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- 
Ng. 
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. But in the annu- 
