Biochemistry and Evolution 61 



ammonia sometimes predominates among the nitrogenous excreta, 

 with urea still playing a substantial part. This seems to indieate a 

 reversion toward wholly aquatic habits, but the persistence of urea 

 production gives present evidence of a one-time truly amphibious or 

 terrestrial past. 



The third group comprises three species within a single genus, 

 Testudo. In the swamp-dwelling T. denticulata the picture is similar 

 to that found in the other amphibious species. In T. graeca, however, 

 a substantial excretion of uric acid makes its appearance and finally, 

 in the desert-living T. elegans, urea no longer appears in more than 

 trivial amounts and uric acid is responsible for practically all the 

 nitrogen accounted for. 



In fairness to Moyle, it should be pointed out here that there were 

 a number of analytical difficulties in this work largely because many 

 of these animals have a habit of excreting a good deal of only partially 

 digested food that is not removed by the usual deproteinization pro- 

 cedures, but even so the results do appear to be pretty clear-cut. It 

 certainly seems that in this group of reptiles we have a series of cases 

 which sum up the biochemical steps which were probably entailed in 

 the evolution of the modern snakes, lizards, and birds. Starting from 

 an amphibious stock of urea producers, they took to progressively 

 drier and drier surroundings, making use at first of the mechanisms 

 they already possessed by reason of their ancestry. Eventually the 

 advantages of uric acid became apparent — its extreme insolubility, 

 the fact that it is relatively innocuous and that little water is required 

 for its excretion — and gradually, probably step by step, there came 

 the switchover from the formation of urea to that of uric acid. This, 

 as Joseph Needham (1931) has argued, must also have tied up with 

 the invention of impermeable eggshells and consequential changes in 

 the conditions of embryonic life. 



Before leaving these chelonian reptiles, it must be mentioned that 

 there are reports in the literature to the effect that the wholly marine 

 turtles excrete only ammonia and no urea or uric acid worth men- 

 tioning. Does this mean, perhaps, that they never had an amphibious 

 phase and so had no use for urea, or that they returned to the water 

 long, long ago and have now altogether forgotten how to make it? 

 Evidently, there is still plenty of room for new work in this field. 



From these studies on amphibia and reptiles some recent research 

 has turned toward another problem of no small interest, viz., the 

 nitrogen excretion of fishes. The positive results obtained by the 

 author so far have been published (Baldwin, 1958. I960), like much 

 of the work already mentioned, but only a few of the negative ones 



