540 BERTRAM G. SMITH 



XII. PHYLOGEXY 



We are here concerned with (a) the origin of the amphibia in 

 general, (b) the origin of the urodeles, and (c) the interrelation- 

 ships of the urodeles with special reference to Cryptobranchus. 

 The evidence may be classified as (a) anatomical, (5) paleontolog- 

 ical, and (c) embryological. In the following survey, no attempt 

 is made to keep these lines of evidence strictly separate; for 

 paleontology is simply an extension of comparative anatomy to 

 fossil forms, with the added element of sequence in time ; while 

 the development during the late embryonic and larval stages 

 gives the key to many adult structures that could not otherwise 

 be homologized. 



It is not within the scope of this paper to enter extensi^^ely 

 into anatomical and paleontological questions; but as a check 

 on possible generalizations derived from the study of amphibian 

 embryology the writer has devoted considerable time to a study 

 of the paleontological material in the American Museum of Nat- 

 ural History, and has endeavored to acquire some degree of 

 familiarity with the results of modern research in this field. In 

 this work he has been greatly aided by Dr. W. K. Gregory, who 

 has in progress a detailed review of the origin of the amphibia 

 (see abstracts in Science, Gregory '11a, 'lib and '12). 



The tetrapoda form a coherent group. The gap between this 

 group and the fishes is one of the weak places in vertebrate phylog- 

 eny, and it is here that the idea of continuity in the descent of 

 the higher vertebrates has been most often attacked; some have 

 maintained a diphyletic origin for the tetrapods and fishes. 



The amphibia undoubtedly include the most primitive known 

 tetrapods. It is on this account that the amphibia possess a 

 peculiar interest from a phylogenetic point of view; the problem of 

 the origin of the amphibia is the problem of the origin of the 

 tetrapods in general. 



The fundamental unity of the gnathostome type leads us to 

 look among the true fishes for the nearest living or fossil repre- 

 sentatives of the ancestral stock of the amphibia. For in fishes 

 and tetrapods the following features, as well as many others, 



