OUR DEBT TO INSECTS. 337 



sible, I tliink, 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 tend- 

 ency for some of the nerve-terminals to respond specially to the os- 

 cillations 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 di- 

 versity 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 gen- 

 eration 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 hypothe- 

 sis 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 in- 

 sects 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 perma- 

 nent and unchangeable home. So, unless insects had something to 

 gain by possessing a color-sense, they could never get one, propheti- 

 cally, 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 



TOL. xxT. — 22 



