Part 16

But Professor Weldon cannot be warned. He told us the “law of dominance conspicuously fails for crosses between certain races.” Thence the start. I venture to give the steps in this impetuous argument. There are exceptions[153]--a fair number if we count the bad ones--there may be more--must be more--_are_ more--no doubt many more: so to the brink. Then the bold leap: may there not be as many cases one way as the other? We have not tried half the sorts of Peas yet. There is still hope. True we know dominance of many characters in some hundreds of crosses, using some twenty varieties--not to speak of other plants and animals--but we _do_ know some exceptions, of which a few are still good. So dominance may yet be all a myth, built up out of the petty facts those purblind experimenters chanced to gather. Let us take wider views. Let us look at fields more propitious--more what we would have them be! Let us turn to eye-colour: at least there is no dominance in that. Thus Professor Weldon, telling us that Mendel “had no proper basis for generalisation about yellow and green peas of any ancestry,” proceeds to this lamentable passage:--

[153] If the “logician-consulting-partner” will successfully apply this _Fallacia acervalis_, the “method of the vanishing heap,” to dominant peas, he will need considerable leisure.

“Now in such a case of alternative inheritance as that of human eye-colour, it has been shown that a number of pairs of parents, one of whom has dark and the other blue eyes, will produce offspring of which nearly one half are dark-eyed, nearly one half are blue-eyed, a small but sensible percentage being children with mosaic eyes, the iris being a patch-work of lighter and darker portions. But the dark-eyed and light-eyed children are not equally distributed among all families; and it would almost certainly be possible, by selecting cases of marriage between men and women of appropriate ancestry, to demonstrate for their families a law of dominance of dark over light eye-colour, or of light over dark. Such a law might be as valid for the families of selected ancestry as Mendel’s laws are for his peas and for other peas of probably similar ancestral history, but it would fail when applied to dark and light-eyed parents in general,--that is, to parents of any ancestry who happen to possess eyes of given colour.”

The suggestion amounts to this: that because there are exceptions to dominance in peas; and because by some stupendous coincidence, or still more amazing incompetence, a bungler might have thought he found dominance of one eye-colour whereas really there was none[154]; therefore Professor Weldon is at liberty to suggest there is a fair chance that Mendel and all who have followed him have either been the victims of this preposterous coincidence not once, but again and again; or else persisted in the same egregious and perfectly gratuitous blunder. Professor Weldon is skilled in the Calculus of Chance: will he compute the probabilities in favour of his hypothesis?

[154] I have no doubt there is no universal dominance in eye-colour. Is it _quite_ certain there is no dominance at all? I have searched the works of Galton and Pearson relating to this subject without finding a clear proof. If there is in them material for this decision may perhaps be pardoned for failing to discover it, since the tabulations are not prepared with this point in view. Reference to the original records would soon clear up the point.

_Ancestry and purity of germ-cells._

To what extent ancestry is likely to elucidate dominance we have now seen. We will briefly consider how laws derived from ancestry stand in regard to segregation of characters among the gametes.

For Professor Weldon suggests that his view of ancestry will explain the facts not only in regard to dominance and its fluctuations but in regard to the purity of the germ-cells. He does not apply this suggestion in detail, for its error would be immediately exposed. In every strictly Mendelian case the _ancestry_ of the pure extracted recessives or dominants, arising from the breeding of first crosses, is identical with that of the impure dominants [or impure recessives in cases where they exist]. Yet the posterity of each is wholly different. The pure extracted forms, in these simplest cases, are no more likely to produce the form with which they have been crossed than was their pure grandparent; while the impure forms break up again into both grand-parental forms.

Ancestry does not touch these facts in the least. They and others like them have been a stumbling-block to all naturalists. Of such paradoxical phenomena Mendel now gives us the complete and final account. Will Professor Weldon indicate how he proposes to regard them?

* * * * *

Let me here call the reader’s particular attention to that section of Mendel’s experiments to which Professor Weldon does not so much as allude. Not only did Mendel study the results of allowing his cross-breds (_DR_’s) to fertilise themselves, giving the memorable ratio

1 _DD_ : 2 _DR_ : 1 _RR_,

but he fertilised those cross-breds (_DR_’s) both with the pure dominant (_D_) and with the pure recessive (_R_) varieties reciprocally, obtaining in the former case the ratio

1 _DD_ : 1 _DR_

and in the latter the ratio

1 _DR_ : 1 _RR_.

The _DD_ group and the _RR_ group thus produced giving on self-fertilisation pure _D_ offspring and pure _R_ offspring respectively, while the _DR_ groups gave again

1 _DD_ : 2 _DR_ : 1 _RR_.

