is a genealogical arrangement, in which we
have to discover the lines of descent by the most permanent characters,
however slight their vital importance may be.

The framework of bones being the same 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 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 intelligible on the view of the gradual modification of parts or
organs, which were alike in the early progenitor of each class. 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 can
clearly see why the embryos of mammals, birds, reptiles, and fishes should
be so closely alike, and should be 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 in 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 tend to reduce an
organ, when it has become useless by changed habits or under changed
conditions {480} of life; and we can clearly 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 of
acting on an organ during early life; hence the organ will not be much
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 reduced, during successive
generations, by disuse or by the tongue and palate having been better
fitted by natural selection to browse without their aid; whereas in the
calf, the teeth have been left untouched by selection or disuse, and on the
principle of inheritance at corresponding ages have been inherited from a
remote period to the present