In the preceding examples, and in many others which must be omitted, the actions in question modify organisms in order to place them in harmony with the conditions of life. Now it is intelligible that when the maximum of possible effect has once been attained, they can only fix the result obtained more fully, but can never determine a change in the opposite direction. The heat, which has by degrees deprived calongo cattle of their hair, will never restore it again; and the cold which has made our pigs woolly, will never deprive them of wool. Here, then, we find _conditions of life_ acting as an agent of preservation and stability. VII. In the preceding passage allusion has been made to natural forces left to themselves. It is to them that the formation is due of the wild races of all the species whose geographical area is very extended, such as the fox, jackal, lion, etc. These races are sometimes so different that they were regarded as distinct species, as long as the intervening geographical and zoological terms were unknown. Frederick Cuvier himself made this mistake in the case of the jackals of India and those of the Senegal. Wild races have, however, never been so numerous or so distinct from each other as domesticated races. Are we to infer from this that man exercises around himself and of himself a kind of magnetic action, as some authors seem to admit? Certainly not. In reality, he only acts upon an animal by setting in action, sometimes intentionally, sometimes unintentionally, the two agents which, hitherto, we have met with everywhere, conditions of life and heredity. By the single fact of domestication, by the confinement which is almost always the result of it, he changes entirely the natural conditions of existence. By leading in his train the animals which he has enslaved, he diversifies still more the influences which act upon them. Prompt to seize every means of rendering them most useful, he profits by the smallest modifications which show the least advantage, pushes them to their utmost limits and produces the _extreme races_, of which our exhibitions of animal races give such curious examples.