188 TROPICAL NATURE, AND OTHER ESSAYS. 



Now the various brown, earthy, ashy, and other neutral 

 tints are those which would be most readily produced, 

 because they are due to an irregular mixture of many 

 kinds of rays ; while pure tints require either rays of 

 one kind only, or definite mixtures in proper proportions 

 of two or more kinds of rays. This is well exemplified 

 by the comparative difficulty of producing definite pure 

 tints by the mixture of two or more pigments ; while a 

 haphazard mixture of a number of these will be almost 

 sure to produce browns, olives, or other neutral or dingy 

 colours. An indefinite or irregular absorption of some 

 rays and reflection of others would, therefore, produce 

 obscure tints ; while pure and vivid colours would re- 

 quire a perfectly definite absorption of one portion of 

 the coloured rays, leaving the remainder to produce the 

 true complementary colour. This being the case we may 

 expect these brown tints to occur when the need of 

 protection is very slight or even when it does not exist at 

 all ; always supposing that bright colours are not in any 

 way useful to the species. But whenever a pure colour is 

 protective, — as green in tropical forests or white among 

 arctic snows, there is no difficulty in producing it, by 

 natural selection acting on the innumerable slight 

 variations of tint which are ever occurring. Such 

 variations may, as we have seen, be produced in a great 

 variety of ways ; either by chemical changes in the 

 secretions, or by molecular changes in surface structure ; 

 and may be brought about by change of food, by the 

 photographic action of light, or by the normal process of 

 generative variation. Protective colours therefore, how- 

 ever curious and complex they may be in certain cases, 

 ofi'er no real difficulties. 



