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.
The variations, or as I provisionally conceive it, that differentiant division among the gametes of which variation (neglecting environment) is the visible expression, has arisen and can arise at one or more points of time, and we have no difficulty in believing it to occur now. In many cases we have clear evidence that it does. Crossing,--dare we call it asymmetrical fertilization?--is _one_ of the causes of the production of heterogeneous gametes--the result of divisions qualitatively differentiant and perhaps asymmetrical[70].
[70] The asymmetries here conceived may of course be combined in an inclusive symmetry. Till the differentiation can be optically recognized in the gametes we shall probably get no further with this part of the problem.
There are other causes and we have to find them. Some years ago I wrote that consideration of the causes of variation was in my judgment premature[71]. Now that through Mendel’s work we are clearing our minds as to the fundamental nature of “gametic” variation, the time is approaching when an investigation of such causes may be not unfruitful.
[71] _Materials for the Study of Variation_, 1894, p. 78.
Of _variation_ as distinct from _transmission_ why does Professor Weldon take no heed? He writes (p. 244):
“If Mendel’s statements were universally valid, even among Peas, the characters of the seeds in the numerous hybrid races now existing should fall into one or other of a few definite categories, which should not be connected by intermediate forms.”
Now, as I have already pointed out, Mendel made no pretence of universal statement: but had he done so, the conclusion, which Professor Weldon here suggests should follow from such a universal statement, is incorrectly drawn. Mendel is concerned with the laws of _transmission of existing characters_, not with _variation_, which he does not discuss.
Nevertheless Professor Weldon has some acquaintance with the general fact of variability in certain peas, which he mentions (p. 236), but the bearing of this fact on the difficulty he enuntiates escapes him.
_Results of crossing in regard to seed characters: normal and exceptional._
The conditions being the same, the question of the characters of the cross-bred zygotes which we will call _AB_’s depends primarily on the specific nature of the varieties which are crossed to produce them. It is unnecessary to point out that if all _AB_’s are to look alike, both the varieties _A_ and _B_ must be _pure_--not in the common sense of descended, as far as can be traced, through individuals identical with themselves, but pure in the Mendelian sense, that is to say that each must be at that moment producing only homogeneous gametes bearing the same characters _A_ and _B_ respectively. Purity of pedigree in the breeder’s sense is a distinct matter altogether. The length of time--or if preferred--the number of generations through which a character of a variety has remained pure, alters the probability of its _dominance_, i.e. its appearance when a gamete bearing it meets another bearing an antagonistic character, no more, so far as we are yet aware, than the length of time a stable element has been isolated alters the properties of the chemical compound which may be prepared from it.
Now when individuals (bearing contrary characters), pure in the sense indicated, are crossed together, the question arises, What will be the appearance of the first cross individuals? Here again, _generally speaking_, when thoroughly green cotyledons are crossed with thoroughly yellow cotyledons, the first-cross seeds will have yellow cotyledons; when fully round peas are crossed with fully wrinkled the first result will _generally speaking_ be _round_, often with slight pitting as Mendel has stated. This has been the usual experience of Correns, Tschermak, Mendel, and myself[72] and, as we shall see, the amount of clear and substantial evidence to the contrary is still exceedingly small. But as any experienced naturalist would venture to predict, there is no _universal_ rule in the matter. As Professor Weldon himself declares, had there been such a universal rule it would surely have been notorious. He might further have reflected that in Mendel’s day, when hybridisation was not the _terra incognita_ it has since become, the assertion of such universal propositions would have been peculiarly foolish. Mendel does not make it; but Professor Weldon perceiving the inherent improbability of the assertion conceives at once that Mendel _must_ have made it, and if Mendel doesn’t say so in words then he must have implied it. As a matter of fact Mendel never treats dominance as more than an incident in his results, merely using it as a means to an end, and I see no reason to suppose he troubled to consider to what extent the phenomenon is or is not universal--a matter with which he had no concern.
[72] The varieties used were _Express_, _Laxton’s Alpha_, _Fillbasket_, _McLean’s Blue Peter_, _Serpette nain blanc_, _British Queen_, _très nain de Bretagne_, Sabre, _mange-tout_ Debarbieux, and a large “grey” sugar-pea, _pois sans parchemin géant à très large cosse_. Not counting the last two, five are round and three are wrinkled. As to cotyledons, six have yellow and four have green. In about 80 crosses I saw no exception to dominance of yellow; but one apparently clear case of dominance of wrinkled and some doubtful ones.
Of course there may be exceptions. As yet we cannot detect the causes which control them, though injury, impurity, accidental crossing, mistakes of various kinds, account for many. Mendel himself says, for instance, that unhealthy or badly grown plants give uncertain results. Nevertheless there seems to be a true residuum of exceptions not to be explained away. I will recite some that I have seen. In my own crosses I have seen green × green give yellow four times. This I incline to attribute to conditions or other disturbance, for the natural pods of these plants gave several yellows. At Messrs Suttons’ I saw second-generation seeds got by allowing a cross of _Sutton’s Centenary_ (gr. wr.) × _Eclipse_ (gr. rd.) to go to seed; the resulting seeds were both green and _yellow_, wrinkled and round. But in looking at a sample of _Eclipse_ I found a few _yellow_ seeds, say two per cent., which may perhaps be the explanation. Green wrinkled × green round _may_ give all wrinkled, and again wrinkled × wrinkled may give _round_[73]. Of this I saw a clear case--supposing no mistake to have occurred--at Messrs Suttons’. Lastly we have the fact that in exceptional cases crossing two forms--apparently pure in the strict sense--may give a mixture in the _first_ generation. There are doubtless examples also of unlikeness between reciprocals, and of this too I have seen one putative case[74].
[73] Professor Weldon may take this as a famous blow for Mendel, till he realizes what is meant by Mendel’s “Hybrid-character.”
[74] In addition to those spoken of later, where the great difference between reciprocals is due to the _maternal_ characters of the seeds.
Such facts thus set out for the first cross-bred generation may without doubt be predicated for subsequent generations.
What then is the significance of the facts?
_Analysis of exceptions._
Assuming that all these “contradictory” phenomena happened truly as alleged, and were not pathological or due to error--an explanation which seems quite inadequate--there are at least four possible accounts of such diverse results--each valid, without any appeal to ancestry.
1. That dominance may exceptionally fail--or in other words be created on the side which is elsewhere recessive. For this exceptional failure we have to seek exceptional causes. The artificial _creation_ of dominance (in a character usually recessive) has not yet to my knowledge been demonstrated experimentally, but experiments are begun by which such evidence may conceivably be obtained.
2. There may be what is known to practical students of evolution as the _false hybridism of Millardet_, or in other words, fertilisation with--from unknown causes--transmission of none or of only some of the characters of one pure parent. The applicability of this hypothesis to the colours and shapes of peas is perhaps remote, but we may notice that it is one possible account of those rare cases where two pure forms give a _mixed_ result in the first generation, even assuming the gametes of each pure parent to be truly monomorphic as regards the character they bear. The applicability of this suggestion can of course be tested by study of the subsequent generations, self-fertilised or fertilised by similar forms produced in the same way. In the case of a _genuine_ false-hybrid the lost characters will not reappear in the posterity.
3. The result may not be a case of transmission at all as it is at present conceived, but of the creation on crossing of something _new_. Our _AB_’s may have one or more characters _peculiar to themselves_. We may in fact have made a distinct “mule” or heterozygote form. Where this is the case, there are several subordinate possibilities we need not at present pursue.
