Chapter 8

“These statements led Gärtner, who was highly sceptical on the subject, carefully to try a long series of experiments; he selected the most constant varieties, and the results conclusively showedthat the colour of the skin of the peais modified when pollen of a differently coloured variety is used.” (The italics are mine.)

In the true spirit of inquiry Professor Weldon doubtless reflected,

“’Tis notAntiquitynorAuthor,That makesTruth Truth, altho’Time’s Daughter”;

but perhaps a word of caution to the reader that another interpretation exists would have been in place. It cannot be without amazement therefore that we find him appropriating these examples as referring to cotyledon-colour, with never a hint that the point is doubtful.

Giltay, without going into details, points out theambiguity85. As Professor Weldon refers to the writings both of Darwin and Giltay, it is still more remarkable that he should regard the phenomenon as clearly one of cotyledon-colour and not coat-colour as Darwin and many other writers have supposed.

Without going further it would be highly improbable that Gärtner is speaking solely or even chiefly of the cotyledons, from the circumstance that these observations are given as evidence of “the influence of foreign pollen on the female organs”; and that Gärtner was perfectly aware of the fact that the coat of the seed was a maternal structure is evident from his statement to that effect on p. 80.

To go into the whole question in detail would require considerable space; but indeed it is unnecessary to labour the point. The reader who examines Gärtner’s account with care, especially the peculiar phenomena obtained in the case of the “grey” pea (macrospermum), with specimens before him, will have no difficulty in recognizing that Gärtner is simply describing the seedsas they looked in their coats, and is not attempting to distinguish cotyledon-characters and coat-characters. If he had peeled them, which in the case of “grey” peas would beabsolutely necessaryto see cotyledon-colour, he must surely have said so.

Had he done so, he would have found the cotyledons full yellow in every ripe seed; for I venture to assert that anyone who tries, as we have, crosses between a yellow-cotyledoned “grey” pea, such as Gärtner’s was, with any pure green variety will see that there is no question whatever as to absolute dominance of the yellow cotyledon-character here, more striking than in any other case. If exceptions are to be looked for, they will not be foundthere; and, except in so far as they show simple dominance of yellow, Gärtner’s observations cannot be cited in this connection at all.

(2)Seton’s case.Another exception given by Professor Weldon is much more interesting and instructive.It is the curious case ofSeton86. Told in the words of the critic it is as follows:—

“Mr Alexander Seton crossed the flowers ofDwarf Imperial, ‘a well-known green variety of the Pea,’ with the pollen of ‘a white free-growing variety.’ Four hybrid seeds were obtained, ‘which did not differ in appearance from the others of the female parent.’ These seeds therefore didnotobey the law of dominance, or if the statement be preferred, greenness became dominant in this case. The seeds were sown, and produced plants bearing ‘green’ and ‘white’ seeds side by side in the same pod. An excellent coloured figure of one of these pods is given (loc. cit.Plate 9, Fig. 1), and is the only figure I have found which illustrates segregation of colours in hybrid Peas of the second generation.”

Now if Professor Weldon had applied to this case the same independence of judgment he evinced in dismissing Darwin’s interpretation of Gärtner’s observations, he might have reached a valuable result. Knowing how difficult it is to give all the points in a brief citation, I turned up the original passage, where I find it stated that the mixed seeds of the second generation “were all completely either of one colour or the other, none of them having an intermediate tint, as Mr Seton had expected.” The utility of this observation of the absence of intermediates, is that it goes some way to dispose of the suggestion of xenia as a cause contributing to the result.

Moreover, feeling perfectly clear, from the fact of the absence of intermediates, that the case must be one of simple dominance in spite of first appearances, I suggest the following account with every confidence that it is the true one. There have been several “Imperials,”thoughDwarf Imperial, in a form which I can feel sure is Seton’s form, I have not succeeded in seeing; but from Vilmorin’s description that the peas when ripe are “franchement verts” I feel no doubt it was a green peawith a green skin. If it had had a transparent skin this description would be inapplicable. Having then a green skin, which may be assumed with every probability of truth, the seeds, even though the cotyledons were yellow, might, especially if examined fresh, be indistinguishable from those of the maternal type. Next from the fact of the mixture in the second generation we learn that thesemi-transparent seed-coat of the paternal form was dominantas a plant-character, and indeed the coloured plate makes this fairly evident. It will be understood that this explanation is as yet suggestive, but from the facts of the second generation, any supposition that there was real irregularity in dominance in this case is out of thequestion87.

(3)Tschermak’s exceptions.These are a much more acceptable lot than those we have been considering. Tschermak was thoroughly alive to the seed-coat question and consequently any exception stated as an unqualified fact on his authority must be accepted. The nature of these cases we shall see. Among the many varieties he used, some beingnotmonomorphic, it would have been surprising if he had not found true irregularities in dominance.

(3a)Buchsbaum case.This variety, growing in the open, gave once a pod in whichevery seed but one was green. In stating this case Professor Weldon refers toBuchsbaumas “a yellow-seeded variety.”Tschermak88, however, describes it as having “gelbes, öfters gelblich-grünes Speichergewebe” (cotyledons); and again says the cotyledon-colour is “allerdings gerade bei Buchsbaum zur Spontanvariation nach gelb-grün neigend!” The (!) is Tschermak’s. Therefore Professor Weldon can hardly claimBuchsbaumas “yellow-seeded” without qualification.

