1892.) Comparative Physiology of Respiration. 823 
take its oxygen and eliminate its carbon dioxide with the same 
directness as its simple prototype, the amæba. 
But in the course of evolution forms appeared with aerial 
respiration; and the insects among these solved the mechani- 
cal difficulty of respiration by a most marvellous system of 
air tubes or tracheæ extending from the free surface, and there- 
fore from the surrounding air, to every organ and tissue. By 
means of this intricate network air is carried and supplied 
almost directly to every particle of living matter. The res- 
piration is not quite direct with the insects, however, for the 
oxygen and carbon dioxide must pass through the membranous 
wall of the air tube before reaching or leaving the living sub- 
stance. 
In the next and final step, the step taken by the highest 
forms, the living material is massed, giving rise not only to 
animals of moderate size, but to the huge creatures that swarm 
in the seas, or walk the earth like the elephant. With all of 
these the step in the differentiation of the respiratory mech- 
anism consists in the great perfection of lungs or gills, and in 
the addition of a complicated circulatory system with a respir- 
atory blood, one of the main purposes being, as the name 
indicates, to subserve in respiration by carrying to each indi- 
vidual cell in the most remote and hidden part of the body 
the vital air, and in the same journey removing the poisonous 
carbon dioxide. 
This has been called Indirect Respiration, because the living 
matter of the body does not take its oxygen directly either 
from air or water, but is supplied by a middle man, so to 
speak, 
The complicated movements by which water is forced over 
the gills or by which the lungs are filled and emptied, and 
the great currents of blood are maintained; that is, the strik- 
ing and easily observed phenomena of respiration are thus 
seen to be only superficial and accessory, only serve as agents 
by which the real and the essential processes that go on in 
silence and obscurity are made possible (see references). 
So far I have attempted to give a brief resumé of the views 
on respiration that have been slowly and laboriously evolved 
