380 OUTLINES OF E VOLUTION AEY BIOLOGY 



from the exercise or the want of exercise of any of our organs, 

 although these differences are not maintained in the individuals 

 of the next generation, for then their results would be much 

 more considerable. 



" I shall show in the second part that when the will determines 

 an animal to some action, the organs which have to execute this 

 action are forthwith stimulated by the affluence of subtle fluids 

 (the nervous fluid) which become the determining cause of the 

 movements which the action in question requires. A multitude 

 of observations establishes this fact, which should no longer be 

 called in question. 



" It results therefrom that numerous repetitions of these acts 

 of organization strengthen, extend, develop and even create 

 the organs which are needed. It is only necessary to observe 

 attentively what is happening everywhere in this respect to 

 convince oneself of the actuality of this cause of organic 

 development and modification. 



" Now, every change acquired in an organ by a habit of use 

 sufficient to have produced it, is maintained afterwards by 

 generation, if it is common to the individuals which have united 

 in the act of fecundation for the reproduction of their species. 

 In short, this modification is propagated, and thus passes to all 

 the individuals which follow and which are subjected to the 

 same environment, without their being obliged to acquire it by 

 the means which really created it. 



" The mingling in reproductive unions, however, between 

 individuals which have different qualities or forms, is necessarily 

 opposed to the constant propagation of those qualities and forms. 

 This is the reason why in man, who is subjected to so many 

 different modifying circumstances, the accidental qualities or 

 defects which he has chanced to acquire are not preserved and 

 propagated by generation. If, when any peculiarities of form or 

 any defects have been acquired, two individuals in this condition 

 should always unite, they would reproduce the same peculiarities, 

 and successive generations confining themselves to similar 

 unions, a special and distinct race would then be formed. But 

 the perpetual mingling between individuals which have not the 

 same peculiarities of form causes all peculiarities acquired as the 

 result of peculiar circumstances of the environment to disappear. 

 Whence one may be certain that if human beings were not 

 separated by the distances of their habitations, the mixed breed- 

 ing would cause the general characters which distinguish the 

 different nations to disappear." l 



In the light of modern knowledge these views on the subject 

 of heredity are, of course, crude and inaccurate enough, but there 



1 Op. cit., Tom. I, pp. 259262. 



