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 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
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