HBARINU. 571 



perilymph. There is a fenestra ovalis, but not a fenestra ro- 

 tunda, though the latter is present in reptiles. In crocodiles 

 and birds the cochlea is tubular, straight, and divided into a 

 scala tympani and a scala vestibuli. The columella of lower 

 forms still persists. In birds and mammals the bone back of 

 the ear is hollowed out to some extent and communicates with 

 the tympanum. Except among the very lowest mammals 

 (Echidna), the ear is such as has been described in detail 

 already. 



Evolution. — The above brief description of the auditory organ 

 in different groups of the animal kingdom will suffice to show 

 that there has been a progressive development or increasing 

 differentiation of structure, while the facts of physiology point 

 to a corresponding progress in function — in other words, there 

 has been an evolution. No doubt natural selection has played 

 a great part. It has been suggested that this is illustrated by 

 cats, that can hear the high tones produced by mice, which 

 would be inaudible to most mammals ; and, as the very exist- 

 ence of such animals must depend on their detecting their prey, 

 it is possible to understand how this principle has operated to 

 determine even what cats shall survive. The author has noticed 

 that terrier dogs also have a very acute sense of hearing, and 

 they also kUl rats, etc. But, unless it be denied that the im- 

 provement from use and the reverse can be inherited, this factor 

 must also be taken into the account. 



There seem to be great differences between hearing as it exists 

 in man and in lower forms. Birds, and at least some horses, 

 possibly some cats and dogs, like music, and give evidence of 

 the possession of a sense of rhythm, as evidenced by the conduct 

 of the steed of the soldier. On the other hand, some dogs seem to 

 greatly dislike music. Certain animals that appear to be devoid 

 of true hearing, as spiders, are nevertheless sensitive to aerial 

 vibrations ; whether by some special undiscovered organ or 

 through the general cutaneous or other kind of sensibility is 

 unknown. It also seems to be more than probable that some 

 groups of insects can hear sounds quite inaudible to us, though 

 by what organs is in great measure unknown. 



The so-called musical ear differs from the non-musical in 

 the ability to discriminate differences in pitch rather than in 

 quality ; in fact, that one defective in the former power may 

 possess the latter in a high degree is a fact that has been some- 

 what lost sight of, both theoretically and practically. It does 



