 When we see leaf @-@ eating insects green , and bark @-@ feeders mottled @-@ grey ; the alpine ptarmigan white in winter , the red @-@ grouse the colour of heather , and the black @-@ grouse that of peaty earth , we must believe that these tints are of service to these birds and insects in preserving them from danger . Grouse , if not destroyed at some period of their lives , would increase in countless numbers ; they are known to suffer largely from birds of prey ; and hawks are guided by eyesight to their prey , so much so , that on parts of the Continent persons are warned not to keep white pigeons , as being the most liable to destruction . Hence I can see no reason to doubt that natural selection might be most effective in giving the proper colour to each kind of grouse , and in keeping that colour , when once acquired , true and constant . 
