KINE TO GENESIS. 363 



low water, which rapidly freshened, and ultimately 

 were desiccated, and respiration by the swallowing of 

 air into the alimentary canal began to take the place 

 of respiration by gills. It is well known that respira- 

 tion by this means may be carried on by fishes of va- 

 rious genera, e. g. Cobitis ; and Professor Gage has 

 shown that the same habit exists in Batrachia and in 

 certain tortoises (Tronychidae). In the middle Car- 

 boniferous shales tracks of land animals occur, and 

 the bones of Batrachia abound in the coal measures. 

 Already in the Permian these Batrachia are accompa- 

 nied by numerous Reptilia, and air breathers of ter- 

 restrial habits had become numerous on the earth. 



The habit of holding in the oesophagus large quan- 

 tities of air while engaged in seeking food in foul 

 water, or on land, on the part of vertebrates which 

 normally oxygenated the blood by means of gills, was 

 probably the mechanical cause of the development of 

 a pouch, and afterwards of a diverticulum of the 

 oesophagus, which became ultimately a swim-bladder 

 or a lung. In vertebrates in which a return to aquatic 

 life became necessary, it became the former; in those 

 which remained for a shorter or longer period of time 

 on land it became the latter.^ It is noteworthy that 

 among fresh-water fishes generally, the swim-bladder 

 is more complex than among marine forms, showing 

 that the varying conditions of shore and fresh-water 

 life have been mainly responsible for its development. 



The development of a lung at once produced a 

 change in the uses to which the various branchial 

 arches were put. The posterior, which supply the 

 lung, would be subjected to greater pressure owing to 

 the increased blood supply demanded by the lung, 



IThis \aew is adopted by C. Morris, American Naturalist, 1892, p. 975. 



