364 AMPHIBIA xiii. 4- 



It must be stated, however, that Watson, who has contributed more 

 than anyone to knowledge of amphibian evolution, believes that it is 

 possible to recognize a number of non-adaptive trends, which are 

 independent of environmental influences. Changes suggested by 

 Watson and others as non-adaptive include flattening of the skull, 

 doubling of the occipital condyles, reduction of the number of roofing 

 bones and loss of ossification in the neurocranium, the changes in the 

 vertebrae already mentioned, and many other features. It is not 

 entirely clear how the 'non-adaptive' nature of these features is estab- 

 lished. SevertzofT has suggested that the flattening of the head is 

 connected with the development of a large mouth for buccal respira- 

 tion. Palaeontology necessarily deals with small points of structure, 

 whose significance for the animal may be difficult to determine, but 

 it does not follow because we are not able to discern the significance 

 of a part that it therefore has none. It is not at all easy in biology to 

 hold the balance between credulous acceptance of a function for 

 every character and a sceptical attitude that insists on regarding the 

 organic world as a jumble of unrelated substances. The only safe rule 

 is to search continually for signs of regular recurrence of similarities 

 of structure and action, and then to make hypotheses about function, 

 which can be tested by experiment. 



5. Newts and Salamanders. Subclass Urodela 



The urodeles, also called Caudata or tailed amphibia, show less 

 deviation from the general form and habitats of the amphibia as a 

 whole than do the specialized anurans (Fig. 209). The adult and larval 

 urodeles differ little from each other, and characters suitable for 

 aquatic life are frequently found in the adult. Indeed, all stages of 

 suitability for land occur, from the terrestrial salamanders, such as 

 Salamandra maculosa, the European salamander, which is viviparous, 

 to the fully aquatic forms, for instance Necturus, the mud-puppy of 

 North America. In many of the aquatic animals there is a tendency to 

 retain in the adult characters usually found in larvae. This process of 

 paedomorphosis has developed to various extents, and independently 

 in several groups. Thus the giant salamander Megalobatrachus, 5! ft 

 in length, in China and Japan, has no eyelids, but loses its gills in 

 the adult. In Cryptobranchus, the hell bender of the United States, 

 the spiracle remains open and is used for the outlet of water during 

 respiration. 



Amphiuma, also from the southern U.S.A., is a very elongate form, 

 with absurdly small legs, no eyelids, and four branchial arches. In 



