78 H. M. JOHNSON 



There may be other factors such as chemical changes by 

 which the deposited substance becomes odorous, etc., but con- 

 sideration of them only increases the presumption against the 

 intensity-difference theory. 



It has been suggested also that the dog may have an acute 

 olfactory sensitivity to the form of the tracks made by his quarry 

 and follow the trail from heel to toe. Certain features of the 

 dog's behavior certainly indicate that he is very sensitive to 

 differences of spatial position of olfactory stimuli. Another sug- 

 gestion is that the smell-substances deposited by the different 

 parts of the foot or body may differ specifically in stimulating 

 quality, and that the dog is affected by this difference. Assuming 

 either of these suggestions as a complete explanation of the dog's 

 hunting behavior would require us to expect a bloodhound 

 striking a man's trail at right angles, to back-track if the man 

 had walked backward instead of forward across the field. 



Dr. P. W. Cobb has suggested a simple hypothesis; that the 

 dog's sense of direction may be due to the trailing of ground 

 smell-substances. For instance: the smell-substances affecting 

 a dog trailing a man who had crossed a mint-bed might be (1) 

 ground + man; (2) ground + man -f mint, the mint being 

 intense; (3) ground + man + mint, the mint-smell-substances 

 diminishing rapidly in the direction the man had taken. The 

 hypothesis impresses the writer as being valuable, although it 

 does not afford a complete explanation of the facts as variously 

 alleged. 



The value of careful field-tests should be apparent. The 

 question may well be raised whether the hunting-behavior of the 

 dog is really an olfactory response. A comparison of the field- 

 behavior of anosmic dogs and normal dogs of the same litter and 

 of a hunting breed, such as the beagle-hound, should prove 

 highly interesting. It would be well worth while to ascertain 

 as a beginning what responses a good hunting dog actually makes 

 when introduced to trails the time and direction of which had 

 previously been ascertained. The effect of numerous disturbing 

 factors which could be introduced, some of which have been 

 suggested above, ought to be quite interesting. It is to be 

 hoped that some one with proper facilities and ample training 

 may become interested enough to make an experimental in- 

 vestigation in this field. 



