446 , GENERAL BIOLOGY 



Ability to distinguish light from darkness might help an 

 organism to adjust itself in position, but much more than 

 this is necessary in order that it should really see anything. 

 In order that a picture of an external object should be formed 

 in the mind, it must first be pictorially impressed on multi- 

 ple receptors, so situated that the rays of light coming from 

 different parts of an object may be spatially arranged there- 

 on. Seeing requires eyes. There is no one of the powers 

 of animals in which they differ more profoundly than in 

 their capacity for light perception. Eyes are of the most 

 diverse structural types, and in each of the main types there 

 exist all degrees of perfection. The camera-like eye of 

 vertebrates, with its inverted retinal picture, differs funda- 

 mentally from the compound eye of an arthropod, with its 

 mosaic pattern and its fine adaptation for the perception of 

 movement. The development of the vertebrate eye is one 

 of the most fascinating chapters in biology, but far too long 

 a story to be recounted here. Whatever its structure, the 

 eye consists essentially of multiple receptors combined into 

 a single organ, and sheltered behind a transparent protec- 

 tive covering, the whole occupying an exposed position at 

 the forward end of the body and in direct communication 

 with the more important nerve centers. 



Taste, smell and hearing give us of themselves hardly any 

 conception of form or of magnitude; nor does touch, for 

 large objects, except in successive impressions as parts are 

 successively explored. But the eye may instantly reveal 

 the whole content of environment, in form, in magnitude, in 

 proportions and in action. Such are the precious powers of 

 this incomparable organ ; and so great are the advantages it 

 confers on its possessor that eyeless animals (save when so 

 small as to be beyond the range of ordinary vision) are prac- 

 tically absent from the lighted places of the earth. 



4. Vibrations of air or of solids produce the sensation of 

 sound. They also, under certain conditions, produce a 



