w groups of 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. It will be seen in my third work that these complex affinities
and the rules for classification receive a rational explanation on the
principle of descent, together with modifications acquired through natural
selection, entailing 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 {12} a single progenitor! So it is, if we look
to the structure of an individual animal or plant, when we see the fore and
hind limbs, the skull and vertebrae, the jaws and legs of a crab, the
petals, stamens, and pistils of a flower, built on the same type or
pattern. During the many changes to which in the course of time all 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 utterly useless condition can, on the
descent-theory, be simply understood. On the principle of modifications
being inherited at the same age in the child, at which each successive
variation first appeared in the parent, we shall see why rudimentary parts
and organs are generally well developed in the individual at a very early
age. On the same principle of inheritance at corresponding ages, and on the
principle of variations not generally supervening at a very early period of
embryonic growth (and both these principles can be shown to be probable
from direct evidence), that most wonderful fact in the whole round of
natural history, namely, the similarity of members of the same great class
in their embryonic condition,--the embryo, for instance, of a mammal, bird,
reptile, and fish being barely distinguishable,--becomes simply
intelligible.

It is the consideration and explanation of such facts as these which has
convinced me that the theory of descent with modification by means of
natural selection is