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for the propagation and fupport of this its native in- 
fed:. If thefe peculiar and diftinguifhing qualities 
reftde, as the chymifts fay, in the effential oil of the 
plant, it will follow, that this, as well as other in- 
feds, fubfifting on vegetables, are by fome means 
or other qualified to extrad, in a nicer manner 
than any chymift can do, the effential oil of their re- 
fpedive plants, nothing elfe therein being of a na- 
ture fufficiently peculiar either to affifl the propaga- 
tion, or fupply the nourifhment, of the infed. 
As to the difficulty, how this plant came to re- 
ceive the eggs of this flie, it is as great in refped 
to the propagation of any other infed on its peculiar 
plant*' The flies of every plant have continual ac- 
cefs to thofe plants, and no doubt are prompted by 
the fight, fmell, or other qualities of their native 
vegetable, which are congenial to them, to propa- 
gate their kind upon them. As this ad is probably 
attended with fome degree of pleafure, it keeps 
them continually bufy in the work of impregnating 
their proper plant. Hence it comes, that before 
the younger plants are removed to a diftance from 
thofe that are more fully grown, they receive fuf- 
ficient colonies from others, already peopled, which 
they extend again to their fuccours, the flies each 
year impregnating all the plants within their reach. 
Whether the thing' happens in this manner, or that 
the eggs of infeds fo fniall are minute enough to 
be carried through the air, and fo difperfed every- 
where, it is neverthelefs a fad, that no vegetable is 
found without its infeds, tho’ propagated by the 
feed. 
