INTRODUCTION. 13 



Each minute item of evidence — often so faint 

 that civihsed senses can no more apprehend 

 it than the unassisted eye can detect the microbes 

 in a drop of water — has not only to be observed 

 but to be weighed, and given its exact value in a 

 lonsf and intricate argument. 



If I may be allowed to digress from the 

 "spoor" of our present argument for a moment, 

 I should like to point out what seems to have 

 been one exceedingly important factor in the 

 development of the human intellect. In the next 

 chapter I shall discuss, in comparing a man's 

 mental processes with a dog's, the probable 

 psychic effect of the comparative size of the 

 olfactory lobe. I mean by the olfactory lobe that 

 part of the brain — so remarkably developed in the 

 canidcB — which receives impressions from the 

 nerves of smell. 



Now why has man no olfactory lobe to speak 

 of? And what may possibly be the outcome of 

 the deficiency ? The answer to the first question 

 is, that man's progenitors were fruit - eating 

 creatures which lived in trees. Now a frugivor- 

 ous animal obviously does not need a keen power 

 of scent for detecting and following prey. It 

 usually discovers its food by means of the eye, 



