Page:Mendel's principles of heredity; a defence.pdf/137

I proceed to the question of dominance which Professor Weldon treats as a prime issue, almost to the virtual concealment of the great fact of gametic purity.

Cross-breds in general, ${\displaystyle AB}$ and ${\displaystyle BA}$, named above, may present many appearances. They may all be indistinguishable from ${\displaystyle A}$, or from ${\displaystyle B}$; some may appear ${\displaystyle A}$'s and some ${\displaystyle B}$'s; they may be patchworks of both; they may be blends presenting one or many grades between the two; and lastly they may have an appearance special to themselves (being in the latter case, as it often happens, "reversionary"), a possibility which Professor Weldon does not stop to consider, though it is the clue that may unravel many of the facts which mystify him now.

Mendel's discovery became possible because he worked with regular cases of the first category, in which he was able to recognize that one of each of the pairs of characters he studied did thus prevail and was "dominant" in the cross-bred to the exclusion of the other character. This fact, which is still an accident of particular cases, Professor Weldon, following some of Mendel's interpreters, dignifies by the name of the "Law of Dominance," though he omits to warn his reader that Mendel states no "Law of Dominance" whatever. The whole question whether one or other character of the antagonistic pair is dominant though of great importance is logically a subordinate one. It depends on the specific nature of the varieties and individuals used, sometimes probably on the influence of