Chapter IV: Professor Weldon’s Collection of “other Evidence Concerning (2)

In the second generation blue wrinkled give only blue wrinkled, and
blue round give blue wrinkled and round, in accordance with general
experience. But we are told that white round give _only_ white round.
This would be true of some white rounds, but not, according to
general experience, of all. Lastly we are told _white wrinkled give
all four classes_. If we had not been just told by Laxton that the
first cross showed dominance of white round, and that blue wrinkled
and blue round give the Mendelian result, I should hesitate in face
of this positive statement, but as it is inconsistent with the rest
of the story I think it is unquestionably an error of statement. The
context, and the argument based on the maple crosses show clearly
also what was in Laxton’s mind. He plainly expected the characters
of the original pure varieties to separate out according to their
original combinations, and this expectation confused his memory and
general impressions. This, at least, until any such result is got
by a fresh observer, using strict methods, is the only acceptable
account.

Of the same nature is the statement given by the late Mr Masters
to Darwin (_Animals and Plants_, I. p. 318) that blue round, white
round, blue wrinkled, and white wrinkled, all reproduced all four
sorts during successive years. Seeing that one sort would give
all four, and two would give two kinds, without special counting
such an impression might easily be produced. There are the further
difficulties due to seed-coat colour, and the fact that the
distinction between round and wrinkled may need some discrimination.
The sorts are not named, and the case cannot be further tested.

[127] See later.

These circumstances do not appear to have been known to Mr Knight, as he seems to have carried on his experiments by continuing to cross his seedlings in the year succeeding their production from a cross and treating the results as reliable; whereas it is probable that the results might have been materially affected by the disturbing causes then in existence arising from the previous cross fertilisation, and which, I consider, would, in all cases where either parent has not become fixed or permanent, lead to results positively perplexing and uncertain, and to variations almost innumerable. I have again selected, and intend to sow, watch, and report; but as the usual climax of variation is nearly reached in the recorded experiment, I do not anticipate much further deviation, except in height and period of ripening--characters which are always very unstable in the pea. There are also important botanical and other variations and changes occurring in cross-fertilised peas to which it is not my province here to allude; but in conclusion I may, perhaps, in furtherance of the objects of this paper, be permitted to inquire whether any light can, from these observations or other means, be thrown upon the origin of the cultivated kinds of peas, especially the “maple” variety, and also as to the source whence the violet and other colours which appear at intervals on the seeds and in the offspring of cross-fertilised purple-flowered peas are derived.”

The reader who has closely followed the preceding passage will begin to appreciate the way in which the new principles help us to interpret these hitherto paradoxical phenomena. Even in this case, imperfectly recorded as it is, we can form a fairly clear idea of what was taking place. If the “round” seeds really occurred as a distinct class, on the heterozygotes as described, it is just possible that the fact may be of great use hereafter.

We are still far from understanding maternal seed-form--and perhaps size--as a dominant character. So far, as Miss Saunders has pointed out to me, it appears to be correlated with a thick and coloured seed-coat.

* * * * *

We have now seen the nature of Professor Weldon’s collection of contradictory evidence concerning dominance in peas. He tells us: “Enough has been said to show the grave discrepancy between the evidence afforded by Mendel’s experiments and that obtained by observers equally trustworthy.”

He proceeds to a discussion of the _Telephone_ and _Telegraph_ group and recites facts, which I do not doubt for a moment, showing that in this group of peas--which have unquestionably been more or less “blend” or “mosaic” forms from their beginning--the “laws of dominance and segregation” do not hold. Professor Weldon’s collection of the facts relating to _Telephone_, &c. has distinct value, and it is the chief addition he makes to our knowledge of these phenomena. The merit however of this addition is diminished by the erroneous conclusion drawn from it, as will be shown hereafter. Meanwhile the reader who has studied what has been written above on the general questions of stability, “purity,” and “universal” dominance, will easily be able to estimate the significance of these phenomena and their applicability to Mendel’s hypotheses.

_D. Miscellaneous cases in other plants and animals_.

Professor Weldon proceeds:

“In order to emphasize the need that the ancestry of the parents,
used in crossing, should be considered in discussing the results of a
cross, it may be well to give one or two more examples of fundamental
inconsistency between different competent observers.”

