And this fact
shows that a character, which is generally of generic value, when it sinks
in value and becomes only of specific value, often becomes variable, though
its physiological importance may remain the same. Something of the same
kind applies to monstrosities: at least Is. Geoffroy St. Hilaire
apparently entertains no doubt, that the more an organ normally differs in
the different species of the same group, the more subject it is to
anomalies in the individuals.
On the ordinary view of each species having been independently created, why
should that part of the structure, which differs from the same part in
other independently created species of the same genus, be more variable
than those parts which are closely alike in the several species? I do not
see that any explanation can be given. But on the view that species are
only strongly marked and fixed varieties, we might expect often to find
them still continuing to vary in those parts of their structure which have
varied within a moderately recent period, and which have thus come to
differ. Or to state the case in another manner: the points in which all
the species of a genus resemble each other, and in which they differ from
allied genera, are called generic characters; and these characters may be
attributed to inheritance from a common progenitor, for it can rarely have
happened that natural selection will have modified several distinct
species, fitted to more or less widely different habits, in exactly the
same manner: and as these so-called generic characters have been inherited
from before the period when the several species first branched off from
their common progenitor, and subsequently have not varied or come to differ
in any degree, or only in a slight degree, it is not probable that they
should vary at the present day.
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