ng existed, to be inherited--we can
understand, on the genealogical view of classification, how it is that
systematists have found rudimentary parts as useful as, or even sometimes
more useful than, parts of high physiological importance. Rudimentary
organs may be compared with the letters in a word, still retained in the
spelling, but become useless in the pronunciation, but which serve as a
clue in seeking for its derivation. On the view of descent with
modification, we may conclude that the existence of organs in a
rudimentary, imperfect, and useless condition, or quite aborted, far from
presenting a strange difficulty, as they assuredly do on the ordinary
doctrine of creation, might even have been anticipated, and can be
accounted for by the laws of inheritance.

_Summary._--In this chapter I have attempted to show, that the
subordination of group to group in all organisms throughout all time; that
the nature of the relationship, by which all living and extinct beings are
united by complex, radiating, and circuitous lines of affinities into one
grand system; the rules followed and the difficulties encountered by
naturalists in their classifications; the value set upon characters, if
constant and prevalent, whether of high vital importance, or of the most
trifling {457} importance, or, as in rudimentary organs, of no importance;
the wide opposition in value between analogical or adaptive characters, and
characters of true affinity; and other such rules;--all naturally follow on
the view of the common parentage of those forms which are considered by
naturalists as allied, together with their modification through natural
selection, with its contingencies of extinction and divergence of
character. In considering this view of classification, it should be borne
in mind that the element of descent has been universally used in ranking
together the sexes, ages, and acknowledged varieties of the same species,
however different they may be in structure. If we extend the use of this
element of descent,--the only certainly known cause of similarity in
organic beings,--we shall understand what is meant by the natural system:
it is genealogical in its attempted arrangement, with the grades of
acquired difference marked by the terms varieties, species, genera,
families, orders, and classes.

On this same view of descent with modification, all the great facts in
Morphology become intelligible,--whether we look to the same patter