1885.] NATURAL SCIENCES OF PHILADELPHIA. 135 



than is required for immediate use. And the small excess gained 

 may be secreted into the air-bladder as a reservoir, to be taken 

 up again by the blood during inactivity of the breathing function. 

 This seems probable from what Cuvier tells us, that when a fish 

 is deprived of the swim-bladder, the product of carbonic acid by 

 the branchiae is very trifling. We cannot imagine such a result 

 unless the bladder in some way supplies oxygen to the blood. If 

 this be the case, the air-bladder still performs, in an indirect 

 manner, its probable original function of a breathing organ. 



If the 113-pothesis here offered be a well-founded one, an inter- 

 esting conclusion as to the process of organic evolution involved 

 may be taken. For we would have the air-breathing function at 

 first performed by the unchanged walls of the oesophagus. Then 

 this became pouched. Then the pouch became constricted off, 

 with a duct of connection. Then the duct disappeared, as the 

 original function vanished, and what was originally a portion of 

 the wall of the intestinal canal, became a separate internal sac. 

 Then this sac decreased in size, until in some instances it became 

 a closed internal bladder, of the size of a pea, far removed from 

 and utterly disconnected with its place of origin. Finally it 

 completely vanished. This process, if correctly drawn, certainly 

 forms a very remarkable organic cycle of development and 

 degeneration, which probably has no counterpart of a similarly 

 striking character in the whole circle of organic life. 



