THE FACULTY OF HEARING. 367 



selection are excluded from the operation of '"convergence." 

 Again, the interpretation of function founded upon this principle 

 of "' convergence '' differs from the explanation drawn from analogy 

 in this, that in estimating analogies, morphological identity is less 

 considered than physiological result, adaptation to purpose 

 being the chief thing sought to be explained. Nevertheless, 

 converging forms of animal, or plant, or of any particular organic 

 structure are the result of adaptation whereby animal matter 

 originally indifferent in endowment, or even if originally possessing 

 special properties and potential capacity of development, may be so 

 influenced by external physical agencies as to approach a common 

 form and constitution. 



To determine, therefore, the anatomical homologies of the insect 

 tympanal organ, it would seem necessary to enquire how far 

 similar structure can arise from protoplasms of the same nature 

 under the influences which acoustic requirements and selective 

 tendencies (including acquired heredity) may exert. 



Physical and Physiological. That function is indicated by 

 structure (wherever the latter is present) has long been an 

 accepted principle of physiology. But structure needs to be 

 interpreted by reference to the physical phenomena, in the 

 production of which it is instrumental : and the characteristics 

 by which we distinguish and define a sensory function are 

 derived solely from the study of the external physical phenomena 

 associated with the sense of their existence. Thus in examining 

 the function of the insect tympanal organ we have first of all to 

 consider what is the physical performance of the tympanal 

 membrane. We understand at once that whatever impressions 

 are received and transmitted must be by means of air waves. 

 Now the study of acoustic phenomena teaches us that intensity 

 of sonorous vibration depends upon the swing (height of 

 excursion) and the pitch of the tone produced upon the number 

 of vibrations, while all the compound tones of a musical note 

 (those rising successively above the fundamental tone) are 



