THE FACULTY OF HEARING. 365 



The question remains however still unsettled, and I therefore 

 venture to offer a few critical observations upon certain points, 

 which may be ranged under the following heads — 



Anatomical. That the insect tympanal organ is a cuticular 

 structure (including the tracheal system with which it is connected,) 

 and that the ganglia and nerves distributed upon it are co-ordinated 

 with it for some special function, towards which the tympanum 

 and resonating air sac concur, are all significant facts. In outward 

 appearance the drum, especially in the Acridia, bears the undoubted 

 impress of an acoustic apparatus, and the existence of a tensor 

 muscle is especially significant, as it implies a reflex action governed 

 by some sense of the vibratory impulses received on the tympanal 

 membrane. This membrane being thin, elastic, and tightly 

 stretched, cannot be a sound-producing organ or a resonator to 

 increase sound, while it is admirably adapted to receive sound. 

 Lastly, the complex development of the highly differentiated nerve 

 ends speaks strongly for the sensory character of their function 

 which, moreover, must be of very subtle order. 



But when the question is of discovering homologies of structure, 

 the difficulty of proving an actual concordance of type between the 

 mammal ear and the insect tympanal organ increases. It is not a 

 little perplexing to find that the most extensive homologies of 

 histologic elements are accepted, as a matter of course, by almost 

 every one who undertakes a special study of the minute anatomy 

 of the several organs of sense. How far the similarity of histologic 

 elements throughout all classes of animals is due solely to identity 

 of origin, is a question seldom discussed. But there is another 

 side to the question, namely, the consideration that external 

 agencies, acting upon an originally indifferent protoplasm, may 

 gradually induce modifications of structural development, and that 

 if these agencies are continued through sufficiently long periods of 

 time, the same or similar structures may eventually appear in all 

 animals subjected to the same plastic influences. In contrast with 

 the doctrine of identity of origin, due to endowment of protoplasm 



