“So the cross Couturier (orange-yellow) ♀ × the green-seeded Express ♂ gave a number of seeds intermediate in colour. (It is not clear from Tschermak’s paper whether all the seeds were of this colour, but certainly some of them were.) The green Plein le Panier [Fillbasket] ♀ × Couturier ♂ in three crosses always gave either seeds of colour intermediate between green and yellow, or some yellow and some green seeds in the same pod. The cross reciprocal to this was not made; but Express ♀ × Couturier ♂ gave 22 seeds of which four were yellowish green[149].
“These facts show first that Mendel’s law of dominance conspicuously fails for crosses between certain races, while it appears to hold for others; and secondly that the intensity of a character in one generation of a race is no trustworthy measure of its dominance in hybrids. The obvious suggestion is that the behaviour of an individual when crossed depends largely upon the characters of its ancestors[150]. When it is remembered that peas are normally self-fertilised, and that more than one named variety may be selected out of the seeds of a single hybrid pod, it is seen to be probable that Mendel worked with a very definite combination of ancestral characters, and had no proper basis for generalisation about yellow and green peas of any ancestry” [which he never made].
Let us pause a moment before proceeding to the climax. Let the reader note we have been told of two groups of cases in which dominance of yellow failed or was irregular. (Why are not Gärtner’s and Seton’s “exceptions” referred to here?) In one of these groups Couturier was always one parent, either father or mother, and were it not for Tschermak’s own obvious hesitation in regard to his own exceptions (see p. [148]), I would gladly believe that Couturier—a form I do not know—may be an exceptional variety. How Professor Weldon proposes to explain its peculiarities by reference to ancestry he omits to tell us. The Buchsbaum case is already disposed of, for on Tschermak’s showing, it is an unstable form.
Happily, thanks to Professor Weldon, we know rather more of the third case, that of Telephone, which, whether as father or mother, was frequently found by Tschermak to give either green, greenish, or patchwork-seeds when crossed with yellow varieties. It behaves, in short, “like a green-seeded pea of exceptional dominance,” as we are now told. For this dominant quality of Telephone’s greenness we are asked to account by appeal to its ancestry. May we not expect, then, this Telephone to be—if not a pure-bred green pea from time immemorial—at least as pure-bred as other green peas which do not exhibit dominance of green at all? Now, what is Telephone? Do not let us ask too much. Ancestry takes a lot of proving. We would not reject him “parce qu’il n’avait que soixante & onze quartiers, & que le reste de son arbre généalogique avait été perdu par l’injure du tems.”
But with stupefaction we learn from Professor Weldon himself that Telephone is the very variety which he takes as his type of a permanent and incorrigible mongrel, a character it thoroughly deserves.
From Telephone he made his colour scale! Tschermak declares the cotyledons to be “yellowish or whitish green, often entirely bright yellow[151].” So little is it a thorough-bred green pea, that it cannot always keep its own self-fertilised offspring green. Not only is this pea a parti-coloured mongrel, but Professor Weldon himself quotes Culverwell that as late as 1882 both Telegraph and Telephone “will always come from one sort, more especially from the green variety”; and again regarding a supposed good sample of Telegraph that “Strange to say, although the peas were taken from one lot, those sown in January produced a great proportion of the light variety known as Telephone. These were of every shade of light green up to white, and could have been shown for either variety,” Gard. Chron. 1882 (2), p. 150. This is the variety whose green, it is suggested, partially “dominates” over the yellow of Pois d’Auvergne, a yellow variety which has a clear lineage of about a century, and probably more. If, therefore, the facts regarding Telephone have any bearing on the significance of ancestry, they point the opposite way from that in which Professor Weldon desires to proceed.
In view of the evidence, the conclusion is forced upon me that the suggestion that “ancestry” may explain the facts regarding Telephone has no meaning behind it, but is merely a verbal obstacle. Two words more on Telephone. On p. [147] I ventured to hint that if we try to understand the nature of the appearance of green in the offspring of Telephone bred with yellow varieties, we are more likely to do so by comparing the facts with those of false hybridisation than with fluctuations in dominance. In this connection I would call the reader’s attention to a point Professor Weldon misses, that Tschermak also got yellowish-green seeds from Fillbasket (green) crossed with Telephone. I suggest therefore that Telephone’s allelomorphs may be in part transmitted to its offspring in a state which needs no union with any corresponding allelomorph of the other gamete, just as may the allelomorphs of “false hybrids.” It would be quite out of place here to pursue this reasoning, but the reader acquainted with special phenomena of heredity will probably be able fruitfully to extend it. It will be remembered that we have already seen the further fact that the behaviour of Telephone in respect to seed-shape was also peculiar (see p. [152]).
Whatever the future may decide on this interesting question it is evident that with Telephone (and possibly Buchsbaum) we are encountering a specific phenomenon, which calls for specific elucidation and not a case simply comparable with or contradicting the evidence of dominance in general.
In this excursion we have seen something more of the “exceptions.” Many have fallen, but some still stand, though even as to part of the remainder Tschermak entertains some doubts, and, it will be remembered, cautions his reader that of his exceptions some may be self-fertilisations, and some did not germinate[152]. Truly a slender basis to carry the coming structure!
But Professor Weldon cannot be warned. He told us the “law of dominance conspicuously fails for crosses between certain races.” Thence the start. I venture to give the steps in this impetuous argument. There are exceptions[153]—a fair number if we count the bad ones—there may be more—must be more—are more—no doubt many more: so to the brink. Then the bold leap: may there not be as many cases one way as the other? We have not tried half the sorts of Peas yet. There is still hope. True we know dominance of many characters in some hundreds of crosses, using some twenty varieties—not to speak of other plants and animals—but we do know some exceptions, of which a few are still good. So dominance may yet be all a myth, built up out of the petty facts those purblind experimenters chanced to gather. Let us take wider views. Let us look at fields more propitious—more what we would have them be! Let us turn to eye-colour: at least there is no dominance in that. Thus Professor Weldon, telling us that Mendel “had no proper basis for generalisation about yellow and green peas of any ancestry,” proceeds to this lamentable passage:—