Part 10

Every case therefore which obeys the Mendelian principle is in direct contradiction to the proposition to which Professor Weldon’s school is committed, and it is natural that he should be disposed to consider the Mendelian principle as applying especially to “alternative” inheritance, while the law of Galton and Pearson is to include the phenomenon of blended inheritance. The latter, he tells us, is “the most usual case,” a view which, if supported by evidence, might not be without value.

It is difficult to blame those who on first acquaintance concluded Mendel’s principle can have no strict application save to alternative inheritance. Whatever blame there is in this I share with Professor Weldon and those whom he follows. Mendel’s own cases were almost all alternative; also the fact of dominance is very dazzling at first. But that was two years ago, and when one begins to see clearly again, it does not look so certain that the real essence of Mendel’s discovery, the purity of germ-cells in respect of certain characters, may not apply also to some phenomena of blended inheritance. The analysis of this possibility would take us to too great length, but I commend to those who are more familiar with statistical method, the consideration of this question: whether dominance being absent, indefinite, or suppressed, the phenomena of heritages completely blended in the zygote, may not be produced by gametes presenting Mendelian purity of characters. A brief discussion of this possibility is given in the Introduction, p. 31.

Very careful inquiry would be needed before such a possibility could be negatived. For example, we know that the Laws based on Ancestry can apply to _alternative_ inheritance; witness the case of the Basset-hounds. Here there is no simple Mendelian dominance; but are we sure there is no purity of germ-cells? The new conception goes a long way and it may well reach to such facts as these.

But for the present we will assume that Mendel’s principle applies only to _certain phenomena of alternative inheritance_, which is as far as our warrant yet runs.

No close student of the recent history of evolutionary thought needs to be told what the attitude of Professor Weldon and his followers has been towards these same disquieting and unwelcome phenomena of alternative inheritance and discontinuity in variation. Holding at first each such fact for suspect, then treating them as rare and negligible occurrences, he and his followers have of late come slowly to accede to the facts of discontinuity a bare and grudging recognition in their scheme of evolution[62].

[62] Read in this connexion Pearson, K., _Grammar of Science_, 2nd ed. 1900, pp. 390–2.

Professor Weldon even now opens his essay with the statement--or perhaps reminiscence--that “it is perfectly possible and indeed probable that the difference between these forms of inheritance [blended, mosaic, and alternative] is only one of degree.” This may be true; but reasoning favourable to this proposition could equally be used to prove the difference between mechanical mixture and chemical combination to be a difference of degree.

Therefore on the announcement of that discovery which once and for all ratifies and consolidates the conception of discontinuous variation, and goes far to define that of alternative inheritance, giving a finite body to what before was vague and tentative, it is small wonder if Professor Weldon is disposed to criticism rather than to cordiality.

* * * * *

We have now seen what is the essence of Mendel’s discovery based on a series of experiments of unequalled simplicity which Professor Weldon does not venture to dispute.

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.”

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.

[63] The whole question as to seed-coat colour is most complex. Conditions of growth and ripening have a great effect on it. Mr Arthur Sutton has shown me samples of _Ne Plus Ultra_ grown in England and abroad. This pea has yellow cotyledons with seed-coats either yellow or “blue.” The foreign sample contained a much greater proportion of the former. He told me that generally speaking this is the case with samples ripened in a hot, dry climate.

Unquestionable Xenia appears occasionally, and will be spoken of later. Moreover to experiment with such a _plant_-character an extra generation has to be sown and cultivated. Consequently the evidence is meagre.

_The normal characters: colour of cotyledons and seed-coats._

Culinary peas (_P. sativum_, omitting purple sorts) can primarily be classified on colour into two groups, yellow and green. In the green certain pigmentary matters persist in the ripe seed which disappear or are decomposed in the yellow as the seed ripens. But it may be observed that the “green” class itself is treated as of two divisions, _green_ and _blue_. In the seedsmen’s lists the classification is made on the _external appearance_ of the seed, without regard to whether the colour is due to the seed-coat, the cotyledons, or both. As a rule perhaps yellow coats contain yellow cotyledons, and green coats green cotyledons, though yellow cotyledons in green coats are common, e.g. _Gradus_, of which the cotyledons are yellow while the seed-coats are about as often green as yellow (or “white,” as it is called technically). Those called “blue” consist mostly of seeds which have green cotyledons seen through transparent skins, or yellow cotyledons combined with green skins. The skins may be roughly classified into thin and transparent, or thick and generally at some stage pigmented. In numerous varieties the colour of the cotyledon is wholly yellow, or wholly green. Next there are many varieties which are constant in habit and other properties but have seeds belonging to these two colour categories in various proportions. How far these proportions are known to be constant I cannot ascertain.

Of such varieties showing mixture of _cotyledon_-colours nearly all can be described as dimorphic in colour. For example in Sutton’s _Nonpareil Marrowfat_ the cotyledons are almost always _either_ yellow _or_ green, with some piebalds, and the colours of the seed-coats are scarcely less distinctly dimorphic. In some varieties which exist in both colours intermediates are so common that one cannot assert any regular dimorphism[64].

