EMBRYOLOGY OF CRYPTOBRANCHUS 557 



eluding that the relationship is very close. In particular we 

 should expect the immediate ancestral stock of Cryptobranchus 

 and the closely related miocene fossil Andrias scheuchzeri (Gadow 

 '01, p. 84) to consist of larger animals. 



According to the view adopted in this paper, the urodeles, 

 very remotely descended from aquatic stock, are primarily ter- 

 restrial, but with aquatic larvae. On the land they were unsuc- 

 cessful in the struggle for existence in the open; they took refuge 

 in sheltered situations, and for this they have paid the penalty 

 of degeneration. Yet, in the main, the result of the arrest of a 

 typical terrestrial adaptive radiation has been the retention of- 

 primitive characters. Some became secondarily aquatic; this 

 is one phase of the tendency toward secluded habits, and involves 

 the retention of larval characters. Added to these more conspic- 

 uous peculiarities we find a great variety of special adaptations to 

 a retired mode of life, some of them correlated with the defense- 

 less condition of the animals. Of all these changes, reversion 

 to an aquatic mode of life is a factor which, cutting across many 

 lines of descent, has done most to disguise the real relationships, 

 and of this we have a conspicuous example in Cryptobranchus. 



