984 The American Naturalist. [December, 
advance, and from that the line of progress is unbroken to 
the more intricate lung of the higher land animals. 
The dorsal position of the bladder and its duct would be 
a difficulty in this inquiry, but for the fact that the duct is 
occasionally ventral. This dorsal position may have arisen 
from the upward pressure of air in the swimming fish, which 
would tend to lift the original pouch. But in the case of fishes 
which made frequent visits to the shore, new influences must 
have come into play. The effect of gravity tended to draw 
the organ and its duct downward, as we find in one family of 
the Ganoids and in all the Dipnoi; and its increased use in 
breathing required a more extended surface. ‘Through this 
requirement came the pouched and cellular lung of the 
Dipnoi. Of every stage of the process here outlined, examples 
exist, and there is great reason to believe that the develop- 
ment of the lung followed the path above pointed out. 
When the carboniferous era opened there may have been 
many lung- and gill-breathing Dipnoi, which spent much of 
their time on land, and some of which, by a gradual improve- 
ment in their organs of locomotion, changed into batrachians. 
But with the appearance of the latter, and of their successors, 
the reptiles, the relations of the fish to the land radically 
changed. The fin, or the simple locomotor organ of the Dipnoi, 
could not compete with the leg and foot as organs of land 
locomotion, and the fish tribe ceased to be lords of the land, 
where instead of feeble prey they now found powerful foes, 
and were driven back to their native habitat, the water. Nor 
did the change end here. In time the waters were invaded 
by the reptiles, numerous swimming forms appearing, which 
it is likely were abundant in the shallower shore line of 
ocean, while they sent many representatives far out to sèa. 
These were actively carnivorous, making the fish their prey, 
the great mass of whom were doubtless driven into the deeper 
waters, beyond the reach of their air-breathing foes. 
In this change of conditions we seem to perceive an adequate - 
cause for the loss of air-breathing habits in those fishes 1- 
which the lung development had not far progressed. It may, 
indeed, have been a leading influence in the development of 
