Bearing in mind that the mutual relation of
organism to organism is of the highest importance, we can see why two
areas, having nearly the same physical conditions, should often be
inhabited by very different forms of life; for according to the length of
time which has elapsed since the colonists entered one of the regions, or
both; according to the nature of the communication which allowed certain
forms and not others to enter, either in greater or lesser numbers;
according or not as those which entered happened to come into more or less
direct competition with each other and with the aborigines; and according
as the immigrants were capable of varying more or less rapidly, there would
ensue in the to or more regions, independently of their physical
conditions, infinitely diversified conditions of life; there would be an
almost endless amount of organic action and reaction, and we should find
some groups of beings greatly, and some only slightly modified; some
developed in great force, some existing in scanty numbers--and this we do
find in the several great geographical provinces of the world.
On these same principles we can understand, as I have endeavoured to show,
why oceanic islands should have few inhabitants, but that of these, a large
proportion should be endemic or peculiar; and why, in relation to the means
of migration, one group of beings should have all its species peculiar, and
another group, even within the same class, should have all its species the
same with those in an adjoining quarter of the world.
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