m ON ANIMAL LIFE 85 



even as regards our own senses we really 

 know or understand very little. Take tlie 

 question of colour. The rainbow is commonly 

 said to consist of seven colours — red, orange, 

 yellow, green, blue, indigo, and violet. 



But it is now known that all our colour 

 sensations are mixtures of three simple col- 

 ours, red, green, and violet. We are, how- 

 ever, absolutely ignorant how we perceive 

 these colours. Thomas Young suggested 

 that we have three different systems of nerve 

 fibres, and Helmholtz regards this as " a not 

 improbable supposition"; but so far as mi- 

 croscopical examination is concerned, there is 

 no evidence whatever for it. 



Or take again the sense of Hearing. The 

 vibrations of the air no doubt play upon the 

 drum of the ear, and the waves thus produced 

 are conducted through a complex chain of 

 small bones to the fenestra ovalis and so to 

 the inner ear or labyrinth. But beyond this 

 all is uncertainty. The labyrinth consists 

 mainly of two parts (1) the cochlea, and (2) 

 the semicircular canals, which are three in 

 number, standing at right angles to one 



