1885.] NATURAL SCIENCES OF PHILADELPHIA. 133 



probably a modern reproduction from the action of similar causes, 

 of the first existing stage of an air-breathing apparatus. And 

 though it is hardly probable that the reproduction is an exact one, 

 yel it may not be very divergent from the original organ. Thus 

 from a simple pouch in the wall of the (esophagus may have 

 arisen, by successive steps, the air-bladder, with its pneumatic 

 duct, its compressing muscle and its plexus of blood capillaries. 

 And this may have unfolded, through further successive steps, 

 several of which vet exist, into a lung like that of Lepidosiren. 

 Thus we seem to possess existing representatives of every impor- 

 tant phase in lung development, from that in which the simple 

 wall of the intestine performed an air-breathing function, to the 

 lung of the batrachian. 



In this view of the cast', the original lung was a simple, smooth- 

 walled bladder, provided with abundant vessels to subserve blood- 

 aeration, with muscles to aid in inhalation and exhalation, and 

 with an air-duct opening into the oesophagus on its dorsal aspect. 

 This dorsal connection may have arisen from the upward pressure 

 of the air in the swimming fish, which would tend to give this 

 position to the original intestinal pouch. But when any fish came 

 to frequently visit the shore two new influences necessarily came 

 into play. The effect of gravity on the growing organ would 

 tend to drag it and its duct from the dorsal to the ventral posi- 

 tion. And the increased use of the bladder in breathing must 

 have required a more extended surface. It first grew cellular, then 

 the cells became laterally-arranged pouches. Finally a constric- 

 tion of the wall separated these Lateral pouches, and two chambers 

 were produced. Of every stage of this process instances still 

 exist, and there is much reason to believe that the development 

 of the lung followed the path here pointed out. 



At the opening of the Carboniferous era there may have been 

 many lung- and gill-breathing Dipnoi, finned Batrachians as we 

 may call them, who spent much of their life on shore. And their 

 habit of land-life would naturally be attended by a gradual change 

 of the fins into better walking organs, from which by a long con- 

 tinued process of evolution, may have arisen the leg and foot of 

 the primordial batrachian. For this purpose to become fully 

 achieved, however, the development of an internal bony skeleton 

 was oecessary, and with the completion of this step of evolu- 

 tion the lung-breathing fish probably directly unfolded into the 



