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Title: Mendel's principles of heredity: A defence
Author: Bateson, William
Language: English
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MENDEL’S PRINCIPLES OF HEREDITY
London: C. J. CLAY AND SONS,
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
AVE MARIA LANE,
AND
H. K. LEWIS, 136, GOWER STREET, W.C.
[Illustration]
Glasgow: 50, WELLINGTON STREET.
Leipzig: F. A. BROCKHAUS.
New York: THE MACMILLAN COMPANY.
Bombay and Calcutta: MACMILLAN AND CO., LTD.
[_All Rights reserved._]
[Illustration:
GREGOR MENDEL
Abbot of Brünn
Born 1822. Died 1884.
_From a photograph kindly supplied by the Very Rev. Dr Janeischek, the
present Abbot._]
MENDEL’S
PRINCIPLES OF HEREDITY
A DEFENCE
BY
W. BATESON, M.A., F.R.S.
_WITH A TRANSLATION OF MENDEL’S ORIGINAL
PAPERS ON HYBRIDISATION._
CAMBRIDGE:
AT THE UNIVERSITY PRESS.
1902
Cambridge:
PRINTED BY J. AND C. F. CLAY,
AT THE UNIVERSITY PRESS.
PREFACE.
In the Study of Evolution progress had well-nigh stopped. The more
vigorous, perhaps also the more prudent, had left this field of science
to labour in others where the harvest is less precarious or the yield
more immediate. Of those who remained some still struggled to push
towards truth through the jungle of phenomena: most were content
supinely to rest on the great clearing Darwin made long since.
Such was our state when two years ago it was suddenly discovered that
an unknown man, Gregor Johann Mendel, had, alone, and unheeded, broken
off from the rest--in the moment that Darwin was at work--and cut a way
through.
This is no mere metaphor, it is simple fact. Each of us who now looks
at his own patch of work sees Mendel’s clue running through it: whither
that clue will lead, we dare not yet surmise.
It was a moment of rejoicing, and they who had heard the news hastened
to spread them and take the instant way. In this work I am proud to
have borne my little part.
But every gospel must be preached to all alike. It will be heard by the
Scribes, by the Pharisees, by Demetrius the Silversmith, and the rest.
Not lightly do men let their occupation go; small, then, would be our
wonder, did we find the established prophet unconvinced. Yet, is it
from misgiving that Mendel had the truth, or merely from indifference,
that no naturalist of repute, save Professor Weldon, has risen against
him?
In the world of knowledge we are accustomed to look for some strenuous
effort to understand a new truth even in those who are indisposed to
believe. It was therefore with a regret approaching to indignation that
I read Professor Weldon’s criticism[1]. Were such a piece from the hand
of a junior it might safely be neglected; but coming from Professor
Weldon there was the danger--almost the certainty--that the small band
of younger men who are thinking of research in this field would take it
they had learnt the gist of Mendel, would imagine his teaching exposed
by Professor Weldon, and look elsewhere for lines of work.
[1] _Biometrika_, I., 1902, Pt. II.
In evolutionary studies we have no Areopagus. With us it is not--as
happily it is with Chemistry, Physics, Physiology, Pathology, and
other well-followed sciences--that an open court is always sitting,
composed of men themselves workers, keenly interested in every new
thing, skilled and well versed in the facts. Where this is the case,
doctrine is soon tried and the false trodden down. But in our sparse
and apathetic community error mostly grows unheeded, choking truth.
That fate must not befall Mendel now.
It seemed imperative that Mendel’s own work should be immediately put
into the hands of all who will read it, and I therefore sought and
obtained the kind permission of the Royal Horticultural Society to
reprint and modify the translation they had already caused to be made
and published in their Journal. To this I add a translation of Mendel’s
minor paper of later date. As introduction to the subject, the same
Society has authorized me to reprint with alterations a lecture on
heredity delivered before them in 1900. For these privileges my warm
thanks are due. The introduction thus supplied, composed originally for
an audience not strictly scientific, is far too slight for the present
purpose. A few pages are added, but I have no time to make it what it
should be, and I must wait for another chance of treating the whole
subject on a more extended scale. It will perhaps serve to give the
beginner the slight assistance which will prepare him to get the most
from Mendel’s own memoir.
* * * * *
The next step was at once to defend Mendel from Professor Weldon. That
could only be done by following this critic from statement to statement
in detail, pointing out exactly where he has gone wrong, what he has
misunderstood, what omitted, what introduced in error. With such
matters it is easy to deal, and they would be as nothing could we find
in his treatment some word of allusion to the future; some hint to the
ignorant that this is a very big thing; some suggestion of what it all
_may_ mean if it _be_ true.
