n are elaborated in
the thirteenth chapter of the _Origin of Species_ (1st ed., 1859). A
good part of this chapter is given up to a discussion of the principles
of classification, only a few pages dealing with morphology proper. But,
as Darwin rightly saw, the two things are inseparable.

We note first that there is no hint of the "scale of beings"--Darwin
conceives the genealogical tree as many branched. Animals can be classed
in "groups under groups," and cannot be arranged in one single series.

He discusses first what kind of characters have the greatest
classificatory value. Certain empirical rules have been recognised, more
or less consciously, by systematists--that analogical characters are
less valuable than homological, that characters of great physiological
importance are not always valuable for classificatory purposes, that
rudimentary organs are often very useful, and so on. He finds that as a
general rule "the less any part of the organisation is concerned with
special habits, the more important it becomes for classification" (p.
414), and adduces in support Owen's remark that the generative organs
afford very clear indications of affinities, since they are unlikely to
be modified by special habits. These rules of classification can be
explained "on the view that the natural system is founded on descent
with modification; that the characters which naturalists consider as
showing true affinity ... are those which have been inherited from a
common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike" (p. 420).

In general, then, homological characters are more valuable for
classificatory purposes because they have a longer pedigree than
analogical characters, which represent recent acquirements of the race.

Coming to morphology proper, Darwin takes up the question of the unity
of type, and the homology of parts, for which the unity of type is but a
general expression.

He treats this on the same lines as E. Geoffroy St Hilaire, and Owen,
referring indeed specifically to Geoffroy's law of connections. "What
can be more curious," he asks, "than that the hand of a man, formed for
grasping, that of a mole for digging, the leg of a horse, the paddle o