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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
the original pattern, or to transpose
The bones of a limb might be shortened and
to any extent,
→and become gradually
enveloped in thick membrane, so as to serve as a fin; or a webbed
might have all its bones, or certain
lengthened to any extent,
the membrane connecting them
→to any extent,
so as to serve as a
yet
all
→this great amount of modification there will be no tendency
to alter the framework of
or the relative connexion of the
parts. If we suppose that
→the ancient progenitor,
the archetype as it may be
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
class. So with the mouths of insects, we have only to suppose that their common progenitor had an upper lip, mandibles, and two
of maxillæ, these parts being perhaps very simple in form; and then natural
→acting on some originally created form, will
account for the infinite diversity in
and
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
and ultimately by the complete abortion of certain parts, by the
of other parts, and by the doubling or multiplication of others,—
which we know to be within the limits of possibility. In the paddles of the
sea-lizards, and in the mouths of certain suctorial crustaceans, the
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