An Introduction to a Biology 



of the matter. When I say that the Mendelian deals with 

 units and the biometrician with masses, I mean, not that 

 the former deals with a few and the latter with many, but 

 that the former first settles what character he is going to 

 treat as a unit and then only deals with it in large numbers 

 when he is sure that the component units of this number 

 are identical, their sameness having reference to such pro- 

 perties as can be discovered by mating them with their 

 like and with their unlike. It is just as necessary for the 

 Mendelian to have a large record of matings as the biometrician 

 to establish his generalisations. But though the Mendelian 

 might allow that the only method of measuring the simi- 

 larity between parents and offspring within a group (such 

 as the unit character Red) was that of the correlation table, 

 he would vehemently maintain that the biometric method 

 overstepped its limits- when it included in such a table more 

 than one such category. What Bateson means when he 

 says that Mendel saw by sure penetration that masses must 

 be avoided, is that the biometric method oversteps its limits 

 when it does this. 



The answer that I should now give to my friend is this : 

 " I fully admit that the only method of measuring the degree 

 of resemblance between a generation and the one which pro- 

 duced it, within such a unit, is the biometric method ; I 

 fully agree with the biometrician when he says that all 

 green peas are not alike in respect of their greenness because 

 all green peas are green, and that the biometric method is 

 the only one to measure their dissimilarity ; but I stoutly 

 maintain that when he puts green and yellow peas together 

 into a correlation table he has started on a path which will 

 not lead to a more intimate knowledge of heredity." 



Biometry furnishes the only means of actually measuring 

 the intensity of heredity within a unit ; Mendelism furnishes 

 the only means by which a fuller knowledge of the properties 

 of these units may be acquired. 1 



1 Exactly the same idea is expressed by Lotsy, :06, p. 143. 



185 



