One horse differs from another not only in the characteristics described, but even to a much greater extent in size. Comparing the smallest pit pony of thirty-six inches with the carriage or the cart horse of eighteen or nineteen hands, it is often difficult to realize that both belong to the same species. Nevertheless the apparently widely different animals are in all essential features, excepting in size, the same. The vast difference in bulk is largely due to the efforts of the breeder in applying the principle of artificial selection. Pit ponies are required to work in mines where the seams are only a few feet high; large horses are wanted for special work or for appearance. Both requirements must be met, and the skill of the breeder, aided by climate and by food, is equal to the demand.