Buchsbaumin fact is in all probability a blend-form and certainly not a true, stable yellow. One of the green seeds mentioned above grew and gave 15yellowsand threegreens, and the result showed pretty clearly, as Tschermak says, that there had been an accidental cross with a tall green.

On another occasionTelephone♀ (another impure green) ×Buchsbaumgave fouryellow smooth andtwogreen wrinkled, but one [? both: the grammar is obscure] of the greens did notgerminate89.

(3b)Telephone cases.Telephone, crossed with at least one yellow variety (Auvergne) gave all or some green or greenish. These I have no doubt are good cases of “defective dominance” of yellow. But it must be noted thatTelephone is an impure green. Nominally a green, it is as Professor Weldon has satisfied himself, very irregular in colour, having many intermediates shading to pure yellow and many piebalds. It is the variety from which alone Professor Weldon made his colour-scale.I desire therefore to call special attention to the fact that Telephone, thoughnot a pure green, Tschermak’s sample being as he says “gelblichweiss grün,” a yellowish-white-green in cotyledon-colour, is the variety which has so far contributed the clearest evidence of the green colour dominating in its crosses with a yellow; and thatBuchsbaumis probably a similar case. To this point we shall return. It may not be superfluous to mention also that one cross betweenFillbasket(a thoroughgreen) andTelephonegave threeyellowishgreen seeds (Tschermak, (36), p. 501).

(3c)Couturier cases.This fully yellow variety in crosses with two fully green sorts gave seeds either yellow or greenish yellow. In one caseFillbasket♀ fertilised byCouturiergave mixed seeds, green and yellow. For any evidence to the contrary, the green in this case may have been self-fertilised. Nevertheless, taking the evidence together, I think it is most likely thatCouturieris a genuine case of imperfect dominance of yellow. If so, it is the only true “exception” in crosses between stable forms.

We have now narrowed down Professor Weldon’s exceptions to dominance of cotyledon-colour to two varieties, one yellow (Couturier), and one yellow “tending to green” (Buchsbaum), which show imperfect dominance of yellow; and one variety,Telephone, an impure and irregular green, which shows occasional but uncertain dominance ofgreen.

What may be the meaning of the phenomenon shown by the unstable or mosaic varieties we cannot tell; but I venture to suggest that when we more fully appreciate the nature and genesis of the gametes, it will be found that the peculiarities of heredity seen in these cases have more in common with those of “false hybridism” (see p.34) than with any true failure of dominance.

Before, however, feeling quite satisfied in regard evento this residuum of exceptions, one would wish to learn the subsequent fate of these aberrant seeds and how their offspring differed from that of their sisters. One only of them can I yet trace, viz. the green seed fromTelephone♀ ×Buchsbaum♂, which proved a veritable “green dominant.” As for the remainder, Tschermak promises in his first paper to watch them. But in his second paper the only passage I can find relating to them declares that perhaps some of the questionable cases he mentioned in his first paper “are attributable to similar isolated anomalies in dominance; some proved themselves by subsequent cultivation to be cases of accidental self-fertilisation; others failed to germinate90‍.” I may warn those interested in these questions, that in estimating changes due to ripening,deadseeds are not available.

B. Seed-coats and shapes.

1.Seed-coats.Professor Weldon lays some stress on the results obtained byCorrens91in crossing a pea having green cotyledons and a thin almost colourless coat (grüne späte Erfurter Folger-erbse) with two purple-flowered varieties. The latter are what are known in England as “grey” peas, though the term grey is not generally appropriate.

In these varieties the cotyledon-colour is yellow andthe coats are usually highly coloured or orange-brown. In reciprocal crosses Correns found no change from the maternal seed-coat-colour or seed-shape. On sowing these peas he obtained plants bearing peas which, using the terminology of Mendel and others, he speaks of as the “first generation.”

These peas varied in the colour of their seed-coats from an almost colourless form slightly tinged with green like the one parent to the orange-brown of the other parent. The seeds varied in this respect not only from plant to plant, but from pod to pod, and from seed to seed, as Professor Correns has informed me.

The peas with more highly-coloured coats were sown and gave rise to plants with seeds showing the whole range of seed-coat-colours again.

Professor Weldon states that in this case neither the law of dominance nor the law of segregation was observed; and the same is the opinion of Correns, who, as I understand, inclines to regard the colour-distribution as indicating a “mosaic” formation. This is perhaps conceivable; and in that case the statement that there was no dominance would be true, and it would also be true that the unit of segregation, if any, was smaller than the individual plant and may in fact be the individual seed.

A final decision of this question is as yet impossible. Nevertheless from Professor Correns I have learnt one point of importance, namely, that the coats of all these seeds werethick, like that of the coloured and as usual dominant form. There is no “mosaic” of coats like one parent and coats like the other, though there may be a mosaic of colours. In regard to the distribution ofcolourhowever the possibility does not seem to me excluded that we are here dealing with changes influenced by conditions.I have grown a “grey” pea and noticed that the seed-coats ripened in my garden differ considerably and not quite uniformly from those received from and probably ripened in France, mine being mostly pale and greyish, instead of reddish-brown. We have elsewhere seen (p.120) that pigments of the seed-coat-colour may be very sensitive to conditions, and slight differences of moisture, for example, may in some measure account for the differences in colour. Among my crosses I have a pod of such “grey” peas fertilised byLaxton’s Alpha(green cotyledons, coat transparent). It contained five seeds, of which four werered-brown on one sideand grey with purple specks on the other. The fifth was of the grey colour on both sides. I regard this difference not as indicating segregation of character but merely as comparable with the difference between the two sides of a ripe apple, and I have little doubt that Correns’ case may be of the samenature92. Phenomena somewhat similar to these will be met with in Laxton’s case of the “maple” seeded peas (see p.161).

