Page:Mendel's principles of heredity; a defence.pdf/134

given ancestry, while the second tells us the percentages of the total offspring which on the average revert to each ancestral type^[1]."

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 considerations 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 offspring. When in such cases individuals bearing opposite characters, ${\displaystyle A}$ and ${\displaystyle B}$, are crossed, the germ-cells of the resulting cross-bred, ${\displaystyle AB}$, are each to be bearers either of character ${\displaystyle A}$ or of character ${\displaystyle B}$, not both.

Consequently when the cross-breds breed either together or with the pure forms, individuals will result of the forms ${\displaystyle AA,AB,BA,BB}$^[2]. Of these the forms ${\displaystyle AA}$ and ${\displaystyle BB}$, 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 ${\displaystyle A}$ type or the ${\displaystyle B}$ 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.