Species partially subject to the rule of man furnish a number of facts which enable us to compare the power of natural forces, when abandoned to their own action, with that of man in modifying a specific type. In all artificial races varieties are infinitely more numerous, more varied and more marked than wild races and varieties. Now the result of these transmutations of organisms has only consisted in the formation of _races_, never in the formation of a _new species_. Darwin himself accepts this conclusion implicitly in his magnificent work on pigeons; for when speaking of the _races of pigeons_ he only says that the difference of form is such that if they had been found in a wild state, we should have been compelled to make at least three or four genera of them. The wild rock pigeons, the original stock of all our domestic pigeons, only differ, on the contrary, in shades of colour. The result is always the same, whenever we can compare the work of nature with our own. When he has anything to do with any vegetable or animal species, _man_ always changes its character, sometimes, after a lapse of some years, the change being much greater than that produced by nature since the species first came into existence. The _effect of the conditions of life_ (_milieu_), of which we will speak presently, the _struggle for existence_ and _natural selection_ understood as I have just described it, the power which man possesses of directing natural forces and changing their resultant, easily explain this superiority of action. Consequently, without leaving the domain of facts, and only judging from what we know, we can say that morphology itself justifies the conclusion that one species has never produced another by means of derivation. To admit the contrary is to call in the _unknown_, and to substitute a _possibility_ for the results of experience. Physiology justifies a still stronger assertion. Upon this ground also man is shown to be as powerful as nature, and for the same reasons. With our cultivated plants and domestic animals, it is not only the primitive form which has undergone change, but certain functions also. If we had only enlarged and deformed the wild carrot and the wild radish, it would not have become more eatable. To render it agreeable to our taste, the production of certain substances had to be reduced, and that of others enlarged, that is to say, nutrition and secretion had to be modified. If the functions in wild animal stocks had remained permanent, we should have had none of those races which are distinguished by a difference in the colour of the hair, in the production of milk, in aptitude for work, or in the production of meat. If instinct itself had not obeyed the action of man, we should not have had in the same kennel, pointers, grey-hounds, truffle dogs and terriers.