218 INTRODUCTION TO ZOOLOGY , 



which gives nothing in vain, has withheld the gift from those 

 species whose dwelling-place is such as to preclude the possi- 

 bility of its exercise. An instance of this is supplied to us 

 from Kentucky, where there is a cavern, known because of its 

 great dimensions by the name of the Mammoth Cave.* It is 

 said to extend to a distance of upwards of twenty miles, and 

 has obviously been excavated by the long continued action of 

 a subterranean river. There is an expanse of this river, about 

 four miles from the entrance, forming a subterranean lake. 

 Here the sense of sight would be useless; and it is found, 

 accordingly, that the fishes which inhabit those gloomy waters 

 are without eyes;t or, to speak more correctly, the visual 

 organs exist only in a rudimentary condition. The capture 

 of these fishes is, nevertheless, difficult, because of the great 

 acuteness of their sense of hearing. 



The eyes of fishes exhibit striking peculiarities. They are 

 without eyelids, properly so called ;J and as the eye is at all 

 times washed by the surrounding water, that gland which 

 supplies moisture to the eye of the higher vertebrate animals 

 is not required, and therefore does not exist. The colours of 

 the eye are of great beauty, varying through various shades 

 of black, blue, red, yellow, and richest orange. 



LOCOMOTION. We now turn to the consideration of the 

 various structural peculiarities, by means of which fishes are 

 enabled to move through the waters with the same, or even 

 greater ease, than the Hawk and the Swallow wing their course 

 through the air. The first bears reference to the weight of 

 the body of the fish, compared with that of the medium in 

 which it lives. The specific gravity, to use the proper term, 

 is nearly the same in both; or, in other words, the weight of 

 the body of the fish is nearly the same as that of an equal 



* There is a popular description of the Mammoth Cave in Chambers's 

 Edinburgh Journal, 1837, vol. vi. and again, in 1843, vol. xii. 



f W. Thompson's Notice of the Blind Fish, Cray-fish, and insects from 

 the Mammoth Cave, Kentucky. Annals of Natural History, vol. xiii. p. 

 112. Some of these blind fish are preserved in the Belfast Museum. Not 

 only the fish, but the Crustacea and insects, are specifically distinct from 

 those found elsewhere ; and in all of them the eyes are apparently wanting, 

 or greatly diminished in size. The "Blind-fish" (Amblyopsis spelceus) 

 is described in Silliman's American Journal of Science, July, 1843, p. 94; 

 and in Annals Nat. Hist. Oct. 1843. 



{ The fold of the skin observed on the eyes of the Dog-fiah and other 

 Sharks, is not generally regarded as a true eyelid. 



