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 consists, it would be needless to observe that it does so 

 in the minuteness 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 resemblance 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 

 icquired 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.). 



