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.
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