tle or no tendency to alter the original pattern, or to transpose
the parts. The bones of a limb might be shortened and flattened to any
extent, becoming at the same time enveloped in thick membrane, so as to
serve as a fin; or a webbed hand might have all its bones, or certain
bones, lengthened to any extent, with the membrane connecting them
increased, so as to serve as a wing; yet all these modifications would
not tend to alter the framework of the bones or the relative connexion
of the parts. If we suppose that an early progenitor--the archetype,
as it may be called--of all mammals, birds and reptiles, 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 class. So with the
mouths of insects, we have only to suppose that their common progenitor
had an upper lip, mandibles, and two pairs of maxillae, 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 reduction and ultimately by the complete abortion of
certain parts, by the fusion 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 gigantic extinct
sea-lizards, and in the mouths of certain suctorial crustaceans, the
general pattern seems thus to have become partially obscured.

There is another and equally curious branch of our subject; namely,
serial homologies, or the comparison of the different parts or organs
in the same individual, and not of the same parts or organs in different
members of the same class. Most physiologists believe that the bones of
the skull are homologous--that is, correspond in number and in relative
connexion--with the elemental parts of a certain number of vertebrae.
The anterior and posterior limbs in all the higher vertebrate classes
are plainly homologous. So it is with the wonderfully complex jaws
and legs of 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