Part 23

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.

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. 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.

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 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.

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. 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.

Professor Weldon may take this as a famous blow for Mendel, till he realizes what is meant by Mendel’s “Hybrid-character.”

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.