58 The Nature of Biological Diversity 



must be emphasized, no matter what animal or animals we choose 

 to study, there is always a possibility or even a likelihood that we 

 shall stumble across some hitherto unknown phenomenon of general 

 or even fundamental importance. 



However, not the whole of comparative biochemistry is in quite 

 this unsatisfactory state. Often facts are scrappy and unsatisfactory 

 because too few species have been studied, but sometimes fairly 

 thorough, systematic studies have been carried out. For example, 

 starting with the work of Stadeler and Frerichs in 1858, evidence has 

 accumulated that the elasmobranch fishes, but not the teleosts or any 

 other group, elaborate urea as their principal nitrogenous excretory 

 end product. This is true also of the holocephalians, which are usually 

 regarded as an aberrant elasmobranch group, so that biochemistry 

 and taxonomy run hand in hand in this case. The function of the 

 urea, long debated, seems now to be certain. It plays a most important 

 role in osmotic regulation in these fishes (Smith, 1936). Here a strik- 

 ing biochemical peculiarity can be correlated with a specific function 

 in a particular kind of environment. 



On a smaller scale, urea formation features also in the dipnoan 

 fishes (Smith, 1930). But when the swamps they inhabit dry up in 

 the dry season, the formation of ammonia — important while they 

 have water to live in — is wholly suppressed in favor of urea produc- 

 tion. In this case urea formation evidently represents a specific device 

 for the detoxication of ammonia, and without some such mechanism 

 the lungfishes would have little or no chance of survival through their 

 periods of estivation. 



A substantially similar case is found in the horned toad, Xenopus 

 laevis. This animal, by all taxonomic standards, is a perfectly good 

 amphibian, but it is one of a number of amphibians that have made 

 a secondary and permanent return to the water. In the ordinary way 

 the bulk of its waste nitrogen is excreted as ammonia, but some urea 

 is nevertheless formed as evidence, presumably, of a truly amphibious 

 ancestry (Balinsky and Baldwin, 1961). It is said that Xenopus can 

 survive periods of estivation during the dry season, and a number of 

 experiments in which specimens were kept under mildly damp in- 

 stead of wholly aquatic conditions have proved that this animal, like 

 the lungfish, switches over to large-scale urea production under these 

 conditions (Balinsky, Cragg, and Baldwin, 19611. Like the lungfish 

 emerging from estivation, Xenopus excretes large amounts of accu- 

 mulated urea when returned to water after being kept out of water 

 for some days (Table 5). Here it seems that these are comparative 

 data of real significance, even though the number of species involved 



