980 The American Naturalist. [December, 
lung is a secondary adaptation, a side issue from its original 
purpose. To this I venture to oppose the theory (which I 
have already offered in the “ Proceedings” of the Academy of 3 
Natural Sciences of Philadelphia) that it is the original pur 
pose, and that its degeneration is due to the disappearance Ce 
the necessity of such a function. As regards the gravitative — 
employment of the bladder, the Teleostean fishes, to which 
this function is confined, are of comparatively modern origin; : 
while the Dipnoi are surviving representatives of a very — 
ancient order of fishes, which flourished in the Devonian age — 
of geology, and in all probability breathed air then as now; — 
and the Ganoids, which approach them in this particular, are 
similarly ancient in origin, and were the ancestors of the — 
Teleosteans. The natural presumption, therefore, is that the 
duty which it subserved in the most ancient fishes was its 
primitive function. - 
The facts of embryology lend strong support to this hypo 
thesis. For the air-bladder is found to arise in a manner very 
similar to the development of the lung. They each begin a8 — 
an outgrowth from the fore-part of the alimentary tract, the only a 
difference being that the air-bladder usually rises dorsally and 5 
the lung ventrally. The fact already cited, that the pneu: 
matic duct is always present in the larval form, in fishes tha 
possess a bladder, is equally significant. All the facts i! 
show that the introduction of external air into the body was 2 
former function of the air-bladder, and that the atrophy of the 
duct in many cases, and the disappearance of the bladder 10 
others, are results of the loss of this function. - 
Such an elaborate arrangement for the introduction of at 
into the body could have, if we may judge from analogy; © 
one purpose, that of breathing, to which purpose the muscular 
and other apparatus for compressing and dilating the bladder, 
now seemingly adapted to gravitative uses, may nave at 
originally applied. The same may be said of the gr 
development of blood capillaries in the inner tune © 
bladder. These may now be used only for the secretion 
gas into its interior, but were perhaps originally employ 
the respiratory secretion of oxygen. In fact, all the © 
