418 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



however, it functions as a part of the auditory mechanism; in others it 

 serves apparently as a closed reservoir for the storage of oxygen secreted 

 from the blood. In certain large groups of fishes it is wholly absent; in 

 others it is completely disconnected, except in the larval stage, with the 

 oesophagus. It lies on the dorsal side of the intestinal canal. Only in a 

 few forms is it clearly an accessory organ of respiration. Many zoolo- 

 gists have regarded its original purpose as not respiratory. Some — as, 

 for example, J. A. Thomson — have stated that there is no demonstrable 

 homology between lung and air-bladder. 27 



That the air-bladder was, however, originally an accessory breathing 

 organ which has now largely lost its use seems conclusive when the evi- 

 dence of paleontology is added to that of zoology. The argument is ad- 

 mirably stated hj Charles Morris 28 and is adopted by Jordan. 29 The air- 

 bladder is not an organ essential for swimming or equilibration, since it 

 is completely absent in the selachians. It is also variable within the same 

 genus, the mackerel having none, yet one exists in Scomber pneuma- 

 tophorus, a species which in every other respect closely resembles the 

 mackerel. It seems, therefore, to be an organ which, if originally neces- 

 sary, has become now unnecessary for the original purpose, and has been 

 retained through its various uses in secondary functions. 



The nature of the original use of the air-bladder is indicated by two 

 lines of evidence : First, in those fish which possess an air-bladder there 

 is a pneumatic duct during the larval stage which connects the air-bladder 

 with the oesophagus. This duct in most cases disappears as the fish de- 

 velops. Second, the ganoids are the ancestors of the modern bony fishes, 

 and in every living ganoid the air-bladder not only connects with the 

 oesophagus, but acts as a supplemental respiratory organ. This attains 

 its maximum utility in the dipnoans, an archaic offshoot from ganoids. 

 In Polypterus the duct, as in amphibians and higher vertebrates, opens 

 furthermore on the ventral side of the oesophagus. 



Morris, following the prevailing conceptions, considers that such a use 

 may have become of advantage to the Devonian ganoids, because they 

 were driven out of the open sea by the selachians, monarchs of the seas. 

 He argues that they were driven into bays, estuaries, shallow coastal 

 waters, and ascended streams, dwelling in inland waters. In such waters, 

 thick with sediment and perhaps poorly aerated, gill-breathing was ren- 

 dered difficult and the plant and animal food of the land tempted to a 

 further independence of the water. 



27 J. A. Thomson : Outlines of zoology. 1899, p. 461. 



28 The origin of lungs, a chapter in evolution. American Naturalist, vol. xxvi, 1892, 

 pp. 975-986. 



89 D. S. Jordan : A guide to the study of fishes, vol. i, 1905, pp. 98-106. 



