X.] IN ANIMALS AND MAN. 57 



terpretation of the apparatus be correct, we can judge of the 

 delicacy of any auditory apparatus by the number of such cells. 

 The greater the number of cells the more delicate will be the 

 hearing of the animal and the wider will be its range. The exact 

 measurement and enumeration of Retzius have shown us that 

 the human cochlea contains 15,500 such cells, that of the cat 

 12,500, that of the rabbit 7,800. Hence man has a more perfect 

 sense of hearing than either of these two animals, but we can- 

 not determine with certainty whether he can better appreciate 

 minute differences, or whether he can hear more notes : pro- 

 bably he is superior in both these respects. There are also 

 individual differences in the number of cells in the human 

 species, although perhaps only within narrow limits. Such 

 differences explain why some individuals do not hear so well, 

 or cannot distinguish so many deep or high notes, as others. 

 I myself possess a rather fine ear, but I can never hear the 

 high notes of certain species of grasshoppers, even when 

 hundreds of them chirp together, although others can hear 

 them easily. 



If then the apparatus by which music is heard in the cat and 

 the rabbit be essentially the same as that of man, only differing 

 in degree, the following question is naturally suggested: — 

 Knowing that ttothing can arise unless it be useful, how has it been 

 possible for this apparatus to originate ? The power of hearing 

 music must have been utterly useless to those animals which 

 do not make music, and hence the origin of their auditory 

 apparatus must have proceeded from other necessities. What 

 can these necessities be ? 



Why has it been useful to Mammalia in the struggle for 

 existence to hear with distinctness all the large number of 

 notes for which their auditory apparatus is fitted, and which 

 renders the hearing of music a possibility ? This question has 

 probably never been asked before, and I must admit that the 

 answer is by no means easy ; at any rate if a complete and 

 detailed explanation be expected. But I believe that it is easy 

 to understand in a general way how the ear of these animals 

 could have been elaborated and raised to so high a pitch by 

 natural selection. Wild animals stand in need of a very fine 

 ear. Beasts of prey, such as cats, must in the first place be 

 able to hear and distinguish between all the sounds made by 



