410 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
product even though this means wasting valuable carbon. Ammonia 
contains no carbon, urea contains one atom of carbon to every two 
of nitrogen excreted, and uric acid contains five carbon atoms for 
every four of nitrogen excreted. But there is less hydrogen in rela- 
tion to nitrogen in a molecule of urea than in ammonia, and still 
less in uric acid. By substituting urea or uric acid for ammonia, 
water is saved, and this is the most important consideration. Fur- 
thermore, both urea and uric acid are nontoxic, and the latter is also 
nearly insoluble in water, so that no free water is needed for its 
elimination. 
The principle may be illustrated by a few examples. Lungfish 
are normally ammonotelic, that is, they excrete ammonia; but when 
they aestivate in dry mud, and water is short, they excrete urea. 
The semiterrestrial Amphibia are ureotelic (excreting urea), but their 
larvae, which are fully aquatic, excrete ammonia, and the African 
genus Xenopus, which is aquatic in the adult stage as well, remains 
ammonotelic throughout its life. Birds and reptiles are uricotelic 
(excreting uric acid). They develop the necessary metabolic ma- 
chinery by necessity in the egg stage, when insoluble uric acid has 
much to be said in its favor, and retain the system throughout their 
lives. Mammals, during their embryonic stages, have the advantage 
of a good supply of water in the blood of the parent, and they use 
urea as the end product of nitrogen metabolism throughout life. Ac- 
cording to Needham (1935), gastropods exemplify the principle very 
well—marine and fresh-water forms store but little uric acid, inter- 
mediate littoral forms store rather more, and terrestrial forms most. 
Insects excrete uric acid. This process may again have been im- 
posed by the cleidoic egg, but it serves these animals in the adult state 
remarkably well, and insects which live in dry surroundings lose 
hardly any water as a result of nitrogen excretion. Spiders use a 
similarly insoluble nitrogen compound, guanin, which serves the same 
purpose. 
Nitrogen excretion, then, is one of the numerous examples which 
demonstrate evolution at a physiological level. But, like so many 
generalizations in biological science, the principle does not cover all 
the known cases. Thus woodlice, which are terrestrial Crustacea, 
and which might therefore be expected to excrete urea or even uric 
acid, do not do so. Most of their waste nitrogen is ammonia. One 
wonders why, particularly as the enzyme chain leading to the forma- 
tion of uric acid is present, for indeed a little uric acid is formed, 
though more is found in the fresh-water genus Aseld/us than in ter- 
restrial isopods. 
TRANSPIRATION 
We may now consider another source of water loss associated with 
life on land: transpiration from the general body surface. In this 
