closely allied in structure, and we can at once understand the fact if they
are closely allied by descent. The succession of the many distinct species
of the same genus throughout the long series of geological formations seems
to have been unbroken or continuous. New species come in gradually one by
one. Ancient and extinct forms of life are often intermediate in character,
like the words of a dead language with respect to its several offshoots or
living tongues. All these facts seemed to me to point to descent with
modification as the means of production of new species.

The innumerable past and present inhabitants of the world are connected
together by the most singular and complex affinities, and can be classed in
groups under groups, in the same manner as varieties can be classed under
species and sub-varieties under varieties, but with much higher grades of
difference. These complex affinities and the rules for classification,
receive a rational explanation on the theory of descent, combined with the
principle of natural selection, which entails divergence of character and
the extinction of intermediate forms. How inexplicable is the similar
pattern of the hand of a man, the foot of a dog, the wing of a bat, the
flipper of a seal, on the doctrine of independent acts of creation! how
simply explained on the principle of the natural selection of successive
slight variations in the diverging descendants from a single progenitor! So
it is with certain parts or organs in the same individual animal or plant,
for instance, the jaws and legs of a crab, or the petals, stamens, and
pistils of a flower. During the many changes to which in the course of time
organic beings have been subjected, certain organs or parts have
occasionally become at first of little use and ultimately superfluous; and
the retention of such parts in a rudimentary and useless condition is
intelligible on the theory of descent. It can be shown that modifications
of structure are generally inherited by the offspring at the same age at
which each successive variation appeared in the parents; it can further be