198 PHILOSOPHY OF ZOOLOGY. 



seems to move, and the earth to be stationary. When 

 bodies move with great velocity, as a cannon ball, they are 

 invisible ; or when the motion is very slow, as the index of 

 a clock, it is likewise imperceptible. 



Although the intimations of external objects obtained by 

 the eye, are, in many cases, apt to mislead, yet, when aid- 

 ed by the sense of touch, and, in some cases, by that of 

 smell, the information communicated to the mind by the 

 organs of vision, is more varied and extensive than that 

 which is derived from any of the other senses. While the 

 errors of the eye may mislead the inferior animals, and man 

 himself, when seeking to supply the wants of existence, yet 

 they have been converted into blessings by the ingenuity 

 of our species, and made subservient to the increase of the 

 pleasure and improvement of civilized society. It must be 

 understood, that we here allude to the deceptive art of Paint- 

 ing. 



The value of the characters furnished by the organs of 

 vision, in the systematical arrangement of animals, is 'sel- 

 dom estimated at a high rate. The differences exhibited 

 in the eyes of animals, otherwise nearly related in form and 

 structure, are so great, that little reliance is placed on the 

 distinctions which they exhibit in the construction of the 

 primary divisions of animals. But in the formation of the 

 inferior groups, the characters of the eye are frequently 

 employed with advantage, as they are remarkable on ac- 

 count of their constancy and obviousness. 



There are many animals in which no trace of organs of 

 vision has been discovered. Thus, eyes are wanting in 

 the pteropodous and acephalous Mollusca, many of the An- 

 nulosa, and all the Radiata. 



In the vertebral and molluscous animals having eyes, 

 these organs are always two in number. Ikit in the annu- 



