he two following chapters I shall discuss, as well as the
difficulty of the subject permits, the several laws which govern
Variability. These may be grouped under the effects of use and disuse,
including changed habits and acclimatisation--arrests of
development--correlated variation--the cohesion of homologous parts--the
variability of multiple parts--compensation of growth--the position of buds
with respect to the axis of the plant--and lastly, analogous variation.
These several subjects so graduate into each other that their distinction
is often arbitrary.

It may be convenient first briefly to discuss that co-ordinating and
reparative power which is common, in a higher or lower degree, to all
organic beings, and which was formerly designated by physiologists as the
_nisus formativus_.

Blumenbach and others[716] have insisted that the principle which
permits a Hydra, when cut into fragments, to develop itself into two or
more perfect animals, is the same with that which causes a wound in the
higher animals to heal by a cicatrice. Such cases as that of the Hydra
are evidently analogous with the spontaneous division or fissiparous
generation of the lowest animals, and likewise with the budding of
plants. Between these extreme cases and that of a mere cicatrice we
have every gradation. Spallanzani,[717] by cutting off the legs and
tail of a Salamander, got in the course of three months six crops of
these members; so that 687 perfect bones were reproduced by one animal
during one season. At whatever {294} point the limb was cut off, the
deficient part, and no more, was exactly reproduced. Even with man, as
we have seen in the twelfth chapter, when treating of polydactylism,
the entire limb whilst in an embryonic state, and supernumerary digits,
are occasionally, though imperfectly, reproduced after amputation. When
a diseased bone has been removed, a new one sometimes "gradually
assumes the regular form, and all the attachments of muscles,
ligaments, &c., become as complete as before."[718]

This power of regrowth does not, however, always act perfectly: the
reproduced tail of a lizard differs in the forms of the scales from the
normal tail: with certain Orthopterous insects the large hind legs are
reproduced of smaller size:[719] the white cicatrice which in the
higher animals unites the edges of a deep wound is not formed of