But the case is very different; and Alph.
de Candolle has well remarked, in his great and admirable work, that floras gain by naturalisation, proportionally with the number of the native genera and species, far more in new genera than in new species.
To give a single instance: in the last edition of Dr.
Asa Gray's "Manual of the Flora of the Northern United States," 260 naturalised plants are enumerated, and these belong to 162 genera.
We thus see that these naturalised plants are of a highly diversified nature.

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