ltaneous in a geological sense.

The fact of the forms of life changing simultaneously, in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. {325} de Verneuil and d'Archiac. After referring to the
parallelism of the palaeozoic forms of life in various parts of Europe, they
add, "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 {326} 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