4 o6 TEE POPULAR SCIENCE MONTHLY 



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 cer- 

 tain position after its hunger is satisfied as before. It would seem suffi- 

 cient 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 com- 

 mon, 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 circu- 

 latory 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, 6 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 fortu- 

 itous. 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 addi- 

 tion 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 plum- 

 age or a pleasing song, both external and internal stimuli work together. 



6 Bechterew, Archiv fur Physiologie (Englemann's), 1905, p. 524. 



