716 Fluctuations 



win, and exercised much influence upon his 

 views at various times. 



Quetelet 's famous discovery of the law of 

 / fluctuating variability changed the entire situa- 

 \ tion and cleared up many difficulties. While a 

 '' clear conception of fluctuations was thus gained, 

 mutations were excluded from consideration, 

 being considered as very rare, or non-existent. 

 They seemed wholly superfluous for the theory 

 of descent, and very little importance was at- 

 tached to their study. Current scientific belief 

 in the matter has changed only in recent 

 years. Mendel's law of varietal hybrids is 

 based upon the principle of unit-characters, and 

 the validity of this conception has thus been 

 brought home to many investigators. 



A study of fluctuating or individual variabil- 

 ity, as it was formerly called, is now carried on 

 chiefly by mathematical methods. It is not my 

 purpose to go into details, as it would re- 

 quire a separate course of lectures. I shall con- 

 sider the limits between fluctuation and muta- 

 tion only, and attempt to set forth an adequate 

 idea of the principles of the first as far as they 

 touch these limits. The mathematical treat- 

 ment of the facts is no doubt of very great value, 

 but the violent discussions now going on be- 

 tween mathematicians such as Pearson, Kap- 

 teyn and others should warn biologists to ab- 



