334 PHILOSOPHY OF ZOOLOGY. 



vious. In other genera the retina is formed from the 

 edges of two long white caudae, in the same manner as it 

 arises in birds from the single white line. 



The eye is one of the most important organs which fishes 

 are known to possess. It enables them to perceive the 

 approach of their foes, and it is the principal instrument by 

 which they obtain their food. The amateur in artificial 

 fly-fishing often tempts the fish with one kind of fly, but 

 in vain ; and, upon substituting another in its place, of a 

 different form or colour, succeeds in the capture. These 

 motions of the fish are all regulated by the eye ; hence some 

 fish will bite as readily at a bit of red cloth as at a piece of 

 flesh. 



As this organ exercises a very powerful influence on the 

 habits of fishes, it should be carefully attended to by the 

 ichthyologist. The characters which are furnished by its 

 form and position are not liable to variations, and they are 

 sufficiently obvious. Those furnished by the colours of 

 the different parts hold a secondary rank. They are not 

 very liable to vary, but they experience great changes after 

 death, and should be used with very great caution. 



It was long known to naturalists, that fishes possessed 

 some means of distinguishing the vibrations of sonorous 

 bodies, or possessed the SENSE of HEARING. Trouts and 

 carp have been taught to come to a particular place of the 

 pond for food upon a bell being rung ; and a drum has 

 sometimes been employed to drive fishes into a net. In 

 general, however, it was supposed that the vibrations com- 

 municated to the water, became sensible to the fish, through 

 the medium of the organs of touch. 



The Abbe NOLLET (in the Hist, de TAcad. R. des 

 Sciences, 1743, p. 26.) ascertained, by conclusive experi- 

 ments, that the human ear was susceptible to the impres- 

 sions of sound, even when immersed in water. This dis- 



