So, in
Australia, the rabbit, though totally unlike any native animal, has
increased so much that it probably outnumbers in individuals all the
native mammals of the country; and in New Zealand the rabbit and the pig
have equally multiplied. Darwin remarks that this "advantage of
diversification of structure 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 body. No physiologist doubts that a stomach adapted
to digest vegetable matter alone, or flesh alone, draws more 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."[39]
_The most closely allied Species inhabit distinct Areas._
One of the curious results of the general action of this principle in
nature is, that the most closely allied species--those whose differences
though often real and important are hardly perceptible to any one but a
naturalist--are usually not found in the same but in widely separated
countries.
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