characterise distinct genera, for they usually
differ much in the species of the same genus, and are highly variable in
the individuals of the same species; we have also seen in the earlier
chapters of this work how variable secondary sexual characters become under
domestication.

_Summary of the three previous Chapters, on the Laws of Variation._

In the twenty-third chapter we have seen that changed conditions
occasionally act in a definite manner on the organisation, so that all, or
nearly all, the individuals thus exposed become modified in the same
manner. But a far more frequent result of changed conditions, whether
acting directly on the organisation or indirectly through the reproductive
system being affected is indefinite and fluctuating variability. In the
three latter chapters we have endeavoured to trace some of the laws by
which such variability is regulated.

Increased use adds the size of a muscle, together with the blood-vessels,
nerves, ligaments, the crests of bone to which these are attached, the
whole bone and other connected bones. So it is with various glands.
Increased functional activity strengthens the sense-organs. Increased and
intermittent pressure thickens the epidermis; and a change in the nature of
the food sometimes modifies the coats of the stomach, and increases or
{353} decreases the length of the intestines. Continued disuse, on the
other hand, weakens and diminishes all parts of the organisation. Animals
which during many generations have taken but little exercise, have their
lungs reduced in size, and as a consequence the bony fabric of the chest,
and the whole form of the body, become modified. With our anciently
domesticated birds, the wings have been little used, and they are slightly
reduced; with their decrease, the crest of the sternum, the scapulae,
coracoids, and furcula, have all been reduced.

With domesticated animals, the reduction of a part from disuse is never
carried so far that a mere rudiment is left, but we have good reason to
believe that this has often occurred under nature. The cause of this
difference probably is that with domestic animals not only sufficient time
has not been granted for so profound a change, but that, from not being
exposed to a severe struggle for life, the principle of the economy of
organisation does not come into action. On the contrary, we sometimes see
that structures which are rudimentary in the parent-species become
partially redev