FRANKINCENSE AND MYRRH 99 



Where scent-glands occur in both sexes among 

 mammals, they may contribute to a mutual sex- 

 appeal, or they may facilitate the recognition 

 of kindred and of well-frequented roadways. In 

 certain cases they may be protectively repellent: 

 thus shrews are in some measure saved from cats 

 by the odoriferous gland which runs along the side 

 of their body. In the homing of many ants odor- 

 iferous particles serve as guide-posts, and the 

 accuracy with which a dog tracks his master's 

 footsteps is one of the marvels of everyday life. 

 Of the chemistry of animal scents little is known, 

 but in insects they include fatty acids, even salicylic 

 acid, free iodine, and in a common millipede of 

 greenhouses actually hydrocyanic acid all of them 

 holding out a promise to the investigator. 



The sense of smell is nearest that of taste, and the 

 two probably merge in some of the fishes. In 

 smelling we are affected by minute particles which 

 are dissolved on the moist surface of the olfactory 

 membrane in our nostrils; in tasting we are 

 affected by substances similarly dissolved on the 

 taste papillae of our tongue. We can smell ex- 

 tremely dilute solutions which we cannot taste. 

 Thus a very minute amount of material coming 

 from a far-off object is sufficient to stimulate our 

 sense of smell, but insufficient to affect taste. 

 Therefore, as Professor Sherrington has put it, 

 our olfactory organs are " distance-receptors," as 

 compared with our gustatory organs. In both 

 cases the stimulus is due to the chemical action of 



