Introduction xxiii 



put forth in detail by J. J. Cunningham in his essay on 

 the " Hormone^ Theoiy of Heredity," in the Archiv fi'ir 

 Eniwicklimgsmechanik (1909), but I have failed to note 

 any direct effect of my essay on the trend of biological 

 thought. 



Among post-Darwinian controversies the one that has 

 latterly assumed the greatest prominence is that of the 

 relative importance of small variations in the way of more 

 or less " fluctuations," and of " discontinuous variations," 

 or " mutations," as De Vries has called them. Darwin, 

 in the first four editions of the " Origin of Species," at- 

 tached more importance to the latter than in subsequent 

 editions ; he was swayed in his attitude, as is well known, 

 by an article of the physicist, Fleeming Jenkin, which 

 appeared in the North British Review. The mathematics 

 of this article were unimpeachable, but they were 

 founded on the assumption that exceptional variations 

 would only occur in single individuals, which is, indeed, 

 often the case among those domesticated races on which 

 Darwin especially studied the phenomena of variation. 

 Darwin was no mathematician or physicist, and we are 

 told in his biography that he regarded every tool-shop rule 

 or optician's thermometer as an instrument of precision : 

 so he appears to have regarded Fleeming Jenkin 's demon- 

 stration as a mathematical deduction which he was bound 

 to accept without criticism. 



Mr. William Bateson, late Professor of Biology in the 

 University of Cambridge, as early as 1894 laid great stress 

 on the importance of discontinuous variations, collecting 

 and collating the known facts in his " Materials for the 

 Study of Variations " ; but this important work, now 

 become rare and valuable, at the time excited so little 

 interest as to be ' remaindered ' within a very few years 

 after publication. 



^ A "hormone" is a chemical substance which, formed in one 

 part of the body, alters the reactions of another part, normally for 

 the good of the organism. 



