islets; and so in small ponds of fresh water. Farmers find
that they can raise most food by a rotation of plants belonging to the
most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any
small piece of ground, could live on it (supposing it not to be in
any 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