One or two illustrations of recognition marks in creatures will be sufficient. A rabbit when alarmed and fleeing to its burrow displays a conspicuous, upturned white tail. The rest of the body is protectively colored. This conspicuous white surface of the upturned tail is a signal flag of danger. The rabbit mostly feeds during moonlight nights, or soon after sunset. The white upturned tails of alarmed rabbits serve as signals and guides to the feeble and young, and also to those at a more remote distance. Thus a number of rabbits, each following the one or two in front, are all able in the quickest manner to reach a place of safety. The spring-bok has a white patch on the face and one on the sides. It also has a curiously well-marked white stripe above the tail. When the animal is at rest this last-mentioned white stripe is very nearly concealed by a fold of skin, but when it is in motion it comes into full view, like the upturned tail of the rabbit, and serves as a guide to friends. There are some animals inhabiting the Arctic regions that are not white,—they are not protectively colored. Such is the musk-sheep. This animal, though living in Arctic regions, is yet brown and conspicuous. Its safety depends upon its association in small herds. It is gregarious. Therefore it is of much more importance to this animal that it should be able to quickly recognize its companions at a distance than that it should be protectively colored and so concealed from its enemies. So long as they keep together in herds they are abundantly able to protect themselves. This is an exception to the rule of local color adaptation that proves the rule. =Sexual Selection.= Among most backboned animals it is the rule that both sexes should be alike in color. This is especially true among the fishes, reptiles, and mammals. But in birds the diversity of sexual coloring is very frequent. It is among this class, therefore, that Sexual Selection can best be studied. One of the most fundamental characteristics of birds is the greater conspicuousness of coloration in the males. In the tropical regions especially are found the most striking examples of divergence in sexual color patterns. In humming birds, the pheasants, the peacocks, chatterers, tanagers, and birds of paradise, the females are exceptionally dull-colored and plain, while the male birds are gorgeously colored and conspicuously attractive. The male birds of paradise, for instance, are not only brilliantly colored, but also have remarkable gorgets, plumes, and crests; whereas the female paradise birds are without these decorations and as plain as our thrushes in their ornamental plumage (Plate VIII).