4. There may be definite _variation_ (distinct from that proper to the “mule”) consequent on causes we cannot yet surmise (see pp. 125 and 128).
The above possibilities are I believe at the present time the only ones that need to be considered in connexion with these exceptional cases[75]. They are all of them capable of experimental test and in certain instances we are beginning to expect the conclusion.
[75] I have not here considered the case in which male and female elements of a pure variety are not homologous and the variety is a _permanent_ monomorphic “mule.” Such a phenomenon, when present, will prove itself in reciprocal crossing. I know no such case in peas for certain.
_The “mule” or heterozygote._
There can be little doubt that in many cases it is to the third category that the phenomena belong. An indication of the applicability of this reasoning will generally be found in the fact that in such “mule” forms the colour or the shape of the seeds will be recognizably peculiar and proper to the specimens themselves, as distinct from their parents, and we may safely anticipate that when those seeds are grown the plants will show some character which is recognizable as novel. The _proof_ that the reasoning may apply can as yet only be got by finding that the forms in question cannot breed true even after successive selections, but constantly break up into the same series of forms[76].
[76] It will be understood that a “mule” form is quite distinct from what is generally described as a “blend.” One certain criterion of the “mule” form is the fact that it cannot be fixed, see p. 25. There is little doubt that Laxton had such a “mule” form when he speaks of “the remarkably fine but unfixable pea, Evolution.” _J. R. Hort. Soc._ XII. 1890, p. 37 (_v. infra_).
This conception of the “mule” form, or “hybrid-character” as Mendel called it, though undeveloped, is perfectly clear in his work. He says that the dominant character may have two significations, it may be either a parental character or a hybrid-character, and it must be differentiated according as it appears in the one capacity or the other. He does not regard the character displayed by the hybrid, whether dominant or other, _as a thing inherited from or transmitted by the pure parent at all, but as the peculiar function or property of the hybrid_. When this conception has been fully understood and appreciated in all its bearings it will be found to be hardly less fruitful than that of the purity of the germ-cells.
The two parents are two--let us say--substances[77] represented by corresponding gametes. These gametes unite to form a new “substance”--the cross-bred zygote. This has its own properties and structure, just as a chemical compound has, and the properties of this new “substance” are _not more strictly_ traceable to, or “inherited” from, those of the two parents than are those of a new chemical compound “inherited” from those of the component elements. If the case be one in which the gametes are pure, the new “substance” is not represented by them, but the compound is again dissociated into its components, each of which is separately represented by gametes.
[77] Using the word metaphorically.
The character of the cross-bred zygote may be anything. It may be something we have seen before in one or other of the parents, it may be intermediate between the two, or it may be something new. All these possibilities were known to Mendel and he is perfectly aware that his principle is equally applicable to all. The first case is his “dominance.” That he is ready for the second is sufficiently shown by his brief reference to time of flowering considered as a character (p. 65). The hybrids, he says, flower at a time _almost exactly intermediate_ between the flowering times of the parents, and he remarks that the development of the hybrids in this case probably happens in the same way as it does in the case of the other characters[78].
[78] “_Ueber die Blüthezeit der Hybriden sind die Versuche noch nicht abgeschlossen. So viel kann indessen schon angegeben werden, dass dieselbe fast genau in der Mitte zwischen jener der Samen- und Pollenpflanze steht, und die Entwicklung der Hybriden bezüglich dieses Merkmales wahrscheinlich in der nämlichen Weise erfolgt, wie es für die übrigen Merkmale der Fall ist._” Mendel, p. 23.
That he was thoroughly prepared for the third possibility appears constantly through the paper, notably in the argument based on the _Phaseolus_ hybrids, and in the statement that the hybrid between talls and dwarfs is generally taller than the tall parent, having increased height as its “hybrid-character.”
All this Professor Weldon has missed. In place of it he offers us the _sententia_ that no one can expect to understand these phenomena if he neglect ancestry. This is the idle gloss of the scribe, which, if we erase it not thoroughly, may pass into the text.
Enough has been said to show how greatly Mendel’s conception of heredity was in advance of those which pass current at the present day; I have here attempted the barest outline of the nature of the “hybrid-character,” and I have not sought to indicate the conclusions that we reach when the reasoning so clear in the case of the hybrid is applied to the pure forms and their own characters.
In these considerations we reach the very base on which all conceptions of heredity and variation must henceforth rest, and that it is now possible for us to attempt any such analysis is one of the most far-reaching consequences of Mendel’s principle. Till two years ago no one had made more than random soundings of this abyss.
I have briefly discussed these possibilities to assist the reader in getting an insight into Mendel’s conceptions. But in dealing with Professor Weldon we need not make this excursion; for his objection arising from the absence of uniform regularity in dominance is not in point.
The soundness of Mendel’s work and conclusions would be just as complete if dominance be found to fail often instead of rarely. For it is perfectly certain that varieties _can_ be chosen in such a way that the dominance of one character over its antagonist is so regular a phenomenon that it _can_ be used in the way Mendel indicates. He chose varieties, in fact, in which a known character _was_ regularly dominant and it is because he did so that he made his discovery[79]. When Professor Weldon speaks of the existence of fluctuation and diversity in regard to dominance as proof of a “grave discrepancy” between Mendel’s facts and those of other observers[80], he merely indicates the point at which his own misconceptions began.
[79] As has been already shown the discovery could have been made equally well and possibly with greater rapidity in a case in which the hybrid had a character distinct from either parent. The cases that would _not_ have given a clear result are those where there is irregular dominance of one or other parent.
[80] Weldon, p. 240.
From Mendel’s style it may be inferred that if he had meant to state universal dominance in peas he would have done so in unequivocal language. Let me point out further that of the 34 varieties he collected for study, he discarded 12 as not amenable to his purposes[81]. He tells us he would have nothing to do with characters which were not sharp, but of a “more or less” description. As the 34 varieties are said to have all come true from seed, we may fairly suppose that the reason he discarded twelve was that they were unsuitable for his calculations, having either ill-defined and intermediate characters, or possibly defective and irregular dominance.
[81] See p. 43.
IV. PROFESSOR WELDON’S COLLECTION OF “OTHER EVIDENCE CONCERNING DOMINANCE IN PEAS.”
_A. In regard to cotyledon colour: Preliminary._
I have been at some pains to show how the contradictory results, no doubt sometimes occurring, on which Professor Weldon lays such stress, may be comprehended without any injury to Mendel’s main conclusions. This excursion was made to save trouble with future discoverers of exceptions, though the existence of such facts need scarcely disturb many minds. As regards the dominance of yellow cotyledon-colour over green the whole number of genuine unconformable cases is likely to prove very small indeed, though in regard to the dominance of round shape over wrinkled we may be prepared for more discrepancies. Indeed my own crosses alone are sufficient to show that in using some varieties irregularities are to be expected. Considering also that the shapes of peas depend unquestionably on more than one pair of allelomorphs I fully expect regular blending in some cases.
As however it may be more satisfactory to the reader and to Professor Weldon if I follow him through his “contradictory” evidence I will endeavour to do so. Those who have even a slight practical acquaintance with the phenomena of heredity will sympathize with me in the difficulty I feel in treating this section of his arguments with that gravity he conceives the occasion to demand.
In following the path of the critic it will be necessary for me to trouble the reader with a number of details of a humble order, but the journey will not prove devoid of entertainment.
