(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.”