CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
5. 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 palaeontologists--for
instance, by Agassiz, Pictet, and 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 at once, the
fact would be fatal to the theory of evolution through natural selection.
For the development by this means of a group of forms, all of which are
descended from some one progenitor, must have been an extremely slow
process; and the progenitors must have lived long before their modified
descendants. But we continually overrate 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.
In all cases positive palaeontological evidence may be implicitly trusted;
negative evidence is worthless, as experience has so often shown. 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 the
United States. We do not make due allowance for the enormous intervals of
time which have elapsed between our consecutive formations, longer perhaps
in many 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 parent-form: and in the succeeding formation, such
groups or 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; and consequently that the
transitional forms would often long remain confined to some one region; but
that, when this adaptation had once 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 spread
rapidly and widely throughout the world. Professor Pictet, in his
excellent Review of this work, in commenting on early transitional forms,
and taking birds as an illustration, cannot see how the successive
modifications of the anterior limbs of a supposed prototype could possibly
have been of any advantage. But look at the penguins of the Southern
Ocean; have not these birds their front limbs in this precise intermediate
state of "neither true arms nor true wings?" Yet these birds hold their
place victoriously in the battle for life; for they exist in infinite
numbers and of many kinds. I do not suppose that we here see the real
transitional grades through which the wings of birds have passed; but what
special difficulty is there in believing that it might profit the modified
descendants of the penguin, first to become enabled to flap along the
surface of the sea like the logger-headed duck, and ultimately to rise from
its surface and glide through the air?