4IO THE GERM-PL.AS.M 



CHAPTER XIV 

 VARIATION 



I. Normal Individual Variation 



Heredity is the transmission of the physical nature of the 

 parent to the offspring. We have seen that this transmission 

 affects the whole organism, and extends to the most trifling 

 details : and we also know that it is never complete, and that 

 the offspring and parent are never identical, but that the former 

 always differs more or less from the latter. These differences 

 give rise to the phenomenon of variation, which thus f on/is 

 an integral part of heredity, for the latter always includes the 

 former. 



A theory of heredity must therefore include a theoretical 

 substantiation of variation, such as I will now attempt. Why 

 is the offspring never an exact duplicate of its parent, even 

 when it possesses but one parent, as is the case in partheno- 

 genesis and reproduction by budding? And what forms the 

 basis of the constant • individual variations ' which, after the 

 precedent of Darwin and Wallace, we regard as the foundation 

 of all processes of natural selection, and as the means which 

 rendered possible such a rich development of organic forms of 

 the most diverse kinds on the face of the earth? 



Darwin himself considered the difference between parent 

 and offspring as due to the diversity of external influences, 

 and I was essentially of the same opinion formerly, and stated 

 that ' all dissimilarities of organisms must depend upon the 

 individuals having been affected by dissimilar external influences 

 during the course of the development of organic nature.' * At 

 that time I attributed to the organism the virtual ' power of 



*'Studien zur Descendenztheorie,' II., Leipzig, 1876, p. 304. English 

 edition, ' Studies in the Theory of Descent," translated and edited by R. 

 Meldola, with a prefatory note by Charles Darwin, London, 1882, Vol. II., 

 p. 677. 



