have suspected how poor a record of the mutations of life, the best
preserved geological section presented, had not the difficulty of our
not discovering innumerable transitional links between the species
which appeared at the commencement and close of each formation, pressed
so hardly on my theory.
_On the sudden appearance of whole groups of Allied Species_. The
abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several pal ontologists, for
instance, by Agassiz, Pictet, and by none more forcibly than by
Professor Sedgwick, as a fatal objection to the belief in the
transmutation of species. If numerous species, belonging to the same
genera or families, have really started into life all at once, the fact
would be fatal to the theory of descent with slow modification through
natural selection. For the development of a group of forms, all of
which have descended from some one progenitor, must have been an
extremely slow process; and the progenitors must have lived long ages
before their modified descendants. But we continually over-rate the
perfection of the geological record, and falsely infer, because certain
genera or families have not been found beneath a certain stage, that
they did not exist before that stage. We continually forget how large
the world is, compared with the area over which our geological
formations have been carefully examined; we forget that groups of
species may elsewhere have long existed and have slowly multiplied
before they invaded the ancient archipelagoes of Europe and of the
United States. We do not make due allowance for the enormous intervals
of time, which have
probably elapsed between our consecutive formations, longer perhaps in
some cases than the time required for the accumulation of each
formation. These intervals will have given time for the multiplication
of species from some one or some few parent-forms; and in the
succeeding formation such species will appear as if suddenly created.
I may here recall a remark formerly made, namely that it might require
a long succession of ages to adapt an organism to some new and peculiar
line of life, for instance to fly through the air; but that when this
had been effected, and a few species had thus acquired a great
advantage over other organisms, a comparatively short time would be
necessary to produce many divergent forms, which would be able to
spread rapidly and widely throughout the world.
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