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If the difficulties be not insuperable in admitting that in the long course of time
individuals of the same species, and likewise of
→allied species,
have proceeded from some one source; then
all the grand leading facts of geographical distribution are explicable on the theory of
→(generally of the more dominant forms of life),
together with subsequent modification and the multiplication of new forms. We can thus understand the high importance of barriers, whether of land or water,
→which separate our
several zoological and botanical provinces. We can thus understand the
of
→sub-genera, genera, and families;
and how it is that under different latitudes, for instance in South America, the inhabitants of the plains and mountains, of the forests, marshes, and deserts, are
→in
so mysterious a
→linked together,
and are likewise linked to the extinct beings which formerly inhabited the same continent. Bearing in mind that the mutual
of organism to organism
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
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
more or less direct competition with each other and with the
and according as the immigrants were capable of varying more or less rapidly, there would ensue in
→different
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
→as we do find,
some groups of beings greatly, and some only slightly
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