Next to the horse (Equus cabattus) comes the ass (Equus asinus) with its varieties, which include the zebras. Naturalists affirm that really wild horses are rare, i.e. horses which have descended directly from parents which have never been domesticated. Wild asses, on the contrary, are common in many parts of the world - in Africa, in Syria, in Persia, in Tartary, in Tibet, up to the frontier of China.
The domestic ass presents some features which require notice. Its size varies in different countries, as also does its colour. The tail is bare of long hairs, excepting the tuft at the end. The ears are longer in comparison with its head than those of the horse, and there are no callosities below the hock joint as there are in the horse. There is commonly a dark stripe running vertically from the top of the shoulder, and another darker in colour extending along the middle of the back, and occasionally there are transverse markings on the legs.
Zebras (Equus zebra) belong to the group of striped asses. There are several varieties, which are distinguished by the length of the ears, by the fulness of the tail and the mane, by the colour and the arrangement of the stripes, by the absence of the callosities on the hind-leg, and by the existence of a modified form on the fore-legs. Quaggas (Equus quagga) are really modified zebras, from which they are chiefly distinguished by the concentration of the stripes on the head and the neck, the markings being less and less distinct from the shoulders back to the haunches, which are perfectly free from stripes. All the varieties of the ass agree in having the horny callosities in a modified form only on the fore-legs. With regard to these bodies, which have attracted so much attention and led to so much diversity of opinion as to their nature and uses in the animal economy, something has to be said in connection with the subject of coloration and skin-markings, of which these curious bodies form an important part.