This group is represented in limited areas on our southern border by two species. One of these, the Gailliard jack rabbit (_Lepus gailliardi_), occurs on the grassy plains of extreme southwestern New Mexico and is succeeded by other white-sided species southward across the Mexican tableland and through interior Oaxaca to the Pacific coast, on the Isthmus of Tehuantepec. The other species, the antelope jack rabbit, occupies a considerable area in southwestern Arizona, and with its geographic races ranges southward through the coastal plains of Sonora and Sinaloa to northern Tepic. All jack rabbits are more or less closely related to the Old World hares, the term “rabbit” having been so generally misapplied to them by the early settlers in the western United States that the name is now fixed by current usage. In Mexico and among the Mexicans of our southwestern border the proper distinction is made and the jack rabbit is termed _liebre_, or hare, and cottontail is called _conejo_, or rabbit. The white-sided species are more widely differentiated from their Old World relatives than the other jack rabbits and are the southernmost representatives of the true hares in America, reaching their limit in the tropics a little beyond the Isthmus of Tehuantepec. The extension of the white on the sides of these species assists in producing one of the most extraordinary examples of directive coloration known among mammals. I had the pleasure of discovering this one day in May, 1895, when hunting on horseback over the grassy plain bordering the Pacific coast of the Isthmus of Tehuantepec. As I rode slowly along, a big jack rabbit hopped deliberately from its form in the grass a few yards away, and by the contraction of a special set of muscles along the back drew the dark-colored dorsal area forward and together so that it formed only a narrow band on the middle of the back, with a corresponding extension of the white area on the rump and sides until, as the animal moved diagonally away, it looked almost entirely white. At a distance of fifty or sixty yards it came to a stop, and expanded and contracted the dark dorsal area, thus producing a “flashing” effect with the changing area of white on the sides and rump. This solved the riddle of the mirror-like white flashes I had often seen as jack rabbits on the tableland had dashed away in the brilliant sunshine. The same habit of “flashing” the white was afterwards observed in the species of southwestern New Mexico and southwestern Arizona, demonstrating the appropriateness of the name, “antelope jack rabbit,” given them by the ranchmen.