[64] Knowing my interest in this subject Professor Weldon was so good as to forward to me a series of his peas arranged to form a scale of colours and shapes, as represented in his Plate I. I have no doubt that the use of such colour-scales will much facilitate future study of these problems.

There are some varieties which have cotyledons green and intermediate shading to greenish yellow, like _Stratagem_ quoted by Professor Weldon. Others have yellow and intermediate shading to yellowish green, such as McLean’s _Best of all_[65]. I am quite disposed to think there may be truly monomorphic varieties with cotyledons permanently of intermediate colour only, but so far I have not seen one[66]. The variety with greatest _irregularity_ (apart from regular dimorphism) in cotyledon-colour I have seen is a sample of “_mange-tout à rames, à grain vert_,” but it was a good deal injured by weevils (_Bruchus_), which always cause irregularity or change of colour.

[65] I notice that Vilmorin in the well-known _Plantes Potagères_, 1883, classifies the intermediate-coloured peas with the _green_.

[66] Similarly though _tall_ and _dwarf_ are Mendelian characters, peas occur of all heights and are usually classified as tall, half-dwarfs, and dwarfs.

Lastly in some varieties there are many piebalds or mosaics.

From what has been said it will be evident that the description of a pea in an old book as having been green, blue, white, and so forth, unless the cotyledon-colour is distinguished from seed-coat colour, needs careful consideration before inferences are drawn from it.

_Shape._

In regard to shape, if we keep to ordinary shelling peas, the facts are somewhat similar, but as shape is probably more sensitive to conditions than cotyledon-colour (not than _seed-coat_ colour) there are irregularities to be perhaps ascribed to this cause. Broadly, however, there are two main divisions, round and wrinkled. It is unquestioned that between these two types every intermediate occurs. Here again a vast number of varieties can be at once classified into round and wrinkled (the classification commonly used), others are intermediate normally. Here also I suspect some fairly clear sub-divisions might be made in the wrinkled group and in the round group too, but I would not assert this as a fact.

I cannot ascertain from botanists what is the nature of the difference between round and wrinkled peas, though no doubt it will be easily discovered. In maize the round seeds contain much unconverted starch, while in the wrinkled or sugar-maize this seems to be converted in great measure as the seed ripens; with the result that, on drying, the walls collapse. In such seeds we may perhaps suppose that the process of conversion, which in round seeds takes place on germination, is begun earlier, and perhaps the variation essentially consists in the premature appearance of the converting ferment. It would be most rash to suggest that such a process may be operating in the pea, for the phenomenon may have many causes; but however that may be, there is evidently a difference of such a nature that when the water dries out of the seed on ripening, its walls collapse[67]; and this collapse may occur in varying degrees.

[67] Wrinkling must of course be distinguished further from the squaring due to the peas pressing against each other in the pod.

In connexion with these considerations I may mention that Vilmorin makes the interesting statement that most peas retain their vitality three years, dying as a rule rapidly after that time is passed, though occasionally seeds seven or eight years old are alive; but that _wrinkled_ peas germinate as a rule less well than round, and do not retain their vitality so long as the round. Vilmorin-Andrieux, _Plantes Potagères_, 1883, p. 423. Similar statements regarding the behaviour of wrinkled peas in India are made by Firminger, _Gardening for India_, 3rd ed. 1874, p. 146.

In respect of _shape_ the seeds of a variety otherwise stable are as a rule fairly uniform, the co-existence of both shapes and of intermediates between them in the same variety is not infrequent. As Professor Weldon has said, _Telephone_ is a good example of an extreme case of mixture of both colours and shapes. _William I._ is another. It may be mentioned that regular dimorphism in respect of shape is not so common as dimorphism in respect of colour. Of great numbers of varieties seen at Messrs Suttons’ I saw none so distinctly dimorphic in shape as _William I._ which nevertheless contains all grades commonly.

So far I have spoken of the shapes of ordinary English culinary peas. But if we extend our observations to the shapes of _large-seeded_ peas, which occur for the most part among the sugar-peas (_mange-touts_), of the “grey” peas with coloured flowers, etc., there are fresh complications to be considered.

Professor Weldon does not wholly avoid these (as Mendel did in regard to shape) and we will follow him through his difficulties hereafter. For the present let me say that the classes _round_ and _wrinkled_ are not readily applicable to those other varieties and are not so applied either by Mendel or other practical writers on these subjects. To use the terms indicated in the Introduction, _seed-shape_ depends on more than one pair of allelomorphs--possibly on several.

