Part 15

[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_.

VI. THE ARGUMENT BUILT ON EXCEPTIONS.

So much for the enormous advance that the Mendelian principles already permit us to make. But what does Professor Weldon offer to substitute for all this? Nothing.

Professor Weldon suggests that a study of ancestry will help us. Having recited Tschermak’s exceptions and the great irregularities seen in the _Telephone_ group, he writes:

“Taking these results together with Laxton’s statements, and with the evidence afforded by the _Telephone_ group of hybrids, I think we can only conclude that segregation of seed-characters is not of universal occurrence among cross-bred peas, and that when it does occur, it may or may not follow Mendel’s law.”

Premising that when pure types are used the exceptions form but a small part of the whole, and that any supposed absence of “segregation” may have been _variation_, this statement is perfectly sound. He proceeds:--

“The law of segregation, like the law of dominance, appears therefore to hold only for races of _particular ancestry_ [my italics]. In special cases, other formulae expressing segregation have been offered, especially by De Vries and by Tschermak for other plants, but these seem as little likely to prove generally valid as Mendel’s formula itself.

“The fundamental mistake which vitiates all work based upon Mendel’s method is the neglect of ancestry, and the attempt to regard the whole effect upon offspring, produced by a particular parent, as due to the existence in the parent of particular structural characters; while the contradictory results obtained by those who have observed the offspring of parents identical in certain characters show clearly enough that not only the parents themselves, but their race, that is their ancestry, must be taken into account before the result of pairing them can be predicted.”

In this passage the Mendelian view is none too precisely represented. I should rather have said that it was from Mendel, first of all men, that we have learnt _not_ to regard the effects produced on offspring “as due to the existence in the parent of particular structural characters.” We have come rather to disregard the particular structure of the parent except in so far as it may give us a guide as to the nature of its gametes.

This indication, if taken in the positive sense--as was sufficiently shown in considering the significance of the “mule” form or “hybrid-character”--we now know may be absolutely worthless, and in any unfamiliar case is very likely to be so. Mendel has proved that the inheritance from individuals of _identical ancestry_ may be entirely different: that from identical ancestry, without new variation, may be produced three kinds of individuals (in respect of each pair of characters), namely, individuals capable of transmitting one type, or another type, or both: moreover that the statistical relations of these three classes of individuals to each other will in a great number of cases be a definite one: and of all this he shows a complete account.

Professor Weldon cannot deal with any part of this phenomenon. He does little more than allude to it in passing and point out exceptional cases. These he suggests a study of ancestry will explain.

As a matter of fact a study of ancestry will give little guide--perhaps none--even as to the probability of the phenomenon of dominance of a character, none as to the probability of normal “purity” of germ-cells. Still less will it help to account for fluctuations in dominance, or irregularities in “purity.”

_Ancestry and Dominance._

In a series of astonishing paragraphs (pp. 241–2) Professor Weldon rises by gradual steps, from the exceptional facts regarding occasional dominance of green colour in _Telephone_ to suggest that the _whole phenomenon of dominance may be attributable to ancestry_, and that in fact one character has no natural dominance over another, apart from what has been created by selection of ancestry. This piece of reasoning, one of the most remarkable examples of special pleading to be met with in scientific literature, must be read as a whole. I reproduce it entire, that the reader may appreciate this curious effort. The remarks between round parenthetical marks are Professor Weldon’s, those between crotchets are mine.

“Mendel treats such characters as yellowness of cotyledons and the like as if the condition of the character in two given parents determined its condition in all their subsequent offspring[147]. Now it is well known to breeders, and is clearly shown in a number of cases by Galton and Pearson, that the condition of an animal does not as a rule depend upon the condition of any one pair of ancestors alone, but in varying degrees upon the condition of all its ancestors in every past generation, the condition in each of the half-dozen nearest generations having a quite sensible effect. Mendel does not take the effect of differences of ancestry into account, but considers that any yellow-seeded pea, crossed with any green-seeded pea, will behave in a certain definite way, whatever the ancestry of the green and yellow peas may have been. (He does not say this in words, but his attempt to treat his results as generally true of the characters observed is unintelligible unless this hypothesis be assumed.) The experiments afford no evidence which can be held to justify this hypothesis. His observations on cotyledon colour, for example, are based upon 58 cross-fertilised flowers, all of which were borne upon ten plants; and we are not even told whether these ten plants included individuals from more than two races.

