The Senses of Animals. 255 



Not much is known concerning the manner in which 

 the end-organs of smell are stimulated. As in the case of 

 taste, it is probably a matter of molecular vibration ; and 

 Professor William Eamsay has suggested that the end- 

 organs are stimulated by vibrations of a lower order than 

 those which give rise to sensations of light and heat. He 

 has also drawn attention to the fact that to produce a 

 sensation of smell, the substance must have a molecular 

 weight at least fifteen times that of hydrogen. 



It is well known that the sense of smell is in some of 

 the mammalia exceedingly acute. The dog can track his 

 master through a crowded thoroughfare. The interesting 

 experiments of Mr. Eomanes * show that, under ordinary 

 conditions of civilized life, the smell of boot-leather is a 

 factor, and the dog tracks his master's boots. In one case, 

 the boots were soaked in oil of aniseed, but this to us 

 powerful scent did not overcome the normal odour of 

 the master's boots. Mr. W. J. Eussell, in a subsequent 

 number of the same periodical, describes how his pug could 

 find a small piece of biscuit by scent, and this odour of 

 biscuit was not overmastered by a strong smell of eau-de- 

 Cologne. Deer-stalkers know well how keen is the sense 

 of smell in the antlered ruminants. 



We must not, however, be too ready to conclude, from 

 these observations, that the olfactory membrane is absolutely 

 more sensitive in such animals than it is in man. It may 

 well be that, though they are so keen to detect certain 

 scents, they are dull to those which affect us power- 

 fully. It is quite possible that the odour of aniseed or 

 eau-de-Cologne is — possibly from the fact that their end- 

 organs are not attuned to these special molecular vibrations 

 — out of their range of smell. Their special interests in 

 life have led to the cultivation of extreme sensibility to 

 special tones of olfactory sensation. Under unusual cir- 

 cumstances, man may cultivate unwonted modes of utilizing 

 the sense of smell. A boy, James Mitchell, who was born 

 blind, deaf, and dumb, and who was mainly dependent on 



* Nature, vol, xxxvi. p. 273. 



