excluded, and the
agency of some specially adaptive cause demonstrated. Again, it is
almost needless to say, no real difficulty is presented (as has been
alleged) by the cases above quoted of seasonal imitations, on the
ground that natural selection could not act alternately on the same
individual. Natural selection is not supposed to act alternately on
the same individual. It is supposed to act always in the same
manner, and if, as in the case of a regularly recurring change in
the colours of the environment, correspondingly recurrent changes
are required to appear in the colours of the animals, natural
selection sets its premium upon those individuals the constitutions
of which best lend themselves to seasonal changes of the needful
kind--probably under the influence of stimuli supplied by the
changes of external conditions (temperature, moisture, &c.).

In the first place, we always find a complete correspondence between
imitative colouring and instinctive endowment. If a caterpillar exactly
resembles the colour of a twig, it also presents the instinct of
habitually reposing in the attitude which makes it most resemble a
twig--standing out from the branch on which it rests at the same angle
as is presented by the real twigs of the tree on which it lives.

Here, again, is a bird protectively coloured so as to resemble stones
upon the rough ground where it habitually lives; and the drawing shows
the attitude in which the bird instinctively reposes, so as still
further to increase its resemblance to a stone. (Fig. 109.)

[Illustration: FIG. 109.--_Oedicnemus crepitans_, showing the
instinctive attitude of concealment. Drawn from a stuffed specimen
in the British Museum, 1/6 nat. size, with appropriate surroundings
supplied.

To take only one other instance, hares and rabbits, like grouse and
partridges--or like the plover just alluded to,--instinctively crouch
upon those surfaces the colours of which they resemble; and I have often
remarked that if, on account of any individual peculiarity of
coloration, the animal is not able thus to secure concealment, it
nevertheless exhibits the instinct of crouching which is of benefit to
all its kind, although, from the accident of its own abnormal colouring,
this instinct is then actually detrimental to the animal itself. For
example, every sportsman must have noticed that the somewhat rare
melanic variety of