The “one or two” run to three, viz. Stocks (hoariness and colour); _Datura_ (character of fruits and colour of flowers); and lastly colours of Rats and Mice. Each of these subjects, as it happens, has been referred to in the forthcoming paper by Miss Saunders and myself. _Datura_ and _Matthiola_ have been subjected to several years’ experiment and I venture to refer the reader who desires to see whether the facts are or are not in accord with Mendel’s expectation and how far there is “fundamental inconsistency” amongst them to a perusal of our work.

But as Professor Weldon refers to some points that have not been explicitly dealt with there, it will be safer to make each clear as we proceed.

1. _Stocks_ (_Matthiola_). Professor Weldon quotes Correns’ observation that glabrous Stocks crossed with hoary gave offspring all hoary, while Trevor Clarke thus obtained some hoary and some glabrous. As there are some twenty different sorts of Stocks[128] it is not surprising that different observers should have chanced on different materials and obtained different results. Miss Saunders has investigated laws of heredity in Stocks on a large scale and an account of her results is included in our forthcoming Report. Here it must suffice to say that the cross hoary ♀ × glabrous ♂ always gave offspring all hoary except once: that the cross glabrous ♀ × hoary ♂ of several types gave all hoary; _but_ the same cross using other hoary types did frequently give a mixture, some of the offspring being hoary, others glabrous. Professor Weldon might immediately decide that here was the hoped for phenomenon of “reversed” dominance, due to ancestry, but here again that hypothesis is excluded. For the glabrous (recessive) cross-breds were _pure_, and produced on self-fertilisation glabrous plants only, being in fact, almost beyond question, “false hybrids” (see p. 34), a specific phenomenon which has nothing to do with the question of dominance.

[128] The number in Haage and Schmidt’s list exceeds 200, counting
colour-varieties.

Professor Weldon next suggests that there is discrepancy between the observations as to flower-colour. He tells us that Correns found _violet_ Stocks crossed with “_yellowish white_” gave violet or shades of violet flaked together. According to Professor Weldon

“On the other hand Nobbe crossed a number of varieties of _M. annua_
in which the flowers were white, violet, carmine-coloured, crimson
or dark blue. These were crossed in various ways, and before a cross
was made the colour of each parent was matched by a mixture of dry
powdered colours which was preserved. In every case the hybrid flower
was of an intermediate colour, which could be matched by mixing the
powders which recorded the parental colours. The proportions in which
the powders were mixed are not given in each [any] case, but it is
clear that the colours blended[129].”

[129] The original passage is in _Landwirths. Versuchstationen_,
1888, XXXV. [_not_ XXXIV.], p. 151.

On comparing Professor Weldon’s version with the originals we find the missing explanations. Having served some apprenticeship to the breeding of Stocks, we, here, are perhaps in a better position to take the points, but it is to me perfectly inexplicable how in such a simple matter as this he can have gone wrong.

Note then

(1) That Nobbe does _not_ specify _which_ colours he crossed together, beyond the fact that _white_ was crossed with each fertile form. The _crimson_ form (_Karmoisinfarbe_), being double to the point of sterility, was not used. There remain then, white, carmine, and two purples (violet, “dark blue”). When _white_ was crossed with either of these, Nobbe says the colour becomes _paler_, whichever sort gave the pollen. Nobbe does not state that he crossed _carmine_ with the purples.

(2) Professor Weldon gives no qualification in his version. Nobbe however states that he found it very difficult to distinguish the result of crossing _carmine with white_ from that obtained by crossing _dark blue or violet with white_[130], thereby nullifying Professor Weldon’s statement that in every case the cross was a simple mixture of the parental colours--a proposition sufficiently disproved by Miss Saunders’ elaborate experiments.

