of which Linnæus has set us the example, as will hereafter be explained.
Generic characters are reckoned by Linnæus of three kinds, the factitious, the essential, and the natural, all founded on the fructification alone, and not on the inflorescence, nor any other part.
The first of these serves only to discriminate genera that happen to come together in the same artificial order or section; the second to distinguish a particular genus, by one striking mark, from all of the same natural order, and consequently from all other plants; and the third comprehends every possible mark common to all the species of one genus.
The factitious character can never stand alone, but may sometimes, commodiously enough, be added to more essential distinctions, as the insertion of the petals in Agrimonia, Engl. Bot. t. 1335, indicating the natural order to which the plant belongs, which character, though essential to that order, here becomes factitious.
Linnæus very much altered his notions of the essential character after he had published
