Investigated the craniology, postcranial skeleton, dentition, external features and other characteristics of the living species of the genus Equus. The great differences between them point to a long separate evolution for each of the 6 groups: horse, ass, hemione, mountain zebra, Burchell zebra, Grevy zebra. Each should be placed in its own subgenus. Hemiones are not closely related to asses: they are one of the most distinctive groups. Burchell and mountain zebras are also well separated from each other. It is difficult to trace any special kinship between any 2 of these groups.