That the mere physiological importance of an organ does not determine its
classificatory value, is almost proved by the fact, that in allied groups,
in which the same organ, as we have every reason to suppose, has nearly the
same physiological value, its classificatory value is widely different. No
naturalist can have worked at any group without being struck with this
fact; and it has been fully acknowledged in the writings of almost every
author. It will suffice to quote the highest authority, Robert Brown, who,
in speaking of certain organs in the Proteaceae, says their generic
importance, "like that of all their parts, not only in this, but, as I
apprehend in every natural family, is very unequal, and in some cases seems
to be entirely lost." Again, in another work he says, the genera of the
Connaraceae "differ in having one or more ovaria, in the existence or
absence of albumen, in the imbricate or valvular aestivation. Any one of
these characters singly is frequently of more than generic importance,
though here even, when all taken together, they appear insufficient to
separate Cnestis from Connarus." To give an example among insects: in one
great division of the Hymenoptera, the antennae, as Westwood has remarked,
are most constant in structure; in another division they differ much, and
the differences are of quite subordinate value in classification; yet no
one will say that the antennae in these two divisions of the same order are
of unequal physiological importance.
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