The theory of evolution teaches us that the special organisation of an animal has been produced by the gradual action which surroundings (climate, soil, food, etc.) have had on the animal's ancestors through the course of ages. As health is the ordinary condition of the body, we may conclude that it can be best preserved by the surroundings which have effected the final result, and in which the animal is popularly said to be in his "natural state" of life. We find this to be true even among varieties of the same species which habitually exist under respective surroundings that are widely different from each other, as we may see among Arctic dogs and those that are natives of tropical climates, neither of which could bear with impunity a mutual change of country. It is evident that an alteration in the natural state of an animal will continue to be more or less detrimental to its health, until the functions of the animal have accommodated themselves to the new surroundings. Although young horses fresh off grass, as a rule, at first feel the bad effects of a change of surroundings, when put into a stable; they become so well accustomed to their new conditions of life, that after some time an abrupt return to the old order of things is not always safe. In our efforts to obtain a maximum of strength or speed, we may succeed to some extent in making special breeds and certain individual horses tolerant, for instance, of food which is much more stimulating than that found by them in their "natural" state. At the same time, experience proves that such apparently tolerant horses are abnormally liable to disease. Therefore, while endeavouring to regulate the stable management of our horses, so as to enable them to meet our civilised requirements, we should keep them in a state as near that of Nature as is practicable. Even among human beings, civilisation is not free from danger to health. The well-intentioned, though extremely ill-advised attempt to make the remnant of Tasmanian natives participate in the supposed blessings of wearing clothes, sleeping in beds, eating cooked food, and practising other forms of comparatively modern hygiene, resulted in their speedy destruction.