THE MENDELIAN CLUE 231 



the inheritance of particular characters through 

 successive generations and to measure quantitatively 

 the degrees of hereditary resemblance. He was 

 led to the law of ancestral inheritance, according 

 to which the average contributions to each inherited 

 faculty are a half from the parents, a quarter from 

 the grandparents, an eighth from the great-grand- 

 parents, and so on backwards in the same diminish- 

 ing ratio (afterwards somewhat modified by Pear- 

 son) ; and another deduction was the law of filial 

 regression or the tendency to approximate to the 

 mean of the stock. It is necessary, however, to bear 

 in mind that these average statistical deductions do 

 not hold in regard to non-blending hereditary 

 characters, and that they do not seem to take 

 sufficient account of the fundamental distinction 

 between inborn variations and individually acquired 

 modifications. The latter are somatic dints due to 

 peculiarities of nurture, and have not been proved 

 to be transmissible; the former are expressions of 

 germinal changefulness, and are in some cases 

 demonstrably transmissible. Of great value, how- 

 ever, has been the statistical demonstration of the 

 heritability of subtle constitutional qualities such 

 as fecundity and longevity and the proof that 

 clearly-defined mental qualities may be handed on 

 to, and distributed among, the offspring just in 

 the same way as bodily characters. 



The first year of this century will be memorable 

 in the annals of biology for the rediscovery of the 

 Mendelian clue by Correns, De Vries, and Tscher- 



