INTRODUCTION. 63 



what heavier than water, their hones, for e^anjple, 

 generally sinking in this fluid ; consequently, had 

 Nature not provided them with a sufficient supply 

 of some substance lighter than water, by which their 

 tendency to sink in this fluid, at least at all ordinary 

 depths might be counteracted, it would have re- 

 quired a constant effort on their parts to keep 

 themselves at any given level. This tendency is 

 accordingly provided against, in a great measure, 

 by the quantity of fat with which fishes are in 

 general furnished, and which, being very nearly m 

 such proportion to the soHd parts as to bring their 

 body, collectively taken, to about the same specific 

 gravity as that of the water which they inhabit, 

 supersedes in them the necessity of making any 

 efibrts, except for the purpose of changing their 

 situation. It is well known of how oleaginous a 

 nature is the flesh of many fishes commonly used 

 at table — the salmon and eel, for example ; and in 

 the internal parts of fishes, in general, the quan- 

 tity of fat is still more remarkable. The gall of 

 many is little else than a kind of oil; and the 

 enormous quantity of this fluid which may be 

 obtained from the liver of the basking shark, the 

 fod, the ling, with several other fishes, is suf- 

 ficiently well known; it is said, that the liver 

 of a single basking shark frequently affords seven 

 or eight barrels of oil. Fishes have no true lungs, 

 which, to all the terrestrial and aerial tribes of 

 animals, as always containing a considerable quan- 

 tity of air, are one great source of buoyancy ; but 



