202 THE AIR-BLADDER OF FISHES. 



the percentage of nitrogen is closely the same at all depths, while 

 oxygen diminishes in quantity from the surface downwards. Thus, 

 if the contents of the air-bladder depended on the relative 

 quantity of gases present, nitrogen should predominate below as 

 well as above. Furthermore, as showing that the bladder still 

 functions as an aerating organ, Cuvier says that when a fish is 

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

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

 air-bladder in some way supplies oxygen to the blood, and thus 

 continues to perform, in an indirect manner, its probable original 

 function of a breathing organ. 



If the hypothesis offered be a well-founded one, an interesting 

 conclusion as to the process of organic evolution may be taken. 

 We should see 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, as the original function vanished, the duct disappeared, and 

 what was originally a portion of the wall of the intestinal canal 

 became a separate internal sac. Then this sac in many instances 

 decreased in size, until in some cases 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 cycle of development and degeneration, which probably 

 has no counterpart (Mr. Morris thinks) of a similarly striking 

 character in the whole circle of organic life. 



Finally, it may be remarked that though many fish use their 

 air-bladder in all probability for gravitative purposes, yet, as many 

 most active fishes have no air-bladder, this organ could not by 

 any known law of evolution have been elaborated for a purpose 

 which actively-swimming fish can so easily dispense with. 



I will only say, in conclusion, that this paper consists of such 

 copious extracts from Mr. Morris, that I have thought it un- 

 necessary to use inverted commas when quoting from his article. 

 Had I done so, my paper would have been, as the old lady found 

 '' Macbeth," too " full of quotations." 



