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.
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