978 The American Naturalist. [December, 
open. In the Physostome order it is longer, and in many 
instances is closed, it being occasionally reduced to a fine fila- 
ment. In the other orders of Teleostean fishes, which embrace 
the great majority of species, the pneumatic duct does not 
exist. Whatever function this duct may have once performed, 
therefore, it seems as a rule to have lost its utility. That its 
function was an essential one in the early stage of fish life is 
rendered evident by the fact that all fishes which have a 
bladder at all possess a pneumatic duct in the larval stage of 
growth, this duct, in most cases, vanishing as they grow older. 
If now we seek to discover the original purpose of this 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 importance 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 a 
importance, or that it has lost its original function. Such we — a 
conceive to be the case with the air-bladder. The fact of its a 
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 shown by the fact that it is useless M i 
some species for the purpose 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 a 
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 GS" 
disappeared. The bladder itself has in many cases partly oF 
wholly disappeared. Where preserved, it seems to be through 
