e Echinodermata, the Mollusca, and the
Arthropoda--we have a repetition of just the same kind of evidence in
favour of continuous descent, with adaptive modification in sundry
lines, as that which I have thus briefly sketched in the case of the
Vertebrata. The roads are different, but the method of travelling is the
same. Moreover, when the embryology of the Worms is closely studied, the
origin of these different roads admits of being clearly traced. So that
when all this mass of evidence is taken together, we cannot wonder that
evolutionists should now regard the science of comparative embryology as
the principal witness to their theory.

CHAPTER V.

PALAEONTOLOGY.

The present Chapter will be devoted to a consideration of the evidence
of organic evolution which has been furnished by the researches of
geologists. On account of its direct or historical nature, this branch
of evidence is popularly regarded as the most important--so much so,
indeed, that in the opinion of most educated persons the whole doctrine
of organic evolution must stand or fall according to the so-called
"testimony of the rocks." Now, without at all denying the peculiar
importance of this line of evidence, I must begin by remarking that it
does not present the denominating importance which popular judgment
assigns to it. For although popular judgment is right in regarding the
testimony of the rocks as of the nature of a history, this judgment, as
a rule, is very inadequately acquainted with the great imperfections of
that history. Knowing in a general way what magnificent advances the
science of geology has made during the present century, the public mind
is more or less imbued with the notion, that because we now possess a
tolerably complete record of the chronological succession of geological
formations, we must therefore possess a correspondingly complete record
of the chronological succession of the forms of life which from time to
time have peopled the globe. Now in one sense this notion is partly
true, but in another sense it is profoundly false. It is partly true if
we have regard only to those larger divisions of the vegetable or animal
kingdoms which naturalists designate by the terms classes and orders.
But the notion becomes progressively more untrue when it is applied to
families and genera, while it is most of all untrue when applied to
species. That this must be so may be rendered apparent by two
considerations.

In the f