1892.] Comparative Physiology of Respiration. 829 
apparently somewhat easier, other things being equal, for it to 
pass from the liquid blood to the water than to the air it seems 
likewise natural that the gills should serve largely for the 
excretion of the carbon dioxide into the water. This is the 
actual condition before us in these, and I believe in all other 
cases, of mixed or of combined aerial and aquatic respiration. 
- And I believe the fundamental law in respiration is, as stated 
above that whenever both water and air are used with corre- 
sponding respiratory organs, the aerial part of the respiration is 
mainly for the supply of oxygen and the aquatic part largely for 
the getting rid of carbon dioxide. 
It is not difficult to see in an actual case like that of the 
Ganoid fishes (Amia and Lepidosteus) the logical steps in its 
evolution, by which this most favorable condition has been 
reached. A condition rendering these fishes capable of living 
in waters of almost all degrees of purity, and thus giving 
them a great advantage in the struggle for existence. But 
what can be said of the soft-shelled turtles, animals belonging 
to a group (Reptilia) in which purely aerial respiration is 
almost exclusively the rule? Standing alone this might be 
exceedingly difficult or impossible of- explanation. The 
-Batrachia (frogs, toads, salamanders, ètc.) all have gills in their 
early or larval stage, and most of them develop in the water, 
and are in the beginning purely aquatic animals. The adults 
must, therefore, in most cases repair to the water at the spawn- 
ing season, and frequently in laying the eggs they must remain 
under the water for considerable intervals. Being under the 
water and the need of oxygen becoming pressing, there seems 
to be, by a sort of organic memory, a revival of the knowledge 
of the way in which respiration was accomplished when as 
larve their natural element was water, and they may take 
“water into the mouth and throat. This may be done by as 
highly a specialized and purely aerial form as the little brown 
tree-frog (Hyla pickeringii) or the yellow-spotted salamander 
(Amblystoma punctatum). Another very interesting form, the 
vermilion-spotted newt (Diemyctylus), after two or three years 
of purely aerial existence, goes to the water on reaching 
maturity, and remains there the rest of its life, regularly 
