280 LAMARCK, AIS LIFE AND WORK 
zation perfects itself and becomes gradually compli- 
cated in a most remarkable way ?”’ 
This leads him to consider what is life, and he re- 
marks (p. xv.) that it does not exist without external 
stimuli. The conditions 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 origi- 
nate creatures becoming gradually more complicated, 
as we see in ascending the animal scale. Then em- 
ploying 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. Hencethe 
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 im- 
portance of the preservation by heredity, though he 
nowhere uses that word, in the new individuals re- 
produced of everything which the results of the life 
and influencing circumstances had caused to be ac- 
quired in the organization of those which have trans- 
mitted existence to them. 
In the Descours préliminazre, referring to the pro- 
