In warning coloration or terrifying attitudes, nothing can be warned or terrified which does not see the warning or terrifying individual. The only question that arises is again whether the warned or terrified one must be possessed of sufficient intelligence to think the matter over and decide according to the evidence presented to it by the visual mechanism or whether it merely obeys the first impulse. Wallace himself proposed this theory of warning coloration, and it must stand or fall on purely theoretical grounds along with the theory of sexual selection, postulating, as it does, the same mechanisms in the warned individual that Darwin's hypothesis postulates in the attracted individual. There is, however, more direct observational evidence in favor of warning coloration than there is in favor of sexual selection. Birds, as Wallace showed, are warned by certain colors of butterflies.
Whitman's[1] observations on pigeons show that their responses and reactions are in many cases purely instinctive, being elicited without previous education or experience, and occur only when the group of conditions is right. Instinct, to me, at least, means a definite response to a definite group of afferent impulses. The afferent impulses once set up, the response of the central nervous system is the same under the same conditions. As Whitman expressed it, "organization shapes behavior," a statement directly in line with Hermann's law of specific response to stimulation. And from this statement of Whitman's dates the transition of the discussion of animal behavior in metaphysical terms to its discussion in terms of biological entities. In animal behavior, organization centers largely, though not wholly,[2] around the central nervous system and its associated afferent and efferent channels and sense organs. Internal secretions of various ductless glands of the body, and afferent nervous impulses from certain of these various glands are, of course, to be considered in animal behavior, and all of them have their influence on the instinctive responses of animals at various seasons of the year. Yet all these things are to be considered as a part of the organization of the individual at any one time. And the conviction is slowly growing, in my own mind, at least, that it is not so much a single afferent impulse as it is groups of afferent impulses that determine the reactions of animals. A few illustrations will make this point clearer.
Hunger has been shown to be associated with definitive movements of the stomach in man, and hence with definitive afferent impulses from the stomach.[3] In the wider sense of the term, hunger is of reflex origin. The feeding reactions of the newly hatched Necturus are accurately and certainly elicited when the animal is hungry, as Whitman showed, but
