An Introduction to a Biology 



APPENDIX G to 4 (b) pp. 184-7 



There is an apparent paradox, in the ideas just expressed, 

 about which I think it is necessary to say a few words, in 

 case the reader should detect it himself and think that it 

 had not occurred to me. 



I have said that biometry deals with masses and Men- 

 dclism with units ; but I have also said that the biometrician 

 exceeds his proper limits when he goes beyond the bomidary 

 of a unit, while the Mendelian is concerned with the mutual 

 properties of numerous units ; in other words, the sphere 

 of the biometrician is within the unit, while that of the 

 Mendelian is outside it. 



The fundamental idea on which the Law of Ancestral 

 Inheritance is based is that set forth in the quotation from. 

 Pearson^ ; 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 in- 

 dividual cases; while the Mendelian happens to have dealt 

 with cases in which prediction was possible. This answer pre- 

 supposes 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. 



1 Vide supra, p. 1G9 

 198 



