itions necessary for
the existence of life are found completely developed in the simplest
organization. We are then led to inquire how this organization, by
reason of certain changes, can give rise to other organisms less simple,
and finally originate creatures becoming gradually more complicated, as
we see in ascending the animal scale. Then employing the two following
considerations, he believes he perceives the solution of the problem
which has occupied his thoughts.

He then cites as factors (1) use and disuse; (2) the movement of
internal fluids by which passages are opened through the cellular tissue
in which they move, and finally create different organs. Hence the
_movement of fluids in the interior of animals_, and the _influence of
new circumstances_ as animals gradually expose themselves to them in
spreading into every inhabitable place, are the two general causes which
have produced the different animals in the condition we now see them.
Meanwhile he perceived the importance of the preservation by heredity,
though he nowhere uses that word, in the new individuals reproduced of
everything which the results of the life and influencing circumstances
had caused to be acquired in the organization of those which have
transmitted existence to them.

In the _Discours preliminaire_, referring to the _progression_ in
organization of animals from the simplest to man, as also to the
successive acquisition of different special organs, and consequently of
as many faculties as new organs obtained, he remarks:

"Then we can perceive how needs (_besoins_), at the outset reduced
to nullity, and of which the number gradually increases, have
produced the inclination (_penchant_) to actions fitted to satisfy
it; how the actions, becoming habitual and energetic, have caused
the development of the organs which execute them; how the force
which excites the organic movements may, in the simplest animals, be
outside of them and yet animate them; how, then, this force has been
transported and fixed in the animal itself; finally, how it then has
become the source of sensibility, and in the end that of acts of
intelligence.

"I shall add that if this method had been followed, then _sensation_
would not have been regarded as the general and immediate cause of
organic movements, and it would not have been said that life is a
series of movements which are executed in virtue of sensations
received by diffe