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.
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