so conspicuous a part in the history of the organic world, almost
inevitably follows on the principle of natural selection; for old forms
will be supplanted by new and improved forms. Neither single species
nor groups of species reappear when the chain of ordinary generation
has once been broken. The gradual diffusion of dominant forms, with the
slow modification of their descendants, causes the forms of life, after
long intervals of time, to appear as if they had changed simultaneously
throughout the world. The fact of the fossil remains of each formation
being in some degree intermediate in character between the
fossils in the formations above and below, is simply explained by their
intermediate position in the chain of descent. The grand fact that all
extinct organic beings belong to the same system with recent beings,
falling either into the same or into intermediate groups, follows from
the living and the extinct being the offspring of common parents. As
the groups which have descended from an ancient progenitor have
generally diverged in character, the progenitor with its early
descendants will often be intermediate in character in comparison with
its later descendants; and thus we can see why the more ancient a
fossil is, the oftener it stands in some degree intermediate between
existing and allied groups. Recent forms are generally looked at as
being, in some vague sense, higher than ancient and extinct forms; and
they are in so far higher as the later and more improved forms have
conquered the older and less improved organic beings in the struggle
for life. Lastly, the law of the long endurance of allied forms on the
same continent, of marsupials in Australia, of edentata in America, and
other such cases, is intelligible, for within a confined country, the
recent and the extinct will naturally be allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the
world to another, owing to former climatal and geographical changes and
to the many occasional and unknown means of dispersal, then we can
understand, on the theory of descent with modification, most of the
great leading facts in Distribution. We can see why there should be so
striking a parallelism in the distribution of organic beings throughout
space, and in their geological succession throughout time; for in both
cases the beings have been connected by the bond of ordinary
generation, and the means of
modification have been the same. We see the full meaning of the
wonderful fact, which must have struck every traveller, namely, that on
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