sible, I think, to suggest a way in which it may have happened. Colors, viewed objectively, consist of ether-waves having different rates of vibration. In an eye devoid of the color-sense, all these ether-waves would doubtless set up the same sort of action in all the ends of the nerves, and would therefore produce exactly the same general sensations. But if in certain eyes there was the slightest tendency for some of the nerve-terminals to respond specially to the oscillations of one particular order, while others of the nerve-terminals responded rather to oscillations of a different order, there would be the first groundwork for the evolution of a color-sense. If this diversity of action in the nerve-ends proved of no service to the animal, it would go no further, because those individuals who possessed it would not be favored beyond those who did not. But if it proved useful, as it undoubtedly would do to flower-haunting insects, natural selection would insure its survival and its constant increase from generation to generation. Even color-blind people among ourselves can be taught by care and attention to discriminate slightly between the hues which they at first confuse; and if we were to choose out, time after time, from a color-blind race, all those individuals who were best able to see these distinctions, we should, no doubt, at last succeed in producing a perfect color-sense. This is just what natural selection seems to have done in the case of bees and butterflies.
Yet it may be urged that insects perhaps had a color-sense before they began to haunt flowers, and that this sense enabled them to pick out the brighter blossoms from the very beginning. Such an hypothesis would make the origin of beautiful flowers a much more simple matter; but we can hardly accept it, for a very good reason. Before the existence of flowers there was probably nothing upon which insects could exert a color-sense. Now, we know that no faculty ever comes into existence until it is practically of use to its possessors. Thus, animals which always live fixed and immovable in one place never develop eyes, because eyes would be quite useless to them; and even those creatures which possess organs of vision in their young and free state lose them as soon as they settle down for life in a permanent and unchangeable home. So, unless insects had something to gain by possessing a color-sense, they could never get one, prophetically, so to speak, against the contingency of flowers at some time or other appearing. Of course, no creature would develop such a sense merely for the sake of admiring the rainbow and the sunset, or of observing gems and shells or other such bright-hued but useless bodies. It is in the insect's practical world of food-hunting and flower-seeking that we must look for the original impulse of the color-sense.
Again, throughout the whole animal world, we see good reasons for concluding that, as a matter of fact, and apart from such deductive reasoning, only those species exhibit evident signs of a color-sense to whom its possession would be an undoubted advantage. Thus, in this
