"The man who habitually and vigorously exercises the organ of his intelligence develops and acquires a great facility of attention, of aptitude for thought, etc., but he has a feeble stomach and strongly limited muscular powers. He, on the contrary, who thinks little does not easily, and then only momentarily fixes his attention, while habitually giving much exercise to his muscular organs, has much vigor, possesses an excellent digestion, and is not given to the abstemiousness of the savant and man of letters. "Moreover, when one exercises long and vigorously an organ or system of organs, the active forces of life (in my opinion, the nervous fluid) have taken such a habit of acting (_porter_) towards this organ that they have formed in the individual an inclination to continue to exercise which it is difficult for it to overcome. "Hence it happens that the more we exercise an organ, the more we use it with facility, the more does it result that we perceive the need (_besoin_) of continuing to use it at the times when it is placed in action. So we remark that the habit of study, of application, of work, or of any other exercise of our organs or of any one of our organs, becomes with time an indispensable need to the individual, and often a passion which it does not know how to overcome. "Thirdly, finally, the effort made by necessity to obtain new faculties is aided by the concurrence of favorable circumstances; they create (_créent_) with time the new organs which are adapted (_propres_) to their faculties, and which as the result develop after long use (_qu'en suite un long emploi développe_). "How important is this consideration, and what light it spreads on the state of organization of the different animals now living! "Assuredly it will not be those who have long been in the habit of observing nature, and who have followed attentively that which happens to living individuals (to animals and to plants), who will deny that a great change in the circumstances of their situation and of their means of existence forces them and their race to adopt new habits; it will not be those, I say, who attempt to contest the foundation of the consideration which I have just exposed. "They can readily convince themselves of the solidity of that which I have already published in this respect.[172] "I have felt obliged to recall to you these great considerations, a sketch of which I traced for you last year, and which I have stated for the most part in my different works, because they serve, as you have seen, as a solution of the problem which interests so many naturalists, and which concerns the determination of _species_ among living bodies.