Part 34

From Telephone he made his colour scale! Tschermak declares the cotyledons to be “yellowish or whitish green, often entirely bright yellow.” 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.

“Speichergewebe gelblich--oder weisslich--grün, manchmal auch vollständig hellgelb.” Tschermak (36), p. 480.

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. Truly a slender basis to carry the coming structure!

In his latest publication on this subject, the notes to the edition of Mendel in Ostwald’s Klassiker (pp. 60–61), Tschermak, who has seen more true exceptions than any other observer, thus refers to them. As to dominance:--“Immerhin kommen vereinzelt auch zweifellose Fälle von Merkmalmischung, d. h. Uebergangsformen zwischen gelber und grüner Farbe, runder und runzeliger Form vor, die sich in weiteren Generationen wie dominantmerkmalige Mischlinge verhalten.” As to purity of the extracted recessives:--Ganz vereinzelt scheinen Ausnahmsfälle vorzukommen."

Küster (22) also in a recent note on Mendelism points out, with reason, that the number of “exceptions” to dominance that we shall find, depends simply on the stringency with which the supposed “law” is drawn. The same writer remarks further that Mendel makes no such rigid definition of dominance as his followers have done.

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--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:--

If the “logician-consulting-partner” will successfully apply this Fallacia acervalis, the “method of the vanishing heap,” to dominant peas, he will need considerable leisure.

“Now in such a case of alternative inheritance as that of human eye-colour, it has been shown that a number of pairs of parents, one of whom has dark and the other blue eyes, will produce offspring of which nearly one half are dark-eyed, nearly one half are blue-eyed, a small but sensible percentage being children with mosaic eyes, the iris being a patch-work of lighter and darker portions. But the dark-eyed and light-eyed children are not equally distributed among all families; and it would almost certainly be possible, by selecting cases of marriage between men and women of appropriate ancestry, to demonstrate for their families a law of dominance of dark over light eye-colour, or of light over dark. Such a law might be as valid for the families of selected ancestry as Mendel’s laws are for his peas and for other peas of probably similar ancestral history, but it would fail when applied to dark and light-eyed parents in general,--that is, to parents of any ancestry who happen to possess eyes of given colour.”

The suggestion amounts to this: that because there are exceptions to dominance in peas; and because by some stupendous coincidence, or still more amazing incompetence, a bungler might have thought he found dominance of one eye-colour whereas really there was none; therefore Professor Weldon is at liberty to suggest there is a fair chance that Mendel and all who have followed him have either been the victims of this preposterous coincidence not once, but again and again; or else persisted in the same egregious and perfectly gratuitous blunder. Professor Weldon is skilled in the Calculus of Chance: will he compute the probabilities in favour of his hypothesis?

I have no doubt there is no universal dominance in eye-colour. Is it quite certain there is no dominance at all? I have searched the works of Galton and Pearson relating to this subject without finding a clear proof. If there is in them material for this decision may perhaps be pardoned for failing to discover it, since the tabulations are not prepared with this point in view. Reference to the original records would soon clear up the point.

Ancestry and purity of germ-cells.

To what extent ancestry is likely to elucidate dominance we have now seen. We will briefly consider how laws derived from ancestry stand in regard to segregation of characters among the gametes.

For Professor Weldon suggests that his view of ancestry will explain the facts not only in regard to dominance and its fluctuations but in regard to the purity of the germ-cells. He does not apply this suggestion in detail, for its error would be immediately exposed. In every strictly Mendelian case the ancestry of the pure extracted recessives or dominants, arising from the breeding of first crosses, is identical with that of the impure dominants [or impure recessives in cases where they exist]. Yet the posterity of each is wholly different. The pure extracted forms, in these simplest cases, are no more likely to produce the form with which they have been crossed than was their pure grandparent; while the impure forms break up again into both grand-parental forms.

Ancestry does not touch these facts in the least. They and others like them have been a stumbling-block to all naturalists. Of such paradoxical phenomena Mendel now gives us the complete and final account. Will Professor Weldon indicate how he proposes to regard them?

* * * * *

Let me here call the reader’s particular attention to that section of Mendel’s experiments to which Professor Weldon does not so much as allude. Not only did Mendel study the results of allowing his cross-breds (DR’s) to fertilise themselves, giving the memorable ratio

1 DD : 2 DR : 1 RR,

but he fertilised those cross-breds (DR’s) both with the pure dominant (D) and with the pure recessive (R) varieties reciprocally, obtaining in the former case the ratio

1 DD : 1 DR

and in the latter the ratio

1 DR : 1 RR.

The DD group and the RR group thus produced giving on self-fertilisation pure D offspring and pure R offspring respectively, while the DR groups gave again

1 DD : 2 DR : 1 RR.

How does Professor Weldon propose to deal with these results, and by what reasoning can he suggest that considerations of ancestry are to be applied to them? If I may venture to suggest what was in Mendel’s mind when he applied this further test to his principles it was perhaps some such considerations as the following. Knowing that the cross-breds on self-fertilisation give

1 DD : 2 DR : 1 RR

three explanations are possible: