Part 24

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. They are all of them capable of experimental test and in certain instances we are beginning to expect the conclusion.

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.

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 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.

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.

“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. 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, he merely indicates the point at which his own misconceptions began.

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.

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. 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.

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.