VARIATION AND HEREDITY 129 



species, e. g. of birds, which differ from their 

 relatives in features similar to those which 

 arise as mutations in experimental breeeding. 



The issue at present seems to be this. The 

 distinctive characteristics of a species may 

 arise in one of two ways, either (1) by the 

 accumulation of fluctuations, or (2) suddenly 

 by mutation. In support of the first theory 

 there are the numerous cases where species 

 are connected by inter-grades. In support 

 of the second theory there is experimental 

 evidence, showing that many characteristics 

 remain integral and refuse to blend. Patient 

 work will be necessary before we can decide 

 as to the relative importance of fluctuations 

 and mutations. 



MENDELISM. One of the most important 

 of recent biological discoveries has been the 

 " law of heredity," stated in 1865 by Gregor 

 Johann Mendel (1822-1884), an Austro- 

 Silesian abbot, who experimented for many 

 years on crossing different varieties of garden 

 peas. His great paper, communicated to the 

 Natural History Society of Briinn, remained 

 practically unknown till 1900, when De Vries 

 in Holland, Correns in Germany, and Tscher- 

 mak in Austria independently, and almost 

 simultaneously, reached experimental results 

 closely resembling Mendel's. This led to a 





