IN THE THEORY OF NATUKAL SELECTION. 267 



only forms, as it were, the nutritive soil at the expense of which the 

 germ-plasm grows, while the latter possessed its characteristic struc- 

 ture from the beginning 1 , viz. before the commencement of growth. 



But the tendencies of heredity, of which the germ-plasm is the 

 bearer, depend upon this very molecular structure, and hence only 

 those characters can be transmitted through successive generations 

 which have been previously inherited, viz. those characters which 

 were potentially contained in the structure of the germ-plasm. It 

 also follows that those other characters which have been acquired by 

 the influence of special external conditions, during the life-time of 

 the parent, cannot be transmitted at all. 



The opposite view has, up to the present time, been maintained, 

 and it has been assumed, as a matter of course, that acquired 

 characters can be transmitted ; furthermore, extremely complicated 

 and artificial theories have been constructed in order to explain how 

 it may be possible for changes produced by the action of external 

 influences, in the course of a life-time, to be communicated to the 

 germ and thus to become hereditary. But no single fact is known 

 which really proves that acquired characters can be transmitted, 

 for the ascertained facts which seem to point to the transmission of 

 artificially produced diseases cannot be considered as a proof; and 

 as long as such proof is wanting we have no right to make this 

 supposition, unless compelled to do so by the impossibility of 

 suggesting a mode in which the transformation of species can take 

 place without its aid. (See Appendix IV, p. 310.) 



It is obvious that the unconscious conviction that we need the 

 aid of acquired characters has hitherto securely maintained the as- 

 sumed axiom of the transmission of such features. It was believed 

 that we could not do without such an axiom in order to explain the 

 transformation of species ; and this was believed not only by those 

 who hold that the direct action of external influences plays an 

 important part in the process, but also by those who hold that the 

 operation of natural selection is the main factor. 



Individual variability forms the most important foundation of 

 the theory of natural selection : without it the latter could not 

 exist, for this alone can furnish the minute differences by the 

 accumulation of which new forms are said to arise in the course of 

 generations. But how can such hereditary individual characters 

 exist if the changes wrought by the action of external influences, 



