_Philosophie zoologique_
in his theory (the second law) accounting for the origin of a new organ,
the result of a new need.

"_First law_: Life, by its proper forces, continually tends to
increase the volume of every body which possesses it, and to
increase the size of its parts, up to a limit which it brings about.

"_Second law_: The production of a new organ in an animal body
results from the supervention of a new want (_besoin_) which
continues to make itself felt, and of a new movement which this want
gives rise to and maintains.

"_Third law_: The development of organs and their power of action
are constantly in ratio to the employment of these organs.

"_Fourth law_: Everything which has been acquired, impressed upon,
or changed in the organization of individuals, during the course of
their life is preserved by generation and transmitted to the new
individuals which have descended from those which have undergone
those changes."

In explaining the second law he says:

"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: