes, it not unlikely 
has a respiratory purpose, the bladder being, as suggested by 
Semper, used as a reservoir for oxygen, to serve the fish when 
sleeping, or when, from any cause, not actively breathing. 
The excess of oxygen is not due to any like excess in the 
gaseous contents of sea-water, for the percentage of oxygen 
decreases from the surface downward, while that of nitrogen 
-` remains nearly unchanged. In all cases, indeed, the bladder 
may preserve a share of its old function, and act as an aid in 
respiration. Speaking of this, Cuvier says: “ With regard to 
the presumed assistance which the swim-bladder affords in 
respiration, it is a fact that, when a fish is deprived of that 
organ, the production of carbonic acid by the branchie is very 
trifling,” thus strongly indicating that the bladder still plays 
a part in the oxygenation of the blood. 
Under the hypothesis here presented, the process of evolu- 
tion involved may be thus summed‘up. Air-breathing in 
fishes was originally performed by the unchanged walls of the 
cesophagus, perhaps at specially vascular localities. Then the 
wall folded inward, and a pouch was finally formed, opening 
to the air. The pouch next became constricted off, with a 
duct of connection. Then the pouch became an air-bladder 
with respiratory function, and finally developed into a simple 
lung. These air-breathing fishes haunted the shores, their fins 