How does Professor Weldon propose to deal with these results, and by what reasoning can he suggest that considerations of ancestry are to be applied to them? If I may venture to suggest what was in Mendel’s mind when he applied this further test to his principles it was perhaps some such considerations as the following. Knowing that the cross-breds on self-fertilisation give

1 _DD_ : 2 _DR_ : 1 _RR_

three explanations are possible:

(_a_) These cross-breds may produce pure _D_ germs of both sexes and pure _R_ germs of both sexes on an average in equal numbers.

(_b_) _Either_ the female, _or_ the male, gametes may be _alone_ differentiated according to the allelomorphs, into pure _D_’s, pure _R_’s, and crosses _DR_ or _RD_, the gametes of the other sex being homogeneous and neutral in regard to those allelomorphs.

(_c_) There may be some neutralisation or cancelling between characters in _fertilisation_ occurring in such a way that the well-known ratios resulted. The absence of and inability to transmit the _D_ character in the _RR_’s, for instance, might have been due not to the original purity of the germs constituting them, but to some condition incidental to or connected with fertilisation.

It is clear that Mendel realized (_b_) as a possibility, for he says _DR_ was fertilised with the pure forms to test the composition of its egg-cells, but the reciprocal crosses were made to test the composition of the pollen of the hybrids. Readers familiar with the literature will know that both Gärtner and Wichura had in many instances shown that the offspring of crosses in the form (_a_ × _b_) ♀ × _c_ ♂ were less variable than those of crosses in the form _a_ ♀ × (_b_ × _c_) ♂, &c. This important fact in many cases is observed, and points to differentiation of characters occurring frequently among the male gametes when it does not occur or is much less marked among the maternal gametes. Mendel of course knew this, and proceeded to test for such a possibility, finding by the result that differentiation was the same in the gametes of both sexes[155].

[155] See Wichura (46), pp. 55–6.

Of hypotheses (_b_) and (_c_) the results of recrossing with the two pure forms dispose; and we can suggest no hypothesis but (_a_) which gives an acceptable account of the facts.

It is the purity of the “extracted” recessives and the “extracted” dominants--primarily the former, as being easier to recognize--that constitutes the real proof of the validity of Mendel’s principle.

Using this principle we reach immediately results of the most far-reaching character. These theoretical deductions cannot be further treated here--but of the practical use of the principle a word may be said. Where-ever there is marked dominance of one character the breeder can at once get an indication of the amount of trouble he will have in getting his cross-bred true to either dominant or recessive character. He can only thus forecast the future of the race in regard to each such pair of characters taken severally, but this is an immeasurable advance on anything we knew before. More than this, it is certain that in some cases he will be able to detect the “mule” or heterozygous forms by the statistical frequency of their occurrence or by their structure, especially when dominance is absent, and sometimes even in cases where there is distinct dominance. With peas, the practical seedsman cares, as it happens, little or nothing for those simple characters of seed-structure, &c. that Mendel dealt with. He is concerned with size, fertility, flavour, and numerous similar characters. It is to these that Laxton (invoked by Professor Weldon) primarily refers, when he speaks of the elaborate selections which are needed to fix his novelties.

We may now point tentatively to the way in which some even of these complex cases may be elucidated by an extension of Mendel’s principle, though we cannot forget that there are other undetected factors at work.

_The value of the appeal to Ancestry._

But it may be said that Professor Weldon’s appeal to ancestry calls for more specific treatment. When he suggests ancestry as “one great reason” for the different properties displayed by different races or individuals, and as providing an account of other special phenomena of heredity, he is perhaps not to be taken to mean any definite ancestry, known or hypothetical. He may, in fact, be using the term “ancestry” merely as a brief equivalent signifying the previous history of the race or individual in question. But if such a plea be put forward, the real utility and value of the appeal to ancestry is even less evident than before.

Ancestry, as used in the method of Galton and Pearson, means a definite thing. The whole merit of that method lies in the fact that by it a definite accord could be proved to exist between the observed characters and behaviour of specified descendants and the ascertained composition of their pedigree. Professor Weldon in now attributing the observed peculiarities of _Telephone_ &c. to conjectural peculiarities of pedigree--if this be his meaning--renounces all that had positive value in the reference to ancestry. His is simply an appeal to ignorance. The introduction of the word “ancestry” in this sense contributes nothing. The suggestion that ancestry might explain peculiarities means no more than “we do not know how peculiarities are to be explained.” So Professor Weldon’s phrase “peas of probably similar ancestral history[156]” means “peas probably similar”; when he speaks of Mendel having obtained his results with “a few pairs of plants of known ancestry[157],” he means “a few pairs of known plants” and no more; when he writes that “the law of segregation, like the law of dominance appears to hold only for races of particular ancestry[158],” the statement loses nothing if we write simply “for particular races.” We all know--the Mendelian, best of all--that particular races and particular individuals may, even though indistinguishable by any other test, exhibit peculiarities in heredity.

