114 A Defence of Mendel's 



given ancestry, while the second tells us the percentages 

 of the total offspring which on the average revert to each 

 ancestral type*." 



With the distinctions between the original Law of 

 Ancestral Heredity, the modified form of the same law, 

 and the Law of Reversion, important as all these considera- 

 tions are, we are not at present concerned. 



For the Mendelian principle of heredity asserts a 

 proposition absolutely at variance with all the laws of 

 ancestral heredity, however formulated. In those cases to 

 which it applies strictly, this principle declares that the 

 cross-breeding of parents need not diminish the purity of 

 their germ-cells or consequently the purity of their off- 

 spring. When in such cases individuals bearing opposite 

 characters, A and J3, are crossed, the germ-cells of the 

 resulting cross-bred, AJ3, are each to be bearers either 

 of character A or of character B, not both. 



Consequently when the cross-breds breed either together 

 or with the pure forms, individuals will result of the forms 

 AA, AB, BA, BB's. Of these the forms A A and BB, 

 formed by the union of similar germs, are stated to be as 

 pure as if they had had no cross in their pedigree, and 

 henceforth their offspring will be no more likely to depart 

 from the A type or the B type respectively, than those of 

 any other originally pure specimens of these types. 



Consequently in such examples it is not the fact that 

 each ancestor must be brought to account as the Galton- 

 Pearson Law asserts, and we are clearly dealing with a 

 physiological phenomenon not contemplated by that Law 

 at all. 



* Grammar of Science, 1900, p. 494. See also Pearson, Proc. Roy. 

 Soc. 1900, LXVI. pp. 142-3. 



t On an average of cases, in equal numbers, as Mendel found. 



