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 Colowrs 
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 Danaida, 
Heliconide, and protected groups of Papilionide, 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 
