410 CONCLUSION 



of the external world are the causes of the origin of varieties, 

 but deny that the same causes have led to the evolution of 

 species. They also abandon that principle when they main- 

 tain that the modification of unicellular forms is governed by 

 other laws than those which regulate that of the mul- 

 ticellular. 



And it is certainly astonishing that this really self-evident 

 truth has never yet been followed to its conclusions, namely, 

 that if the same fundamental laws hold throughout the 

 organic world, and if species are only a collection of 

 individuals, and genera a collection of species, families 

 only a collection of genera, and so on, groups which have dif- 

 ferentiated themselves from a number of originally similar 

 individuals, that then the same laws must hold for the 

 development of these groups as for that of the individual, 

 that the causes which modify an individual within the 

 boundaries of a species must be the same as those which 

 modify it beyond these boundaries. Finally, that therefore 

 also the special laws of growth which influence and modify 

 an individual during its life, must be essentially those which 

 underlie the manifold variety of the whole world of organic 

 forms. 



Any one who thus completely renders allegiance to the 

 supremacy of the principle of the unity of the organic world, 

 who rejects everything which contradicts this principle, can- 

 not help admitting that in truth, as I assert, the ultimate 

 origin of the various kinships in the animal and vegetable 

 kingdom is to be traced to individual differences, and that the 

 differences between the former, like the latter, must be 

 essentially determined by external conditions, by the modi- 

 fications of organic growth. 



Only in this way also is it possible to explain in a natural 

 way, as I have attempted to do, the origin and unity of all 

 forms of mental action. 