Now exceptions are always interesting and suggestive things, and sometimes hold a key to great mysteries. Still when a few exceptions are found disobeying rules elsewhere conformed to by large classes of phenomena it is not an unsafe course to consider, with such care as the case permits, whether the exceptions may not be due to exceptional causes, or failing such causes whether there may be any possibility of error. But to Professor Weldon, an exception is an exception--and as such may prove a very serviceable missile; so he gathers them as they were “smooth stones from the brook.”
Before examining the quality of this rather miscellaneous ammunition I would wish to draw the non-botanical reader’s attention to one or two facts of a general nature.
For our present purpose the seed of a pea may be considered as consisting of two parts, the _embryo with its cotyledons_, enclosed in a _seed-coat_. It has been known for about a century that this coat or skin is a _maternal_ structure, being part of the mother plant just as much as the pods are, and consequently not belonging to the next generation at all. If then any changes take place in it consequent on fertilisation, they are to be regarded not as in any sense a transmission of character by heredity, but rather as of the nature of an “infection.” If on the other hand it is desired to study the influence of hereditary transmission on seed-coat characters, then the crossed seeds must be sown and the seed-coats of their seeds studied. Such infective changes in maternal tissues have been known from early times, a notable collection of them having been made especially by Darwin; and for these cases Focke suggested the convenient word _Xenia_. With this familiar fact I would not for a moment suppose Professor Weldon unacquainted, though it was with some surprise that I found in his paper no reference to the phenomenon.
For as it happens, xenia is not at all a rare occurrence with _certain varieties_ of peas; though in them, as I believe is generally the case with this phenomenon, it is highly irregular in its manifestations, being doubtless dependent on slight differences of conditions during ripening.
The coats of peas differ greatly in different varieties, being sometimes thick and white or yellow, sometimes thick and highly pigmented with green or other colours, in both of which cases it may be impossible to judge the cotyledon-colour without peeling off the opaque coat; or the coats may be very thin, colourless and transparent, so that the cotyledon-colour is seen at once. It was such a transparent form that Mendel says he used for his experiments with cotyledon-colour. In order to see xenia a pea with a _pigmented_ seed-coat should be taken as seed-parent, and crossed with a variety having a different cotyledon-colour. There is then a fair chance of seeing this phenomenon, but much still depends on the variety. For example, _Fillbasket_ has green cotyledons and seed-coat green except near the hilar surface. Crossed with _Serpette nain blanc_ (yellow cotyledons and yellow coat) this variety gave three pods with 17 seeds in which the seed-coats were almost full yellow (xenia). Three other pods (25 seeds), similarly produced, showed slight xenia, and one pod with eight seeds showed little or none.
On the other hand _Fillbasket_ fertilised with _nain de Bretagne_ (yellow cotyledons, seed-coats yellow to yellowish green) gave six pods with 39 seeds showing slight xenia, distinct in a few seeds but absent in most.
Examples of xenia produced by the contrary proceeding, namely fertilising a yellow pea with a green, may indubitably occur and I have seen doubtful cases; but as by the nature of the case these are _negative_ phenomena, i.e. the seed-coat remaining greenish and _not_ going through its normal maturation changes, they must always be equivocal, and would require special confirmation before other causes were excluded.
Lastly, the special change (xenia) Mendel saw in “grey” peas, appearance or increase of purple pigment in the thick coats, following crossing, is common but also irregular.
If a _transparent_ coated form be taken as seed-parent there is no appreciable xenia, so far as I know, and such a phenomenon would certainly be paradoxical[82].
[82] In some transparent coats there is pigment, but so little as a rule that xenia would be scarcely noticeable.
In this connection it is interesting to observe that Giltay, whom Professor Weldon quotes as having obtained purely Mendelian results, got no xenia though searching for it. If the reader goes carefully through Giltay’s numerous cases, he will find, _almost_ without doubt, that none of them were such as produce it. _Reading Giant_, as Giltay states, has a _transparent_ skin, and the only xenia likely to occur in the other cases would be of the peculiar and uncertain kind seen in using “grey” peas. Professor Weldon notes that Giltay, who evidently worked with extreme care, _peeled_ his seeds before describing them, a course which Professor Weldon, not recognizing the distinction between the varieties with opaque and transparent coats, himself wisely recommends. The coincidence of the peeled seeds giving simple Mendelian results is one which might have alarmed a critic less intrepid than Professor Weldon.
Bearing in mind, then, that the coats of peas may be transparent or opaque; and in the latter case may be variously pigmented, green, grey, reddish, purplish, etc.; that in any of the latter cases there may or may not be xenia; the reader will perceive that to use the statements of an author, whether scientific or lay, to the effect that on crossing varieties he obtained peas of such and such colours _without specifying at all whether the coats were transparent or whether the colours he saw were coat- or cotyledon-colours_ is a proceeding fraught with peculiar and special risks.
(1) _Gärtner’s cases._ Professor Weldon gives, as exceptions, a series of Gärtner’s observations. Using several varieties, amongst them _Pisum sativum macrospermum_, a “grey” pea, with coloured flowers and seed-coats[83], he obtained results partly Mendelian and partly, as now alleged, contradictory. The latter consist of seeds “dirty yellow” and “yellowish green,” whereas it is suggested they should have been simply yellow.
[83] Usually correlated characters, as Mendel knew.
Now students of this department of natural history will know that these same observations of Gärtner’s, whether rightly or wrongly, have been doing duty for more than half a century as stock illustrations of xenia. In this capacity they have served two generations of naturalists. The ground nowadays may be unfamiliar, but others have travelled it before and recorded their impressions. Darwin, for example, has the following passage[84]:
[84] _Animals and Plants_, 2nd ed. 1885, p. 428.
“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 showed _that the colour of the skin of the pea_ is 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 not _Antiquity_ nor _Author_, That makes _Truth 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 the ambiguity[85]. 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.
[85] “_Eine andere Frage ist jedoch, ob der Einfluss des Pollens auf den Keim schon äusserlich an diesen letzteren sichtbar sein kann. Darwin führt mehrere hierher gehörige Fälle an, und wahrscheinlich sind auch die Resultate der von Gärtner über diesen Gegenstand ausgeführten Experimente hier zu erwähnen, wenn es auch nicht ganz deutlich ist, ob der von Gärtner erwähnte directe Einfluss des Pollens sich nur innerhalb der Grenzen des Keimes merklich macht oder nicht._” p. 490.
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 seeds _as 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 be _absolutely necessary_ to 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 found _there_; 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 of Seton[86]. Told in the words of the critic it is as follows:--
“Mr Alexander Seton crossed the flowers of _Dwarf 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 did _not_ obey 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.”
[86] Appendix to paper of Goss, _Trans. Hort. Soc._ v. 1822, pub. 1824 (_not_ 1848, as given by Professor Weldon), p. 236.
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_,” though _Dwarf 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 pea _with 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 the _semi-transparent seed-coat of the paternal form was dominant_ as 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 the question[87].
[87] Since the above passage was written I find the “_Imperials_” described in “Report of Chiswick Trials,” _Proc. R. Hort. Soc._ 1860, I. p. 340, as “skin thick”; and on p. 360 “skin thick, blue”; which finally disposes of this “exception.”
(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 being _not_ monomorphic, it would have been surprising if he had not found true irregularities in dominance.
(3 _a_) _Buchsbaum case._ This variety, growing in the open, gave once a pod in which _every seed but one was green_. In stating this case Professor Weldon refers to _Buchsbaum_ as “a yellow-seeded variety.” Tschermak[88], 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 claim _Buchsbaum_ as “yellow-seeded” without qualification.
