We must, as Barrande has
remarked, look to some special law. We shall see this more clearly when we
treat of the present distribution of organic beings, and find how slight is
the relation between the physical conditions of various countries and the
nature of their inhabitants.
This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species
are formed by having some advantage over older forms; and the forms, which
are already dominant, or have some advantage over the other forms in their
own country, give birth to the greatest number of new varieties or
incipient species. We have distinct evidence on this head, in the plants
which are dominant, that is, which are commonest and most widely diffused,
producing the greatest number of new varieties. It is also natural that
the dominant, varying and far-spreading species, which have already
invaded, to a certain extent, the territories of other species, should be
those which would have the best chance of spreading still further, and of
giving rise in new countries to other new varieties and species. The
process of diffusion would often be very slow, depending on climatal and
geographical changes, on strange accidents, and on the gradual
acclimatization of new species to the various climates through which they
might have to pass, but in the course of time the dominant forms would
generally succeed in spreading and would ultimately prevail.
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