INTRODUCTORY OBSERVATIONS. XV 



tlicir numbers in comparifon to invifible animal-. 

 cula and vermiculi ? One hundred and fifty 

 millions have been computed in the milt of a 

 fingle fifh. 



How docs it happen that the earth is not over- 

 run by animals, and that they find food fufEcient 

 for the prefervation of life ? In a day, an immenfe 

 legion will fometimes arife, carrying famine and 

 defolation along with it. — To prefen^e the re- 

 quifite balance, there mufr be deftrudion propor- 

 tioned to propagation ; and the wifdom of Nature 

 feems to have provided for it in a certain degree. 

 The animal, its egg, or the young, are all liable 

 to perifli ; and the more flages it has to pafs 

 through before maturity, the greater hazard does 

 it undergo. Many females produce thoufa^" Js 

 of eggs, without any commerce with the male ; 

 it often happens that thefe are never fecundated, 

 or a very fmall portion of them, if external fe- 

 cundation Ihould take place. The young, in 

 their tender ftate, may either be deftroyed by the 

 elements, or become a prey to thofe ftronger than 

 themfelves. From their various metamorphofes, 

 they are liable to perilh by difeafe, or from un~ 

 fuitable fituations. Therefore, comparatively 

 fpeaking, few come to maturity. The number 

 pf butterflies is very inconliderable, m proportion 



to 



