Page:Popular Science Monthly Volume 84.djvu/410

406 not when the hunger is satisfied. There has been much debate as to why the animal no longer responds as well to the sight of food held in a certain position after its hunger is satisfied as before. It would seem sufficient to remember that the afferent impulses coming from a full stomach differ greatly from the afferent impulses coming from an empty one; and it is not, in my opinion, necessary to postulate a mysterious psychical change, of obscure origin, in the animal to explain its failure to take food after its hunger is satisfied. While all the external conditions may be the same, the afferent impulses from the stomach, and hence the group of afferent impulses concerned in the feeding response, is changed after hunger is satisfied. Two different groups of impulses, although they may have certain impulses in common, are not necessarily integrated to the same motor response.

The "Geschlechtstrieb" is similarly associated with a particular group of afferent impulses, dependent from their origin, among other things, upon changes in internal secretions, and changes in the circulatory conditions in particular local regions. A review of the various influences operative in exciting sexual desire of higher animals is found in a paper by von Bechterew, and the argument need not be pursued at length here. Sufficient has been said to show that both the feeding and the sexual act have their driving force in particular groups of afferent impulses, some of which are of internal (proprioceptive or interoceptive) origin. The state of hunger or of sexual desire once established, the consummation of either the feeding or the sexual act is dependent upon external conditions which may be more or less fortuitous. The sight of food when the animal is hungry is an incitement to take food. If the sight of a peculiarly colored butterfly warns the bird, we will say, against taking it as food, we have the afferent impulses of internal and of external origin acting in opposition, and it will readily be seen that if the color of the butterfly is to protect it, that color must convey a very strong stimulus to the bird.

In the case of the sexual act, the conditions under which Darwin imagined that pleasing song or beauty of plumage might be operative are somewhat different. The female is ready for the sexual act, and awaits the coming of the male. To the internal stimuli, there may conceivably be added an external group in the nature of a color pattern that attracts her attention or a song that mingles with the mood or more properly, with the group of other afferent impulses, and the addition of these simple elements, not necessarily powerful in themselves, may be just sufficient to turn the balance in the favor of the male possessing them. In the case of warning coloration, the warning color must act as a powerful deterrent agent, but in the case of a bright plumage or a pleasing song, both external and internal stimuli work together.