370 THE FACULTY OF HEARING. 



aggregate auditory organ Into parts so distinct as the cochlea, semi- 

 circular canals, vestibular sac, Sec, we must recognise a correspon- 

 ding division of labour. And in accordance with this principle 

 if we credit the insect tympanal organ with an acoustic function 

 at all, we must assign to it the particular office which best 

 corresponds with its capacity of performance as estimated by its 

 structural and physical characteristics, instead of fixing upon it 

 the diverse burden of duties which 'a general organ of hearing 

 must perform. 



We justly assume that the higher articulata are endowed with 

 sensory faculties which place them in adequate relation with their 

 external surroundings. But in considering the widely varying 

 conditions of life, we cannot but see that the various adaptations 

 must tend to obscure both outward resemblance and internal 

 characteristics of typical sensory organs, if indeed any single type 

 of auditory organ prevails. While therefore the presence cf a 

 tympanal ear in the chirping orthoptera appears to indicate some 

 natural relation of development between the sound-uttering and 

 hearing insect organs, we need not be surprised at the absence of 

 this particular structure in numberless insects in whom some 

 other means of hearing (for the possession of some such faculty 

 can be scarcely doubted) must exist ; as for instance hearing hairs or 

 special skin appendages whose responsive vibratory movements may 

 be regulated by varied length and stiffness of their growth. And for 

 such a function the antenuce as feelers of sound seem well adapted 

 by the rapidity with which they can be moved In every .'direction, and 

 by the various degrees of rigidity which they may assume by varied 

 muscular contraction. But in order to distinguish between what 

 is heard through a special organ of hearing, and what is felt through 

 organs of feeling distributed in the integument, we must consider 

 more particularly what is understood by special and general 

 sensation. We know that from one and the same external 

 stimulus, sensations of radically different quality arise according 

 to the nature of the instrument affected. The vibrations w^hich 