[147] Mendel, on the contrary, disregards the “condition of the character” in the parent altogether; but is solely concerned with the nature of the characters of the _gametes_.

“The many thousands of individuals raised from these ten plants afford an admirable illustration of the effect produced by crossing a few pairs of plants of known ancestry; but while they show this perhaps better than any similar experiment, they do not afford the data necessary for a statement as to the behaviour of yellow-seeded peas in general, whatever their ancestry, when crossed with green-seeded peas of any ancestry. [Mendel of course makes no such statement.]

“When this is remembered, the importance of the exceptions to dominance of yellow cotyledon-colour, or of smooth and rounded shape of seeds, observed by Tschermak, is much increased; because although they form a small percentage of his whole result, they form a very large percentage of the results obtained with peas of certain races. [Certainly.] The fact that _Telephone_ behaved in crossing on the whole like a green-seeded race of exceptional dominance shows that something other than the mere character of the parental generation operated in this case. Thus in eight out of 27 seeds from the yellow _Pois d’Auvergne_ ♀ × _Telephone_ ♂ the cotyledons were yellow with green patches; the reciprocal cross gave two green and one yellow-and-green seed out of the whole ten obtained; and the cross _Telephone_ ♀ × (yellow-seeded) _Buchsbaum_[148] ♂ gave on one occasion two green and four yellow seeds.

[148] Regarding this “exception” see p. 146.

“So the cross _Couturier_ (orange-yellow) ♀ × the green-seeded _Express_ ♂ gave a number of seeds intermediate in colour. (It is not clear from Tschermak’s paper whether _all_ the seeds were of this colour, but certainly some of them were.) The green _Plein le Panier_ [_Fillbasket_] ♀ × _Couturier_ ♂ in three crosses always gave either seeds of colour intermediate between green and yellow, or some yellow and some green seeds in the same pod. The cross reciprocal to this was not made; but _Express_ ♀ × _Couturier_ ♂ gave 22 seeds of which four were yellowish green[149].

[149] See p. 148.

“These facts show _first_ that Mendel’s law of dominance conspicuously fails for crosses between certain races, while it appears to hold for others; and _secondly_ that the intensity of a character in one generation of a race is no trustworthy measure of its dominance in hybrids. The obvious suggestion is that the behaviour of an individual when crossed depends largely upon the characters of its ancestors[150]. When it is remembered that peas are normally self-fertilised, and that more than one named variety may be selected out of the seeds of a single hybrid pod, it is seen to be probable that Mendel worked with a very definite combination of ancestral characters, and had no proper basis for generalisation about yellow and green peas of any ancestry” [which he never made].

[150] Where was that “logician,” the “consulting-partner,” when this piece of reasoning passed the firm?

Let us pause a moment before proceeding to the climax. Let the reader note we have been told of _two_ groups of cases in which dominance of yellow failed or was irregular. (Why are not Gärtner’s and Seton’s “exceptions” referred to here?) In one of these groups _Couturier_ was always one parent, either father or mother, and were it not for Tschermak’s own obvious hesitation in regard to his own exceptions (see p. 148), I would gladly believe that _Couturier_--a form I do not know--may be an exceptional variety. _How_ Professor Weldon proposes to explain its peculiarities by reference to ancestry he omits to tell us. The _Buchsbaum_ case is already disposed of, for on Tschermak’s showing, it is an unstable form.

Happily, thanks to Professor Weldon, we know rather more of the third case, that of _Telephone_, which, whether as father or mother, was frequently found by Tschermak to give either green, greenish, or patchwork-seeds when crossed with yellow varieties. It behaves, in short, “like a green-seeded pea of exceptional dominance,” as we are now told. For this dominant quality of _Telephone’s_ greenness we are asked to account _by appeal to its ancestry_. May we not expect, then, this _Telephone_ to be--if not a pure-bred green pea from time immemorial--at least as pure-bred as other green peas which do _not_ exhibit dominance of green at all? Now, what is _Telephone_? Do not let us ask too much. Ancestry takes a lot of proving. We would not reject him “_parce qu’il n’avait que soixante & onze quartiers, & que le reste de son arbre généalogique avait été perdu par l’injure du tems_.”

