36 Darbishire, Laws of Heredity. 



The fundamental idea on which the Law of Ancestral In- 

 heritance is based is that set forth in the quotation from Pearson 

 on p. 4 of this essay ; it is that a knowledge of the characters of 

 the parents does not enable us to predict the character of the 

 offspring in individual cases. 



The fundamental idea in Mendelian theory is that the 

 ascertainable gametic characters of the parents do enable us to 

 predict the character of the offspring in individual cases. 



How are these two diametrically opposite ideas about 

 heredity to be reconciled? The answer which most naturally 

 suggests itself is that the biometrician happens to have dealt 

 with cases about which it was impossible to predict in individual 

 cases ; while the Mendelian happens to have dealt with cases in 

 which prediction was possible. This answer presupposes the 

 existence of two sets of phenomena in heredity, those about which 

 it is possible to predict, and those about which it is not. Now 

 let us grant for the moment that the Mendelian theory (which I 

 think by no means proven yet) that the characters of an organism 

 consist of a number of separate character-units is true. What 

 relation, if any, do the two sets of hereditary phenomena — the 

 predicable and the non-predicable bear to these units? Just 

 this. The non-predicable phenomenon is the incomplete 

 correlation between the degree in which any character x is 

 exhibited by a parent, in a single case, and the degree in which 

 that same character is exhibited in its child. The predicable 

 phenomenon is the result of the union of x with x., or of x with y. 



Chemistry furnishes a parallel. The chemist cannot predict 

 the rate at which any given atom in a litre of oxygen is travelling ; 

 he can only deal with 'statistics of average conduct'; but he can 

 predict the result of passing an electric spark in a vessel con- 

 taining oxygen and hydrogen. Yet he who deals with the 

 properties of the elements may be said to deal with units, and 

 he who deals with the component atoms — and one can only 

 deal with them in large numbers — may be said to deal with 

 masses. 



It is true that the biometrician possesses the only means of 