[130] “_Es ist sogar sehr schwierig, einen Unterschied in der Farbe
der Kreuzungsprodukte von Karmin und Weiss gegenüber Dunkelblau oder
Violett und Weiss zu erkennen._”

(3) Lately the champion of the “importance of small variations,” Professor Weldon now prefers to treat the distinctions between established varieties as negligible fluctuations instead of specific phenomena[131]. Therefore when Correns using “_yellowish white_” obtained one result and Nobbe using “_white_” obtained another, Professor Weldon hurries to the conclusion that the results are comparable and therefore contradictory. Correns however though calling his flowers _gelblich-weiss_ is careful to state that they are described by Haage and Schmidt (the seed-men) as “_schwefel-gelb_” or sulphur-yellow. The topics Professor Weldon treats are so numerous that we cannot fairly expect him to be personally acquainted with all; still had he _looked_ at Stocks before writing, or even at the literature relating to them, he would have easily seen that these yellow Stocks are a thoroughly distinct form[132]; and in accordance with this fact it would be surprising if they had not a distinctive behaviour in their crosses. To use our own terminology their colour character depends almost certainly on a _compound_ allelomorph. Consequently there is no evidence of contradiction in the results, and appeal to ancestry is as unnecessary as futile.

[131] See also the case of _Buchsbaum_, p. 146, which received
similar treatment.

[132] One of the peculiarities of most _double_ “sulphur” races is
that the singles they throw are _white_. See Vilmorin, _Fleurs de
pleine Terre_, 1866, p. 354, _note_. In _Wien. Ill. Gartenztg._ 1891,
p. 74, mention is made of a new race with singles also “sulphur,”
cp. _Gartenztg._ 1884, p. 46. Messrs Haage and Schmidt have kindly
written to me that this new race has the alleged property, but that
six other yellow races (two distinct colours) throw their singles
white.

2. _Datura._ As for the evidence on _Datura_, I must refer the reader again to the experiments set forth in our Report.

The phenomena obey the ordinary Mendelian rules with accuracy. There are (as almost always where discontinuous variation is concerned) occasional cases of “mosaics,” a phenomenon which has nothing to do with “ancestry.”

3. _Colours of Rats and Mice._ Professor Weldon reserves his collection of evidence on this subject for the last. In it we reach an indisputable contribution to the discussion--a reference to Crampe’s papers, which together constitute without doubt the best evidence yet published, respecting colour-heredity in an animal. So far as I have discovered, the only previous reference to these memoirs is that of Ritzema Bos[133], who alludes to them in a consideration of the alleged deterioration due to in-breeding.

[133] _Biol. Cblt._ XIV. 1894, p. 79.

Now Crampe through a long period of years made an exhaustive study of the peculiarities of the colour-forms of Rats, white, black, grey and their piebalds, as exhibited in Heredity.

Till the appearance of Professor Weldon’s article Crampe’s work was unknown to me, and all students of Heredity owe him a debt for putting it into general circulation. My attention had however been called by Dr Correns to the interesting results obtained by von Guaita, experimenting with crosses originally made between albino _mice_ and piebald Japanese waltzing mice. This paper also gives full details of an elaborate investigation admirably carried out and recorded.

In the light of modern knowledge both these two researches furnish material of the most convincing character demonstrating the Mendelian principles. It would be a useful task to go over the evidence they contain and rearrange it in illustration of the laws now perceived. To do this here is manifestly impossible, and it must suffice to point out that the albino is a simple recessive in both cases (the waltzing character in mice being also a recessive), and that the “wild grey” form is one of the commonest heterozygotes--there appearing, like the yellow cotyledon-colour of peas, _in either of two capacities_, i.e. as a pure form, or as the heterozygote form of one or more combinations[134].

[134] The various “contradictions” which Professor Weldon suggests
exist between Crampe, von Guaita and Colladon can almost certainly be
explained by this circumstance. For Professor Weldon “wild-coloured”
mice, however produced, are “wild-coloured” mice and no more (see
Introduction).

Professor Weldon refers to both Crampe and von Guaita, whose results show an essential harmony in the fact that both found _albino_ an obvious recessive, pure almost without exception, while the coloured forms show various phenomena of dominance. Both found heterozygous colour-types. He then searches for something that looks like a contradiction. Of this there is no lack in the works of Johann von Fischer (11)--an authority of a very different character--whom he quotes in the following few words:

“In both rats and mice von Fischer says that piebald rats crossed
with albino varieties of their species, give piebald young if the
father only is piebald, white young if the mother only is piebald.”

But this is doing small justice to the completeness of Johann von Fischer’s statement, which is indeed a proposition of much more amazing import.

