have been such as to permit of the
deposit of an unbroken, or but rarely interrupted, series of strata
through a long period of time; in which the group of animals to be
investigated has existed in such abundance as to furnish the requisite
supply of remains; and in which, finally, the materials composing the
strata are such as to ensure the preservation of these remains in a
tolerably perfect and undisturbed state.

It so happens that the case which, at present, most nearly fulfils
all these conditions is that of the series of extinct animals which
culminates in the horses; by which term I mean to denote not merely the
domestic animals with which we are all so well acquainted, but their
allies, the ass, zebra, quagga, and the like. In short, I use "horses"
as the equivalent of the technical name _Equidae,_ which is applied to
the whole group of existing equine animals.

The horse is in many ways a remarkable animal; not least so in the fact
that it presents us with an example of one of the most perfect pieces
of machinery in the living world. In truth, among the works of human
ingenuity it cannot be said that there is any locomotive so perfectly
adapted to its purposes, doing so much work with so small a quantity
of fuel, as this machine of nature's manufacture--the horse. And, as
a necessary consequence of any sort of perfection, of mechanical
perfection as of others, you find that the horse is a beautiful
creature, one of the most beautiful of all land-animals. Look at the
perfect balance of its form, and the rhythm and force of its action. The
locomotive machinery is, as you are aware, resident in its slender fore
and hind limbs; they are flexible and elastic levers, capable of being
moved by very powerful muscles; and, in order to supply the engines
which work these levers with the force which they expend, the horse
is provided with a very perfect apparatus for grinding its food and
extracting therefrom the requisite fuel.

Without attempting to take you very far into the region of osteological
detail, I must nevertheless trouble you with some statements respecting
the anatomical structure of the horse; and, more especially, will it be
needful to obtain a general conception of the structure of its fore and
hind limbs, and of its teeth. But I shall only touch upon those points
which are absolutely essential to our inquiry.

Let us turn in the first place to the fore-limb. In most quadrupeds, as
in ourselves, the