448 ANIMAL PHYSIOLOGY. 
the latter in either of the senses to which we have alluded 
above, is a matter on which there is very little evidence. It is 
perhaps best to assume, at least, the possibility of the truth of. 
both of them. 
Uric Acid.—This substance can be oxidized in the laboratory 
to urea, thus: 
C;H.N.O; + H.0 + O = C.N.H.O, + CN.H,.O, 
Uric acid. Alloxan. Urea. 
so that it has been assumed that uric acid in the body is a stage 
short of urea, and this seemed the more plausible, since it re- 
places the latter in the cold-blooded animals. But this is not 
entirely the case, for in the frog urea is found in the urine, 
and our knowledge of this secretion in most of them is very 
incomplete; moreover, in the birds, representing the very great- 
est degree of activity and the highest oxidative capacity, uric 
acid is the principal nitrogenous body of the urine, and not 
urea, 
Pathological When there is excessive indulgence by man 
in proteid foods, etc., the uric acid, normally small in quantity, 
is increased greatly, and may give rise to depositions of urates 
about the joints. 
It seems best to regard uric acid as the result of proteid 
metabolism when of a certain type, and urea as the outcome 
of the vital processes of animals of a distinct physiological 
type. 
Evolution —There is a good deal of paleontological evidence 
which points to a phylogenetic (ancestral) relation between 
birds and reptiles; hence the many points of functional resem- 
blance between these groups of creatures now so different in 
form and, in some respects, in functions. The excessive pro- 
duction of uric acid (uric-acid diathesis) can be understood in 
the light of physiological reversion. It is well known that this 
diathesis is hereditary—that is to say, the metabolic habit of 
excessive production of uric acid may be imparted to offspring. 
Hippuric Acid.—Among the herbivora hippuric acid may be 
said to replace uric acid. In the laboratory this acid may be 
made from benzoic acid and glycocol (glycin), thus: 
C.H..COOH + Fi) > CH, = CH.< (GG OH + HO, 
Benzoic acid. Glycin. Hippuric acid. 
It is interesting to note that, when benzoic acid is swallowed 
by man, hippuric acid appears in the urine; and it is said that 
