ch 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 degree 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
"shark" moth, Xylina vetusta. This form bears a most deceptive
resemblance to a piece of rotten wood, and the appearance is greatly
increased by the modification of the innate impulse to flight common to
so many animals, which has here been transformed into an almost contrary
instinct. This moth does not fly away from danger, but "feigns death,"
that is, it draws antennae, legs and wings close to the body, and
remains perfectly motionless. It may be touched, picked up, and thrown
down again, and still it does not move. This remarkable instinct must
surely have developed simultaneously with the wood-colouring; at all
events, both cooperating variations are now present, and prove that both
the external and the most minute internal structure have undergone a
process of adaptation.

The case is the same with all structural variations of animal parts,
which are not absolutely insignificant. When the insects acquired wings
they must also have acquired the mechanism with which to move them--the
musculature, and the nervou