Apart from sexual selection, color in animals mainly serves two functions. It is either "protective" or "warning." The object of the first is to render the animal inconspicuous, the object of the second is the opposite--to make it conspicuous. Why it should be an object with some animals to be palpably exposed will be apparent from the following familiar instance of "warning" coloration. There are two great families of butterflies, the _Danaidæ_ and _Acraiedæ_, which are inedible owing to the presence in their bodies of acrid and unwholesome juices. Now to swallow one of these creatures--and birds, monkeys, lizards, and spiders are very fond of butterflies--would be gratuitous. It would be disappointing to the eater, who would have to disgorge his prey immediately, and it would be an unnecessary sacrifice of the subject of the experiment. These butterflies, therefore, must have their disagreeableness in some way advertised, and so they dress up with exceptional eccentricity, distinguishing themselves by loud patterns and brilliant colorings, so that the bird, the monkey, and the rest can take in the situation at a glance. These animated danger-signals float serenely about the forests with the utmost coolness in the broadest daylight, leisureliness, defiance and self-complacency marking their every movement, while their duskier brethren have to hurry through the glades in terror of their lives. For the same reason, well-armed or stinging insects are always conspicuously ornamented with warning colors. The expense of eating a wasp, for instance, is too great to lead to a second investment in the same insect, and wasps therefore have been rendered as showy as possible so that they may be at once seen and as carefully avoided. The same law applies to bees, dragonflies, and other gaudy forms; and it may be taken as a rule that all gayly-colored insects belong to one or other of these two classes: that is, that they are either bad eating or bad-stingers. Now the remarkable fact is that all these brilliant and unwholesome creatures are closely imitated in outward apparel by other creatures not themselves protected by acrid juices, but which thus share the same immunity. That these are cases of mimicry is certain from many considerations, not the least striking of which is that frequently one of the sexes is protectively colored and not the other.