2.Seed-shapes.Here Professor Weldon has three sets of alleged exceptions to the rule of dominance of round shape over wrinkled. The first are Rimpau’s cases, the second are Tschermak’s cases, the third group are cases of “grey” peas, which we will treat in a separate section (see pp.153and158).

(a)Rimpau’s cases.Professor Weldon quotes Rimpau as having crossed wrinkled and roundpeas93and foundthe second hybrid generation dimorphic as usual. The wrinkled peas were selected and sown and gave wrinkled peasand roundpeas, becoming “true” to the wrinkled character in one case only in the fifth year, while in the second case—that of aTelephonecross—there was a mixture of round and wrinkled similarly resulting fromwrinkledseed for two years, but the experiment was not continued.

These at first sight look like genuine exceptions. In reality, however, they are capable of a simple explanation. It must be remembered that Rimpau was working in ignorance of Mendel’s results, was not testing any rule, and was not on the look out for irregularities. Now all who have crossed wrinkled and round peas on even a moderate scale will have met with the fact that there is frequentlysomewrinkling in the cross-bred seeds. Though round when compared with the true wrinkled, these are often somewhat more wrinkled than the round type, and in irregular degrees. For my own part I fully anticipate that we may find rare cases of complete blending in this respect though I do not as yet know one.

Rimpau gives a photograph of eight peas (Fig. 146) which he says represent the wrinkled form derived from this cross. It is evident that these are not fromone podbut a miscellaneous selection. On close inspection it will be seen that while the remainder are shown with theircotyledon-surfaces upwards, the two peas at the lower end of the row are represented with theirhilar-surfaces upwards. Remembering this it will be recognized that these two lower peas are in factnotfully wrinkled peas but almost certainlyround“hybrids,” and the depression is merely that which is often seen in round peas (such asFillbasket), squared by mutual pressure. Such peas, when sown, might of course give some round.

As Tschermak writes ((37), p. 658), experience has shown him that cross-bred seeds with character transitional between “round” and “wrinkled” behave as hybrids, and have both wrinkled and round offspring, and he now reckons them accordingly with the round dominants.

Note further the fact that Rimpau found the wrinkled form came true in thefifthyear, while the round gave at first more, later fewer, wrinkleds, not coming true till theninthyear. This makes it quite clear that therewasdominance of the round form, but that the heterozygotes were not so sharply distinguishable from the two pure forms as to be separated at once by a person not on the look-out for the distinctions. Nevertheless therewassufficient difference to lead to a practical distinction of the cross-breds both from the pure dominants and from the pure recessives.

TheTelephonecase may have been of the same nature; though, as we have seen above, this pea is peculiar in its colour-heredity and may quite well have followed a different rule in shape also. As stated before, the wrinkled offspring were not cultivated after the third year, but theroundseeds are said to have still given some wrinkleds in the eighth year after the cross, as would be expected in a simple Mendelian case.

(b)Tschermak’s cases.The cases Professor Weldon quotes from Tschermak all relate to crosses withTelephoneagain, and this fact taken with the certainty that the colour-heredity ofTelephoneis abnormal makes it fairly clear that there is here something of a really exceptional character. What the real nature of the exception is, and how far it is to be taken as contradicting the “law of dominance,” is quite another matter.

3.Other phenomena, especially regarding seed-shapes, in the case of “grey” peas. Modern evidence.Professor Weldon quotes from Tschermak the interesting facts about the “grey” pea,Graue Riesen, but does not attempt to elucidate them. He is not on very safe ground in adducing these phenomena as conflicting with the “law of dominance.” Let us see whither we are led if we consider these cases. On p.124I mentioned that the classes round and wrinkled do not properly hold if we try to extend them to large-seeded sorts, and that these cases require separate consideration. In many of such peas, which usually belong either to the classes of sugar-peas (mange-touts) or “grey” peas (with coloured flowers), the seeds would be rather described as irregularly indented, lumpy orstony94, than by any use of the terms round or wrinkled. One sugar-pea (Debarbieux) which I have used has large flattish, smooth, yellow seeds with white skins, and this also in its crossings follows the rules about to be described for the large-seeded “grey” peas.

In the large “grey” peas the most conspicuous feature is the seed-coat, which is grey, brownish, or of a bright reddish colour. Such seed-coats are often speckled with purple, and on boiling these seed-coats turn dark brown. They are in fact the very peas used by Mendel in making up his third pair of characters. Regarding them ProfessorWeldon, stating they may be considered separately, writes as follows:—

“Tschermak has crossedGraue Riesenwith five races ofP. sativum, and he finds that the form of the first hybrid seedsfollows the female parent, so that if races ofP. sativumwith round smooth seeds be crossed withGraue Riesen(which has flattened, feebly wrinkled seeds) the hybrids will be round and smooth or flattened and wrinkled, as theP. sativumor theGraue Riesenis used as femaleparent95. There is here a more complex phenomenon than at first sight appears; because if the flowers of the first hybrid generation are self-fertilised, the resulting seeds of the second generation invariably resemble those of theGraue Riesenin shape, although in colour they follow Mendel’s law of segregation!”

