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 Milne Edwards. No physiologist doubts that a stomach by
being 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 perfectly the animals and plants are
diversified for different habits of life, so will a greater number of
individuals be capable of there supporting themselves.
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