an at once see
why their crossed offspring should follow the same complex laws in their
degrees and kinds of resemblance to their parents,--in being absorbed into
each other by successive crosses, and in other such points,--as do the
crossed offspring of acknowledged varieties. On the other hand, these would
be strange facts if species have been independently created, and varieties
have been produced by secondary laws.

If we admit that the geological record is imperfect in an extreme degree,
then such facts as the record gives, support the theory of descent with
modification. New species have come on the stage slowly and at successive
intervals; and the amount of change, after equal intervals of time, is
widely different in different groups. The extinction of species and of
whole groups of species, which has played 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 {476} 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