368 THE FACULTY OF HEARING. 



produced simultaneously with its fundamental note. Whatever 

 perception of sound may result therefrom, it seems certain that 

 the limits of impressions which may be heard, depend primarily 

 upon the capacity of the drum of the ear for swinging largely or 

 vibrating quickly, and not upon hearing power of the nerve. 

 Perhaps it might fairly be inferred that as the perception of sound 

 is not likely to be in excess of the mechanical means of its 

 transmission, the significance of the tympanal membrane with its 

 tensor muscle in the insect organ is established by the fact that it 

 is the only conceivable means of sound conduction unless we 

 suppose the nerve to be set in vibration by direct communication 

 through the hard insect integument. The significance of the 

 insect tympanum considered as the basis of an auditory function 

 is emphasised by the absence of an otolith sac, and by the equally 

 significant fact that the tympanum is absent in aquatic animals in 

 whom the otolith sac is of regular occurrence. 



Seeing then, how exactly the mode of action of the tympanum 

 corresponds with the actual acoustic requirements of an auditory 

 function, we may consistently infer that the primary anatomical 

 differentiation of the growing protoplasm, which has resulted in 

 the development of an elastic membrane stretched in a firm 

 resistent frame, and supplied with special muscle and nerve has 

 been from the first persistently influenced by physical agencies 

 (including the acquired heredity.) And whatever difficulties beset 

 the anatomical demonstration of identity of type, when comparing 

 the tympanal organs of an acridium and a locust, a much greater 

 difficulty attends the rejection of the function attributed to both, 

 on the ground of their physical character and mode of action. 

 For it is impossible to assign any other office without ignoring the 

 physical evidence upon which physiologists have always relied in 

 the determination of every known sensory organ. 



The physical states and acts which concern the remaining parts 

 of these tympanal organs are less readily investigated. The direct 

 contact of nerve ends with the tympanum in the acridium type, 



