660 The Colors of Animals and Plants. [ November, 
more pigments, while a hap-hazard mixture of a number of these 
will be almost sure to produce browns, olives, or other neutral or 
dirty colors. An indefinite or irregular absorption of some rays 
and reflection of others would, therefore, produce obscure tints ; 
while pure and vivid colors would require a perfectly definite ab- 
sorption of one portion of the colored rays, leaving the remainder 
to produce the true complementary color. This being the case, 
we may expect these brown tints to occur when the need of pro- 
tection is very slight, or even when it does not exist at all, al- 
` ways supposing that bright colors are not in any way useful to 
the species. But whenever a pure color 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 varia- 
tions may, as we have seen, be produced in a great variety of 
ways, either by chemical changes in the secretions or by molec- 
ular 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 colors, there- 
fore, however curious and complex they may be in certain cases, 
offer no real difficulties. 
Theory of Warning Colors.—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 color; hence 
we find among these groups some of the most exquisitely colored 
objects in nature. Many of the uneatable caterpillars are strik- 
ingly beautiful; while the Danaidie, Heliconidx, and protec 
groups of Papilionide comprise a series of butterflies of the most 
brilliant and contrasted colors. The bright colors of many of 
the sea-anemones and sea-slugs will probably be found to be in 
this sense protective, serving as a warning of their uneatable- 
ness. On our theory none of these colors offer any difficulty. 
Conspicuousness being useful, every variation tending to brighter 
and purer colors was selected, the result being the beautiful vari- 
ety and contrast we find. 
But when we come to those groups which gain protection 
solely by being mistaken for some of these brilliantly colored but 
uneatable creatures, a difficulty really exists, and to many min 
is so great as to be insuperable. It will be well, therefore, tO 
endeavor to explain how the resemblance in question may have 
