les followed and
difficulties met with being the same. The existence of genera, families,
orders, &c., and their mutual relations, naturally ensues from
extinction going on at all periods amongst the diverging descendants of
a common stock. These terms of affinity, relations, families, adaptive
characters, &c., which naturalists cannot avoid using, though
metaphorically, cease being so, and are full of plain signification.

CHAPTER VIII

UNITY OF TYPE IN THE GREAT CLASSES; AND MORPHOLOGICAL STRUCTURES

_Unity of Type_{454}.

{454} _Origin_, Ed. i. p. 434, vi. p. 595. Ch. VIII corresponds to
a section of Ch. XIII in the _Origin_, Ed. i.

Scarcely anything is more wonderful or has been oftener insisted on than
that the organic beings in each great class, though living in the most
distant climes and at periods immensely remote, though fitted to widely
different ends in the economy of nature, yet all in their internal
structure evince an obvious uniformity. What, for instance, is more
wonderful than that the hand to clasp, the foot or hoof to walk, the
bat's wing to fly, the porpoise's fin{455} to swim, should all be built
on the same plan? and that the bones in their position and number should
be so similar that they can all be classed and called by the same names.
Occasionally some of the bones are merely represented by an apparently
useless, smooth style, or are soldered closely to other bones, but the
unity of type is not by this destroyed, and hardly rendered less clear.
We see in this fact some deep bond of union between the organic beings
of the same great classes--to illustrate which is the object and
foundation of the natural system. The perception of this bond, I may
add, is the evident cause that naturalists make an ill-defined
distinction between true and adaptive affinities.

{455} _Origin_, Ed. i. p. 434, vi. p. 596. In the _Origin_, Ed. i.
these examples occur under the heading _Morphology_; the author
does not there draw much distinction between this heading and that
of _Unity of Type_.

_Morphology._

There is another allied or rather almost identical class of facts
admitted by the least visionary naturalists and included under the name
of Morphology. These facts show that in an individual organic being,
several of its organs consist of some other organ metamorphosed{456}:
thus the sepals, petals, stamens, pistils, &c. of every plant can be
shown to be metam