alised
even at the suggestion of change.

The difficulty of indulging in variety is incomparably greater among
the rest of the animal world. If a pea-hen should take it into her
head that bars would be prettier than eyes in the tail of her spouse,
she could not possibly get what she wanted. It would require
hundreds of generations in which the pea-hens generally concurred in
the same view before sexual selection could effect the desired
alteration. The feminine delight of indulging her caprice in matters
of ornament is a luxury denied to the females of the brute world,
and the law that rules changes in taste, if studied at all, can only
be ascertained by observing the alternations of fashion in civilised
communities.

There are long sequences of changes in character, which, like the
tunes of a musical snuff-box, are regulated by internal mechanism.
They are such as those of Shakespeare's "Seven Ages," and others due
to the progress of various diseases. The lives of birds are
characterised by long chains of these periodic sequences. They are
mostly mute in winter, after that they begin to sing; some species
are seized in the early part of the year with so strong a passion
for migrating that if confined in a cage they will beat themselves
to death against its bars; then follow courtship and pairing,
accompanied by an access of ferocity among the males and severe
fighting for the females. Next an impulse seizes them to build nests,
then a desire for incubation, then one for the feeding of their young.
After this a newly-arisen tendency to gregariousness groups them
into large flocks, and finally they fly away to the place whence they
came, goaded by a similar instinct to that which drove them forth a
few months previously. These remarkable changes are mainly due to
the conditions of their natures, because they persist with more or
less regularity under altered circumstances. Nevertheless, they are
not wholly independent of circumstance, because the period of
migration, though nearly coincident in successive years, is modified
to some small extent by the weather and condition of the particular
year.

The interaction of nature and circumstance is very close, and it is
impossible to separate them with precision. Nurture acts before birth,
during every stage of embryonic and pre-embryonic existence, causing
the potential faculties at the time of birth to be in some degree
the effect of nurture. We need not, however, be hy