THE FACULTY OF HEARING. 373 



(or as some physiologists would put it, the outcome of its aggregate 

 modes of motion) we should expect to meet in the individual 

 development of the higher animal a gradually differentiating and 

 ascending scale recognisable by a more decided departure from the 

 Simpler organ, and an ever increasing qualitative distinction of 

 function. From this it would follow that each added function 

 conditions greater concentration of action and closer aggregation of 

 the structural elements. Thus a highly developed sensory 

 function implies a gathering together of co-ordinated organs 

 which are not necessarily existent in the lower animal, and 

 probably not located together so as to exemplify the same 

 special type. But it happens that the common ideal of a sensory 

 organ is founded upon its highest types in ourselves and the 

 vertebrata generally, and we are apt to estimate those of the 

 invertebrata by a standard which makes it difficult to recognise 

 resemblance with the multiple functions and complex apparatus in 

 which they are concentrated. We have only to imagine the 

 decentralisation of such organs, the breaking up of united depart- 

 ments, and their re-distribution in other places, in short a process 

 of reversion to earlier forms. And we see how a simple auditory 

 function in some more primitive organ located in some unexpected 

 position might readily be explained on the principle of adaptation 

 to physical requirements determined by the life habits and 

 conditions of the animal possessing it. Nor is it difficult to com- 

 prehend the co-existence of two or more specifically different 

 organs subserving divided auditory functions in different species, 

 or even the same species of insect. 



