﻿Manchester Memoirs, Vol. xlix. (1905), No. <>. 17 



-either theory to a set of facts in which the conditions, on 

 which that theory is true, do not obtain ; and the manner 

 in which materials for the study of heredity are collected 

 by Mendelians is so different from those employed by 

 biometricians that this is very rarely, if ever, the case. 



From this it follows that the naturalist who sets out 

 to attack the problem of heredity will not as in the past 

 collect his facts and then see whether they fit the one 

 theory or the other, but will make it his first duty to 

 decide whether he will attack it from the point of view of 

 the physicist or the demon, from the outside or from the 

 inside. If he decides on the former, he may if he wishes, 

 breed his material, but he will find a great deal ready to 

 hand in the records of matings of, for example, grey- 

 hounds, racehorses, and men. If he decides on the latter, 

 it is almost indispensable that he should breed his material 

 for himself. That is why biometricians are concerned 

 with ' ancestry/ and Mendelians with ' posterity.' Yet 

 these are not two things, but one thing, looked at from 

 opposite ends. But there is a difference between ancestry 

 and posterity, namely that the latter only can be dealt 

 with by the method of experiment. 



A confirmatory sidelight on the truth of my comparison 

 is thrown by the consideration that of the two men 

 whom I have quoted as representing the rival theories of 

 heredity, the biometer is a mathematician, while the 

 Mendelian is a zoologist : and it is entirely in accord 

 with expectation that the former regards the phenomena 

 of heredity from that point of view which does not 

 presuppose knowledge of the unit, while the latter is 

 concerned with the properties of the individual organism. 



If we could imagine the demon and the physicist 

 incapable of appreciating each others point of view we 

 could understand the contempt each would have for the ' 

 clumsy methods and erroneous opinions of the other. 



