counted 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 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 pattern
displayed in the homologous organs, to whatever purpose applied, of the
different species of a class; or to the homologous parts constructed on
the same pattern in each individual animal and plant.

On the principle of successive slight variations, not necessarily
or generally supervening at a very early period of life, and being
inherited at a corresponding period, we can understand the great leading
facts in Embryology; namely, the resemblance in an individual embryo of
the homologous parts, which when matured will become widely different
from each other in structure and function; and the resemblance in
different species of a class of the homologous parts or organs, though
fitted in the adult members for purposes as different as po