828 The American Naturalist. [October, 
confirmed by the repetition of old experiments and by many 
new ones during the present summer; it is also confirmed by 
the experiments made on Lepidosteus in a different way by 
Dr. E. L. Mark (9), and published in 1890. I therefore feel 
confident that this is the expression of a general physiological 
law in nature. 
From the standpoint of evolution we must suppose that all 
forms originated from aquatic ancestors, ancestors whose only 
source of oxygen was that dissolved in the water. As the 
water is everywhere covered with the limitless supply of oxy- 
gen in the air, there being 209 parts of oxygen in 1000 parts 
of air as contrasted with the 6 parts of oxygen dissolved in 
1000 parts of water, it is not difficult to conceive that in the 
infinite years the animals found by necessity and experience 
that the needed oxygen was more abundant in the overlying 
air, and that some at least would try more and more to make 
use of it. And as any thin membrane with a plentiful blood 
supply may serve as a respiratory organ to furnish the blood 
with oxygen, it is not impossible to suppose that such a mem- 
brane, as in the throat, could modify itself, little by little, with 
ever increasing efficiency; and that a part might become 
especially folded to form a gill and another might become 
sacular or lung-like to contain air. While I am no believer 
in the purely mechanical physiology which sees no need of 
more than physics and chemistry to render possible and 
explain all the phenomena of life, yet it is patent to everyone 
that although vital energy is something above and beyond 
the energies of physics and chemistry, still it makes use of 
those; and certainly dead matter forms the material from 
which living is built. So given a living thing, it in most 
cases moves along lines of least rather than greatest resistance, — 
therefore if practically a limitless supply of oxygen may be 
obtained from the air and only a limited amount from the 
water, if any thing that might serve as a lung is present, most 
naturally the animal will take the oxygen from the air where — 
‘itisin greater abundance and most easily obtained. On the — 
other hand carbon dioxide is so soluble in water that practi- — 
cally a limitless amount may be excreted into it; and as it is 
