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 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. 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 do find, a less strict degree of parallelism in the
succession of the productions of the land than with those of the sea.

Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the world,
accords well with the principle of new species having been formed by
dominant species spreading widely and varying; the new species thus
produced being themselves dominant, owing to their having had some
advantage over their already dominant parents, as well as over other
species; and again spreading, varying, and producing new forms. The
old forms which are beaten and which yield their places to the new and
victorious forms, will generally be allied in groups, from