76 INTRODUCTION. 



that the motions of their spine, or of their several 

 fins, may be abundantly instrumental to the same 

 end. It is a very vicious line of reasoning which 

 leads us to the question, that any alleged object is 

 effected, in certain animals, by any given organ, 

 because the same object is, in other animals, ef- 

 fected without it. 



The same organ, which to man is the instrument 

 of touch, is, to the quadruped that of support 

 to the bird that of flight to the fish that of 

 swimming ; whereas touch, which is in man seated 

 in the hand, is, in other mammiferous animals, 

 seated sometimes in the root of the whiskers, some- 

 times in the snout, sometimes in the tip of the 

 wings, sometimes in the tail ; while, in the duck, 

 its seat is the bill. 



It is probable that all fishes, with very few 

 exceptions, rise occasionally to the surface of the 

 water ; but to what depth they are capable of de- 

 scending with impunity, remains undecided. It is 

 universally known, that the atmosphere exercises a 

 pressure on every thing exposed to it, which goes 

 on, progressively increasing from above, downwards, 

 so that it is the greatest at the surface of the earth ; 

 and that the water exercises a similar pressure, 

 which, in like manner, becomes, progressively 

 greater from the top to the bottom of the mass, 

 so that it is the greatest in immediate contact 

 with the base of the reservoir in which it is con- 

 tained. 



But although near its surface, water exercises 