From this account who would not infer that we have here some mystery which does not accord with the Mendelian principles? As a matter of fact the case is dominance in a perfectly obvious if distinct form.

Graue Riesen, a large grey sugar-pea, thepois sans parchemin géantof the French seedsmen, has full-yellow cotyledons and a highly coloured seed-coat of varying tints. In shape the seed is somewhat flattened with irregular slight indentations, lightly wrinkled if the term be preferred. Tschermak speaks of it in his first paper as “Same flach, zusammengedrückt”—a flat, compressed seed; in his second paper as “flache, oft schwach gerunzelte Cotyledonen-form,” or cotyledon-shape, flat, often feebly wrinkled, as Professor Weldon translates.

First-crosses made from this variety, each with a different form ofP. sativum, are stated on the authority of Tschermak’s five cases, to follow exclusively the maternal seed-shape. From “schwach gerunzelte,” “feebly wrinkled,” Professor Weldon easily passes to “wrinkled,” and tells usthat according as a roundsativumor theGraue Riesenis used as mother, the first-cross seeds “will be round and smooth or flattened and wrinkled.”

As a matter of fact, however, the seeds ofGraue Riesenthoughslightlywrinkled do not belong to the “wrinkled” class; but if the classification “wrinkled” and “round” is to be extended to such peas at all, they belong to theround. Mendel is careful to state that hisroundclass are “either spherical or roundish, the depressions on the surface, when there are any, always slight”; while the “wrinkled” class are “irregularly angular, deeplywrinkled96.”

On this description alone it would be very likely thatGraue Riesenshould fall into theroundclass, and as such it behaves in its crosses,being dominant over wrinkled(see Nos. 3 and 6, below). I can see that in this case Professor Weldon has been partly misled by expressions of Tschermak’s, but the facts of the second generation should have aroused suspicion. Neither author notices that as all five varieties crossed by Tschermak withGraue Riesenwereround, the possibilities are not exhausted. Had Tschermak tried a really wrinkledsativumwithGraue Riesenhe would have seen this obvious explanation.

As some of my own few observations of first-crosses bear on this point I may quote them, imperfect though they are.

I grew the purple-flowered sugar-pea “Pois sans parchemin géant à très large cosse,” a soft-podded “mange-tout” pea, flowers and seed-coats coloured, from Vilmorin’s, probably identical withGraue Riesen.

1. One flower of this variety fertilised withPois très nain de Bretagne(very small seed; yellow cotyledons; veryround) gave seven seeds indistinguishable (in their coats) from those of the mother, save for a doubtful increase in purple pigmentation of coats.2. Fertilised byLaxton’s Alpha(green; wrinkled; coats transparent), two flowers gave 11 seeds exactly as above, the purple being in this case clearly increased.In the following the purple sugar-pea wasfather.3.Laxton’s Alpha(green; wrinkled; coats transparent) fertilised by the purple sugar-pea gave one pod of four seeds with yellow cotyledons androundform.4.Fillbasket(green; smooth but squared; coats green) fertilised by thepurplesugar-pea gave one pod with six seeds, yellowcotyledons97;Fillbasketsize and shape; but the normally green coat yellowed nearthe hilumby xenia.5.Express(“blue”-green cotyledons and transparent skins; round) fertilised withpurple sugar-peagave one pod with four seeds, yellow cotyledons, shape round, much as inFillbasket.6.British Queen(yellow cotyledons, wrinkled, white coats) ♀ × purple sugar-pea gave two pods with seven seeds, cotyledons yellow, coatstinged greenish(xenia?), allround.So much for the “Purple” sugar-pea.I got similar results withMange-tout Debarbieux. This is a soft-poddedMange-toutor sugar-pea, with white flowers, large, flattish, smooth seeds, scarcely dimpled; yellow cotyledons.7.Debarbieuxfertilised bySerpette nain blanc(yellow cotyledons; wrinkled; white skin; dwarf) gave one pod with six seeds, size and shape ofDebarbieux, with slight dimpling.8.Debarbieuxbynain de Bretagne(very small; yellow cotyledons; very round) gave three pods, 12 seeds, all yellow cotyledons, of which two pods had eight seeds identical in shape withDebarbieux, while the third had four seeds likeDebarbieuxbut more dimpled. The reciprocal cross gave two seeds exactly likenain de Bretagne.

2. Fertilised byLaxton’s Alpha(green; wrinkled; coats transparent), two flowers gave 11 seeds exactly as above, the purple being in this case clearly increased.

In the following the purple sugar-pea wasfather.

3.Laxton’s Alpha(green; wrinkled; coats transparent) fertilised by the purple sugar-pea gave one pod of four seeds with yellow cotyledons androundform.

4.Fillbasket(green; smooth but squared; coats green) fertilised by thepurplesugar-pea gave one pod with six seeds, yellowcotyledons97;Fillbasketsize and shape; but the normally green coat yellowed nearthe hilumby xenia.

5.Express(“blue”-green cotyledons and transparent skins; round) fertilised withpurple sugar-peagave one pod with four seeds, yellow cotyledons, shape round, much as inFillbasket.

6.British Queen(yellow cotyledons, wrinkled, white coats) ♀ × purple sugar-pea gave two pods with seven seeds, cotyledons yellow, coatstinged greenish(xenia?), allround.

So much for the “Purple” sugar-pea.

I got similar results withMange-tout Debarbieux. This is a soft-poddedMange-toutor sugar-pea, with white flowers, large, flattish, smooth seeds, scarcely dimpled; yellow cotyledons.

