120 
The Evolution of the Horse. 
In turning to the consideration of the anatomical structure 
of the horse, it will only be possible, in the space allowed for 
this paper, to speak briefly upon certain points of special interest 
connected with its present mode of life, its evolution, and its 
relation to other animal forms. 
I will first take an obvious external characteristic — the 
so-called " chestnuts," or " mallenders " and " sallenders," as 
they are designated in old books. These are certain patches 
upon the inner side of the limbs of all horses, definite in form 
and definite in position, in which the structure of the skin is 
different from what it is in other places. The papillae of the 
derm or true skin are enlarged, and an abundant and thickened 
epidermal covering, which becomes dry and horny, and sometimes 
accumulates in considerable quantity on the surface, takes the 
place of the usual hair. Their structure, in fact, is much like 
that of a wart or corn ; but they are not a pathological product, 
though often treated as such in old books on veterinary surgery. 
They exist at birth, are equally developed in both sexes, and 
their constancy of form, size, and position (as mentioned before), 
give one of the characteristic distinctions of the species Equus 
caballus. They differ in form in the two limbs, but are in both 
placed upon the inner surface, and nearer to the hinder than 
the front border. That on the fore-limb is above the carpal, or 
wrist-joint (commonly called "knee"), that on the hind limb, 
below the ankle, or " hock " joint. In the donkeys and zebras 
the hinder one is absent, but the one upon the fore-limb exists, 
though in a modified form. In the zebra it is a large, circular 
patch of bare skin, of a black colour, but scarcely elevated above 
the surrounding surface. 
The signification and utility of these structures are a com- 
plete puzzle. Various suggestions have been made, none of 
which will bear examination. One of the most plausible, 
especially in the light of modern comparative anatomy, is that 
that they are rudiments or vestiges of the inner toe — the thumb 
or pollex of the fore-limb, the great toe or hallux of the hind 
limb — which, as already indicated, and will be more fully shown 
presently, is not otherwise represented in the horse. There are, 
however, many objections to this theory. The inner toe is always 
the first to disappear in all mammals, and no traces of it are found 
in any Ungulate, either Perissodactyle or Art iodactyle, except the 
most ancient forms. It is, therefore, most unlikely that anything 
of this digit should remain in the horse after the complete dis- 
appearance of 1 he second, four! h, and fifth. In the next place, 
there is nothing beneath the modified patch of skin showing any 
trace of the structure of a toe, and the resemblance of this patch 
