of protective coloration in connection with sexual selection, since it is clear that the same objection, namely, the need of intelligence, may be urged against the assumption of warning coloration. And warning coloration is effective against some animals with distance receptors, but not necessarily against animals without such sense organs.
Visual and auditory mechanisms make their appearance well down in the animal scale, and hence the possibility of reactions through these mechanisms also arises rather low in the scale. The question then shifts to the central nervous system, and we must inquire whether in any of the lower forms, before the onset of intelligence, reactions to color are possible.
Since the work of Sumner[1] on the change of color pattern in flounders when placed in an aquarium with a certain color pattern in the bottom, we have known that reactions to color do occur under certain conditions. There are, of course, certain limitations to the process, and the mechanism of the response may still require investigation, but the response of the fish to visual impressions is undoubted, and it would be a far cry to postulate intelligence in the process. If a flounder responds to changes in the color pattern of the bottom of an aquarium, it is apparently not a more serious offense to postulate a response to a peculiar color pattern in another flounder. Other observations of the same kind on other animals lead to similar conclusions with regard to response to color. In the terminology of the physiologist, the response is a reflex process, involving an afferent or sensory impulse, the mediation of the central nervous system, and some efferent or outgoing nervous channel to the pigment cells of the skin. The medieval debate as to whether a reflex must always occur through the spinal cord, or even through the palæencephalon[2] is quite beside the point.
Another instance in which the appeal of color or odor is of importance is the attraction of bees and insects generally by flowers. The debate on the color vision of bees is far from closed, but that color plays some role will probably be admitted without prolonged argument. Whether bees have a sense of smell may perhaps be an open question, but that flowers attract bees seems clear. Both flower and insect visitor often have developed extraordinary modifications of the primitive type of structures so that neither insect nor flower can get along without the other. The yucca plant and the Pronuba moth constitute a case in point. Often but one species of insect can fertilize a particular flower, and it may happen that an insect can successfully visit but one species of flower. The question is not so much how the insect knows enough to visit that particular flower, but what particular features of the flower appeal to specific sense organs of the insect.
