" M. Barrande has made forcible remarks to precisely
the same effect. It is, indeed, quite futile to look to changes of
currents, climate, or other physical conditions, as the cause of these
great mutations in the forms of life throughout the world, under the
most different climates. 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 new varieties arising, which have
some advantage over older forms; and those forms, which are already
dominant, or have some advantage over the other forms in their own
country, would naturally oftenest give rise to new varieties or
incipient species; for these latter must be victorious in a still
higher degree in order to be preserved and to survive. We have
distinct evidence on this head, in the plants which are dominant, that
is, which are commonest in their own homes, and are most widely
diffused, having produced the greatest number of new varieties. It is
also natural that the dominant, varying, and far-spreading species,
which already have 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 new
varieties and species.
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