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 from the
principle of natural selection; for old forms are supplanted by new and
improved forms. Neither single species nor groups of species reappear when
the chain of ordinary generation is once 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 beings can be classed with all recent beings, naturally
follows from the living and the extinct being the offspring of common
parents. As species have generally diverged in character during their long
course of descent and modification, we can understand why it is that the
more ancient forms, or early progenitors of each group, so often occupy a
position in some degree intermediate between existing groups. Recent forms
are generally looked upon as being, on the whole, higher in the scale of
organisation than ancient forms; and they must be higher, in so far as the
later and more improved forms have conquered the older and less improved
forms in the struggle for life; they have also generally had their organs
more specialised for different functions.
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