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.