III ON ANIMAL LIFE 85 
even as regards our own senses we really 
know or understand very little. Take the 
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 