7.Debarbieuxfertilised bySerpette nain blanc(yellow cotyledons; wrinkled; white skin; dwarf) gave one pod with six seeds, size and shape ofDebarbieux, with slight dimpling.

8.Debarbieuxbynain de Bretagne(very small; yellow cotyledons; very round) gave three pods, 12 seeds, all yellow cotyledons, of which two pods had eight seeds identical in shape withDebarbieux, while the third had four seeds likeDebarbieuxbut more dimpled. The reciprocal cross gave two seeds exactly likenain de Bretagne.

But it may be objected that the shape of this large grey pea is verypeculiar98; and that it maintains its type remarkably when fertilised by many distinct varieties though its pollen effects little or no change in them; for, so long as round varieties ofsativumare used as mothers, this is true as we have seen. But when once it is understood that inGraue Riesenthere is no question of wrinkling, seeing that the variety behaves as aroundvariety, the shape and especially the size of the seed must be treated as a maternal property.

Whythe distinction between the shape ofGraue Riesenand that of ordinary round peas should be a matter of maternal physiology we do not know. The question is one for the botanical chemist. But there is evidently very considerable regularity, the seeds borne by thecross-bredsexhibiting the form of the “grey” pea, which is then a dominant character as much as the seed-coat charactersare. And that is what Tschermak’sGraue Riesencrosses actually did, thereby exhibiting dominance in a very clear form. To interject these cases as a mystery without pointing out how easily they can be reconciled with the “law of dominance” may throw an unskilled reader into gratuitous doubt.

Finally, sincethe wrinkled peas,Laxton’s AlphaandBritish Queen,pollinated by a large flat mange-tout, witness Nos. 3 and 6 above, became round in both cases where this experiment was made, we here merely see the usual dominance of the non-wrinkled character; though of course if around-seeded mother be used there can be no departure from the maternal shape, as far as roundness is concerned.

Correns’ observations on the shapes of a “grey” pea crossed with a round shelling pea, also quoted by Professor Weldon as showing no dominance of roundness, are of course of the same nature as those just discussed.

C. Evidence of Knight and Laxton.

In the last two sections we have seen that in using peas of the “grey” class, i.e. with brown, red, or purplish coats, special phenomena are to be looked for, and also that in the case of large “indented” peas, the phenomena of size and shape may show some divergence from that simple form of the phenomenon of dominance seen when ordinary round and wrinkled are crossed. Here the fuller discussion of these phenomena must have been left to await further experiment, were it not that we have other evidence bearing on the same questions.

The first is that of Knight’s well-known experiments, long familiar but until now hopelessly mysterious. I have not space to quote the various interpretations which Knight and others have put upon them, but as the Mendelianprinciple at once gives a complete account of the whole, this is scarcely necessary, though the matter is full of historical interest.

Crossing a white pea with a very large grey purple-flowered form Knight (21) found that the peas so produced “were not in any sensible degree different from those afforded by other plants of the same [white] variety; owing, I imagine, to the external covering of the seed (as I have found in other plants) being furnished entirely by thefemale99.” All grew verytall100, and had colours of maleparent101. The seeds they produced were darkgrey102.

“I had frequent occasion to observe, in this plant [the hybrid], a stronger tendency to produce purple blossoms, and coloured seeds, than white ones; for when I introduced the farina of a purple blossom into a white one, the whole of the seeds in the succeeding year became coloured [viz.DR×DgivingDDandDR]; but, when I endeavoured to discharge this colour, by reversing the process, a part only of them afforded plants with white blossoms; this part sometimes occupying one end of the pod, and being at times irregularly intermixed with those which, when sown, retained their colour” [viz.DR×RgivingDRandRR] (draws conclusions, now obviouslyerroneous103).

In this account we have nothing not readily intelligible in the light of Mendel’s hypothesis.

The next evidence is supplied by an exceptionally complete record of a most valuable experiment made byLaxton104. The whole story is replete with interest, and as it not only carries us on somewhat beyond the point reached by Mendel, but furnishes an excellent illustration of how his principles may be applied, I give the whole account in Laxton’s words, only altering the paragraphing for clearness, and adding a commentary. The paper appears inJour. Hort. Soc.N.S.III.1872, p. 10, and very slightly abbreviated inJour. of Hort.XVIII.1870, p. 86. Some points in the same article do not specially relate to this section, but for simplicity I treat the whole together. It is not too much to say that two years ago the whole of this story would have been a maze of bewildering confusion. There are still some points in it that we cannot fully comprehend, for the case is one of far more than ordinary complexity, but the general outlines are now clear. In attempting to elucidate the phenomena it will be remembered that there are no statistics (those given being inapplicable), and the several offspring are only imperfectly referred to the several classes of seeds. This being so, our rationale cannot hope to be complete. Laxton states that as the seeds of peas are liable to change colour with keeping, for this and other reasons he sent to the Society a part of the seeds resulting from his experiment before it was brought to a conclusion.

