Evidences of Theory of Natural Selection. 319 
In the first place, we always find a complete cor- 
respondence 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.) 
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 
natural selection cons’sts, it would be needless to observe that it does so 
in the mdzuteness of the protective resemblance which in so many 
cases is presented. Of course where the resemblance is only very general, 
the phenomena might be ascribed to mere coincidence, of which the 
instincts of the animal have taken advantage. But in the measure 
that the 1esemblance becomes minutely detailed, the supposition 
of mere coincidence is 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 sclection 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.), 
