as the bases of their classifications when these were
consciously artificial, were selected because it seemed that they were
the structures most likely to prove of use in subsequent attempts at
working out the natural system.

This general principle of natural affinity, of which all naturalists
have seen more or less well-marked evidence in organic nature, and after
which they have all been feeling, has sometimes been regarded as
natural, but more often as supernatural. Those who regarded it as
supernatural took it to consist in a divine ideal of creation according
to types, so that the structural affinities of organisms were to them
expressions of an archetypal plan, which might be revealed in its
entirety when all organisms on the face of the earth should have been
examined. Those, on the other hand, who regarded the general principle
of affinity as depending on some natural causes, for the most part
concluded that these must have been utilitarian causes; or, in other
words, that the fundamental affinities of structure must have depended
upon fundamental requirements of function. According to this view, the
natural classification would eventually be found to stand upon a basis
of physiology. Therefore all the systems of classification up to the
earlier part of the present century went upon the apparent axiom, that
characters which are of most importance to the organisms presenting them
must be characters most indicative of natural affinities. But the truth
of the matter was eventually found to be otherwise. For it was
eventually found that there is absolutely no correlation between these
two things; that, therefore, it is a mere chance whether or not organs
which are of importance to organisms are likewise of importance as
guides to classification; and, in point of fact, that the general
tendency in this matter is towards an inverse instead of a direct
proportion. More often than not, the greater the value of a structure
for the purpose of indicating natural affinities, the less is its value
to the creatures presenting it.

Enough has now been said to show three things. First, that long before
the theory of descent was entertained by naturalists, naturalists
perceived the fact of natural affinities, and did their best to
construct a natural system of classification for the purpose of
expressing such affinities. Second, that naturalists had a kind of
instinctive belief in some one principle running through the whole