[88] (36), p. 502 and (37), p. 663.
_Buchsbaum_ in 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 15 _yellows_ and three _greens_, and the result showed pretty clearly, as Tschermak says, that there had been an accidental cross with a tall green.
On another occasion _Telephone_ ♀ (another impure green) × _Buchsbaum_ gave four _yellow smooth and_ two _green wrinkled_, but one [? both: the grammar is obscure] of the greens did not germinate[89].
[89] Professor Weldon should have alluded to this. _Dead_ seeds have no bearing on these questions, seeing that their characters may be pathological. The same seeds are later described as “_wie Telephone selbst_,” so, apart from the possibility of death, they may also have been self-fertilised.
(3 _b_) _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 that _Telephone 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, though not 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 that _Buchsbaum_ is probably a similar case. To this point we shall return. It may not be superfluous to mention also that one cross between _Fillbasket_ (a thorough _green_) and _Telephone_ gave three _yellowish_ green seeds (Tschermak, (36), p. 501).
(3 _c_) _Couturier cases._ This fully yellow variety in crosses with two fully green sorts gave seeds either yellow or greenish yellow. In one case _Fillbasket_ ♀ fertilised by _Couturier_ gave 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 that _Couturier_ is 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 of _green_.
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 even to 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 from _Telephone_ ♀ × _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 germinate_[90].” I may warn those interested in these questions, that in estimating changes due to ripening, _dead_ seeds are not available.
[90] “_Vielleicht sind einige der l.c. 507 bis 508 erwähnten fraglichen Fälle auf ähnliche vereinzelte Anomalien der Merkmalswerthigkeit zu beziehen; einige erwiesen sich allerdings beim Anbau als Producte ungewollter Selbstbefruchtung, andere keimten nicht._”
_B. Seed-coats and shapes._
1. _Seed-coats._ Professor Weldon lays some stress on the results obtained by Correns[91] in 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.
[91] Regarding this case I have to thank Professor Correns for a good deal of information which he kindly sent me in response to my inquiry. I am thus able to supplement the published account in some particulars.
In these varieties the cotyledon-colour is yellow and the 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 were _thick_, 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 of _colour_ however 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 by _Laxton’s Alpha_ (green cotyledons, coat transparent). It contained five seeds, of which four were _red-brown on one side_ and 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 same nature[92]. Phenomena somewhat similar to these will be met with in Laxton’s case of the “maple” seeded peas (see p. 161).
[92] Mr Hurst, of Burbage, tells me that in varieties having coats green or white, e.g. _American Wonder_, the white coats are mostly from early, the green from later pods, the tints depending on conditions and exposure.
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. 153 and 158).
(_a_) _Rimpau’s cases._ Professor Weldon quotes Rimpau as having crossed wrinkled and round peas[93] and found the second hybrid generation dimorphic as usual. The wrinkled peas were selected and sown and gave wrinkled peas _and round_ peas, becoming “true” to the wrinkled character in one case only in the fifth year, while in the second case--that of a _Telephone_ cross--there was a mixture of round and wrinkled similarly resulting from _wrinkled_ seed for two years, but the experiment was not continued.
[93] In the first case _Knight’s Marrow_ with _Victoria_, both ways; in the second _Victoria_ with _Telephone_, both ways.
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 frequently _some_ wrinkling 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 from _one pod_ but a miscellaneous selection. On close inspection it will be seen that while the remainder are shown with their _cotyledon_-surfaces upwards, the two peas at the lower end of the row are represented with their _hilar_-surfaces upwards. Remembering this it will be recognized that these two lower peas are in fact _not_ fully wrinkled peas but almost certainly _round_ “hybrids,” and the depression is merely that which is often seen in round peas (such as _Fillbasket_), 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 the _fifth_ year, while the round gave at first more, later fewer, wrinkleds, not coming true till the _ninth_ year. This makes it quite clear that there _was_ dominance 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 there _was_ sufficient difference to lead to a practical distinction of the cross-breds both from the pure dominants and from the pure recessives.
The _Telephone_ case 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 the _round_ seeds 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 with _Telephone_ again, and this fact taken with the certainty that the colour-heredity of _Telephone_ is 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. 124 I 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 or stony[94], 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.
[94] Gärtner’s _macrospermum_ was evidently one of these, though from the further account (p. 498) it was probably more wrinkled. There are of course _mange-touts_ which have perfectly round seeds. Mendel himself showed that the _mange-tout_ character, the soft constricted pod, was transferable. There are also _mange-touts_ with fully wrinkled seeds and “grey” peas with small seeds (see Vilmorin-Andrieux, _Plantes Potagères_, 1883).
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 Professor Weldon, stating they may be considered separately, writes as follows:--
“Tschermak has crossed _Graue Riesen_ with five races of _P. sativum_, and he finds that the form of the first hybrid seeds _follows the female parent_, so that if races of _P. sativum_ with round smooth seeds be crossed with _Graue Riesen_ (which has flattened, feebly wrinkled seeds) the hybrids will be round and smooth or flattened and wrinkled, as the _P. sativum_ or the _Graue Riesen_ is used as female parent[95]. 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 the _Graue Riesen_ in shape, although in colour they follow Mendel’s law of segregation!”
[95] Correns found a similar result.
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, the _pois sans parchemin géant_ of 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 of _P. 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 us that according as a round _sativum_ or the _Graue Riesen_ is used as mother, the first-cross seeds “will be round and smooth or flattened and wrinkled.”
As a matter of fact, however, the seeds of _Graue Riesen_ though _slightly_ wrinkled 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 the _round_. Mendel is careful to state that his _round_ class are “either spherical or roundish, the depressions on the surface, when there are any, always slight”; while the “wrinkled” class are “irregularly angular, deeply wrinkled[96].”
[96] “_Entweder kugelrund oder rundlich, die Einsenkungen, wenn welche an der Oberfläche vorkommen, immer nur seicht, oder sie sind unregelmässig kantig, tief runzlig_ (_P. quadratum_).”
On this description alone it would be very likely that _Graue Riesen_ should fall into the _round_ class, 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 with _Graue Riesen_ were _round_, the possibilities are not exhausted. Had Tschermak tried a really wrinkled _sativum_ with _Graue Riesen_ he 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 with _Graue Riesen_.
1. One flower of this variety fertilised with _Pois très nain de Bretagne_ (very small seed; yellow cotyledons; very round) gave seven seeds indistinguishable (in their coats) from those of the mother, save for a doubtful increase in purple pigmentation of coats.
2. Fertilised by _Laxton’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 was _father_.
3. _Laxton’s Alpha_ (green; wrinkled; coats transparent) fertilised by the purple sugar-pea gave one pod of four seeds with yellow cotyledons and _round_ form.
4. _Fillbasket_ (green; smooth but squared; coats green) fertilised by the _purple_ sugar-pea gave one pod with six seeds, yellow cotyledons[97]; _Fillbasket_ size and shape; but the normally green coat yellowed near _the hilum_ by xenia.
[97] The colour is the peculiarly deep yellow of the “grey” _mange-tout_.
5. _Express_ (“blue”-green cotyledons and transparent skins; round) fertilised with _purple sugar-pea_ gave one pod with four seeds, yellow cotyledons, shape round, much as in _Fillbasket_.
6. _British Queen_ (yellow cotyledons, wrinkled, white coats) ♀ × purple sugar-pea gave two pods with seven seeds, cotyledons yellow, coats _tinged greenish_ (xenia?), all _round_.
