Part 11

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.