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 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,