So much for the “_Purple_” sugar-pea.
I got similar results with _Mange-tout Debarbieux_. This is a soft-podded _Mange-tout_ or sugar-pea, with white flowers, large, flattish, smooth seeds, scarcely dimpled; yellow cotyledons.
7. _Debarbieux_ fertilised by _Serpette nain blanc_ (yellow cotyledons; wrinkled; white skin; dwarf) gave one pod with six seeds, size and shape of _Debarbieux_, with slight dimpling.
8. _Debarbieux_ by _nain 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 with _Debarbieux_, while the third had four seeds like _Debarbieux_ but more dimpled. The reciprocal cross gave two seeds exactly like _nain de Bretagne_.
But it may be objected that the shape of this large grey pea is very peculiar[98]; 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 of _sativum_ are used as mothers, this is true as we have seen. But when once it is understood that in _Graue Riesen_ there is no question of wrinkling, seeing that the variety behaves as a _round_ variety, the shape and especially the size of the seed must be treated as a maternal property.
[98] It is certainly subject to considerable changes according to conditions. Those ripened in my garden are without exception much larger and flatter than Vilmorin’s seeds (now two years old) from which they grew. The colour of the coats is also much duller. These changes are just what is to be expected from the English climate--taken with the fact that my sample of this variety was late sown.
_Why_ the distinction between the shape of _Graue Riesen_ and 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 the _cross-breds_ exhibiting the form of the “grey” pea, which is then a dominant character as much as the seed-coat characters are. And that is what Tschermak’s _Graue Riesen_ crosses 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, since _the wrinkled peas_, _Laxton’s Alpha_ and _British 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 a _round_-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 Mendelian principle 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 the female[99].” All grew very tall[100], and had colours of male parent[101]. The seeds they produced were dark grey[102].
[99] Thus avoiding the error of Seton, see p. 144. There is no xenia perhaps because the seed-coat of mother was a transparent coat.
[100] As heterozygotes often do.
[101] Dominance of the purple form.
[102] Dominance of the grey coat as a maternal character.
“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_ × _D_ giving _DD_ and _DR_]; 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_ × _R_ giving _DR_ and _RR_] (draws conclusions, now obviously erroneous[103]).
[103] Sherwood’s view (_J. R. Hort. Soc._ XXII. p. 252) that this was the origin of the “Wrinkled” pea, seems very dubious.
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 by Laxton[104]. 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 in _Jour. Hort. Soc._ N.S. III. 1872, p. 10, and very slightly abbreviated in _Jour. 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.
[104] It will be well known to all practical horticulturalists that Laxton, originally of Stamford, made and brought out a large number of the best known modern peas. The firm is now in Bedford.
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 about 2-1/2 ft. 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” seeds[105].
[105] A round white ♀ × grey ♂ giving the usual result, round, “white” (yellow) seeds.
In 1867 I sowed these seeds, and all five produced tall purple-flowered purplish-stemmed plants[106], and the seeds, with few exceptions, had all maple or brownish-streaked envelopes of various shades; the remainder had entirely violet or deep purple-coloured envelopes[107]: in shape the peas were partly indented; but a few were round[108]. 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 very large[109].
[106] Tall heterozygotes, with normal dominance of purple flowers.
[107] Here we see dominance of the _pigmented_ seed-coat as a maternal character over _white_ seed-coat. The colours of the seed-coats are described as essentially two: maple or brown-streaked, and violet, the latter being a small minority. As the sequel shows, the latter are heterozygotes, not breeding true. Now Mendel found, and the fact has been confirmed both by Correns and myself, that crossing a grey pea which is capable of producing purple leads to such production as a form of xenia.
We have here therefore in the purple seeds the union of dissimilar gametes, with production of xenia. But as the brown-streaked seeds are also in part heterozygous, the splitting of a compound allelomorph has probably taken place, though without precise statistics and allotment of offspring among the several seeds the point is uncertain. The colour of seed-coats in “grey” peas and probably “maples” also is, as was stated on p. 150, sensitive to conditions, but the whole difference between “maples” and purple is too much to attribute safely to such irregularity. “Maple” is the word used to describe certain seed-coats which are pigmented with intricate brown mottlings on a paler buff ground. In French they are _perdrix_.
[108] This is not, as it stands, explicable. It seems from this point and also from what follows that if the account is truly given, some of the plants may have been mosaic with segregation of characters in particular flowers; but see subsequent note.
[109] As, commonly, in heterozygotes when fertile.
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 white seeds[110]. 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 partially indented[111]. 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 the remainder[112]. 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 of seed[113]. 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-coloured seeds[114].
[110] Recessive in flower-colour, seed-coat colour, and in seed-shape as a maternal character: pure recessives as the sequel proved.
[111] These are then a mixture of pure dominants and cross-bred dominants, and are now inextricably confused. This time the round seeds may have been all on particular plants--showing recessive seed-shape as a maternal character. It seems just possible that this fact suggested the idea of “round” seeds on the _coloured_ plants in the last generation. Till that result is confirmed it should be regarded as very doubtful on the evidence. But we cannot at the present time be sure how much difference there was between these round seeds and the _normal_ maples in point of shape; and on the whole it seems most probable that the roundness was a mere fluctuation, such as commonly occurs among the peas with large indented seeds.
[112] Is this really evidence of segregation of characters, the flower being the unit? In any case the possibility makes the experiment well worth repeating, especially as Correns has seen a phenomenon conceivably similar.
[113] Being a mixture of heterozygotes (probably involving several pairs of allelomorphs) and homozygotes.
[114] This looks as if the violet colour was merely due to irregularity of xenia.
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 white seeds[115]. Some of the coloured seeds, which I had expected would produce purple-flowered plants, produced plants with white flowers and round white seeds only[116]; the majority, however, brought plants with purple flowers and with seeds principally marked with purple or grey, the maple- or brown-streaked being in the minority[117]. 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, all violet[118]; but those plants which bore maple-coloured seeds seemed the most constant and fixed in character of the purple-flowered seedlings[119], and the purplish and grey peas, being of intermediate characters, appeared to vary most[120]. 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.
[115] Pure recessives.
[116] Pure recessives in coats showing maternal dominant character.
[117] Now recognized as pure homozygotes.
[118] This seems almost certainly segregation by flower-units, and is as yet inexplicable on any other hypothesis. Especially paradoxical is the presence of “white” seeds on these plants. The impression is scarcely resistible that some remarkable phenomenon of segregation was really seen here.
[119] Being now homozygotes.
[120] Being heterozygotes exclusively.
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 never changed[121]. 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 are unaltered[122]; 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-white colour[123], and the seeds nearly round[124] and larger or smaller according as there may have been a difference in the size of the seeds of the two varieties[125].
[121] The nature of this mistake is now clear; for as stated above xenia is only likely to occur when the maternal seed-coat is pigmented. The violet coats in this experiment are themselves cases of xenia.
[122] Knight, it was seen, crossed round ♀ × indented ♂ and consequently got no change of form.
[123] Cotyledons seen through coat.
[124] Ordinary dominance of round.
[125] This is an extraordinary statement to be given as a general truth. There are sometimes indications of this kind, but certainly the facts are not usually as here stated.
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 wrinkled seeds[126]. This would 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 the course 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 be produced[127]), although a reversion to the characters of either parent, or of any one of the ancestors, may take place at an earlier period.
