124 ZOOLOGICAL PHILOSOPHY 



strong, and powerful, it is alleged that its habitual use has nothing to 

 do with it, that its continued disuse involves it in no loss, and finally, 

 that this organ has always been the same since the creation of the species 

 to which the animal belongs, then I ask. Why can our domestic ducks 

 no longer fly like wild ducks ? I can, in short, cite a multitude of in- 

 stances among ourselves, which bear witness to the differences that 

 accrue to us from the use or disuse of any of our organs, although these 

 differences are not preserved in the new individuals which arise by 

 reproduction : for if they were their effects would be far greater. 



I shall show in Part II., that when the will guides an animal to any 

 action, the organs which have to carry out that action are immediately 

 stimulated to it by the influx of subtle fluids (the nervous fluid), 

 which become the determining factor of the movements required. 

 This fact is verified by many observations, and cannot now be called 

 in question. 



Hence it follows that numerous repetitions of these organised 

 activities strengthen, stretch, develop and even create the organs 

 necessary to them. We have only to watch attentively what is 

 happening all around us, to be convinced that this is the true cause 

 of organic development and changes. 



Now every change that is wrought in an organ through a habit 

 of frequently using it, is subsequently preserved by reproduction, 

 if it is common to the individuals who unite together in fertilisation 

 for the propagation of their species. Such a change is thus handed 

 on to all succeeding individuals in the same environment, without 

 their having to acquire it in the same way that it was actually created. 



Furthermore, in reproductive imions, the crossing of individuals 

 who have different quaUties or structures is necessarily opposed to 

 the permanent propagation of these quahties and structures. Hence 

 it is that in man, who is exposed to so great a diversity of environment, 

 the accidental quaUties or defects which he acquires are not preserved 

 and propagated by reproduction. If, when certain peculiarities 

 of shape or certain defects have been acquired, two individuals who 

 are both affected were always to unite together, they would hand on 

 the same peculiarities ; and if successive generations were limited 

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

 perpetual crossings between individuals, who have not the same 

 peculiarities of shape, cause the disappearance of all pecuUarities 

 acquired by special action of the environment. Hence, we may be sure 

 that if men were not kept apart by the distances of their habitations, 

 the crossing in reproduction would soon bring about the disappear- 

 ance of the general characteristics distinguishing different nations. 



If I intended here to pass in review all the classes, orders, genera 



