CHAPTER II. 

 LIGHT-SENSE. 



THE most primitive visual function is the perception of 

 light, and the eyes of some animals are so constructed that 

 this would seem to be the only purpose for which they 

 exist. 



According to Prof. Elliot Smith, 1 in the forerunners of 

 the Mammalia, and in the primitive mammals themselves, 

 the senses of sight and hearing were but poorly developed 

 compared with that of smell. This was due," he says, 

 'not only to the fact that the sense of smell had already 

 installed its instruments in, and taken possession of, the 

 cerebral hemisphere, long before the advent in this dominant 

 part of the brain of any adequate representation of the other 

 senses, but also, and chiefly, because to a small land-grubbing 

 animal the guidance of smell impressions, whether in search 

 for food or as a means of recognition of friends or enemies, 

 was much more serviceable than all other senses. Thus 

 the small creature's mental life was lived essentially in the 

 atmosphere of odors, and every object in the outside world 

 was judged primarily and predominantly by its smell, the 

 sense of touch, vision and hearing being merely auxiliary 

 to the compelling influence of smell." 



The smallest and least developed eyes in existing mammals 

 are found in moles, hedgehogs, insectivorous bats and 

 cave-rats. All of them are nocturnal in their habits and 

 have onlv rods in their retinae no cones. The visual 



