822 The American Naturalist. [October, 
respiration?” by saying it is the taking up of oxygen and the 
giving out of carbon dioxide by living matter. This is the 
universal and essential fact with all living things, whether 
they are animals or plants, whether they live in the water or 
on the land. But the ways by which this fundamental life 
process is made possible, the mechanisms employed to bring 
the oxygen in contact with the living matter and to remove 
the carbon dioxide from it are almost as varied as the groups 
of animals, each group seeming to have worked out the prob- 
lem in accordance with its special needs. It is possible, how- 
ever, in tracing out these complex and varied methods and 
mechanisms to discover two great methods, the Direct and the 
Indirect. (See 8 of references.) 
In the first there is the direct assumption of oxygen from 
the surrounding medium, and the excretion of carbon dioxide 
directly into it. The best examples of this are presented by 
unicellular forms like the amœba where the living substance 
is small in amount and everywhere laved by the respiratory 
medium. But as higher and higher forms were destined to 
appear, evidently the minute, organless amceba could not in 
itself realize the great aim toward which Nature was moving. 
There must be an aggregation of amcebas, some of them serv- 
ing for one purpose and some for another. Like human 
society, as civilization advances, each individual does fewer 
things, becomes in some ways less independent, but in a nar- 
row sphere acquires a marvellous proficiency. Or to use the 
technical language of science, in order to advance there-must 
be aggregation of mass, differentiation of structure and spec- 
ialization of function. Evidently, however, if there is an 
aggregation of mass, some of the mass is liable to be so far 
removed from the supply of oxygen and the space into which 
carbon dioxide can be eliminated that itis liable to be starved 
for the one and poisoned by the other. Nature adopted two 
simple ways to obviate this, first to form its aggregated masses 
in the shape of a network or sponge, with intervening channels 
through which a constant stream of fresh water may be made 
to circulate, so that each individual cell of the mass could 
