tem
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 (_creent_) 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 developpe_).

"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.

"Indeed, if in ascending in the series of animals from the most
simply organized animalcule, as from the monad, which seems to be
only an animated point, up to the animals the most perfect, or whose
structure is the most complicated--in a word, up to animals with
mammae--yo