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.”
III. The Facts in regard to Dominance of Characters in Peas.
Professor Weldon refers to no experiments of his own and presumably has made none. Had he done so he would have learnt many things about dominance in peas, whether of the yellow cotyledon-colour or of the round form, that might have pointed him to caution.
In the year 1900 Messrs Vilmorin-Andrieux & Co. were kind enough to send to the Cambridge Botanic Garden on my behalf a set of samples of the varieties of Pisum and Phaseolus, an exhibit of which had greatly interested me at the Paris Exhibition of that year. In the past summer I grew a number of these and made some preliminary cross-fertilizations among them (about 80 being available for these deductions) with a view to a future study of certain problems, Mendelian and others. In this work I had the benefit of the assistance of Miss Killby of Newnham College. Her cultivations and crosses were made independently of my own, but our results are almost identical. The experience showed me, what a naturalist would expect and practical men know already, that a great deal turns on the variety used; that some varieties are very sensitive to conditions while others maintain their type sturdily; that in using certain varieties Mendel’s experience as to dominance is regularly fulfilled, while in the case of other varieties irregularities and even some contradictions occur. That the dominance of yellow cotyledon-colour over green, and the dominance of the smooth form over the wrinkled, is a general truth for Pisum sativum appears at once; that it is a universal truth I cannot believe any competent naturalist would imagine, still less assert. Mendel certainly never did. When he speaks of the “law” or “laws” that he has established for Pisum he is referring to his own discovery of the purity of the germ-cells, that of the statistical distribution of characters among them, and the statistical grouping of the different germ-cells in fertilization, and not to the “Law of Dominance” which he never drafted and does not propound.
The issue will be clearer if I here state briefly what, as far as my experience goes, are the facts in regard to the characters cotyledon-colour and seed-shapes in peas. I have not opportunity for more than a passing consideration of the seed-coats of pure forms[63]; that is a maternal character, a fact I am not sure Professor Weldon fully appreciates. Though that may be incredible, it is evident from many passages that he has not, in quoting authorities, considered the consequences of this circumstance.
The normal characters: colour of cotyledons and seed-coats.