1885.] NATURAL SCIENCES OF PHILADELPHIA. 127 



paradise us is without an air-bladder, while all other species of 

 the genus have one. The same condition occurs in related 

 genera. Thus in the species of Sebastes the air-bladder is very 

 large, while in the uexl genus of the family it is scarcely the 

 size of a pea. 



These examples will serve to show the great diversity in the 

 shape, size and condition of this organ. And it may be said 

 here that these variations have no appreciable effect upon the 

 velocit}' and activity of the fish. Those that have no air-bladder 

 seem in no respect at a disadvantage, as compared with those 

 that have one. Again, it ma} r be said that no animal organ 

 whose function is of known importance presents such extraor- 

 dinary modifications. In the heart, lungs, brain, etc., there is 

 one shape, position and condition of greatest efficiency, and 

 throughout the lower forms we find a steady and undeviating 

 advance towards this condition. There is in all these organs a 

 persistent movement towards homogeneit3 r ; not towards hetero- 

 geneity, such as we find in the air-bladder. The natural conclusion 

 from this would be that the air-bladder is not an organ of func- 

 tional importance, while its absence from many fish, and great 

 diversity in others, indicates that it is of minor value to the fish 

 tribe. If it is of absolute necessity to any fish as a gravitating 

 organ, why is it not necessaiy to all, and why has it not 

 developed into some shape and condition of greatest efficiency? 

 The existence of the air-bladder is proof that it has had, at some 

 time, a function of considerable importance ; but its many 

 variations go to prove that it has ceased to perform any 

 essential function, and is on the road towards extinction. On 

 no other theory can we explain its great diversity in nearly 

 related species. 



That the air-bladder is degenerating we have evidence in cases 

 like that above mentioned, where it is no larger than a pea. It 

 is difficult to imagine that this minute organ is of any use to the 

 animal. But no process of evolution can take place, except the 

 organ is of use at every stage of its development. The natural 

 conclusion is that the air-bladder evolved long ago, under some 

 influence not now active, and is now on the road towards extinc- 

 tion, being retained only in those forms where it serves some 

 minor purpose, but being nearly or quite obliterated in forms in 

 which it is put to no practical use. This secondary use of 



