1894.] 



THE BRITISH NATURALIST. 



67 



leave no difficulties in the way of even the most doubting disciple of the 

 doctrine of descent. 



As previously stated, the lungs arise as a growth from the ventral wall 

 of the oesophagus ; the primitive lung-sac communicates with the 

 oesophagus directly ; but in the higher forms it is ?< as it were, pushed 

 back, and the communication takes place through a tube, the trachea or 

 wind pipe, of the higher animals. The condition of a simple sac 

 communicating almost directly (that is, there is no true wind pipe) with 

 the oesophagus occurs in the lowest existing Dipnoan, the Australian 

 mud fish, Ceratodus ; though even in Ceratodus, the lining membrane 

 forms bands and net-works, giving a greater aerating surface than if it 

 was quite smooth, and thus showing that even in this mud fish the lung is 

 of too high an order to be regarded as similar to the first lung that 

 was ever developed. In the other two existing species of this remarkable 

 group of fishes, the lungs are distinctly paired posteriorly. 



In the batrachian group, there are always two lungs ; that is, the 

 process of sub-division in the primitive lung-sac has been carried further 

 in all the batrachians than in any of the Dipnoi, although in some 

 (Menobranchus and Proteus) the inner surface is distinctly smooth, not 

 forming bands and net-works as in the Dipnoi, and thus presenting 

 almost certain evidence of retrogression. As a rule, in batrachians, the 

 two lungs are equal in size ; but in Amphiuma, Siren, and the Caecilians, 

 whose elongated bodies present a very snake-like appearance, the right 

 lung is usually much longer than the left ; in fact, in some Caecilians, the 

 left lung is almost atrophied. A similar variation occurs in snakes, and 

 may be regarded as an adaptation of the respiratory organs to the 

 elongated form of the body. Snakes — as is now generally admitted — 

 are degenerate lizards ; in other words, a snake is a lizard which has 

 lost its limbs, and this retrogression is not confined to the skeleton, but 

 affects the lungs also, the left lung being in most snakes quite 

 rudimentary, and occasionally even absent. Speaking generally, the 

 lungs of lizards are not so highly developed as is the case in other 

 reptilian groups. Anyone who remembers that in the Crocodilia the 

 heart is more complex than in any other reptiles will not be surprised 

 that the lungs exhibit a correspondingly higher organization ; but it is 

 remarkable that in most chelonians the lungs are as highly specialized 

 as in crocodiles. 



The general direction of specialization of the lungs in reptiles is 

 indicated, if we regard the lizard's lung as typical of that of the earliest 

 true reptiles ; the wind pipe leads into two bronchi, which open directly, 

 one into each lung ; the walls of the lungs are produced internally into 

 numerous septa, which increase the aerating surface ; these septa are 

 most numerous near the bronchial opening ; at the posterior extremity of 

 the lung the surface is smooth and but little vascular ; the ophidian 



