THE I'ACULTY OF HEARING. 355 



of structure and function we come across theories of type, origin, 

 and evolution, which are generahsations raised upon the long 

 series of anatomical facts learnt by comparative analysis of 

 organic matter developed into special structures under given 

 conditions. 



But in ordinary observation when called upon to say whether 

 any special sensory organ, whose presence is not at once manifest 

 from outward evidence, really exist or not in the creature we are 

 examining, we often decide upon grounds which are more apparent 

 than real, or upon facts which are mistaken to possess the force of 

 true generalisations. For the naturalist seldom appeals to anatomical 

 facts or takes interest in them, except when he wants their support 

 in his biological speculations. And when he does want them he 

 is in danger of misconstruing their meaning from his want of 

 familiarity with the generalisations of comparative science. On 

 the other hand a seemingly easy and certain approach to the 

 solution of his difficulty offers itself in looking for such proof of 

 adaptation of the bodily organism to its physical surrounding, as 

 appears to warrant the opinion which he may form. For instance, 

 if a drumiike expansion of the skin be noticed in an insect or any 

 other member of the Articulata, he might conclude that it signified 

 the presence of an organ of hearing : whereas the fact is capable of 

 various explanation, and to mistake it for a general physiological 

 indication is to ignore a multitude of opposing facts supplied by 

 comparative anatomy, and moreover exhibit a total miscomprehen- 

 sion of the physical conditions under which an animal membrane 

 must be placed in order to receive and transmit vibratory im- 

 pressions. To prove the existence of an auditory apparatus and an 

 equivalent sense of hearing, the anatomical details of the presumed 

 sensory organ must be compared with those of other known organs, 

 and a due relation of form and function be established between 

 them, as well as correspondence in essential physical properties. 

 Hence, in following up an enquiry of this kind, it is necessary first 

 of all to fix in our minds the general doctrine before dealing with 



