These cases of an animal's color and markings harmonizing with the usual environment are called instances of _protective resemblance_; that is, they are resemblances for a purpose, that purpose being to render the animal indistinguishable from its surroundings and thus to aid it in escaping its enemies. Such protective resemblances are obviously of great value to animals, and, like other advantageous modifications, have been produced by the action of natural selection. Those individuals of a species most conspicuous and hence most readily perceived by enemies are the first (under ordinary circumstances) to be captured and eaten. The less conspicuous live and produce young like themselves. Of these young the least conspicuous are again saved and so over and over again through thousands of generations until this natural selecting of the protectively colored results in the production of the wonderfully specialized examples of resemblance to which attention is called in the following paragraphs. =General, variable, and special protective resemblance.=--In the brooks most fishes are dark olive or greenish above and white below. To the birds and other enemies which look down on them they are colored like the bottom. To their fish-enemies which look up from below they are like the white light above them in color and their forms are not clearly seen. The green tree-frogs and tree-snakes which live habitually among green foliage; the mottled gray and tawny lizards and birds and small mammals of the plains and deserts, and the white hares and foxes and owls and ptarmigan of the snowy arctic regions--all show a general protective resemblance. [Illustration: FIG. 162.--The twig or walking-stick insect, _Diapheromera femorata_. (From specimen.)] Sometimes an animal changes color when its surroundings change. Certain hares and grouse of northern latitudes are white in winter when the snow covers all the ground, but in summer when much of the snow melts, revealing the brown and gray rocks and withered leaves, they put on a grayish and brownish coat of hair or feathers. A small insect called the toad-bug (_Galgulus_) lives abundantly on the banks of a pond on the campus of Stanford University. The shores of this pond are covered in some places with bits of bluish rock, in others with bits of reddish rock, and in still others with sand. Specimens of the toad-bug collected from the blue rocks are bluish or leaden in color, those from the red rocks are reddish, and those from the sand are sand-colored. Changes of color to suit the surroundings can be quickly made by some animals. The chameleons of the tropics change momentarily from green to brown, blackish, or golden. There is a little fish (_Oligocottus snyderi_) common in the tide-pools of the Bay of Monterey in California whose color changes quickly to harmonize with the rocks it happens to rest above. Such changing coloration to suit the surroundings may be called _variable protective resemblance_.