uity of descent,--the only
known cause of the similarity of organic beings,--is the bond, hidden as it
is by various degrees of {414} modification, which is partially revealed to
us by our classifications.

Let us now consider the rules 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 have to 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." So with plants, how remarkable
it is that the organs of vegetation, on which their whole life depends, are
of little signification, excepting in the first main divisions; whereas the
organs of reproduction, with their product the seed, are of paramount
importance!

We must not, therefore, in classifying, trust to resemblances in parts of
the organisation, however important {415} they may be for the welfare of
the being 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 great