“The seeds exhibited were derived from a single experiment. Amongst these seeds will be observed some of several remarkable colours, including black, violet, purple-streaked and spotted, maple, grey, greenish, white, and almost every intermediate tint, the varied colours being apparently produced on the outer coat or envelope of the cotyledons only.The peas were selected for their colours, &c., from the third year’s sowing in 1869 of the produce of a cross in 1866 of the early round white-seeded and white-flowered garden variety “Ringleader,” which is about21/2ft. in height, fertilised by the pollen of the common purple-flowered “maple” pea, which is taller than “Ringleader,” and has slightly indented seeds. I effected impregnation by removing the anthers of the seed-bearer, and applying the pollen at an early stage. This cross produced a pod containing five round white peas, exactly like the ordinary “Ringleader”seeds105.In 1867 I sowed these seeds, and all five produced tall purple-flowered purplish-stemmedplants106, and the seeds, with few exceptions, had all maple or brownish-streaked envelopes of various shades; the remainder had entirely violet or deep purple-colouredenvelopes107: in shape the peas were partly indented;but a few wereround108. Some of the plants ripened off earlier than the “maple,” which, in comparison with “Ringleader,” is a late variety; and although the pods were in many instances partially abortive, the produce was verylarge109.In 1868 I sowed the peas of the preceding year’s growth, and selected various plants for earliness, productiveness, &c. Some of the plants had light-coloured stems and leaves; these all showed white flowers, and produced round whiteseeds110. Others had purple flowers, showed the purple on the stems and at the axils of the stipules, and produced seeds with maple, grey, purple-streaked, or mottled, and a few only, again, with violet-coloured envelopes. Some of the seeds were round, some partiallyindented111. The pods on each plant, in the majority of instances, contained peas of like characters; but in a few cases the peas in the same pod varied slightly, and in some instances a pod or two on the same plant contained seeds all distinct from theremainder112. The white-flowered plants were generally dwarfish,of about the height of “Ringleader”; but the coloured-flowered sorts varied altogether as to height, period of ripening, and colour and shape ofseed113. Those seeds with violet-coloured envelopes produced nearly all maple- or parti-coloured seeds, and only here and there one with a violet-coloured envelope; that colour, again, appeared only incidentally, and in a like degree in the produce of the maple-colouredseeds114.In 1869 the seeds of various selections of the previous year were again sown separately; and the white-seeded peas again produced only plants with white flowers and round whiteseeds115. Some of the coloured seeds, which I had expected would produce purple-flowered plants, produced plants with white flowers and round white seedsonly116; the majority, however, brought plants with purple flowers and with seeds principally marked with purple or grey, the maple- or brown-streaked being in theminority117. On some of the purple-flowered plants were again a few pods with peas differing entirely from the remainder on the same plant. In some pods the seeds were all white, in others all black, and in a few, again, allviolet118; but those plants which bore maple-coloured seeds seemed the most constant and fixed in character of the purple-floweredseedlings119, and the purplish and grey peas, being of intermediate characters, appearedto varymost120. The violet-coloured seeds again produced almost invariably purplish, grey, or maple peas, the clear violet colour only now and then appearing, either wholly in one pod or on a single pea or two in a pod. All the seeds of the purple-flowered plants were again either round or only partially indented; and the plants varied as to height and earliness. In no case, however, does there seem to have been an intermediate-coloured flower; for although in some flowers I thought I found the purple of a lighter shade, I believe this was owing to light, temperature, or other circumstances, and applied equally to the parent maple. I have never noticed a single tinted white flower nor an indented white seed in either of the three years’ produce. The whole produce of the third sowing consisted of seeds of the colours and in the approximate quantities in order as follows,—viz.: 1st, white, about half; 2nd, purplish, grey, and violet (intermediate colours), about three-eighths; and, 3rd, maple, about one-eighth.From the above I gather that the white-flowered white-seeded pea is (if I may use the term) an original variety well fixed and distinct entirely from the maple, that the two do not thoroughly intermingle (for whenever the white flower crops out, the plant and its parts all appear to follow exactly the characters of the white pea), and that the maple is a cross-bred variety which has become somewhat permanent and would seem to include amongst its ancestors one or more bearing seeds either altogether or partly violet- or purple-coloured; for although this colour does not appear on the seed of the “maple,” it is very potent in the variety, and appears in many parts of the plant and its offspring from cross-fertilised flowers, sometimes on the external surface or at the sutures of the pods of the latter, at others on the seeds and stems, and very frequently on the seeds; and whenever it shows itself on any part of the plant, the flowers are invariably purple. My deductions have been confirmed by intercrosses effected between the various white-, blue-, some singularly bright green-seeded peas which I have selected, and the maple- and purple-podded and the purple-flowered sugar peas, and by reversing those crosses.I have also deduced from my experiments, in accordance with the conclusions of the late Mr Knight and others, that the colours of the envelopes of the seeds of peas immediately resulting from a cross are neverchanged121. I find, however, that the colour and probably the substance of the cotyledons are sometimes, but not always, changed by the cross fertilisation of two different varieties; and I do not agree with Mr Knight that the form and size of the seeds produced areunaltered122; for I have on more than one occasion observed that the cotyledons in the seeds directly resulting from a cross of a blue wrinkled pea fertilised by the pollen of a white round variety have been of a greenish-whitecolour123, and the seeds nearlyround124and larger or smaller according as there may have been a difference in the size of the seeds of the twovarieties125.I have also noticed that a cross between a round white and a blue wrinkled pea will in the third and fourth generations (second and third years’ produce) at times bring forth blue round, blue wrinkled, white round and white wrinkled peas in the same pods, that the white round seeds, when again sown, will produce only white round seeds, that the white wrinkled seeds will, up to the fourth or fifth generation, produce both blue and white wrinkled and round peas, that the blue round peas will produce blue wrinkled and round peas, but that the blue wrinkled peas will bear only blue wrinkledseeds126. Thiswould seem to indicate that the white round and the blue wrinkled peas are distinct varieties derived from ancestors respectively possessing one only of those marked qualities; and, in my opinion, the white round peas trace their origin to a dwarfish pea having white flowers and round white seeds, and the blue wrinkled varieties to a tall variety, having also white flowers but blue wrinkled seeds. It is also noticeable, that from a single cross between two different peas many hundreds of varieties, not only like one or both parents and intermediate, but apparently differing from either, may be produced in thecourse of three or four years (the shortest time which I have ascertained it takes to attain the climax of variation in the produce of cross-fertilised peas, and until which time it would seem useless to expect a fixed seedling variety to beproduced127), although a reversion to the characters of either parent, or of any one of the ancestors, may take place at an earlier period.These circumstances do not appear to have been known to Mr Knight, as he seems to have carried on his experiments by continuing to cross his seedlings in the year succeeding their production from a cross and treating the results as reliable; whereas it is probable that the results might have been materially affected by the disturbing causes then in existence arising from the previous cross fertilisation, and which, I consider, would, in all cases where either parent has not become fixed or permanent, lead to results positively perplexing and uncertain, and to variations almost innumerable. I have again selected, and intend to sow, watch, and report; but as the usual climax of variation is nearly reached in the recorded experiment, I do not anticipate much further deviation, except in height and period of ripening—characters which are always very unstable in the pea. There are also important botanical and other variations and changes occurring in cross-fertilised peas to which it is not my province here to allude; but in conclusion I may, perhaps, in furtherance of the objects of this paper, be permitted to inquire whether any light can, from these observations or other means, be thrown upon the origin of the cultivated kinds of peas, especially the “maple” variety, and also as to the source whence the violet and other colours which appear at intervals on the seeds and in the offspring of cross-fertilised purple-flowered peas are derived.”

