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 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. ("The Evolution Theory",
London, 1904, page 144.)

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