[156] See above, p. 192.

[157] See above, p. 187.

[158] See above, p. 184.

But though on analysis those introductions of the word “ancestry” are found to add nothing, yet we can feel that as used by Professor Weldon they are intended to mean a great deal. Though the appeal may be confessedly to ignorance, the suggestion is implied that if we did know the pedigrees of these various forms we should then have some real light on their present structure or their present behaviour in breeding. Unfortunately there is not the smallest ground for even this hope.

As Professor Weldon himself tells us[159], conclusions from pedigree must be based on the conditions of the several ancestors; and even more categorically (p. 244), “_The degree to which a parental character affects offspring depends not only upon its development in the individual parent, but on its degree of development in the ancestors of that parent._” [My italics.] Having rehearsed this profession of an older faith Professor Weldon proceeds to stultify it in his very next paragraph. For there he once again reminds us that _Telephone_, the mongrel pea of recent origin, which does not breed true to seed characters, has yet manifested the peculiar power of stamping the recessive characters on its cross-bred offspring, though pure and stable varieties that have exhibited the same characters in a high degree for generations have _not_ that power. As we now know, the presence or absence of a character in a progenitor _may_ be no indication whatever as to the probable presence of the character in the offspring; for the characters of the latter depend on gametic and not on zygotic differentiation.

[159] See above, p. 186.

The problem is of a different order of complexity from that which Professor Weldon suggests, and facts like these justify the affirmation that if we could at this moment bring together the whole series of individuals forming the pedigree of _Telephone_, or of any other plant or animal known to be aberrant as regards heredity, we should have no more knowledge of the nature of these aberrations; no more prescience of the moment at which they would begin, or of their probable modes of manifestation; no more criterion in fact as to the behaviour such an individual would exhibit in crossing[160], or solid ground from which to forecast its posterity, than we have already. We should learn then--what we know already--that at some particular point of time its peculiar constitution was created, and that its peculiar properties then manifested themselves: how or why this came about, we should no more comprehend with the full ancestral series before us, than we can in ignorance of the ancestry. Some cross-breds follow Mendelian segregation; others do not. In some, palpable dominance appears; in others it is absent.

[160] Beyond an indication as to the homogeneity or “purity” of its gametes at a given time.

If there were no ancestry, there would be no posterity. But to answer the question _why_ certain of the posterity depart from the rule which others follow, we must know, not the ancestry, but how it came about _either_ that at a certain moment a certain gamete divided from its fellows in a special and unwonted fashion; _or_, though the words are in part tautological, the reason why the union of two particular gametes in fertilisation took place in such a way that gametes having new specific properties resulted[161]. No one yet knows how to use the facts of ancestry for the elucidation of these questions, or how to get from them a truth more precise than that contained in the statement that a diversity of specific consequences (in heredity) may follow an apparently single specific disturbance. Rarely even can we see so much. The appeal to ancestry, as introduced by Professor Weldon, masks the difficulty he dare not face.

[161] May there be a connection between the extraordinary fertility and success of the _Telephone_ group of peas, and the peculiar frequency of a blended or mosaic condition of their allelomorphs? The conjecture may be wild, but it is not impossible that the two phenomena may be interdependent.

In other words, it is the _cause of variation_ we are here seeking. To attack that problem no one has yet shown the way. Knowledge of a different order is wanted for that task; and a compilation of ancestry, valuable as the exercise may be, does not provide that particular kind of knowledge.

Of course when once we have discovered by experiment that--say, _Telephone_--manifests a peculiar behaviour in heredity, we can perhaps make certain forecasts regarding it with fair correctness; but that any given race or individual will behave in such a way, is a fact not deducible from its ancestry, for the simple reason that organisms of identical ancestry may behave in wholly distinct, though often definite, ways.

It is from this hitherto hopeless paradox that Mendel has begun at last to deliver us. The appeal to ancestry is a substitution of darkness for light.

VII. THE QUESTION OF ABSOLUTE PURITY OF GERM-CELLS.

But let us go back to the cases of defective “purity” and consider how the laws of ancestry stand in regard to them. It appears from the facts almost certain that purity may sometimes be wanting in a character which elsewhere usually manifests it.

Here we approach a question of greater theoretical consequence to the right apprehension of the part borne by Mendelian principles in the physiology of heredity. We have to consider the question whether the purity of the gametes in respect of one or other antagonistic character is or is likely to be in case of _any_ given character a _universal_ truth? The answer is unquestionably--No--but for reasons in which “ancestry” plays no part[162].

