in relation to the outer world. Perhaps from this cause it has
partly arisen, that almost all naturalists lay the greatest stress on
resemblances in organs of high vital or physiological importance. No
doubt this view of the classificatory importance of organs which
are important is generally, but by no means always, true. But their
importance for classification, I believe, depends on their greater
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 most 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; 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 wil