_Stability and Variability._

Generally speaking peas which when seen in bulk are monomorphic in colour and shape, will give fairly true and uniform offspring (but such strict monomorphism is rather exceptional). Instances to the contrary occur, and in my own brief experience I have seen some. In a row of _Fill-basket_ grown from selected seed there were two plants of different habit, seed-shape, etc. Each bore pods with seeds few though large and round. Again _Blue Peter_ (blue and round) and _Laxton’s Alpha_ (blue and wrinkled), grown in my garden and left to nature uncovered, have each given a considerable proportion of seeds with _yellow_ cotyledons, about 20% in the case of _Laxton’s Alpha_. The distribution of these on the plants I cannot state. The plants bearing them in each case sprang from green-cotyledoned seeds taken from samples containing presumably unselected green seeds only. A part of this exceptional result may be due to crossing, but heterogeneity of conditions[68] especially in or after ripening is a more likely cause, hypotheses I hope to investigate next season. Hitherto I had supposed the crossing, if any, to be done by _Bruchus_ or Thrips, but Tschermak also suspects _Megachile_, the leaf-cutter bee, which abounds in my garden.

[68] Cotyledon-colour is not nearly so sensitive to ordinary changes in conditions as coat-colour, provided the coat be uninjured. But even in monomorphic _green_ varieties, a seed which for any cause has burst on ripening, has the exposed parts of its cotyledons _yellow_. The same may be the case in seeds of green varieties injured by _Bruchus_ or birds. These facts make one hesitate before denying the effects of conditions on the cotyledon-colour even of uninjured seeds, and the variation described above may have been simply weathering. The seeds were gathered very late and many were burst in _Laxton’s Alpha_. I do not yet know they are alive.

Whatever the cause, these irregularities may undoubtedly occur; and if they be proved to be largely independent of crossing and conditions, this will in nowise vitiate the truth of the Mendelian principle. For in that case it may simply be variability. Such true variation, or sporting, in the pea is referred to by many observers. Upon this subject I have received most valuable facts from Mr Arthur Sutton, who has very kindly interested himself in these inquiries. He tells me that several highly bred varieties, selected with every possible care, commonly throw a small but constant proportion of poor and almost vetch-like plants, with short pods and small round seeds, which are hoed out by experienced men each year before ripening. Other high-class varieties always, wherever grown, and when far from other sorts, produce a small percentage of some one or more definite “sports.” Of these peculiar sports he has sent me a collection of twelve, taken from as many standard varieties, each “sport” being represented by eight seeds, which though quite distinct from the type agree with each other in almost all cases.

In two cases, he tells me, these seed-sports sown separately have been found to give plants identical with the standard type and must therefore be regarded as sports in _seed characters_ only; in other cases change of plant-type is associated with the change of seed-type.

In most standard varieties these definite sports are not very common, but in a few they are common enough to require continual removal by selection[69].

[69] It is interesting to see that in at least one case the same--or practically the same--variety has been independently produced by different raisers, as we now perceive, by the fortuitous combination of similar allelomorphs. _Sutton’s Ringleader_ and _Carter’s First Crop_ (and two others) are cases in point, and it is peculiarly instructive to see that in the discussion of these varieties when they were new, one of the points indicating their identity was taken to be the fact that they produced _the same “rogues.”_ See _Gard. Chron._ 1865, pp. 482 and 603; 1866, p. 221; 1867, pp. 546 and 712.

Rimpau quotes Blomeyer (_Kultur der Landw. Nutzpflanzen_, Leipzig, 1889, pp. 357 and 380) to the effect that _purple_-flowered plants with _wrinkled_ seeds may spring as direct sports from peas with _white_ flowers and _round_ seeds. I have not seen a copy of Blomeyer’s work. Probably this “wrinkling” was “indentation.”

I hope before long to be able to give statistical details and experiments relating to this extraordinarily interesting subject. As de Vries writes in his fine work _Die Mutationstheorie_ (I. p. 580), “a study of the seed-differences of inconstant, or as they are called, ‘still’ unfixed varieties, is a perfect treasure-house of new discoveries.”

Let us consider briefly the possible significance of these facts in the light of Mendelian teaching. First, then, it is clear that as regards most of such cases the hypothesis is not excluded that these recurring sports may be due to the fortuitous concurrence of certain scarcer hypallelomorphs, which may either have been free in the original parent varieties from which the modern standard forms were raised, or may have been freed in the crossing to which the latter owe their origin (see p. 28). This possibility raises the question whether, if we could make “_pure_ cultures” of the gametes, any variations of this nature would ever occur. This may be regarded as an unwarrantable speculation, but it is not wholly unamenable to the test of experiments.

But variability, in the sense of division of gonads into heterogeneous gametes, may surely be due to causes other than crossing. This we cannot doubt. Cross-fertilization of the zygote producing those gametes is _one_ of the causes of such heterogeneity among them. We cannot suppose it to be the sole cause of this phenomenon.

When Mendel asserts the purity of the germ-cells of cross-breds he cannot be understood to mean that they are _more pure_ than those of the original parental races. These must have varied in the past. The wrinkled seed arose from the round, the green from the yellow (or _vice versâ_, if preferred), and probably numerous intermediate forms from both.