way peculiar in its nature), and may be said to be striving to the
utmost to live there; but, it is seen, 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
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 can gain some crude idea in what manner some of the
natives would have had to be modified, in order to have gained an
advantage over the other natives; and we may, I think, at least safely
infer that diversification of structure, amounting to new generic
differences, would have been 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
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