An Introduction to a Biology 



true of the whole population was also true of the individual 

 a mistake which, I believe, was due to an attempt to 

 discover whether certain phenomena were evidence in favour 

 of the one or the other of two theories without appreciating 

 the essential character of either theory and much less their 

 mutual relation ; to a failure, in short, to realise that a bio- 

 metric formula of heredity is true only of large masses, the 

 component units of which in most cases unite at random, 

 while the Mendelian theory is an attempt to account for 

 the hereditary phenomena exhibited by the union of in- 

 dividuals carefully selected, by a theory of the constitution 

 of their germ-cells. It is perhaps not unnatural, though it 

 is certainly unjustifiable, that when an experimenter is think- 

 ing of a set of facts before him now in terms of the one theory 

 and now in terms of the other without having clearly fixed 

 the peculiar characters of each theory to its proper owner, 

 he should get these characters misplaced ; that he should 

 add to the real character of the biometric theory one which 

 it does not possess that of applicability to the individual. 

 I have discussed this particular error of judgment at some 

 length because it illustrates the kind of mistake a student 

 of heredity at the present time may make unless he realises 

 the exact nature, at which I have so far only hinted, of the 

 two theories whose compatibility with fact he is testing. 

 Were I not persuaded that mine is not the only case in which 

 this or a similar kind of error has been made, I should not 

 have described it ; and it is because I believe that the mis- 

 understandings and arguments at cross purposes, which 

 have lately characterised discussions on heredity, will be 

 things of the past when the relation between biometric and 

 Mendelian theory is clearly seen, that I set forth the following 

 considerations. 



Ill 



From a point of view which commands a wide range of 

 our experience, our knowledge may be divided into two 



