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 papilla 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 
