1892.} The Origin of Lungs. 981 
stances mentioned—the similarity in larval development 
between bladder and lung, the larval existence of the pneu- 
matic duct, the arrangements for compressing and dilating 
the bladder, and the capillary vessels on its inner tunic— 
point to the breathing of air as its original purpose. 
It is probable that the Ganoid, as well as the Dipnoid, 
bladder, is to some extent still used in breathing. The 
Dipnoi have both lungs and gills, and probably breathe with 
the latter in ordinary cases, but use their lungs when the 
inland waters in which they live become thick and muddy, or 
are charged with gases from decomposing organic matter. 
The Ganoid fishes to some extent breathe the air. ln Polyp- 
terus the air-bladder resembles the Dipnoid lung in having 
lateral divisions, and a ventral connection with the esophagus, 
while in Lepidosteus (the American Gar Pike) it is cellular 
and lung-like. This fish keeps near the surface, and may be 
seen to emit air-bubbles, probably taking in a fresh supply of 
air. The American Bow-Fin or Mud Fish (Amia) has a 
bladder of the same lung-like character, and has been seen by 
Wilder to come to the surface, open its jaws widely, and 
apparently swallow a large quantity of air. He considers 
that both Lepidosteus and Amia inhale and exhale air at some- 
what regular intervals, resembling in this the salamanders and 
tadpoles, “which, as the gills shrink and the lungs increase, 
come more frequently to the surface for air.” 
As the facts stand there is no evident line of demarcation 
between the gas-containing bladders of many of the Teleo- 
steans, the air-containing bladders of others and of the Ganoids, 
and the lung of the Dipnoi, and the indications are in favor 
of their having originally had the same function, and of this 
being the breathing of air. 
If now we ask, what were the conditions of life under which 
this organ was developed, and what the later conditions which 
rendered it of no utility as a lung, some definite answer may 
be given. The question takes us back to the Devonian and 
Silurian geological periods, during which the original develop- 
ment of the bladder probably took place. In this era the seas 
were thronged with fishes of two distinct orders, the Elasmo- 