[126] If we were obliged to suppose that this is a matured conclusion based on detailed observation it would of course constitute the most serious “exception” yet recorded. But it is clear that the five statements are not mutually consistent. We have dominance of round white in first cross.
In the second generation blue wrinkled give only blue wrinkled, and blue round give blue wrinkled and round, in accordance with general experience. But we are told that white round give _only_ white round. This would be true of some white rounds, but not, according to general experience, of all. Lastly we are told _white wrinkled give all four classes_. If we had not been just told by Laxton that the first cross showed dominance of white round, and that blue wrinkled and blue round give the Mendelian result, I should hesitate in face of this positive statement, but as it is inconsistent with the rest of the story I think it is unquestionably an error of statement. The context, and the argument based on the maple crosses show clearly also what was in Laxton’s mind. He plainly expected the characters of the original pure varieties to separate out according to their original combinations, and this expectation confused his memory and general impressions. This, at least, until any such result is got by a fresh observer, using strict methods, is the only acceptable account.
Of the same nature is the statement given by the late Mr Masters to Darwin (_Animals and Plants_, I. p. 318) that blue round, white round, blue wrinkled, and white wrinkled, all reproduced all four sorts during successive years. Seeing that one sort would give all four, and two would give two kinds, without special counting such an impression might easily be produced. There are the further difficulties due to seed-coat colour, and the fact that the distinction between round and wrinkled may need some discrimination. The sorts are not named, and the case cannot be further tested.
[127] See later.
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 the _Telephone_ and _Telegraph_ group 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 to _Telephone_, &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:
“In order to emphasize the need that the ancestry of the parents, used in crossing, should be considered in discussing the results of a cross, it may be well to give one or two more examples of fundamental inconsistency between different competent observers.”
The “one or two” run to three, viz. Stocks (hoariness and colour); _Datura_ (character of fruits and colour of flowers); and lastly colours of Rats and Mice. Each of these subjects, as it happens, has been referred to in the forthcoming paper by Miss Saunders and myself. _Datura_ and _Matthiola_ have been subjected to several years’ experiment and I venture to refer the reader who desires to see whether the facts are or are not in accord with Mendel’s expectation and how far there is “fundamental inconsistency” amongst them to a perusal of our work.
But as Professor Weldon refers to some points that have not been explicitly dealt with there, it will be safer to make each clear as we proceed.
1. _Stocks_ (_Matthiola_). Professor Weldon quotes Correns’ observation that glabrous Stocks crossed with hoary gave offspring all hoary, while Trevor Clarke thus obtained some hoary and some glabrous. As there are some twenty different sorts of Stocks[128] it is not surprising that different observers should have chanced on different materials and obtained different results. Miss Saunders has investigated laws of heredity in Stocks on a large scale and an account of her results is included in our forthcoming Report. Here it must suffice to say that the cross hoary ♀ × glabrous ♂ always gave offspring all hoary except once: that the cross glabrous ♀ × hoary ♂ of several types gave all hoary; _but_ the same cross using other hoary types did frequently give a mixture, some of the offspring being hoary, others glabrous. Professor Weldon might immediately decide that here was the hoped for phenomenon of “reversed” dominance, due to ancestry, but here again that hypothesis is excluded. For the glabrous (recessive) cross-breds were _pure_, and produced on self-fertilisation glabrous plants only, being in fact, almost beyond question, “false hybrids” (see p. 34), a specific phenomenon which has nothing to do with the question of dominance.
[128] The number in Haage and Schmidt’s list exceeds 200, counting colour-varieties.
Professor Weldon next suggests that there is discrepancy between the observations as to flower-colour. He tells us that Correns found _violet_ Stocks crossed with “_yellowish white_” gave violet or shades of violet flaked together. According to Professor Weldon
“On the other hand Nobbe crossed a number of varieties of _M. annua_ in which the flowers were white, violet, carmine-coloured, crimson or dark blue. These were crossed in various ways, and before a cross was made the colour of each parent was matched by a mixture of dry powdered colours which was preserved. In every case the hybrid flower was of an intermediate colour, which could be matched by mixing the powders which recorded the parental colours. The proportions in which the powders were mixed are not given in each [any] case, but it is clear that the colours blended[129].”
[129] The original passage is in _Landwirths. Versuchstationen_, 1888, XXXV. [_not_ XXXIV.], p. 151.
On comparing Professor Weldon’s version with the originals we find the missing explanations. Having served some apprenticeship to the breeding of Stocks, we, here, are perhaps in a better position to take the points, but it is to me perfectly inexplicable how in such a simple matter as this he can have gone wrong.
Note then
(1) That Nobbe does _not_ specify _which_ colours he crossed together, beyond the fact that _white_ was crossed with each fertile form. The _crimson_ form (_Karmoisinfarbe_), being double to the point of sterility, was not used. There remain then, white, carmine, and two purples (violet, “dark blue”). When _white_ was crossed with either of these, Nobbe says the colour becomes _paler_, whichever sort gave the pollen. Nobbe does not state that he crossed _carmine_ with the purples.
(2) Professor Weldon gives no qualification in his version. Nobbe however states that he found it very difficult to distinguish the result of crossing _carmine with white_ from that obtained by crossing _dark blue or violet with white_[130], thereby nullifying Professor Weldon’s statement that in every case the cross was a simple mixture of the parental colours--a proposition sufficiently disproved by Miss Saunders’ elaborate experiments.
[130] “_Es ist sogar sehr schwierig, einen Unterschied in der Farbe der Kreuzungsprodukte von Karmin und Weiss gegenüber Dunkelblau oder Violett und Weiss zu erkennen._”
(3) Lately the champion of the “importance of small variations,” Professor Weldon now prefers to treat the distinctions between established varieties as negligible fluctuations instead of specific phenomena[131]. Therefore when Correns using “_yellowish white_” obtained one result and Nobbe using “_white_” obtained another, Professor Weldon hurries to the conclusion that the results are comparable and therefore contradictory. Correns however though calling his flowers _gelblich-weiss_ is careful to state that they are described by Haage and Schmidt (the seed-men) as “_schwefel-gelb_” or sulphur-yellow. The topics Professor Weldon treats are so numerous that we cannot fairly expect him to be personally acquainted with all; still had he _looked_ at Stocks before writing, or even at the literature relating to them, he would have easily seen that these yellow Stocks are a thoroughly distinct form[132]; and in accordance with this fact it would be surprising if they had not a distinctive behaviour in their crosses. To use our own terminology their colour character depends almost certainly on a _compound_ allelomorph. Consequently there is no evidence of contradiction in the results, and appeal to ancestry is as unnecessary as futile.
[131] See also the case of _Buchsbaum_, p. 146, which received similar treatment.
[132] One of the peculiarities of most _double_ “sulphur” races is that the singles they throw are _white_. See Vilmorin, _Fleurs de pleine Terre_, 1866, p. 354, _note_. In _Wien. Ill. Gartenztg._ 1891, p. 74, mention is made of a new race with singles also “sulphur,” cp. _Gartenztg._ 1884, p. 46. Messrs Haage and Schmidt have kindly written to me that this new race has the alleged property, but that six other yellow races (two distinct colours) throw their singles white.
2. _Datura._ As for the evidence on _Datura_, I must refer the reader again to the experiments set forth in our Report.
The phenomena obey the ordinary Mendelian rules with accuracy. There are (as almost always where discontinuous variation is concerned) occasional cases of “mosaics,” a phenomenon which has nothing to do with “ancestry.”
