RESPONSIVE LIFE OF ORGANISMS 



445 



selection, and may be made to serve other perceptive func- 

 tions, such as the locating of enemies or the recognition of 

 friends and kindred in terrestrial animals. Dogs, and many 

 wild vertebrates, depend upon this sense apparently for a 

 very large part of the knowledge of the world they live in. 



The sense of smell in the human species is not at its best 

 development. Anyone may convince himself of this by 

 the most casual observation of the actions of his own dog. 

 We cannot know, of course, what smells are like to a dog, 

 any more than we may know of any other sense impression 

 of which we have had no experience ; but we cannot doubt 

 that they are to him a means of fine discrimination. 



Since chemical substances must be in solution, or in 

 gaseous form, in order that they may affect the organs of 

 taste and smell, the receptors of these organs in terrestrial 

 animals are withdrawn into moistened cavities within the 

 body, where they are protected from evaporation. 



Since the animal body is a sort of chemical engine, it is 

 not strange that chemical stimuli are frequently the actuat- 

 ing causes that call forth various responses within the body. 

 It is by means of chemical reactions set a-going within the 

 retina of the eye, that the weak stimuli of light vibrations 

 are reinforced and made effective in proportion to their 

 importance. 



3. Rays of light produce the sensation of vision. At its 

 beginning, vision is nothing more, perhaps, than a dim 

 awareness of a difference existing between light and dark- 

 ness. The eye doubtless has had a long line of antecedents. 

 It may have begun as a red "eye spot" like that of Euglena, 

 and may have owed its earliest efficiency as a receptor of 

 vibrations to the greater absorbing power for radiant energy 

 of the lower colors of the spectrum. And it may have been 

 responsive to heat rays rather than to light rays at first. 



