circuitous. We see why certain
characters are far more serviceable than others for classification; why
adaptive characters derived from rudimentary parts, though of no service
to the beings, are often of high classificatory value; and why
embryological characters are often the most valuable of all. The real
affinities of all organic beings, in contradistinction to their adaptive
resemblances, are due to inheritance or community of descent. The
Natural System is a genealogical arrangement, with the acquired grades
of difference, marked by the terms, varieties, species, genera,
families, etc.; and we have to discover the lines of descent by the most
permanent characters, whatever they may be, and of however slight vital
importance.

The similar framework of bones in the hand of a man, wing of a bat, fin
of the porpoise, and leg of the horse--the same number of vertebrae
forming the neck of the giraffe and of the elephant--and innumerable
other such facts, at once explain themselves on the theory of descent
with slow and slight successive modifications. The similarity of pattern
in the wing and in the leg of a bat, though used for such different
purpose--in the jaws and legs of a crab--in the petals, stamens, and
pistils of a flower, is likewise, to a large extent, intelligible on
the view of the gradual modification of parts or organs, which were
aboriginally alike in an early progenitor in each of these classes. On
the principle of successive variations not always supervening at an
early age, and being inherited at a corresponding not early period of
life, we clearly see why the embryos of mammals, birds, reptiles, and
fishes should be so closely similar, and so unlike the adult forms. We
may cease marvelling at the embryo of an air-breathing mammal or bird
having branchial slits and arteries running in loops, like those of a
fish which has to breathe the air dissolved in water by the aid of
well-developed branchiae [gills].

Disuse, aided sometimes by natural selection, will often have reduced
organs when rendered useless under changed habits or conditions of life;
and we can understand on this view the meaning of rudimentary organs.
But disuse and selection will generally act on each creature, when it
has come to maturity and has to play its full part in the struggle for
existence, and will thus have little power in an organ during early
life; hence the organ will not be reduced or rendered rudimentary at
this early