ark" 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
cooeperating 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 nervous apparatus necessary for its
automatic regulation. All instincts depend upon compound reflex
mechanisms and are just as indispensable as the parts they have to set
in motion, and all may have arisen through processes of selection if
the reasons which I have elsewhere given for this view are
correct.[54]

Thus there is no lack of adaptations within the organism, and
particularly in its most important and complicated parts, so that we
may say that there is no actively functional organ that has not
undergone a process of adaptation relative to its function and the
requirements of the organism. Not only is every gland structurally
adapted, down to the very minutest histological details, to its
function, but the function is equally minutely adapted to the needs of
the body. Every cell in the mucous lining of the intestine is exactly
regulated in its relation to the different nutritive substances, and
behaves in quite a different way towards the fats, and towards
nitrogenous substances, or peptones.

I have elsewhere called attention to the many adaptations of the whale
to the surrounding medium, and have pointed out--what has long been
known, but is not universally admitted, even now--that in it a great
number of important organs have been transformed in adaptation to the
peculiar conditions of aquatic life, although the ancestors of the
whale must have lived, like other hair-covered mammals, on land. I
cited