LAMARCK, 125 
short as to give rise to the vulgar and false hypothesis 
of stability and immutability. The transformation is 
effected by the obligation of the individual to accom- 
modate itself to the altered conditions of life. Fresh 
circumstances elicit fresh requirements and fresh activi- 
ties. Great weight must be laid on the use or disuse 
of organs. “In every animal still in the course of de- 
velopment, the more frequent and sustained use of an 
organ gradually fortifies, developes and enlarges it, and 
endows it with strength proportional to the duration of 
this use ; while the persistent disuse of an organ imper- 
ceptibly weakens and deteriorates it, diminishes its effi- 
ciency in an increasing ratio, and ultimately destroys it.” 
“And thus,” he says, “nature exhibits living beings 
merely as individuals succeeding one another in genera- 
tions; species have only a relative stability, and are 
only transiently immutable.” 
Lamarck touches upon the struggle of each against all 
(I. 99, and elsewhere), but does not discover the term 
Natural Selection. He is fully conscious of the two 
factors, heredity and adaptation, but his theories and 
convictions lack the emphasis of detailed evidence. 
Yet his subtle apprehension of life may be evinced by 
his interpretation of instinct. According to him, all 
acts of instinct are effected by incitement, exercised 
upon the nervous system by acquired inclinations 
(penchans acquis); and these acts, not being the product 
of deliberation, choice, or judgment, certainly and un- 
erringly satisfy the requirements experienced and the 
inclinations resulting from habit. But if these inclina- 
tions to maintain the habit and renew the actions 
related to them, are once acquired, they are henceforward 
