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.”
But in Mendelian cases we are concerned primarily not with the product
of gametes of blended character, but with the consequences of the
union of gametes already discontinuously dissimilar. The existence of
pure Mendelian gametes for given characters is perfectly compatible
with the existence of blended or mosaic gametes for similar characters
elsewhere, but this principle enables us to form a comprehensive and
fruitful conception of the relation of the two phenomena to each other.
As I also pointed out, through the imperfection of our method which
does not yet permit us to _see_ the differentiation among the gametes
though we know it exists, we cannot yet as a rule obtain certain proof
of the impurity of the gametes (except perhaps in the case of mosaics)
as distinct from evidence of imperfect dominance. If however the case
be one of a “mule” form, distinct from either parent, and not merely
of dominance, there is no _a priori_ reason why even this may not be
possible; for we should be able to distinguish the results of breeding
first crosses together into _four_ classes: two pure forms, one or
more blend or mosaic forms, and “mule” forms. Such a study could as
yet only be attempted in simplest cases: for where we are concerned
with a compound allelomorph capable of resolution, the combinations
of the integral components become so numerous as to make this finer
classification practically inapplicable.
But in many cases--perhaps a majority--though by Mendel’s statistical
method we can perceive the fluctuations in the numbers of the several
products of fertilisation, we shall not know whether abnormalities in
the distribution of those products are due to a decline in dominance,
or to actual impurity of the gametes. We shall have further to
consider, as affecting the arithmetical results, the possibility of
departure from the rule that each kind of gamete is produced in equal
numbers[164]; also that there may be the familiar difficulties in
regard to possible selection and assortative matings among the gametes.
[164] In dealing with cases of decomposition or resolution of
compound characters this consideration is of highest importance.
I have now shown how the mosaic and blend-forms are to be regarded in
the light of the Mendelian principle. What has Professor Weldon to say
in reference to them? His suggestion is definite enough--that a study
of ancestry will explain the facts: _how_, we are not told.
In speaking of the need of study of the characters of the _race_ he
is much nearer the mark, but when he adds “that is their ancestry,”
he goes wide again. When _Telephone_ does not truly divide the
antagonistic characters among its germ-cells this fact is in nowise
simply traceable to its having originated in a cross--a history it
shares with almost all the peas in the market--but to its own peculiar
nature. In such a case imperfect dominance need not surprise us.
What we need in all these phenomena is a knowledge of the properties
of each race, or variety, as we call it in peas. We must, as I have
often pleaded, study the properties of each form no otherwise than the
chemist does the properties of his substances, and thus only can we
hope to work our way through these phenomena. _Ancestry_ holds no key
to these facts; for the same ancestry is common to own brothers and
sisters endowed with dissimilar properties and producing dissimilar
posterity. To the knowledge of the properties of each form and the laws
which it obeys there are no short cuts. We have no periodic law to
guide us. Each case must as yet be separately worked out.
We can scarcely avoid mention of a further category of phenomena
that are certain to be adduced in opposition to the general truth
of the purity of the extracted forms. It is a fact well known to
breeders that a highly-bred stock may, unless selections be continued,
“degenerate.” This has often been insisted on in regard to peas. I
have been told of specific cases by Messrs Sutton and Sons, instances
which could be multiplied. Surely, will reply the supporters of the
theory of Ancestry, this is simply impurity in the extracted stocks
manifesting itself at last. Such a conclusion by no means follows, and
the proof that it is inapplicable is obtained from the fact that the
“degeneration,” or variation as we should rather call it, need not
lead to the production of any proximate ancestor of the selected stock
at all, but immediately to a new form, or to one much more remote--in
the case of some high class peas, _e.g._, to the form which Mr Sutton
describes as “vetch-like,” with short pods, and a very few small round
seeds, two or three in a pod. Such plants are recognized by their
appearance and are rigorously hoed out every year before seeding.
To appreciate the meaning of these facts we must go back to what was
said above on the nature of compound characters. We can perceive that,
as Mendel showed, the integral characters of the varieties can be
dissociated and re-combined in any combination. More than that; certain
integral characters can be resolved into further integral components,
by _analytical_ variations. What is taking place in this process of
resolution we cannot surmise, but we may liken the consequences of
that process to various phenomena of analysis seen elsewhere. To
continue the metaphor we may speak of return to the vetch-like type as
a _synthetical_ variation: well remembering that we know nothing of
any _substance_ being subtracted in the former case or added in the
latter, and that the phenomenon is more likely to be primarily one of
alteration in arrangement than in substance.
A final proof that nothing is to be looked for from an appeal to
ancestry is provided by the fact--of which the literature of variation
contains numerous illustrations--that such newly synthesised forms,
instead of themselves producing a large proportion of the high class
variety which may have been their ancestor for a hundred generations,
may produce almost nothing but individuals like themselves. A subject
fraught with extraordinary interest will be the determination whether
by crossing these newly synthesised forms with their parent, or
another pure form, we may not succeed in reproducing a great part
of the known series of components afresh. The pure parental form,
produced, or extracted, by “analytical” breeding, would not in ordinary
circumstances be capable of producing the other components from which
it has been separated; but by crossing it with the “synthesised”
variety it is not impossible that these components would again
reappear. If this can be shown to be possible we shall have entirely
new light on the nature of variation and stability.