les--first, that
modifications usually supervene late in the life of the individual; and
second, that such modifications tend to be inherited by the offspring at
a corresponding, not early, age (p. 444).

Thus, applying these principles to a hypothetical case of the origin of
new species of birds from a common stock, he writes:--"... from the many
slight successive steps of variation having supervened at a rather late
age and having been inherited at a corresponding age, the young of the
new species of our supposed genus will manifestly tend to resemble each
other much more closely than do the adults, just as we have seen in the
case of pigeons"[355] (pp. 446-7).

Since the embryo shows the generalised type, the structure of the embryo
is useful for classificatory purposes. "For the embryo is the animal in
its less modified state; and in so far it reveals the structure of its
progenitor" (p. 449)--the embryological archetype reveals the ancestral
form. "Embryology rises greatly in interest, when we thus look at the
embryo as a picture, more or less complete, of the parent form of each
great class of animals" (p. 450)--a prophetic remark, in view of the
enormous subsequent development of phylogenetic speculation.

We may sum up by saying that Darwin interpreted von Baer's law
phylogenetically.

The rest of the chapter is devoted to a discussion of abortive and
vestigial organs, whose existence Darwin naturally turns to great
advantage in his argument for evolution. Throughout the whole chapter
Darwin's preoccupation with the problems of classification is clearly
manifest.

On the question as to whether descent was monophyletic or polyphyletic
Darwin expressed no dogmatic opinion. "I believe that animals have
descended from at most only four or five progenitors, and plants from an
equal or lesser number.... I should infer from analogy that probably all
the organic beings which have ever lived on this earth have descended
from one primordial form, into which life was first breathed" (p. 484).

Darwin rightly laid much stress upon the morphological evidence for
evolution,[356] which he considered to be weighty. It probably contributed
greatly to the success of his theory. Though he himself did little or no
work in pure morphology, he was alive to the importance of such work,[357]
and followed with interest the progress of evolutionary morphology,
incorporating some of its results in later editions of the _Origin_, and