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 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 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,