Biochemistry ami Evolution 59 



is small. But of course it would be well to have the results either con- 

 firmed or contradicted by investigations on other species with similar 

 habits and habitats. 



Table 5. Nitrogen excretion of Xenopus in 

 water after 19 days in damp (not wet) condi- 

 tions. 



source: Balinsky, Cragg. and Baldwin (1961). 



The results so far mentioned fit in with much other information 

 so precisely that we may perhaps be excused for seeming to jump to 

 conclusions over only one or two species. The basic fact behind all 

 this is that ammonia, familiar enough as a household article, is never- 

 theless extremely toxic, as was conclusively shown by Sumner many 

 years ago. No animal, probably, could survive for long on dry land 

 without having some means of converting ammonia into a more 

 innocuous material such as urea. Indeed, Xenopus apart, all the 

 anuran amphibians so far examined excrete about 80 per cent or 

 more of their waste nitrogen as urea (see Table 6). and the habit 



Table 6. Nitrogen excretion of amphibians: Anura 



Amphibian 



Xenopus laevis 

 Rana esculenta 

 R. temporaria 

 Hyla arborea 

 Bufo calami ta 



SOURCE: Cragg, Balinsky, and Baldwin (1961). 



of urea production has been inherited by the mammals, including 

 even those primitive forms that still lay eggs (Cragg, Balinsky, and 

 Baldwin, 1961). However, the majority of the reptiles, together with 



