It may metaphorically be said that natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations: rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, _whenever and wherever opportunity offers_, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of Time has marked the lapse of ages, and then so imperfect is our view into long-past geological ages that we see only that the forms of life are now different from what they formerly were. * * * * * [Page 66.] Although natural selection can act only through and for the good of each being, yet characters and structures, which we are apt to consider as of very trifling importance, may thus be acted on. When we see leaf-eating insects green and bark-feeders mottled-gray, the Alpine ptarmigan white in winter, the red-grouse the color of heather, 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 eye-sight 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 natural selection might be effective in giving the proper color to each kind of grouse, and in keeping that color, when once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any particular color would produce little effect: we should remember how essential it is in a flock of white sheep to destroy a lamb with the faintest trace of black. WHY HABITS AND STRUCTURE ARE NOT IN AGREEMENT. [Origin of Species, page 142.]