"The foundation of this law derives its proof from the third, in which the facts known allow of no doubt; for, if the forces of action of an organ, by their increase, further develop this organ--namely, increase its size and power, as is constantly proved by facts--we may be assured that the forces by which it acts, just originated by a new want felt, would necessarily give birth to the organ adapted to satisfy this new want, if this organ had not before existed. "In truth, in animals so low as not to be able to _feel_, it cannot be that we should attribute to a felt want the formation of a new organ, this formation being in such a case the product of a mechanical cause, as that of a new movement produced in a part of the fluids of the animal. "It is not the same in animals with a more complicated structure, and which are able to _feel_. They feel wants, and each want felt, exciting their inner feeling, forthwith sets the fluids in motion and forces them towards the point of the body where an action may satisfy the want experienced. Now, if there exists at this point an organ suitable for this action, it is immediately cited to act; and if the organ does not exist, and only the felt want be for instance pressing and continuous, gradually the organ originates, and is developed on account of the continuity and energy of its employment. "If I had not been convinced: 1, that the thought alone of an action which strongly interests it suffices to arouse the _inner feeling_ of an individual; 2, that a felt want can itself arouse the feeling in question; 3, that every emotion of _inner feeling_, resulting from a want which is aroused, directs at the same instant a mass of nervous fluid to the points to be set in activity, that it also creates a flow thither of the fluids of the body, and especially nutrient ones; that, finally, it then places in activity the organs already existing, or makes efforts for the formation of those which would not have existed there, and which a continual want would therefore render necessary--I should have had doubts as to the reality of the law which I have just indicated. "But, although it may be very difficult to verify this law by observation, I have no doubt as to the grounds on which I base it, the necessity of its existence being involved in that of the third law, which is now well established.