xiii. 9 AMPHIBIAN EVOLUTION 367 



that is characteristic of any group of animals when closely studied. 

 Besides the main types that can be distinguished, countless lesser 

 variations will be found, and one realizes that the characteristics of the 

 populations are still today in process of continual and perhaps rapid 

 change. Anyone trying to discover the relationships of the various 

 derivatives of ranids or bufonids must be impressed by the presence 

 of series of parallel lines of development, so that it is impossible to 

 disentangle the relationships. Evolution viewed at close quarters by 

 the student of abundant modern animals looks very different from 

 the simple picture seen by the lucky collector of a few rare fossils, 

 who can arrange his types in genealogical trees and is apt to forget that 

 they represent only an infinitesimally small sample of abundant and 

 varied populations. 



We can perhaps find certain tendencies in the modern amphibian 

 populations that are similar to the tendencies of the fossil series. Many 

 return to the water, especially among the urodeles. Others become 

 more fully terrestrial, either by climbing trees or by burrowing into 

 the earth. Both these habits have been independently adopted many 

 times by recently evolved lines and, no doubt, still more often in the 

 past by creatures that have died out, leaving no trace. 



9. Can Amphibia be said to be higher animals than fishes? 



It is not easy to decide whether there is a clear sense in which 

 amphibians can be said to have advanced over their fish ancestors. 

 They have moved from the water into environments that are in a 

 sense less suitable for life. In order to maintain a watery system, such 

 as a frog or toad, outside the water, various special structures and 

 methods of behaviour have been evolved. The presence of such 

 additional systems can be said to add complexity to the organization. 

 It is difficult to make a count of the number of 'parts' involved in the 

 organization of any animal. Amphibia possess many special devices, 

 for instance, for respiration without loss of moisture, for control of 

 water intake and water loss, for return to water to breed, and so on. 

 Even without making a proper quantitative computation it seems 

 reasonable to say that these add up to make an organization more 

 complicated than that of a fish. The integration of the action of so 

 many parts requires an elaborate nervous system, and there is evi- 

 dently some connexion between the increased size and importance of 

 the nervous system and the development of this more complicated 

 organization that enables life to continue in a different environment. 



Considering the matter in this way it is hardly sensible to ask the 