The peas were selected for their colours, &c., from the third year’s sowing in 1869 of the produce of a cross in 1866 of the early round white-seeded and white-flowered garden variety “Ringleader,” which is about21/2ft. in height, fertilised by the pollen of the common purple-flowered “maple” pea, which is taller than “Ringleader,” and has slightly indented seeds. I effected impregnation by removing the anthers of the seed-bearer, and applying the pollen at an early stage. This cross produced a pod containing five round white peas, exactly like the ordinary “Ringleader”seeds105.

In 1867 I sowed these seeds, and all five produced tall purple-flowered purplish-stemmedplants106, and the seeds, with few exceptions, had all maple or brownish-streaked envelopes of various shades; the remainder had entirely violet or deep purple-colouredenvelopes107: in shape the peas were partly indented;but a few wereround108. Some of the plants ripened off earlier than the “maple,” which, in comparison with “Ringleader,” is a late variety; and although the pods were in many instances partially abortive, the produce was verylarge109.

In 1868 I sowed the peas of the preceding year’s growth, and selected various plants for earliness, productiveness, &c. Some of the plants had light-coloured stems and leaves; these all showed white flowers, and produced round whiteseeds110. Others had purple flowers, showed the purple on the stems and at the axils of the stipules, and produced seeds with maple, grey, purple-streaked, or mottled, and a few only, again, with violet-coloured envelopes. Some of the seeds were round, some partiallyindented111. The pods on each plant, in the majority of instances, contained peas of like characters; but in a few cases the peas in the same pod varied slightly, and in some instances a pod or two on the same plant contained seeds all distinct from theremainder112. The white-flowered plants were generally dwarfish,of about the height of “Ringleader”; but the coloured-flowered sorts varied altogether as to height, period of ripening, and colour and shape ofseed113. Those seeds with violet-coloured envelopes produced nearly all maple- or parti-coloured seeds, and only here and there one with a violet-coloured envelope; that colour, again, appeared only incidentally, and in a like degree in the produce of the maple-colouredseeds114.

In 1869 the seeds of various selections of the previous year were again sown separately; and the white-seeded peas again produced only plants with white flowers and round whiteseeds115. Some of the coloured seeds, which I had expected would produce purple-flowered plants, produced plants with white flowers and round white seedsonly116; the majority, however, brought plants with purple flowers and with seeds principally marked with purple or grey, the maple- or brown-streaked being in theminority117. On some of the purple-flowered plants were again a few pods with peas differing entirely from the remainder on the same plant. In some pods the seeds were all white, in others all black, and in a few, again, allviolet118; but those plants which bore maple-coloured seeds seemed the most constant and fixed in character of the purple-floweredseedlings119, and the purplish and grey peas, being of intermediate characters, appearedto varymost120. The violet-coloured seeds again produced almost invariably purplish, grey, or maple peas, the clear violet colour only now and then appearing, either wholly in one pod or on a single pea or two in a pod. All the seeds of the purple-flowered plants were again either round or only partially indented; and the plants varied as to height and earliness. In no case, however, does there seem to have been an intermediate-coloured flower; for although in some flowers I thought I found the purple of a lighter shade, I believe this was owing to light, temperature, or other circumstances, and applied equally to the parent maple. I have never noticed a single tinted white flower nor an indented white seed in either of the three years’ produce. The whole produce of the third sowing consisted of seeds of the colours and in the approximate quantities in order as follows,—viz.: 1st, white, about half; 2nd, purplish, grey, and violet (intermediate colours), about three-eighths; and, 3rd, maple, about one-eighth.

