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.

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 on an organ during early
life; hence the organ will not be reduced or rendered rudimentary at
this early age. The calf, for instance, has inherited teeth, which never
cut through the gums of the upper jaw, from an early progenitor having
well-developed teeth; and we may believe, that the teeth in the mature
animal were formerly reduced by disuse owing to the tongue and palate,
or lips, having become excellently fit