dd, "If struck by this strange sequence, we turn our
attention to North America, and there discover a series of analogous
phenomena, it will appear certain that all these modifications of
species, their extinction, and the introduction of new ones, cannot be
owing to mere changes in marine currents or other causes more or less
local and temporary, but depend on general laws which govern the whole
animal kingdom." 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. The process of diffusion
may often be very slow, being dependent on climatal and geographical
changes, or on strange accidents, but in the long run the dominant
forms will generally succeed in spreading. The diffusion would, it
is probable, be slower with the terrestrial inhabitants of distinct
continents than with the marine inhabitants of the continuous sea. We
might therefore expect to find, as we apparently do find, a less strict
degree of parallel succ