eet of man, the hands of the
ape, the paws of the beast of prey, the hoof of the horse and of the ox,
the paws of the mole, the fins of the seal and of the whale, the
wing-membranes of the flying-squirrel, correspond to one another in their
smallest parts and ossicles, and can all be registered with the same
numbers and letters; _i.e._, they are homologous to one another even to the
minutest detail. The _ideal_ plan and connection in the organisms,
disclosed by these facts, and long ago acknowledged and admired, receives
at the same time its simple _material_ basis through the acceptance of a
common descent. {73}

A similar relation is observed in _rudimentary organs_.

Many of them, as the nipples of males, point, if not to a common descent
from a lower form, at least to a common plan of the sexes. But when the
embryo of the whale still has its teeth in the jaw, the grown up whale its
hip-bones, when the eye of man still has its winking membrane, the ear and
many portions of the skin their rudimentary muscles of motion, the end of
the vertebral column its rudimentary tail, the intestinal canal its blind
intestine; when sightless animals, living in the dark, still have their
rudimentary eyes, blind worms their shoulder-blades; when in like manner
the plants, especially in their parts of fecundation, show in great number
such rudimentary organs as are entirely useless for the functions of life,
but which are never misleading in determining their relationship with other
plants:--how simply are all these facts explained by the descent theory,
how not at all without it!

Finally, if we now mention the _history of the development of animals_, we
shall have to postpone to the next section the consideration of the most
essential facts furnished by this science; for the individual development
of animals is a process which could speak not only for a descent of the
species, but also for a descent of them through gradual development. But
where, as in the present section, we treat the descent theory apart from
the evolution theory, we have also to think of the possibility that the
species or groups of species are not originated through gradual
development, but nevertheless do originate through descent--namely, in
leaps through metamorphosis of germs or a heterogenetic generation; and for
such an idea we find confirmation in the {74} observation of the history of
development of animals, which we call _change of generation_ or
_met