vii] Law of Ancestral Heredity 1 29 



It is curious that the one example to which a partially correct 

 system of analysis was applied before Mendelian methods 

 were rediscovered, should have been of this remarkably 

 exceptional order. 



There is little reason to anticipate, as we once did, that a 

 distinct group of cases obeying a Law of Ancestral Heredity 

 will have to be recognized. That principle in certain cases 

 gives an epitome of the consequences of the Mendelian 

 process, and in all likelihood its applicability to any pheno- 

 mena of natural inheritance is due to this fact. The Law 

 of Ancestral Heredity takes of course no account of domi- 

 nance, or of segregation with all the consequences it entails ; 

 but as describing the results to be witnessed among a 

 population interbreeding at random, its predictions would 

 frequently approximate to the truth. In particular it is to 

 be observed that the arithmetical results of DR x R and of 

 DR x DR are correctly predicted by the Law of Ancestral 

 Heredity. Some of the phenomena of blending are also ex- 

 pressed with accuracy. Besides this, inasmuch as dominant 

 individuals which have both parents dominants will in a 

 mixed population frequently be pure dominants, statistical 

 phenomena which could be mistaken for an effect of 

 ancestral composition will often occur*. 



For instance if purple Sweet Peas in F 3 were bred from 

 promiscuously, F 3 would consist of purples and other colours, 

 and the excess of purples in the mass would be greater than 

 it was in F z . If the F 3 purples were again separately saved, 

 the proportion of purples in F t would be still greater, and 

 so on. This gradual increase in the proportion of purples 

 might carelessly be mistaken for a consequence of the fact 

 that each generation had more purples in its ancestry. As 

 we now know, that conclusion would be quite incorrect. 

 The increase is in reality due to the appearance of actually 

 pure purples in F^ and in subsequent generations, and to 

 the effect which their presence has' on the composition of 

 the population. The impure purples in each generation 



* Darbishire's conclusions in regard to mice (Biometrika, 1904, in. 

 pp. 23-5) were obviously based on a mistake of this kind,, as he has 

 himself since admitted. (See Mem. Manchester Lit. Phil. Soc. 1905, 

 No. 6, p. 7.) 



B. H. 9 