3. _Colours of Rats and Mice._ Professor Weldon reserves his collection of evidence on this subject for the last. In it we reach an indisputable contribution to the discussion--a reference to Crampe’s papers, which together constitute without doubt the best evidence yet published, respecting colour-heredity in an animal. So far as I have discovered, the only previous reference to these memoirs is that of Ritzema Bos[133], who alludes to them in a consideration of the alleged deterioration due to in-breeding.
[133] _Biol. Cblt._ XIV. 1894, p. 79.
Now Crampe through a long period of years made an exhaustive study of the peculiarities of the colour-forms of Rats, white, black, grey and their piebalds, as exhibited in Heredity.
Till the appearance of Professor Weldon’s article Crampe’s work was unknown to me, and all students of Heredity owe him a debt for putting it into general circulation. My attention had however been called by Dr Correns to the interesting results obtained by von Guaita, experimenting with crosses originally made between albino _mice_ and piebald Japanese waltzing mice. This paper also gives full details of an elaborate investigation admirably carried out and recorded.
In the light of modern knowledge both these two researches furnish material of the most convincing character demonstrating the Mendelian principles. It would be a useful task to go over the evidence they contain and rearrange it in illustration of the laws now perceived. To do this here is manifestly impossible, and it must suffice to point out that the albino is a simple recessive in both cases (the waltzing character in mice being also a recessive), and that the “wild grey” form is one of the commonest heterozygotes--there appearing, like the yellow cotyledon-colour of peas, _in either of two capacities_, i.e. as a pure form, or as the heterozygote form of one or more combinations[134].
[134] The various “contradictions” which Professor Weldon suggests exist between Crampe, von Guaita and Colladon can almost certainly be explained by this circumstance. For Professor Weldon “wild-coloured” mice, however produced, are “wild-coloured” mice and no more (see Introduction).
Professor Weldon refers to both Crampe and von Guaita, whose results show an essential harmony in the fact that both found _albino_ an obvious recessive, pure almost without exception, while the coloured forms show various phenomena of dominance. Both found heterozygous colour-types. He then searches for something that looks like a contradiction. Of this there is no lack in the works of Johann von Fischer (11)--an authority of a very different character--whom he quotes in the following few words:
“In both rats and mice von Fischer says that piebald rats crossed with albino varieties of their species, give piebald young if the father only is piebald, white young if the mother only is piebald.”
But this is doing small justice to the completeness of Johann von Fischer’s statement, which is indeed a proposition of much more amazing import.
That investigator in fact began by a study of the cross between the albino Ferret and the Polecat, as a means of testing whether they were two species or merely varieties. The cross, he found, was in colour and form a blend of the parental types. Therefore, he declares, the Ferret and the Polecat are two distinct species, because, “as everybody ought to know,”
“_The result of a cross between albino and normal [of one species] is always a constant one, namely an offspring like the father at least in colour_[135],”
[135] “Das Resultat einer Kreuzung zwischen Albino- und Normal-form ist stets, also, constant, ein dem Vater mindestens in der Färbung gleiches Junge.” This law is predicated for the case in which both parents belong to the same species.
whereas in _crosses_ (between species) this is _not_ the case.
And again, after reciting that the Ferret-Polecat crosses gave intermediates, he states:
“But all this is _not_ the case in crosses between albinos and normal animals within the species, in which always and without any exception the young resemble the father in colour[136].”
[136] “Dieses Alles ist aber _nie_ der Fall bei Kreuzungen unter Leucismen und normalen Thieren innerhalb der Species, bei denen _stets und ohne jede Ausnahme die Jungen in Färbung dem Vater gleichen_.”
These are admirable illustrations of what is meant by a “_universal_” proposition. But von Fischer doesn’t stop here. He proceeds to give a collection of evidence in proof of this truth which he says “ought to be known to everyone.” He has observed the fact in regard to albino mole, albino shrew (_Sorex araneus_), melanic squirrel (_Sciurus vulgaris_), albino ground-squirrel (_Hypudaeus terrestris_), albino hamster, albino rats, albino mice, piebald (grey-and-white or black-and-white) mice and rats, partially albino sparrow, and we are even presented with two cases in Man. No single exception was known to von Fischer[137].
[137] He even withdraws two cases of his own previously published, in which grey and albino mice were alleged to have given mixtures, saying that this result must have been due to the broods having been accidentally mixed by the servants in his absence.
In his subsequent paper von Fischer declares that from matings of rats in which the mothers were grey and the fathers albino he bred 2017 pure albinos; and from albino mothers and grey fathers 3830 normal greys. “Not a single individual varied in any respect, or was in any way intermediate.”
With piebalds the same result is asserted, save that certain melanic forms appeared. Finally von Fischer repeats his laws already reached, giving them now in this form: _that if the offspring of a cross show only the colour of the father, then the parents are varieties of one species; but if the colour of the offspring be intermediate or different from that of the father, then the parents belong to distinct species_.
The reader may have already gathered that we have here that bane of the advocate--the witness who proves too much. But why does Professor Weldon confine von Fischer to the few modest words recited above? That author has--so far as colour is concerned--a complete law of heredity supported by copious “observations.” Why go further?
Professor Weldon “brings forth these strong reasons” of the rats and mice with the introductory sentence:
“Examples might easily be multiplied, but as before, I have chosen rather to cite a few cases which rest on excellent authority, than to quote examples which may be doubted. I would only add one case among animals, in which the evidence concerning the inheritance of colour is affected by the ancestry of the varieties used.”
So once again Professor Weldon suggests that his laws of ancestry will explain even the discrepancies between von Fischer on the one hand and Crampe and von Guaita on the other but he does not tell us how he proposes to apply them.
In the cross between the albino and the grey von Fischer tells us that both colours appear in the offspring, but always, without exception or variation, that of the father only, in 5847 individuals.
Surely, the law of ancestry, if he had a moment’s confidence in it, might rather have warned Professor Weldon that von Fischer’s results were wrong somewhere, of which there cannot be any serious doubt. The precise source of error is not easy to specify, but probably carelessness and strong preconception of the expected result were largely responsible, though von Fischer says he did all the recording most carefully himself.
Such then is the evidence resting “on excellent authority”: may we some day be privileged to see the “examples which may be doubted”?
The case of mice, invoked by Professor Weldon, has also been referred to in our Report. Its extraordinary value as illustrating Mendel’s principles and the beautiful way in which that case may lead on to extensions of those principles are also there set forth (see the present Introduction, p. 25). Most if not all of such “conflicting” evidence can be reconciled by the steady application of the Mendelian principle that the progeny will be constant when--and only when[138]--_similar_ gametes meet in fertilisation, apart from any question of the characters of the parent which produces those gametes.
[138] Excluding “false hybridisations.”
V. PROFESSOR WELDON’S QUOTATIONS FROM LAXTON.
In support of his conclusions Professor Weldon adduces two passages from Laxton, some of whose testimony we have just considered. This further evidence of Laxton is so important that I reproduce it in full. The first passage, published in 1866, is as follows:--
“The results of experiments in crossing the Pea tend to show that the colour of the immediate offspring or second generation sometimes follows that of the female parent, is sometimes intermediate between that and the male parent, and is sometimes distinct from both; and although at times it partakes of the colour of the male, it has not been ascertained by the experimenter ever to follow the exact colour of the male parent[139]. In shape, the seed frequently has an intermediate character, but as often follows that of either parent. In the second generation, in a single pod, the result of a cross of Peas different in shape and colour, the seeds are sometimes all intermediate, sometimes represent either or both parents in shape or colour, and sometimes both colours and characters, with their intermediates, appear. The results also seem to show that the third generation or the immediate offspring of a cross, frequently varies from its parents in a limited manner--usually in one direction only, but that the fourth generation produces numerous and wider variations[140]; the seed often reverting partly to the colour and character of its ancestors of the first generation, partly partaking of the various intermediate colours and characters, and partly sporting quite away from any of its ancestry.”
