The Australian Shepherd, often known simply as the 'Aussie', is a medium-sized breed of dog that was, despite its name, developed on ranches in the United States during the 19th century. The Australian Shepherd most likely is the descendant of Carea Leonés, Pyrenean Shepherd and Old German Herding dogs that were brought over by shepherds. Merled eyes occur, as well, where one color is mixed in and swirled with another. Any combination of eye color is acceptable in the breed standard, so long as the eyes are healthy. The same pigment that is lacking in the ear can also be lacking in the eyes, affecting its development. Although many believe that only dogs with blue eyes have eye problems, it is not correct. Due to the contrast between eye problems and blue eyes, eye conditions in blue-eyed dogs are much easier to spot.