metimes 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 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 old doctrine of creation, might even have been anticipated in
accordance with the views here explained.

SUMMARY.

In this chapter I have attempted to show that the arrangement of all
organic beings throughout all time in groups under groups--that the
nature of the relationships by which all living and extinct organisms
are united by complex, radiating, and circuitous lines of affinities
into a few grand classes--the rules followed and the difficulties
encountered by naturalists in their classifications--the value set upon
characters, if constant and prevalent, whether of high or of the
most trifling importance, or, as with 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 if we admit the common parentage of allied forms,
together with their modification through variation and natural
selection, with the 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, dimorphic forms, and acknowledged
varieties of the same species, however much they may differ from each
other in structure. If we extend the use of this element of descent--the
one 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, most of the great facts
in Morphology become intelligible--whether we look to the same pattern
displayed by the different species of the same class in their homologous
organs, to whatever purpose applied, or to the serial and lateral
homologies in each individual animal and plant.

On the principle of su