360 H. D. REED 



of auditory element found to exist in each taxonomic group; b) 

 that the possibility of neoteny, as the explanation of certain 

 features in such forms as Necturus, Cryptobranchus and others, 

 cannot be overlooked, and, c) that the morphology of these organs 

 has a significant bearing upon the relationships and descent of 

 the principal groups of caudates. 



It is contended by some that structures so intimately associ- 

 ated in function with the habits of the animal are likely to be 

 affected by the environment and, therefore, unreliable in showing 

 affinities or tracing descent. It is true that these structures are 

 not only affected by the environment and needs of the animal, 

 but show a decided adaptation in this respect. There are prob- 

 ably few internal structures in animals which are not ultimately 

 affected by environment, and the differences of form and rela- 

 tions of internal parts are, to a considerable extent, a reflection of 

 the life and habits of the animal itself. Form and relation of 

 parts alone, therefore, do not always constitute reliable criteria 

 for passing judgment, either with regard to morphology or line- 

 age. It is exactly to this error that some of the confusion men- 

 tioned above can be traced. It has been apparent from the be- 

 ginning that, so far as this system of organs is concerned, the only 

 safe basis for homology resides in the principle that two struc- 

 tures, however much they may resemble each other in form and 

 function, are different, unless comprising the same combination 

 of elements. For example, the fenestral plate and stylus of Nec- 

 turus and Cryptobranchus, in form and skeletal relations, appear 

 identical. Development shows them to be entirely different in 

 the elements which combine to form them, and they are, there- 

 fore, not homologous structures and not indicative of close affin- 

 ity of the animals possessing them. 



It is mentioned in the summary of the first part of this paper 

 that two morphologic types of sound-transmitting apparatus 

 occur in the tailed amphibia. When these types in the various 

 groups are reviewed with the above-mentioned principle in mind, 

 it will be found that the different families of Caudata become 

 grouped into two legions, each of which represents a line of de- 

 scent, judged by the nature of the variations of the combining 



