26 BULLETIN OF THE 



function upon the alimentary canal, or one of its dependencies. The 

 water in which such fishes lived may have been at times incapable of 

 furnishing the necessary amount of oxygen, but sufficiently serviceable 

 as a means of removing, through the gills, the carbon dioxide. The 

 solubility of carbon dioxide in water, as compared with that of oxygen, 

 favors such an explanation. 



I have not succeeded in finding many analyses of the gases held 

 in solution by water which seem capable of throwing light on this 

 question. 



A. Morren ('41, pp. 471, 478-480, and '44, p. 12), who conducted 

 numerous experiments to ascertain the effect of light and of green or- 

 ganisms on the composition of the gases dissolved in water, has recorded 

 some interesting facts which seem to me to bear upon the problem. He 

 ascertained that when the per cent of oxygen in the gas extracted (by 

 boiling) from the water fell below 18, 19, or even 20, the fish contained 

 in his experimental reservoir began to languish, and many of them died. 

 He also deduced from his experiments these conclusions : that water 

 which [is stagnant or] flows slowly over a slimy bottom is subject to 

 conditions which serve to explain why it may be habitually less oxy- 

 genated than water which runs rapidly over a sandy bottom, and why 

 it undergoes greater variations in the composition of its dissolved 

 gases. 



Morren also found that the oxygen in the gas contained in the waters 

 of the river Marne fell as low as 18 per cent on the 18th of June, 1835, 

 when there was a remarkable mortality among the fishes in the river, 

 and he ascribed this mortality to the want of oxygen in the water. 



If such gas is incapable of supporting the life of fishes, it might occur 

 under certain circumstances that the proportion of oxygen would be 

 considerably below the normal 32 per cent, and still be far from pro- 

 ducing asphyxia. Under such conditions, which presumably happen 

 more often in the stagnant water of shaded swamps than elsewhere, 

 fishes which could avail themselves of the oxygen in the atmosphere 

 would be able to survive when others could not. They might still 

 employ their gills for the elimination of carbon dioxide into the water, 

 — 100 volumes of which can absorb about 120 volumes of the gas, — 

 but would have to depend largely upon the atmosphere for their supply 

 of oxygen. 



One might conclude, then, that in the transfer of the respiratory 

 function from the gills to the prospective lungs the two components of 

 the respiratory process were separated from each other, and that the 



