HORSES. 
49 1 
shown at C, which represents the jaw of a six-year-old horse, in which the third 
incisor is partly worn, although still retaining a large mark. Up to five years 
the age of a horse can consequently be determined with accuracjq and also approxi¬ 
mately for some years longer. As a rule, the mark disappears in the first 
incisor of the lower jaw at six years, in the second at seven., and in the third at 
eight, while in the corresponding upper teeth it persists about two years longer. 
D shows the upper jaw of a horse about eight years of age, when the mark has 
nearly disappeared in the first incisor. After the mark has been lost in all the 
incisors no indications of age are afforded. In old horses, as in E, a kind of spurious 
mark is, however, produced, owing to the tooth having become so much worn down 
that the pulp-cavity of its basal portion is exposed. Such spurious marks have, 
however, no ring of enamel, and cannot be made to counterfeit the true mark, 
although attempts to make them pass for this were, and perhaps still are, made by 
unscrupulous dealers. When the spurious mark makes its appearance, the section 
of each incisor forms a wide triangle, the broad and flattened crown having been 
completely worn away; and in extreme old age, when the teeth are ground down 
to their very roots, as at F, they become very narrow. 
The Horse (Equus caballus). 
The horse differs from the other members of the genus in having the tail 
thickly covered with long hairs from the root to the extremity, and also by the 
mane being longer and more flowing. It has also a bare callosity on the inner 
side of the hind-limb a little below the heel-joint, or hock, so that such callosities 
are present in all the four limbs. Moreover, the head is smaller, the ears are 
shorter, the limbs proportionately more elongated, and the hoofs broader than in 
any of the other species. In colour, domesticated horses vary greatly, but they 
seldom show any definite markings beyond a more or less distinct dappling. The 
wild horses of the Asiatic steppes are, however, of a dun colour; and since domesti¬ 
cated dun-coloured individuals—especially in India and Argentina—frequently show 
a dark streak down the middle of the back, and sometimes two or even three 
transverse shoulder-stripes, and likewise dark bands on the limbs, it has been 
inferred that originally the horse was a dun-coloured animal, more or less marked 
with dark stripes. The height among the domesticated breeds is no less varied than 
the coloration. Thus, while cart-horses frequently attain the height of 17 or 18 
hands (5 feet 8 inches or 6 feet) at the withers, the Shetland pony seldom 
exceeds 11 hands (3 feet 8 inches), and is occasionally as low as 8| hands (2 
feet 10 inches). The Asiatic wild horses are of medium stature. 
From what is known of the present wild or half-wild races, it is probable that 
the horse was originally an inhabitant of open steppes, where it dwelt in large 
droves headed by an old stallion. And from the habit displayed by domestic horses 
of clearing away the snow from their pasture in winter by scraping with the front 
hoof, Darwin was of opinion that the original habitat of the species was in 
regions where the ground is covered during a portion of the year with snow. 
So far as we know at present, the true horse in its original wild 
state was mainly confined to Europe and Asia, although it extended 
Distribution. 
