n, that where
they come into the closest competition with each other, the advantages of
diversification of structure, with the accompanying differences of habit
and constitution, determine that the inhabitants, which thus jostle each
other most closely, shall, as a general rule, belong to what we call
different genera and orders.

The same principle is seen in the naturalisation of {115} plants through
man's agency in foreign lands. It might have been expected that the plants
which have succeeded in becoming naturalised in any land would generally
have been closely allied to the indigenes; for these are commonly looked at
as specially created and adapted for their own country. It might, also,
perhaps have been expected that naturalised plants would have belonged to a
few groups more especially adapted to certain stations in their new homes.
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. They differ, moreover, to a large extent from the
indigenes, for out of the 162 genera, no less than 100 genera are not there
indigenous, and thus a large proportional addition is made to the genera of
these States.

By considering the nature of the plants or animals which have struggled
successfully with the indigenes of any country, and have there become
naturalised, we may gain some crude idea in what manner some of the natives
would have to be modified, in order to gain an advantage over the other
natives; and we may at least safely infer that diversification of
structure, amounting to new generic differences, would be profitable to
them.

The advantage of diversification in the inhabitants of the same region is,
in fact, the same as that of the physiological division of labour in the
organs of the same individual body--a subject so well elucidated by Milne
{116} Edwards. No physiologist doubts that a stomach adapted to digest
vegetable matter alone, or flesh alone, draws most nutriment from these
substances. So in the general economy of any land, the more widely and
pe