OF NATURAL HISTORY. 141 



any change, Is called its vis inertiaey and is always propor- 

 tional to the quantity of matter in any particular body. 

 When we double or triple a body, we uniformly find, that 

 the force reqiiilite to move it with equal celerity muft like- 

 wife be doubled or tripled. Thefe, and fimilar faifts, which 

 are refults of perpetual experience, fliow that body is equal- 

 ly indifferent to motion and reft ; that this indifference 

 feems to be the natural confequence q£ the moft abfolute 

 inactivity •, and that the power of beginning motion is pe- 

 culiar to aiTbive and intelligent beings. Leaving, therefore, 

 all metaphyfical fpeculations on this fubjeiSt, we fhall give 

 fome remarks upon the motions of animals. 



In general, all the progrefUve motions of animals are per- 

 formed by the inftrumentality of mufcles, tendons, and arti- 

 culations. The operation of mufcles depends upon fome 

 unknown influence derived to them from the brain and 

 nerves. Hence the brain and nerves are the fources of every 

 motion as well as of every fenfation. With regard to the 

 caufes which determine the a6i:ions of animals, thefe muft be 

 referred to fenfation, and the confequent exertions of intel- 

 le£l. The firft impreflion an objedl makes upon our fenfa- 

 tions ftimulates us either to approach or retire from it, ac- 

 cording as it excites afFe^ion or averfion. Thefe motions 

 neceffarily refult from the firli imprelEon made by the ob- 

 ]eO:. But man, and many other animals, have the power of 

 reiifting thefe original motives to acStion, and of remaining 

 at reft, without either retiring or approaching. < If a man,' 

 fays the Count de BufFon, < were deprived of fight, he 



< would make no movement to gratify his eyes. The fame 



< thing would happen, if he were deprived of any of the 



< other fenfes ; and, if deprived of every fenfe, he would 



< remain perpetually at reft, and no obje(5l would excite him 



< to move, though, by natural conformation, he were fully 



< capable of motion.* Natural wants, as that of taking nour- 



S 



