v COLOURS OF ANIMALS 361 



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, there- 

 fore, produce obscure tints ; while pure and vivid colours 

 would require 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 occur- 

 ing. 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 physiological 

 action of light, or by the normal process of generative varia- 

 tion. Protective colours therefore, however curious and com- 

 plex they may be in certain cases, offer no real difficulties. 



Theory of Warning Colours 



These differ greatly from the last class, inasmuch as they 

 present us with a variety of brilliant hues, often of the 

 greatest purity, and combined in striking contrasts and 

 conspicuous patterns. Their use depends upon their bold- 

 ness and visibility, not on the presence of any one colour ; 

 hence we find among these groups some of the most ex- 

 quisitely-coloured objects in nature. Many of the uneatable 

 caterpillars are strikingly beautiful ; while the Danaidae, 

 Heliconidae, and protected groups of Papilionidae, comprise a 

 series of butterflies of the most brilliant and contrasted 

 colours. The bright colours of many of the sea -anemones 

 and sea -slugs will probably be found to be in this sense 



