104 The Organs of Respiration 



air-breathing in fishes was originally performed, as it probably 

 often is now, by the unchanged walls of the oesophagus. Then 

 these walls expanded inwardly, forming a simple cavity, partly 

 closed by a fold of membrane, like that of the Ophiocephalidae. 

 A step further reduced this membranous fold to a narrow open- 

 ing, leading to an inner pouch. As the air-breathing function 

 developed, the opening became a tube, and the pouch a simple 

 lung, with compressing muscles and capillary vessels. By a con- 

 tinuation of the process the smooth-walled pouch became saccu- 

 lated, its surface being increased by folding into breathing cells. 

 Finally, a longitudinal constriction divided it into two lateral 

 pouches, such as we find in the lung of the Dipnoans. This 

 brings us to the verge of the lung of the amphibians, which is 

 but a step in advance, and from that the line of progress is un- 

 broken 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 occa- 

 sionally 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 the Crossopterygians and in all 

 the Dipnoans, and its increased use in breathing required a more 

 extended surface. Through this requirement came the pouched 

 and cellular lung of the Dipnoans. Of every stage of the process 

 here outlined examples exist, and there is great reason to be- 

 lieve that the development of the lung followed the path above 

 pointed out. 



" When the carboniferous era opened there may have been 

 many lung- and gill-breathing fishes which spent much of their 

 time on land, and some of which, by a gradual improvement of 

 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 Dipnoans could not com- 

 pete 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 



