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. 



" I 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. 



" It 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^n^ration) 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 



