2 74 SIGXIFICANCE OF SEXUAL REPRODUCTION- [V. 



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 b}'^ the impossibility of suggesting 

 a mode in which the transformation of species can take place 

 without its aid. (See Appendix IV, p. 319.) 



It is obvious that the unconscious conviction that we need 

 the aid of acquired characters has hitherto securely maintained 

 the assumed 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 dilTerences 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, during the life of an individual, cannot be trans- 

 mitted? We are clearly compelled to find some other source 

 of hereditary individual differences, or the theory of natural 

 selection would collapse, as it certainly would if hereditary 

 individual variations did not exist. If, on the other hand, 

 acquired difi'erences are transmitted, this would prove that 

 there must be something wrong in the theory of the continuity 

 of the germ-plasm, as above described, and in the non-trans- 

 mission of acquired characters which results from this theory. 

 But I believe that it is possible to suggest that the origin of 

 hereditary individual characters takes place in a manner quite 

 different from any which has been as yet brought forward. To 

 explain this origin is the task which I am about to undertake in 

 the following pages. 



The origin of individual variability has been hitherto repre- 



