An Introduction to a Biology 



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, gre3^hounds, racehorses, and men. 

 If he decides on the latter, it is almost indispensable that 

 he should breed his material for liimself. That is why 

 biometricians are concerned with " ancestry," and Men- 

 delians 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 in- 

 capable of appreciating each other's point of vicw^, we could 

 understand the contempt each w^ould have for the clumsy 

 methods and erroneous opinions of the other. 



And though we can perhaps understand the Mendelian 

 declaring as he slides the latch of his breeding-pen that 

 " Operating among such phenomena the gross statistical 

 method is a misleading instrument ; and, applied to these 

 intricate discriminations, the imposing Correlation Table 

 into which the biometrical Procrustes fits his arrays of un- 

 analysed data is still no substitute for the common sieve of 

 a trained judgment " ; and that " nothing but minute 

 analysis of the facts by an observer thoroughly conversant 

 with the particular plant or animal, its habits^and properties, 



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