[139] This is of course on account of the maternal seed characters. Unless the coat-characters are treated separately from the cotyledon-characters Laxton’s description is very accurate. Both this and the statements respecting the “shape” of the seeds, a term which as used by Laxton means much more than merely “wrinkled” and “smooth,” are recognizably true as general statements.
[140] Separation of hypallelomorphs.
Here Professor Weldon’s quotation ceases. It is unfortunate he did not read on into the very next sentence with which the paragraph concludes:--
“These sports appear to become fixed and permanent in the next and succeeding generations; and the tendency to revert and sport thenceforth seems to become checked if not absolutely stopped[141].”
[141] The combinations being exhausted. Perhaps Professor Weldon thought his authority was here lapsing into palpable nonsense!
Now if Professor Weldon instead of leaving off on the word “ancestry” had noticed this passage, I think his article would never have been written.
Laxton proceeds:--
“The experiments also tend to show that the height of the plant is singularly influenced by crossing; a cross between two dwarf peas, commonly producing some dwarf and some tall [? in the second generation]; but on the other hand, a cross between two tall peas does not exhibit a tendency to diminution in height.
“No perceptible difference appears to result from reversing the parents; the influence of the pollen of each parent at the climax or fourth generation producing similar results[142].”
[142] Laxton constantly refers to this conception of the “climax” of--as we now perceive--analytical variation and recombination. Many citations could be given respecting his views on this “climax” (cp. p. 167).
The significance of this latter testimony I will presently discuss.
Professor Weldon next appeals to a later paper of Laxton’s published in 1890. From it he quotes this passage:
“By means, however, of cross-fertilisation alone, and unless it be followed by careful and continuous selection, the labours of the cross-breeder, instead of benefiting the gardener, may lead to utter confusion,”
Here again the reader would have gained had Professor Weldon, instead of leaving off at the comma, gone on to the end of the paragraph, which proceeds thus:--
“because, as I have previously stated, the Pea under ordinary conditions is much given to sporting and reversion, for when two dissimilar old or fixed varieties have been cross-fertilised, three or four generations at least must, under the most favourable circumstances, elapse before the progeny will become fixed or settled; and from one such cross I have no doubt that, by sowing every individual Pea produced during the three or four generations, hundreds of different varieties may be obtained; but as might be expected, I have found that where the two varieties desired to be intercrossed are unfixed, confusion will become confounded[143], and the variations continue through many generations, the number at length being utterly incalculable.”
[143] Further subdivision and recombination of hypallelomorphs.
Professor Weldon declares that Laxton’s “experience was altogether different from that of Mendel.” The reader will bear in mind that when Laxton speaks of fixing a variety he is not thinking particularly of seed-characters, but of all the complex characters, fertility, size, flavour, season of maturity, hardiness, etc., which go to make a serviceable pea. Considered carefully, Laxton’s testimony is so closely in accord with Mendelian expectation that I can imagine no chance description in non-Mendelian language more accurately stating the phenomena.
Here we are told in unmistakable terms the breaking up of the original combination of characters on crossing, their re-arrangement, that at the fourth or fifth generation the possibilities of sporting [sub-division of compound allelomorphs and re-combinations of them?] are exhausted, that there are then definite forms which if selected are thenceforth fixed [produced by union of similar gametes?] that it takes longer to select some forms [dominants?] than others [recessives?], that there may be “mule” forms[144] or forms which cannot be fixed at all[145] [produced by union of dissimilar gametes?].
[144] For instance the _talls_ produced by crossing _dwarfs_ are such “mules.” Tschermak found in certain cases distinct increase in height in such a case, though not always (p. 531).
[145] “The remarkably fine but unfixable pea _Evolution_.” Laxton, p. 37.
But Laxton tells us more than this. He shows us that numbers of varieties may be obtained--hundreds--“incalculable numbers.” Here too if Professor Weldon had followed Mendel with even moderate care he would have found the secret. For in dealing with the crosses of _Phaseolus_ Mendel clearly forecasts the conception of _compound characters themselves again consisting of definite units_, all of which may be separated and re-combined in the possible combinations, laying for us the foundation of the new science of Analytical Biology.
How did Professor Weldon, after reading Mendel, fail to perceive these principles permeating Laxton’s facts? Laxton must have seen the very things that Mendel saw, and had he with his other gifts combined that penetration which detects a great principle hidden in the thin mist of “exceptions,” we should have been able to claim for him that honour which must ever be Mendel’s in the history of discovery.
When Laxton speaks of selection and the need for it, he means, what the raiser of new varieties almost always means, the selection of _definite_ forms, not impalpable fluctuations. When he says that without selection there will be utter confusion, he means--to use Mendelian terms--that the plant which shows the desired combination of characters must be chosen and bred from, and that if this be not done the grower will have endless combinations mixed together in his stock. If however such a selection be made in the fourth or fifth generation the breeder may very possibly have got a fixed form--namely, one that will breed true[146]. On the other hand he may light on one that does not breed true, and in the latter case it may be that the particular type he has chosen is not represented in the gametes and will _never_ breed true, though selected to the end of time. Of all this Mendel has given us the simple and final account.
[146] Apart from fresh original variations, and perhaps in some cases imperfect homozygosis of some hypallelomorphs.
At Messrs Sutton and Sons, to whom I am most grateful for unlimited opportunities of study, I have seen exactly such a case as this. For many years Messrs Sutton have been engaged in developing new strains of the Chinese Primrose (_Primula sinensis_, hort.). Some thirty thoroughly distinct and striking varieties (not counting the _Stellata_ or “Star” section) have already been produced which breed true or very nearly so. In 1899 Messrs Sutton called my attention to a strain known as “Giant Lavender,” a particularly fine form with pale magenta or lavender flowers, telling me that it had never become fixed. On examination it appeared that self-fertilised seed saved from this variety gave some magenta-reds, some lavenders, and some which are white on opening but tinge with very faint pink as the flower matures.
On counting these three forms in two successive years the following figures appeared. Two separately bred batches raised from “Giant Lavender” were counted in each year.
Magenta Lavender White red faintly tinged
1901 1st batch 19 27 14 " 2nd " 9 20 9 1902 1st " 12 23 11 " 2nd " 14 26 11 -- -- -- 54 96 45
The numbers 54 : 96 : 45 approach the ratio 1 : 2 : 1 so nearly that there can be no doubt we have here a simple case of Mendelian laws, operating without definite dominance, but rather with blending.
When Laxton speaks of the “remarkably fine but unfixable pea _Evolution_” we now know for the first time exactly what the phenomenon meant. It, like the “Giant Lavender,” was a “mule” form, not represented by germ-cells, and in each year arose by “self-crossing.”
This is only one case among many similar ones seen in the Chinese Primrose. In others there is no doubt that more complex factors are at work, the subdivision of compound characters, and so on. The history of the “Giant Lavender” goes back many years and is not known with sufficient precision for our purposes, but like all these forms it originated from crossings among the old simple colour varieties of _sinensis_.
