er constancy throughout
large groups of species; and this constancy depends on such organs having
generally been subjected to less change in the adaptation of the species to
their conditions of life. That the mere physiological importance of an
organ does not determine its classificatory value, is almost shown by the
one 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
amongst insects, in one great division of the Hymenoptera, the antennae, as
Westwood has remarked, are most constant in structure; {416} in another
division they differ much, and the differences are of quite subordinate
value in classification; yet no one probably will say that the antennae in
these two divisions of the same order are of unequal physiological
importance. Any number of instances could be given of the varying
importance for classification of the same important organ within the same
group of beings.

Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of high value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants, and
certain rudimentary bones of the leg, are highly serviceable in exhibiting
the close affinity between Ruminants and Pachyderms. Robert Brown has
strongly insisted on the fact that the rudimentary florets are of the
highest importance in the classification of the Grasses.

Numerous instances could be given of character