That investigator in fact began by a study of the cross between the albino Ferret and the Polecat, as a means of testing whether they were two species or merely varieties. The cross, he found, was in colour and form a blend of the parental types. Therefore, he declares, the Ferret and the Polecat are two distinct species, because, “as everybody ought to know,”

“_The result of a cross between albino and normal [of one species] is
always a constant one, namely an offspring like the father at least
in colour_[135],”

[135] “Das Resultat einer Kreuzung zwischen Albino- und Normal-form
ist stets, also, constant, ein dem Vater mindestens in der Färbung
gleiches Junge.” This law is predicated for the case in which both
parents belong to the same species.

whereas in _crosses_ (between species) this is _not_ the case.

And again, after reciting that the Ferret-Polecat crosses gave intermediates, he states:

“But all this is _not_ the case in crosses between albinos and normal
animals within the species, in which always and without any exception
the young resemble the father in colour[136].”

[136] “Dieses Alles ist aber _nie_ der Fall bei Kreuzungen unter
Leucismen und normalen Thieren innerhalb der Species, bei denen
_stets und ohne jede Ausnahme die Jungen in Färbung dem Vater
gleichen_.”

These are admirable illustrations of what is meant by a “_universal_” proposition. But von Fischer doesn’t stop here. He proceeds to give a collection of evidence in proof of this truth which he says “ought to be known to everyone.” He has observed the fact in regard to albino mole, albino shrew (_Sorex araneus_), melanic squirrel (_Sciurus vulgaris_), albino ground-squirrel (_Hypudaeus terrestris_), albino hamster, albino rats, albino mice, piebald (grey-and-white or black-and-white) mice and rats, partially albino sparrow, and we are even presented with two cases in Man. No single exception was known to von Fischer[137].

[137] He even withdraws two cases of his own previously published,
in which grey and albino mice were alleged to have given mixtures,
saying that this result must have been due to the broods having been
accidentally mixed by the servants in his absence.

In his subsequent paper von Fischer declares that from matings of rats in which the mothers were grey and the fathers albino he bred 2017 pure albinos; and from albino mothers and grey fathers 3830 normal greys. “Not a single individual varied in any respect, or was in any way intermediate.”

With piebalds the same result is asserted, save that certain melanic forms appeared. Finally von Fischer repeats his laws already reached, giving them now in this form: _that if the offspring of a cross show only the colour of the father, then the parents are varieties of one species; but if the colour of the offspring be intermediate or different from that of the father, then the parents belong to distinct species_.

The reader may have already gathered that we have here that bane of the advocate--the witness who proves too much. But why does Professor Weldon confine von Fischer to the few modest words recited above? That author has--so far as colour is concerned--a complete law of heredity supported by copious “observations.” Why go further?

Professor Weldon “brings forth these strong reasons” of the rats and mice with the introductory sentence:

“Examples might easily be multiplied, but as before, I have chosen
rather to cite a few cases which rest on excellent authority, than to
quote examples which may be doubted. I would only add one case among
animals, in which the evidence concerning the inheritance of colour
is affected by the ancestry of the varieties used.”

So once again Professor Weldon suggests that his laws of ancestry will explain even the discrepancies between von Fischer on the one hand and Crampe and von Guaita on the other but he does not tell us how he proposes to apply them.

In the cross between the albino and the grey von Fischer tells us that both colours appear in the offspring, but always, without exception or variation, that of the father only, in 5847 individuals.

Surely, the law of ancestry, if he had a moment’s confidence in it, might rather have warned Professor Weldon that von Fischer’s results were wrong somewhere, of which there cannot be any serious doubt. The precise source of error is not easy to specify, but probably carelessness and strong preconception of the expected result were largely responsible, though von Fischer says he did all the recording most carefully himself.

Such then is the evidence resting “on excellent authority”: may we some day be privileged to see the “examples which may be doubted”?

The case of mice, invoked by Professor Weldon, has also been referred to in our Report. Its extraordinary value as illustrating Mendel’s principles and the beautiful way in which that case may lead on to extensions of those principles are also there set forth (see the present Introduction, p. 25). Most if not all of such “conflicting” evidence can be reconciled by the steady application of the Mendelian principle that the progeny will be constant when--and only when[138]--_similar_ gametes meet in fertilisation, apart from any question of the characters of the parent which produces those gametes.

[138] Excluding “false hybridisations.”