1905] BARNES—THE THEORY OF RESPIRATION 97 
dioxid and alcohol. Indeed, lactic seems an equally characteristic 
though transient product. The fact that hydrogen has also been 
often recognized among them supports the interpretation of the 
function of oxygen just suggested, and accords thoroughly with the 
theory of hydroxylation. In that process hydrogen atoms from the 
dissociation of water would be left free in case there was insufficient 
oxygen to form H,O,,. 
Fourth: But if the organism can get an adequate supply of oxy- 
gen, the katabolism continues, some of the most complex previous 
products breaking up by hydroxylation and thermal cleavage. 
Among the fragments are undoubtedly some that lose in part those 
very groups in which sugars, alcohols, fatty acids, etc., are peculiarly 
rich. These are rebuilt at the expense of such foods, which there- 
fore disappear as a result of respiration. That ethyl alcohol does 
not persist when oxygen is present may mean either that it is decom- 
posed, or that in its nascent state it is assimilated in the rebuilding 
of proteids, for we have seen how easily acetic acid, one of its oxida- 
tion products, can be converted into an amido-acid, glycocoll, and 
be thus in direct line for reconstructive metabolism. 
This, in its fundamental features, is the theory I have presented 
in lectures to advanced classes since 1898, though always as more or 
less a speculation. For various details I am indebted to the recent 
literature, already cited. Because it is capable of explaining the 
observed facts, which are now sufficiently numerous to demand a 
coherent explanation, I conceive it to be entitled to the dignity of a 
theory. Time forbids the discussion of details, and many points 
have been considered that cannot be here presented. 
This theory maintains the direct relation of aerobic and anaerobic 
respiration, whose genetic connection was long since advocated by 
PFEFFER. Anaerobic respiration is the primary process in all organ- 
isms. Whether aerobic respiration occurs or not depends upon the 
availability of oxygen. The relation of fermentation to the process 
is not wholly clear; for although fermentation gives rise to the same 
products as anaerobic respiration, this may depend in part upon 
respiratory decompositions, such as have been described, and in part 
upon digestion, which, as IwANowsKY and Mazé think, render the 
alcohol from sugars available for assimilation. I am inclined to 
