The Organs of Respiration 99 



organ, there is abundant reason to believe that it had nothing 

 to do with swimming. Certainly the great family of the sharks, 

 which have no bladder, are at no disadvantage in changing their 

 depth or position in the water. Yet if the bladder is necessary 

 to any fish as an aid in swimming, why not to all? And if this 

 were its primary purpose, how shall we explain its remarkable 

 variability? No animal organ with a function of essential im- 

 portance presents such extraordinary modifications in related 

 species and genera. In the heart, brain, and other organs there 

 is one shape, position, and condition of greatest efficiency, and 

 throughout the lower forms we find a steady advance towards 

 this condition. Great variation, on the other hand, usually 

 indicates that the organ is of little functional importance, or 

 that it has lost its original function. Such we conceive to be 

 the case with the air-bladder. The fact of its absence from some 

 and its presence in other fishes of closely related species goes 

 far to prove that it is a degenerating organ; and the same is 

 shewn by the fact that it is useless in some species for the pur- 

 pose to which it is applied in others. That it had, at some time 

 in the past, a function of essential importance there can be no 

 question. That it exists at all is proof of this. But its modern 

 variations strongly indicate that it has lost this function and 

 is on the road towards extinction. Larval conditions show that 

 it had originally a pneumatic duct as one of its essential parts, 

 but this has in most cases disappeared. The bladder itself 

 has in many cases partly or wholly disappeared. Where pre- 

 served, it seems to be through its utility for some secondary 

 purpose, such as an aid in swimming or in hearing. That its 

 evolution began very long ago there can be no question; and 

 the indications are that it began long ago to degenerate, through 

 the loss of its primitive function. 



" What was this primitive function ? In attempting to answer 

 this question we must first consider the air-bladder in relation 

 to the fish tribe as a whole. No shark or ray possesses the air- 

 bladder. In some few sharks, indeed, there is a diverticulum 

 of the pharynx which may be a rudimentary approach to the 

 air-bladder ; but this is very questionable. The conditions of its 

 occurrence in the main body of modern fishes, the Teleostean, we 

 have already considered. But in the most ancient living orders 



