IO2 The Organs of Respiration 



considers that both Lepisosteus and Amia inhale and exhale air 

 at somewhat regular intervals, resembling in this the salaman- 

 ders and tadpoles, 'which, as the gills shrink and the lungs in- 

 crease, 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 Teleosteans, 

 the air-containing bladders of the others and the Ganoids, and 

 the lung of the Dipnoans, 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 development of the 

 bladder probably took place. In this era the seas were thronged 

 with fishes of several classes, the Elasmobranchs among others, 

 followed by the Dipnoi and Crossopterygians. The sharks were 

 without, the Dipnoans and Crossopterygians doubtless with, an 

 air-bladder a difference in organization w r hich was most likely 

 due to some marked difference in their life-habits. The Elasmo- 

 branchs were the monarchs of the seas, against whose incursions 

 the others 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 lung-bearing fishes, driven by 

 their foes into bays and estuaries, and the waters of shallow 

 coasts, ascending streams and dwelling in inland waters. Here 

 two influences probably acted on them. The waters they dwelt 

 in 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 as a strong induce- 

 ment 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 nu- 

 merous foes to fear, many fishes seek food on the shore, and 



