982 The American Naturalist. [December, 
branchs and the Dipnoi, with the Ganoids as a branch of the 
true fishes. The former were without, the latter with, an air- 
bladder; a difference in organization which was most likely 
due to some marked difference in their life habits. The 
Elasmobranchs were the monarchs of the seas, against whose 
incursions the Ganoids put on a thick protective armor, and 
probably sought the shallow shore waters, while their foes 
held chief possession of the deeper waters without. 
We seem, then, to perceive the Ganoid fishes driven by their 
foes into bays and estuaries and the waters of shallow coasts, | 
ascending streams, and dwelling in inland waters. Heretwo 
influences probably acted on them. The waters they dwelt in a 
were often thick with sediment, and were doubtless in many 
instances poorly aerated, rendering gill-breathing difficult. 
And the land presented conditions likely to serve asa strong 
inducement to fishes to venture on shore. Its plant life was | 
abundant, while its only animal inhabitants seem to have 
been insects, worms, and snails. There can be little doubt 
that the active fish forms of that period, having no enemies to 
fear on the land, and much to gain, made active efforts to 
obtain a share of this vegetable and animal food. Even — 
to-day, when they have numerous foes to fear, many fishes = 
seek food on the shore, and some even climb trees for this @ 
purpose. Under the conditions of the period mentioned there 
was a powerful inducement for them to assume this habit. 
Such conditions must have strongly tended to induce fishes . 
to breathe the air, and have acted to develop an organ for this 
purpose. In addition to the influences of foul or muddy water 
and of visits to land, may be named that of the drying-out of ai 
pools, by which fishes are sometimes left in the moist mud th 
the recurrence of rains, or are even buried in the dried mud a 
during the rainless season. This is the case with the modern 
Dipnoi, which use their lungs under such circumstances. In a 
certain other fresh-water fishes, of the family Ophiocephalide, 
air is breathed while the mud continues soft enough for tne 
fish to come to the surface, but during the dry period the oe 
animal remains in a torpid state. These fishes have no lungs, 
but breathe the air into a simple cavity in the pharynx, whow : 
