4-0 T H E P H I L O S O P H Y 



die at ftated periods, to make way for the exiftence of others. The 

 fame inconvenience would happen, if the produdion of all animals, 

 and particularly that vaft number of fpecies, and that immenfe pro- 

 fufion of individuals, to which the infedl tribes give birth, were to 

 take place at one period. The air would be fo crowded with noxi- 

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

 bly exift. Befides, the fpecies which feed upon particular plants, if 

 they were produced at a time when thefe plants did not fiourifh, 

 would infallibly perifh for want of food. In Lapland, where the 

 duration of heat is extremely fhort, the whole infedls which inhabit 

 . that dreary and barren region are produced in a few weeks. Though 

 the number of fpecies, compared with thofe of the more prolific cli- 

 mates, be very limited, the inconvenience is feverely felt. But eve- 

 ry natural evil is accompanied with fome advantage. The rein- 

 deer, upon which the exiftence of the Laplanders chiefly depends, 

 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 rein-deer feed during the hot feafon, and return to the 

 vallies after the cold has deftroyed the myriads of infeds. This 

 forced migration has two good effeds : 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 peculiar to 

 the vegetable kingdom. But M. Trembley found, that feveral fpe- 

 cies of the frefh-water polypus could fuccefsfully undergo this won- 

 derful procefs. Since his time, it has been difcovered, that the ac- 

 tinia, or fea-nettle, is likewife capable of being engrafted to an in- 

 dividual of the fame or of a different fpecies. In all thefe inftances, 

 the portions of the divided animals grow together, and become di- 

 ftind individuals. 



2 Having 



