LAMARCK’S THEORY OF DESCENT 303 
the effects of its inner feelings ; which I shall constantly 
prove by known facts. 
“Thus, to arrive at a knowledge of the true 
causes of so many different forms and so many dif- 
ferent habits of which the known animals offer us 
examples, it is necessary to consider that circum- 
stances infinitely diversified, but all slowly changing, 
into which the animals of each race are successively 
thrown, have caused, for each of them, new wants 
and necessarily changes in their habits. Moreover, 
this truth, which cannot be denied, being once rec- 
ognized, it will be easy to see how the new needs 
have been able to be satisfied, and the new habits 
formed, if any attention be given to the two follow- 
ing laws of nature, which observation always confirms: 
“First Law. 
“In every animal which has not exceeded the term 
of its development, the more frequent and sustained 
use of any organ gradually strengthens this organ, de- 
velops and enlarges it, and gives it a strength propor- 
tioned to the length of time of such use; while the 
constant lack of use of such an organ imperceptibly 
weakens it, causes it to become reduced, progressively 
diminishes its faculties, and ends in its disappearance. 
“ Second Law. 
“Everything which nature has caused _ individ- 
uals to acquire or lose by the influence of the 
circumstances to which their race may be for a long 
time exposed, and consequently by the influence of 
the predominant use of such an organ, or by that of 
the constant lack of use of such part, it preserves by 
heredity (géxération) and passes on to the new indi- 
viduals which descend from it, provided that the 
changes thus acquired are common to both sexes, or 
