f
the porpoise, and the wing of the bat, should all be constructed on the
same pattern, and should include similar bones, in the same relative
positions? Geoffroy St Hilaire has strongly insisted on the high
importance of relative position or connection in homologous parts; they
may differ to almost any extent in form and size, and yet remain
connected together in the same invariable order" (p. 434).

The unity of plan cannot be explained on teleological grounds, as Owen
has admitted in his _Nature of Limbs_, nor is it explicable on the
hypothesis of special creation (p. 435). It can be understood only on
the theory that animals are descended from one another and retain for
innumerable generations the essential organisation of their ancestors.
"The explanation is to a large extent simple on the theory of the
selection of successive slight modifications--each modification being
profitable in some way to the modified form, but often affecting by
correlation other parts of the organisation. In changes of this nature,
there will be little or no tendency to alter the original pattern or to
transpose the parts.... If we suppose that the ancient progenitor, the
archetype as it may be called, of all animals, had its limbs constructed
on the existing general pattern, for whatever purpose they served, we
can at once perceive the plain significance of the homologous
construction of the limbs throughout the whole class" (p. 435).

We may note three important points in this passage--first, the
identification of the archetype with the common progenitor; second, the
view that progressive evolution is essentially adaptive, and dominated
by natural selection; and third, the _petitio principii_ involved in the
assumption that adaptive modification brings inevitably in its train the
necessary correlative changes.

In his section on morphology Darwin shows clearly the influence of Owen,
and through him of the transcendental anatomists. He refers to the
transcendental idea of "metamorphosis," as exemplified in the vertebral
theory of the skull and the theory of the plant appendage, and shows
how, on the hypothesis of descent with modification, "metamorphosis" may
now be interpreted literally, and no longer figuratively merely (p.
439).

Very great interest attaches to Darwin's treatment of development, for
post-Darwinian morphology was based to a very large extent on the
presumed relation between the development of the individual and th