II. Mendel and the Critic’s Version of him.
The “Law of Dominance.”
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, AB and BA, named above, may present many appearances. They may all be indistinguishable from A, or from B; some may appear A’s and some 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 external conditions and on other factors we cannot now discuss. There is as yet no universal law here perceived or declared.
Professor Weldon passes over the proof of the purity of the germ-cells lightly enough, but this proposition of dominance, suspecting its weakness, he puts prominently forward. Briefest equipment will suffice. Facing, as he supposes, some new pretender—some local Theudas—offering the last crazy prophecy,—any argument will do for such an one. An eager gathering in an unfamiliar literature, a scrutiny of samples, and he will prove to us with small difficulty that dominance of yellow over green, and round over wrinkled, is irregular even in peas after all; that in the sharpness of the discontinuity exhibited by the variations of peas there are many grades; that many of these grades co-exist in the same variety; that some varieties may perhaps be normally intermediate. All these propositions are supported by the production of a collection of evidence, the quality of which we shall hereafter consider. “Enough has been said,” he writes (p. 240), “to show the grave discrepancy between the evidence afforded by Mendel’s own experiments and that obtained by other observers, equally competent and trustworthy.”
We are asked to believe that Professor Weldon has thus discovered “a fundamental mistake” vitiating all that work, the importance of which, he elsewhere tells us, he has “no wish to belittle.”