6 AVERS. [Vol. VI. 



more new, problems are yet to be solved, but I think they are 

 of secondary importance as compared with the main facts estab- 

 lishing the genesis of the ear in a canal organ group, and its 

 retreat to a place by itself deeper within the head, where it has 

 developed, in its own peculiar and highly functional way, into 

 one of the three main mechanisms controlling our acquisition 

 of knowledge ; into one of the principal instruments on which 

 we must depend in our exploration of the vast unknown. 



To combine the results of the anatomical investigations with 

 those from the embryological field, and to deduce from this 

 accumulation of facts the general law of development of the 

 vertebrate ear is the purpose of this paper. How well I may 

 succeed I leave time to decide by bringing new and completer 

 knowledge than we now possess of many of the details of the 

 developmental processes, as also of many matters of adult rela- 

 tions of the organ in question which still need elucidation. 



For many favors and substantial aid in carrying on my inves- 

 tigations I wish to make grateful acknowledgment to the follow- 

 ing gentlemen : — 



To Dr. G. D. Ladd and Dr. E. M. Connell for valued assist- 

 ance. To Professor G. W. Peckham for the use of a fine immer- 

 sion lens. To Messrs. Cudahy for much valuable material on 

 the mammalian ear. To the director of the Marine Biological 

 Laboratory, Professor C. O. Whitman, and the gentlemen com- 

 prising his staff, for very efficient aid in securing the needed 

 marine vertebrates, and also for the excellent library and labora- 

 tory facilities accorded me during a short stay in June-July, 

 1 89 1. And to Mr. E. P. Allis for the gift of living adult and 

 embryonic material of our native marsupial, the Virginia Opos- 

 sum, which has enabled me to give some account of the audi- 

 tory anatomy of one member of a group which in this respect 

 is entirely new to science. 



The following list includes the forms which have served as 

 the basis for my conclusions. Some species were examined for 

 the solution of one or more doubtful points ; but most of them 

 have been carefully anatomized and studied in histological detail 

 as well. As far as possible the material has been studied in 

 the living condition, and I have endeavored by using several 

 methods to reduce the errors necessarily introduced into the 



