50 THE PHILOSOPHY 



fore wifely ordained, that the earth fhould ahvays be cover- 

 ed with plants : But flie has alfo ordained, that particular 

 tribes fhould die at flated periods, to make way for the ex- 

 iftence of others. The fame inconvenience would happen 

 if the produ<Stion of all animals, and particularly that vaft 

 number of fpecies, and that immenfe profufion of individu- 

 als, to which the infe£l tribes give birth^ were to take place 

 at one period. The air would be fo crowded with noxious 

 creatures, that neither man nor the larger animals could poffi- 

 bly exift. Beiides, the fpecies which feed upon particular 

 plants, if they were produced at a time when thefe plants did 

 not flourlfh, would Infallibly perlfli for want of food. In 

 Lapland, where the duration of heat Is extremely fhort, the 

 whole Infe6ls which inhabit that dreary and barren region 

 are produced In a few weeks. Though the number of fpe- 

 cies, compared with thofe of the more prolific climates, be 

 very limited, the inconvenience is feverely felt. But every 

 natural evil is accompanied with fome advantage. The rein- 

 deer, upon which the exiftence of the Laplanders chiefly de- 

 pends, are tormented by the fwarms Of flies. To avoid their 

 numberlefs enemies, thefe animals leave the vallies, and 

 afcend the mountains, where the cold Is too great for the 

 flies to follow. In thefe lofty regions, the reln-deer feed 

 during the hot feafon, and return to the vallies after the cold 

 has deflroyed the myriads of InfecSls. This forced migration 

 has too good effects : It both preferves the health of the 

 rein-deer, and the vegetables In the vallies, which otherwife 

 would have been prematurely exhaufted. 



The operation of engrafting was long thought to be pecu- 

 Har to the vegetable kingdom. But M. Trembley found, 

 that feveral fpecies of the frefh-water polypus could fuccefs- 

 fully undergo this wonderful procefs. Since his time, it has 

 been difcovered, that the aEllnia^ or fea-nettle. Is likewlfe 

 capable of being engrafted to an individual of the fame or of 