[162] This discussion leaves “false hybridism” for separate consideration.

Hoping to interest English men of science in the Mendelian discoveries I offered in November 1900 a paper on this subject to “Nature.” The article was of some length and exceeded the space that the Editor could grant without delay. I did not see my way to reduce it without injury to clearness, and consequently it was returned to me. At the time our own experiments were not ready for publication and it seemed that all I had to say would probably be common knowledge in the next few weeks, so no further attempt at publication was made.

In that article I discussed this particular question of the absolute purity of the germ-cells, showing how, on the analogy of other bud-variations, it is almost certain that the germ-cells, even in respect to characters normally Mendelian, may on occasion present the same mixture of characters, whether apparently blended or mosaic, which we know so well elsewhere. Such a fact would in nowise diminish the importance of Mendel’s discovery. The fact that mosaic peach-nectarines occur is no refutation of the fact that the _total_ variation is common. Just as there may be trees with several such mosaic fruits, so there may be units, whether varieties, individual plants, flowers or gonads, or other structural units, bearing mosaic egg-cells or pollen grains. Nothing is more likely or more in accordance with analogy than that by selecting an individual producing germs of blended or mosaic character, a race could be established continuing to produce such germs. Persistence of such blends or mosaics in _asexual_ reproduction is well-known to horticulturists; for example “bizarre” carnations, oranges streaked with “blood”-orange character, and many more. In the famous paper of Naudin, who came nearer to the discovery of the Mendelian principle than any other observer, a paper quoted by Professor Weldon, other examples are given. These forms, once obtained, can be multiplied _by division_; and there is no reason why a zygote formed by the union of mosaic or blended germs, once arisen, should not in the cell-divisions by which its gametes are formed, continue to divide in a similar manner and produce germs like those which united to form that zygote. The irregularity, once begun, may continue for an indefinite number of divisions.

I am quite willing to suppose, with Professor Weldon (p. 248), that the pea _Stratagem_ may, as he suggests, be such a case. I am even willing to accept provisionally as probable that when two gametes, themselves of mosaic or blended character, meet together in fertilisation, they are more likely to produce gametes of mosaic or blended character than of simply discontinuous character. Among Messrs Sutton’s Primulas there are at least two striking cases of “flaked” or “bizarre” unions of bright colours and white which reproduce themselves by seed with fair constancy, though Mendelian purity in respect of these colours is elsewhere common in the varieties (I suspect mosaics of “false hybridism” among allelomorphs in some of these cases). Similarly Galton has shown that though children having one light-eyed and one dark-eyed parent generally have eyes either light or dark, the comparatively rare medium eye-coloured persons when they mate together frequently produce children with medium eye-colour.

In this connection it may be worth while to allude to a point of some practical consequence. We know that when pure dominant--say yellow--is crossed with pure recessive--say green--the dominance of yellow is seen; and we have every reason to believe this rule generally (_not_ universally) true for pure varieties of peas. But we notice that in the case of a form like the pea, depending on human selection for its existence, it might be possible in a few years for the races with pure seed characters to be practically supplanted by the “mosaicized” races like the _Telephone_ group, if the market found in these latter some specially serviceable quality. In the maincrop peas I suspect this very process is taking place[163]. After such a revolution it might be possible for a future experimenter to conclude that _Pisum sativum_ was by nature a “mosaicized” species in these respects, though the mosaic character may have arisen once in a seed or two as an exceptional phenomenon. When the same reasoning is extended to wild forms depending on other agencies for selection, some interesting conclusions may be reached.

[163] Another practical point of the same nature arises from the great variability which these peas manifest in plant- as well as seed-characters. Mr Hurst of Burbage tells me that in _e.g._ _William the First_, a pea very variable in seed-characters also, tall plants may be so common that they have to be rogued out even when the variety is grown for the vegetable market, and that the same is true of several such varieties. It seems by no means improbable that it is by such roguing that the unstable mosaic or blend-form is preserved. In a thoroughly stable variety such as _Ne Plus Ultra_ roguing is hardly necessary even for the seed-market.

Mr N. N. Sherwood in his useful account of the origin and races of peas (_Jour. R. Hort. Soc._ XXII. 1899, p. 254) alludes to the great instability of this class of pea. To Laxton, he says, “we are indebted for a peculiar type of Pea, a round seed with a very slight indent, the first of this class sent out being _William the First_, the object being to get a very early blue-seeded indented Pea of the same earliness as the Sangster type with a blue seed, or in other words with a Wrinkled Pea flavour. This type of Pea is most difficult to keep true on account of the slight taint of the Wrinkled Pea in the breed, which causes it to run back to the Round variety.”