OUR FACE FROM FISH TO MAN 



backward into the forebrain, of which indeed they 

 form the dominant part. If favorable signals are 

 transmitted by the smelling nerves, the eyes turn 

 toward the source of the odor and by means of the 

 locomotor machinery the whole "ship" is steered 

 in the right direction. The two olfactory capsules, 

 rather widely separated from each other on either 

 side of the head, not only double the chance of 

 picking up a trail of olfactory value, but doubtless 

 also serve as directional organs. The bilateral 

 arrangement of the other sense organs may have 

 a similar significance. 



The resemblances of the shark nose to the human 

 nose are fundamental and the subsequent changes 

 in this organ are relatively not great. The ultimate 

 mystery with regard to all the sense organs of 

 vertebrates is decidedly not what are the broad 

 stages of their evolution from fish to man, but 

 what physical and chemical forces acting upon the 

 primitive vertebrate skin caused one set of epi- 

 thelial cells to become sensitive to olfactory 

 stimulations, another set to respond to light, others 

 to physical vibrations of different rates, and still 

 others to be deaf and blind to all other stimuli 



except those coming from within the organism; 



156 



