All the foregoing rules and aids and difficulties in classification may be
explained, if I do not greatly deceive myself, on the view that the natural
system is founded on descent with modification--that the characters which
naturalists consider as showing true affinity between any two or more
species, are those which have been inherited from a common parent, all true
classification being 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.
But I must explain my meaning more fully. I believe that the ARRANGEMENT
of the groups within each class, in due subordination and relation to each
other, must be strictly genealogical in order to be natural; but that the
AMOUNT of difference in the several branches or groups, though allied in
the same degree in blood to their common progenitor, may differ greatly,
being due to the different degrees of modification which they have
undergone; and this is expressed by the forms being ranked under different
genera, families, sections or orders. The reader will best understand what
is meant, if he will take the trouble to refer to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied genera
existing during the Silurian epoch, and descended from some still earlier
form.
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