246 THE INFLUENCE OF LIGHT ON LIFE. 



with which are not connected with our subject; but we may just 

 remark that there does not appear to be any very prominent reason 

 why we should not ascribe the blackness of skin to be due to a 

 long-continued habitation in sunny regions. At any rate, we know 

 that we can obtain brown hues from the sun's influence, and that 

 anyone who is for long shut out from the light, as a prisoner in a 

 dark cell or a workman in a mine, becomes pasty-looking and 

 very pale-faced. 



Now, leaving out of consideration altogether Darwin's theory 

 of Natural and Sexual Selection as the cause of colours in animals, 

 we can fairly consider it established that light has to a certain 

 extent a direct influence on the pigment. Professor Semper 

 calls attention to a most interesting influence which light exerts on 

 some animals in an indirect way. The animals referred to are 

 those which have the power of changing their colour to suit the 

 surroundings, and the Professor asserts that this is brought about 

 by the influence of the surrounding light acting upon the retina of 

 the eye. For instance, if a fish possessing this power rests over 

 red sand, then the impression of the red through the eye upon the 

 brain will cause the brain to effect certain contractions in the 

 pigment cells on the surface of the creature's body, whereby all the 

 different coloured pigment-cells will be contracted except the red 

 ones, and the fish will consequently bear then a similarity to the 

 red sand. That, at any rate, is the explanation which has been 

 put forward, and by that theory it is supposed that the red light is 

 unable to excite the red pigment-cells (chromatophores) to con- 

 tract. And so with other colours. If the Hght is blue, then all 

 the pigment-cells except the blue ones will contract, and the skin 

 will thus be blue. 



It is somewhat interesting to find Darwin, in his Descent of 

 Matiy explaining that the colours of the shells of some MoUusca 

 are no doubt influenced, to a certain extent, by the amount of 

 light, and pointing out that the lower surface of the shells is 

 generally less highly coloured than the upper and exposed surface. 

 When we thus pass in review the influence which light, or the 

 absence of light, has in altering colours, in changing, it may be, 

 the manner of a creature's hfe, in causing the degeneration of the 

 organ of vision, and even in preventing development or causing 



