VARIATION 411 



giving rise by multiplication only to exact copies of itself ; but 

 this capacity, in my opinion, did not produce accurate results, 

 because the organism is also capable of reacting to external 

 influences, and may therefore deviate from the inherited ten- 

 dency in one or another direction, according to the nature of 

 these influences. 



Variation would consequently not depend upon a special 

 force existing in the organism, but would simply be the result 

 of external influences, which, either directly or indirectly, are 

 capable of preventing the organism from keeping strictly to the 

 inherited course of development. 



Although I still consider this view to be in general correct, 

 the origin of individual variation, on which the transformation 

 of species is based, cannot be deduced so easily from the action 

 of dissimilar external influences as seemed possible when I wrote 

 the passage quoted above. I have discussed this question in 

 full on a previous occasion, and will here only briefly refer 

 to it.* 



At that time, no one supposed that any difference existed 

 between the modifications which may be brought about in the 

 soma and those which proceed from the germ-plasm. Since 

 then, however, we have been compelled — at least in my opinion 

 — to consider that only those variations which are ' blastogenic^ 

 and not those which are 'somatogenic,'' can be transmitted. We 

 can no longer regard the direct influence of external impressions 

 on the soma as a means of producing hereditary individual 

 variations. It therefore remains to be seen what is the origin 

 of these variations, upon the existence of which we imagine the 

 entire development of organic nature to depend. 



This development could be accounted for most easily on 

 Nageli's hypothesis, according to which the idioplasm is so 

 constituted, that in the course of generations it could exert a 

 definite and regular transforming influence upon itself, and by 

 this means could convert one species into another. Many 

 reasons may, however, be urged against this hypothesis. In 

 the first place, development by internal forces only is contra- 

 dictory to the close adaptations of organisms to their conditions 

 of life ; and secondly, we should not make use of unknown 



*C/. ' Die Bedeutung der sexuellen Fortpflanzung fiir die Selections- 

 theorie,' Jena, 1886. English Edition, Oxford, 1889, p. 255. 



