followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms most
like each other. It might have been thought (and was in ancient times
thought) that those parts of the structure which determined the habits
of life, and the general place of each being in the economy of nature,
would be of very high importance in classification. Nothing can be more
false. No one regards the external similarity of a mouse to a shrew, of
a dugong to a whale, of a whale to a fish, as of any importance. These
resemblances, though so intimately connected with the whole life of the
being, are ranked as merely "adaptive or analogical characters;" but to
the consideration of these resemblances we shall recur. It may even be
given as a general rule, that the less any part of the organisation
is concerned with special habits, the more important it becomes for
classification. As an instance: Owen, in speaking of the dugong, says,
"The generative organs, being those which are most remotely related to
the habits and food of an animal, I have always regarded as affording
very clear indications of its true affinities. We are least likely in
the modifications of these organs to mistake a merely adaptive for an
essential character." With plants how remarkable it is that the organs
of vegetation, on which their nutrition and life depend, are of little
signification; whereas the organs of reproduction, with their product
the seed and embryo, are of paramount importance! So again, in formerly
discussing certain morphological characters which are not functionally
important, we have seen that they are often of the highest service in
classification. This depends on their constancy throughout many allied
groups; and their constancy chiefly depends on any slight deviations not
having been preserved and accumulated by natural selection, which acts
only on serviceable characters.

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