INTRODUCTORY OBSERVATIONS, xliil 



leeches furviving three years in water without 

 any vifible food. This inflance, and that of the 

 hydrachn^, indeed, re different from total abfti- 

 nence ; for birds which drink feldom, and even 

 carnivorous animals, have fubfifled on water a- 

 lonc for a very coiifidcrable time. Whether in- 

 vifibie animalcuia might be the aliment of thofe 

 animals, 1 cannot tell, but I have feen mails, 

 which were kept months in water only, void 

 very fenfible excretions, and increafe the fize of 

 their faeils, though thefe contmued uncommonly 

 tranfparent, nor -zoiila the excretions be the re- 

 inains of food in the ftomnch, for the fnails had 

 never ate ; they were young ; and I had bred 

 them from the eesr. Iiowevcr, this is little more 

 furprifmg than that plants fhould vegecate and 

 attain confiderable fize in water, for both they 

 and animals extraG: fome nutritive part, which, 

 by various decompofitions, fecretions, and affimi- 

 iations, is converted into their own fubftance. 

 Now, indeed, it is fuppofed that the excretions of 

 plants are difcovercd. 



If daily experience did not teftify the reverfe, 

 it might reafonably be conceived that all the 

 parts of an animal were of equal ufe to it ; and 

 that privation of one would be as injurious as 

 privation of any other. But we are acquainted 

 with fome animals which can lofe a portion of 



the 



