e modified form, but often affecting by correlation of growth
other parts of the organisation. In changes of this nature, there will be
little or no tendency to modify the original pattern, or to transpose
parts. The bones of a limb might be shortened and widened to any extent,
and become gradually enveloped in thick membrane, so as to serve as a fin;
or a webbed foot might have all its bones, or 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 maxillae, these parts being perhaps very simple in form; and
then natural selection, acting on some originally created form, 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 {436} 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 vertebrae. 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