Nowhere, or at no time, can man boast of improving the horse by the arts
of breeding. What is the reason of this?

The horse, the ox, the hog, and the sheep comprise the four great
classes of domesticated farm animals. In certain directions man has
improved these three last. These improvements have made them more
valuable. The ox has been bred to make more flesh from the same amount
of food, and to lay on fat at an earlier age; the cow has been bred to
give instead of a supply of milk barely large enough to sustain her
young, a bountiful yield, and of a richer quality; the hog has been bred
into a veritable machine to convert food into pork; the sheep has been
bred to yield more wool, and of a finer texture, and to make more
mutton. All these changes have been beneficial because the value of the
animal lay in its production of beef, milk, pork, wool, or mutton, as
the case might be. It is true that these changes have been accomplished
at the expense of vigor and endurance. These two qualities are important
in the hog, ox, or sheep, but those that have been developed so far
overshadow their lessening that on the whole we can say that the arts of
man have improved our kine, swine, and sheep.

But it is not so with the horse. Its value does not depend upon the
quantity and quality of its flesh, milk, or bodily covering. Unlike the
others its value depends upon the work it can do. Hence vigor and
endurance are the prime essentials of a good horse. But as man has
lessened the vigor and endurance of the hog, ox, and sheep, so he has of
the horse. This is the invariable result of human art. Whenever man
tampers with the work of nature he is certain to lessen bodily vigor. It
could not be otherwise. For the course of nature, undisturbed and
undeflected, is always towards the greatest health. Man changes the
course of nature and the result is lessened vigor and endurance.