ng, consistent with what is required for the maintenance of the
species.

Everything depends upon adaptation! We have spoken much of adaptation
in colouring, in connection with the examples brought into prominence
by Darwin, because these are conspicuous, easily verified, and at the
same time convincing for the theory of selection. But is it only
desert and polar animals whose colouring is determined through
adaptation? Or the leaf-butterflies, and the mimetic species, or the
terrifying markings, and "warning-colours" and a thousand other kinds
of sympathetic colouring? It is, indeed, never the colouring alone
which makes up the adaptation; the structure of the animal plays a
part, often a very essential part, in the protective disguise, and
thus _many_ variations may cooperate towards _one_ common end. And it
is to be noted that it is by no means only external parts that are
changed; internal parts are _always_ modified at the same time--for
instance, the delicate elements of the nervous system on which depend
the _instinct_ of the insect to hold its wings, when at rest, in a
perfectly definite position, which, in the leaf-butterfly, has the
effect of bringing the two pieces on which the marking occurs on the
anterior and posterior wing into the same direction, and thus
displaying as a whole the fine curve of the midrib on the seeming
leaf. But the wing-holding instinct is not regulated in the same way
in all leaf-butterflies; even our indigenous species of Vanessa, with
their protective ground-colouring, have quite a distinctive way of
holding their wings so that the greater part of the anterior wing is
covered by the posterior when the butterfly is at rest. But the
protective colouring appears on the posterior wing and on the tip of
the anterior, _to precisely the distance to which it is left
uncovered_. This occurs, as Standfuss has shown, in different degrees
in our two most nearly allied species, the uncovered portion being
smaller in _V. urticae_ than in _V. polychloros_. In this case, as in
most leaf-butterflies, the holding of the wing was probably the
primary character; only after that was thoroughly established did the
protective marking develop. In any case, the instinctive manner of
holding the wings is associated with the protective colouring, and
must remain as it is if the latter is to be effective. How greatly
instincts may change, that is to say, may be adapted, is shown by the
case of the Noctuid "sh