LAMARCK’S THEORY OF DESCENT 319 
differences might not be maintained in the individuals 
which follow them genetically, for then their products 
would be still more considerable. 
“]T shall prove, in the second part, that when the 
will urges an animal to any action, the organs which 
should execute this action are immediately provoked 
by the affluence of subtile fluids (the nervous fluid), 
which then become the determining cause which calls 
for the action in question. A multitude of observa- 
tions prove this fact, which is now indisputable. 
“Tt results that the multiplied repetitions of these 
acts of organization strengthen, extend, develop, and 
even create the organs which are necessary. It is 
only necessary attentively to observe that which is 
everywhere occurring to convince ourselves of the 
well-grounded basis of this cause of organic develop- 
ments and changes. 
‘“‘ Moreover, every change acquired in an organ by a 
habit of use sufficient to. have produced it is then 
preserved by heredity (g¢xération) if it is common to 
the individuals which, in fecundation, unite in the 
reproduction of their species. Finally, this change is 
propagated, and thus is transmitted to all the indi- 
viduals which succeed and which are submitted to the 
same circumstances, unless they have been obliged to 
acquire it by the means which have in reality created 
it 
“ Besides, in reproductive unions the crossings be- 
tween the individuals which have different qualities 
or forms are necessarily opposed to the continuous 
propagation of these qualities and these forms. We 
see that in man, who is exposed to so many diverse 
circumstances which exert an influence on him, the 
qualities or the accidental defects which he has been 
in the way of acquiring, are thus prevented from being 
preserved and propagated by generation. If, when 
some particular features of form or any defects are 
acquired, two individuals under this condition should 