From the above I gather that the white-flowered white-seeded pea is (if I may use the term) an original variety well fixed and distinct entirely from the maple, that the two do not thoroughly intermingle (for whenever the white flower crops out, the plant and its parts all appear to follow exactly the characters of the white pea), and that the maple is a cross-bred variety which has become somewhat permanent and would seem to include amongst its ancestors one or more bearing seeds either altogether or partly violet- or purple-coloured; for although this colour does not appear on the seed of the “maple,” it is very potent in the variety, and appears in many parts of the plant and its offspring from cross-fertilised flowers, sometimes on the external surface or at the sutures of the pods of the latter, at others on the seeds and stems, and very frequently on the seeds; and whenever it shows itself on any part of the plant, the flowers are invariably purple. My deductions have been confirmed by intercrosses effected between the various white-, blue-, some singularly bright green-seeded peas which I have selected, and the maple- and purple-podded and the purple-flowered sugar peas, and by reversing those crosses.

I have also deduced from my experiments, in accordance with the conclusions of the late Mr Knight and others, that the colours of the envelopes of the seeds of peas immediately resulting from a cross are neverchanged121. I find, however, that the colour and probably the substance of the cotyledons are sometimes, but not always, changed by the cross fertilisation of two different varieties; and I do not agree with Mr Knight that the form and size of the seeds produced areunaltered122; for I have on more than one occasion observed that the cotyledons in the seeds directly resulting from a cross of a blue wrinkled pea fertilised by the pollen of a white round variety have been of a greenish-whitecolour123, and the seeds nearlyround124and larger or smaller according as there may have been a difference in the size of the seeds of the twovarieties125.

I have also noticed that a cross between a round white and a blue wrinkled pea will in the third and fourth generations (second and third years’ produce) at times bring forth blue round, blue wrinkled, white round and white wrinkled peas in the same pods, that the white round seeds, when again sown, will produce only white round seeds, that the white wrinkled seeds will, up to the fourth or fifth generation, produce both blue and white wrinkled and round peas, that the blue round peas will produce blue wrinkled and round peas, but that the blue wrinkled peas will bear only blue wrinkledseeds126. Thiswould seem to indicate that the white round and the blue wrinkled peas are distinct varieties derived from ancestors respectively possessing one only of those marked qualities; and, in my opinion, the white round peas trace their origin to a dwarfish pea having white flowers and round white seeds, and the blue wrinkled varieties to a tall variety, having also white flowers but blue wrinkled seeds. It is also noticeable, that from a single cross between two different peas many hundreds of varieties, not only like one or both parents and intermediate, but apparently differing from either, may be produced in thecourse of three or four years (the shortest time which I have ascertained it takes to attain the climax of variation in the produce of cross-fertilised peas, and until which time it would seem useless to expect a fixed seedling variety to beproduced127), although a reversion to the characters of either parent, or of any one of the ancestors, may take place at an earlier period.

These circumstances do not appear to have been known to Mr Knight, as he seems to have carried on his experiments by continuing to cross his seedlings in the year succeeding their production from a cross and treating the results as reliable; whereas it is probable that the results might have been materially affected by the disturbing causes then in existence arising from the previous cross fertilisation, and which, I consider, would, in all cases where either parent has not become fixed or permanent, lead to results positively perplexing and uncertain, and to variations almost innumerable. I have again selected, and intend to sow, watch, and report; but as the usual climax of variation is nearly reached in the recorded experiment, I do not anticipate much further deviation, except in height and period of ripening—characters which are always very unstable in the pea. There are also important botanical and other variations and changes occurring in cross-fertilised peas to which it is not my province here to allude; but in conclusion I may, perhaps, in furtherance of the objects of this paper, be permitted to inquire whether any light can, from these observations or other means, be thrown upon the origin of the cultivated kinds of peas, especially the “maple” variety, and also as to the source whence the violet and other colours which appear at intervals on the seeds and in the offspring of cross-fertilised purple-flowered peas are derived.”

The reader who has closely followed the preceding passage will begin to appreciate the way in which the new principles help us to interpret these hitherto paradoxical phenomena. Even in this case, imperfectly recorded as it is, we can form a fairly clear idea of what was taking place.If the “round” seeds really occurred as a distinct class, on the heterozygotes as described, it is just possible that the fact may be of great use hereafter.

We are still far from understanding maternal seed-form—and perhaps size—as a dominant character. So far, as Miss Saunders has pointed out to me, it appears to be correlated with a thick and coloured seed-coat.

We have now seen the nature of Professor Weldon’s collection of contradictory evidence concerning dominance in peas. He tells us: “Enough has been said to show the grave discrepancy between the evidence afforded by Mendel’s experiments and that obtained by observers equally trustworthy.”

He proceeds to a discussion of theTelephoneandTelegraphgroup and recites facts, which I do not doubt for a moment, showing that in this group of peas—which have unquestionably been more or less “blend” or “mosaic” forms from their beginning—the “laws of dominance and segregation” do not hold. Professor Weldon’s collection of the facts relating toTelephone, &c. has distinct value, and it is the chief addition he makes to our knowledge of these phenomena. The merit however of this addition is diminished by the erroneous conclusion drawn from it, as will be shown hereafter. Meanwhile the reader who has studied what has been written above on the general questions of stability, “purity,” and “universal” dominance, will easily be able to estimate the significance of these phenomena and their applicability to Mendel’s hypotheses.

D. Miscellaneous cases in other plants and animals.

Professor Weldon proceeds:

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