All the northern weasels become pure white in winter when they live in a region where the snow lies continuously and the cold is steady; but south of that line they do not change color. The change from the summer brown to the winter white—when they become "ermines"—is produced by an actual loss of color in the hair; but the spring change back to brown is effected by shedding the old white hair and getting a new brown coat. In the Middle Ages ermine fur was permitted to be worn only by royalty, and later by judges on the bench. A somewhat different, and strictly American, species is the mink. It is somewhat less slender than the weasels, and is semiaquatic in habits, dwelling always near streams, where it feeds on earthworms, frogs, and fishes. Having this kind of food, and being keen-witted and secretive, it has been able to continue to exist in the midst of civilization, and the vast number of its dark pelts that come to the fur market are nearly all got by farmers' boys in traps set near home.