INTRODUCTORY OBSERVATIONS. XIX 



dther reptiles. No evident curvature is feen in 

 the body of a fnail ; biit in the belly are fuc- 

 ceflive undulations, by which its peculiar pro- 

 greffion is performed. It is more convincing, 

 that animalcula have mufcular parts, and perhaps 

 joints formed of membrane, or a fimilar fub- 

 ftance, vi^hen we can adually obferve fins, feet, 

 or fibrilli. Many which we fuppofe deprived 

 of them, really are not fo ; and in others, 

 they are fo minute, flender, or tranfparent, 

 or fo much of the fame colour as the fluid 

 they inhabit, a5 feldom or never to be via- 

 ble. Indeed we often fee particles, in an infu- 

 fion, carried along, at a diftance from the animal- 

 cula, by fome invifible hair. 



When an animal is called imperfect, we mean, 

 that it wants organs with the ufe of which we 

 are acquainted. E:jtternal impreffions are cer- 

 tainly the origin of ideas ; and there is rea* 

 fon to beheve, that mind originates entirely 

 with the ufe that can be made of the fenfes. 

 Undoubtedly we can form no conception of any 

 objed, without the intervention of fome of the 

 fenfes : and, although the mind may wander 

 through all the immeafurable field of imagina^ 

 tion, ftill it can invent no new idea that is with- 

 out any chain or hnk to what is fuggefted by the 

 fenfes. Thus, if it is pofTible to conceive that a 

 c 2 man 



