The truth of the principle that the greatest amount of life can be
supported by great diversification of structure, is seen under many natural
circumstances. In an extremely small area, especially if freely open to
immigration, and where the contest between individual and individual must
be very severe, we always find great diversity in its inhabitants. For
instance, I found that a piece of turf, three feet by four in size, which
had been exposed for many years to exactly the same conditions, supported
twenty species of plants, and these belonged to eighteen genera and to
eight orders, which shows how much these plants differed from each other.
So it is with the plants and insects on small and uniform islets: also in
small ponds of fresh water. Farmers find that they can raise more 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 its nature not to be in any way peculiar), 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, 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.
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