l groups
that the affinities between the two groups admit of being best detected.
And it is obvious that this is just what ought to be the case on the
theory of descent with divergent modification; while, upon the
alternative theory of special creation, no reason can be assigned why
the lowest or the oldest types should thus combine the characters which
afterwards become severally distinctive of higher or newer types.

Again, I have already alluded to the remarkable fact that there is no
correlation between the value of structures to the organisms which
present them, and their value to the naturalist for the purpose of
tracing natural affinity; and I have remarked that up to the close of
the last century it was regarded as an axiom of taxonomic science, that
structures which are of most importance to the animals or plants
possessing them must likewise prove of most importance in any natural
system of classification. On this account, all attempts to discover the
natural classification went upon the supposition that such a direct
proportion must obtain--with the result that organs of most
physiological importance were chosen as the bases of systematic work.
And when, in the earlier part of the present century, De Candolle found
that instead of a direct there was usually an inverse proportion between
the functional and the taxonomic value of a structure, he was unable to
suggest any reason for this apparently paradoxical fact. For, upon the
theory of special creation, no reason can be assigned why organs of
least importance to organisms should prove of most importance as marks
of natural affinity. But on the theory of descent with progressive
modification the apparent paradox is at once explained. For it is
evident that organs of functional importance are, other things equal,
the organs which are most likely to undergo different modifications in
different lines of family descent, and therefore in time to have their
genetic relationships in these different lines obscured. On the other
hand, organs or structures which are of no functional importance are
never called upon to change in response to any change of habit, or to
any change in the conditions of life. They may, therefore, continue to
be inherited through many different lines of family descent, and thus
afford evidence of genetic relationship where such evidence fails to be
given by any of the structures of vital importance, which in the course
of many generations hav