22 



If the air-bladder enable the animal to rise and sink, at pleasure, 

 it is clear, that the one •without such organ, and which can yet 

 perform the same office, must either have some compensating 

 power, hitherto undetected, or the air-bladder is not necessary for 

 the purpose to which it is commonly presumed applicable. No 

 scrutiny has been able to discover any equivalent, in those genera 

 which have it not : and it would seem, at least, very anomalous, 

 to be requisite in the progression of one, and not of another, 

 unless the structural difference were far more considerable than 

 it is found to bo, in many instances. Chemical analysis has 

 proved, that the air contained in these bladders, is not atmospheric 

 air ; nor is it universally the same in all fishes — nitrogen being 

 in excess in some species, and oxygen in others ; which would 

 lead to the deduction, that it was not a mere swimming apparatus; 

 for they are filled with an animal fluid ; and I am not aware 

 that in any branch of physiology, a natural secretion is elaborated, 

 except in connexion v ith some vital function; which the simply 

 moving upward, or downward, in the water, cannot be considered. 

 We must, therefore, look for some faculty in those, to whom has 

 been granted this additional organ, which is not held in common 

 with such as are deprived of it : for in all animate nature, where 

 the organism is varied, so I believe, are the demonstrations. 



For such reasons as these, I am induced to conclude, with 

 Dr. Priestley and the ancients, that the air-bladder in fishes, is 

 designed for other purposes, than merely ascending, or descending, 

 through the water. 



The discovery of what these may be, is still to be made. 