But with stupefaction we learn from Professor Weldon himself that _Telephone_ is the very variety which he takes _as his type of a permanent and incorrigible mongrel_, a character it thoroughly deserves.

From _Telephone_ he made his colour scale! Tschermak declares the cotyledons to be “yellowish or whitish green, often entirely bright yellow[151].” So little is it a thorough-bred green pea, that it cannot always keep its own self-fertilised offspring green. Not only is this pea a parti-coloured mongrel, but Professor Weldon himself quotes Culverwell that as late as 1882 both _Telegraph_ and _Telephone_ “will always come from one sort, more especially from the green variety”; and again regarding a supposed good sample of _Telegraph_ that “Strange to say, although the peas were taken from one lot, those sown in January produced a great proportion of the light variety known as _Telephone_. These were of every shade of light green up to white, and could have been shown for either variety,” _Gard. Chron._ 1882 (2), p. 150. This is the variety whose green, it is suggested, partially “dominates” over the yellow of _Pois d’Auvergne_, a yellow variety which has a clear lineage of about a century, and probably more. If, therefore, the facts regarding _Telephone_ have any bearing on the significance of ancestry, they point the opposite way from that in which Professor Weldon desires to proceed.

[151] “_Speichergewebe gelblich--oder weisslich--grün, manchmal auch vollständig hellgelb._” Tschermak (36), p. 480.

In view of the evidence, the conclusion is forced upon me that the suggestion that “ancestry” may explain the facts regarding _Telephone_ has no meaning behind it, but is merely a verbal obstacle. Two words more on _Telephone_. On p. 147 I ventured to hint that if we try to understand the nature of the appearance of green in the offspring of _Telephone_ bred with yellow varieties, we are more likely to do so by comparing the facts with those of false hybridisation than with fluctuations in dominance. In this connection I would call the reader’s attention to a point Professor Weldon misses, that Tschermak _also got yellowish-green seeds from Fillbasket (green) crossed with Telephone_. I suggest therefore that _Telephone’s_ allelomorphs may be in part transmitted to its offspring in a state which needs no union with any corresponding allelomorph of the other gamete, just as may the allelomorphs of “false hybrids.” It would be quite out of place here to pursue this reasoning, but the reader acquainted with special phenomena of heredity will probably be able fruitfully to extend it. It will be remembered that we have already seen the further fact that the behaviour of _Telephone_ in respect to seed-shape was also peculiar (see p. 152).

Whatever the future may decide on this interesting question it is evident that with _Telephone_ (and possibly _Buchsbaum_) we are encountering a _specific_ phenomenon, which calls for specific elucidation and not a case simply comparable with or contradicting the evidence of dominance in general.

In this excursion we have seen something more of the “exceptions.” Many have fallen, but some still stand, though even as to part of the remainder Tschermak entertains some doubts, and, it will be remembered, cautions his reader that of his exceptions some may be self-fertilisations, and some did not germinate[152]. Truly a slender basis to carry the coming structure!

[152] In his latest publication on this subject, the notes to the edition of Mendel in Ostwald’s _Klassiker_ (pp. 60–61), Tschermak, who has seen more true exceptions than any other observer, thus refers to them. As to dominance:--“_Immerhin kommen vereinzelt auch zweifellose Fälle von Merkmalmischung, d. h. Uebergangsformen zwischen gelber und grüner Farbe, runder und runzeliger Form vor, die sich in weiteren Generationen wie dominantmerkmalige Mischlinge verhalten._” As to purity of the extracted recessives:--_Ganz vereinzelt scheinen Ausnahmsfälle vorzukommen._"

Küster (22) also in a recent note on Mendelism points out, with reason, that the number of “exceptions” to dominance that we shall find, depends simply on the stringency with which the supposed “law” is drawn. The same writer remarks further that Mendel makes no such rigid definition of dominance as his followers have done.