certain bones, lengthened to any extent, and the membrane connecting
them increased to any extent, so as to serve as a wing: yet in all this
great amount of modification there will be no tendency to alter the
framework of bones or the relative connexion of the several parts. If
we suppose that the ancient progenitor, the archetype as it may be
called, of all mammals, had its limbs constructed on the existing
general pattern, for whatever purpose they served, we can at once
perceive the plain signification of the homologous construction of the
limbs throughout the whole class. So with the mouths of insects, we
have only to
suppose that their common progenitor had an upper lip, mandibles, and
two pair of maxill , these parts being perhaps very simple in form; and
then natural selection will account for the infinite diversity in
structure and function of the mouths of insects. Nevertheless, it is
conceivable that the general pattern of an organ might become so much
obscured as to be finally lost, by the atrophy and ultimately by the
complete abortion of certain parts, by the soldering together of other
parts, and by the doubling or multiplication of others, variations
which we know to be within the limits of possibility. In the paddles of
the extinct gigantic sea-lizards, and in the mouths of certain
suctorial crustaceans, the general pattern seems to have been thus to a
certain extent obscured.
There is another and equally curious branch of the present subject;
namely, the comparison not of the same part in different members of a
class, but of the different parts or organs in the same individual.
Most physiologists believe that the bones of the skull are homologous
with that is correspond in number and in relative connexion with the
elemental parts of a certain number of vertebr . The anterior and
posterior limbs in each member of the vertebrate and articulate classes
are plainly homologous. We see the same law in comparing the
wonderfully complex jaws and legs in crustaceans. It is familiar to
almost every one, that in a flower the relative position of the sepals,
petals, stamens, and pistils, as well as their intimate structure, are
intelligible on the view that they consist of metamorphosed leaves,
arranged in a spire. In monstrous plants, we often get direct evidence
of the possibility of one organ being transformed into another; and we
can actually see in embryonic crustaceans and in many other animals,
and in flowers, that organs, which when mature
become extremely different, are at an early stage of growth exactly
326