Both to expose each error and to supply effectively what is wanting,
within the limits of a brief article, written with the running pen,
is difficult. For simplicity I have kept almost clear of reference to
facts not directly connected with the text, and have foregone recital
of the now long list of cases, both of plants and animals, where the
Mendelian principles have already been perceived. These subjects are
dealt with in a joint Report to the Evolution Committee of the Royal
Society, made by Miss E. R. Saunders and myself, now in the Press. To
Miss Saunders who has been associated with me in this work for several
years I wish to express my great indebtedness. Much of the present
article has indeed been written in consultation with her. The reader
who seeks fuller statement of facts and conceptions is referred to the
writings of other naturalists who have studied the phenomena at first
hand (of which a bibliography is appended) and to our own Report.
I take this opportunity of acknowledging the unique facilities
generously granted me, as representative of the Evolution Committee,
by Messrs Sutton and Sons of Reading, to watch some of the many
experiments they have in progress, to inspect their admirable records,
and to utilise these facts for the advancement of the science of
heredity. My studies at Reading have been for the most part confined to
plants other than those immediately the subject of this discussion, but
some time ago I availed myself of a kind permission to examine their
stock of peas, thus obtaining information which, with other facts since
supplied, has greatly assisted me in treating this subject.
* * * * *
I venture to express the conviction, that if the facts now before us
are carefully studied, it will become evident that the experimental
study of heredity, pursued on the lines Mendel has made possible, is
second to no branch of science in the certainty and magnitude of the
results it offers. This study has one advantage which no other line of
scientific inquiry possesses, in that the special training necessary
for such work is easily learnt in the practice of it, and can be learnt
in no other way. All that is needed is the faithful resolve to scamp
nothing.
If a tenth part of the labour and cost now devoted by leisured persons,
in this country alone, to the collection and maintenance of species of
animals and plants which have been collected a hundred times before,
were applied to statistical experiments in heredity, the result in a
few years would make a revolution not only in the industrial art of the
breeder but in our views of heredity, species and variation. We have at
last a brilliant method, and a solid basis from which to attack these
problems, offering an opportunity to the pioneer such as occurs but
seldom even in the history of modern science.
We have been told of late, more than once, that Biology must become an
_exact_ science. The same is my own fervent hope. But exactness is not
always attainable by numerical precision: there have been students of
Nature, untrained in statistical nicety, whose instinct for truth yet
saved them from perverse inference, from slovenly argument, and from
misuse of authorities, reiterated and grotesque.
The study of variation and heredity, in our ignorance of the causation
of those phenomena, _must_ be built of statistical data, as Mendel
knew long ago; but, as he also perceived, the ground must be prepared
by specific experiment. The phenomena of heredity and variation are
specific, and give loose and deceptive answers to any but specific
questions. That is where our _exact_ science will begin. Otherwise we
may one day see those huge foundations of “biometry” in ruins.
But Professor Weldon, by coincidence a vehement preacher of precision,
in his haste to annul this first positive achievement of the precise
method, dispenses for the moment even with those unpretending forms of
precision which conventional naturalists have usefully practised. His
essay is a strange symptom of our present state. The facts of variation
and heredity are known to so few that anything passes for evidence; and
if only a statement, or especially a conclusion, be negative, neither
surprise nor suspicion are aroused. An author dealing in this fashion
with subjects commonly studied, of which the literature is familiar and
frequently verified, would meet with scant respect. The reader who has
the patience to examine Professor Weldon’s array of objections will
find that almost all are dispelled by no more elaborate process than a
reference to the original records.
With sorrow I find such an article sent out to the world by a Journal
bearing, in any association, the revered name of Francis Galton,
or under the high sponsorship of Karl Pearson. I yield to no one in
admiration of the genius of these men. Never can we sufficiently regret
that those great intellects were not trained in the profession of the
naturalist.
Mr Galton suggested that the new scientific firm should have a
mathematician and a biologist as partners, and--soundest advice--a
logician retained as consultant[2]. Biologist surely must one partner
be, but it will never do to have him sleeping. In many well-regulated
occupations there are persons known as “knockers-up,” whose thankless
task it is to rouse others from their slumber, and tell them work-time
is come round again. That part I am venturing to play this morning, and
if I have knocked a trifle loud, it is because there is need.
_March, 1902._
[2] _Biometrika_, I. Pt. I. p. 5.
CONTENTS.
INTRODUCTION.
THE PROBLEMS OF HEREDITY AND THEIR SOLUTION, pp. 1–39.
Preliminary statement of Mendel’s principles, 8. Relation of Mendel’s
discovery to the law of Ancestral Heredity, 19. _Heterozygote_ and
_Homozygote_, 23. New conceptions necessitated by Mendel’s discovery,
26. Simple alternative characters, or _allelomorphs_, 27. _Compound
allelomorphs_ and their components, 29. Analytical Variations, 29.
Relation of Mendel’s principle to continuous variation, 32. Dominance,
32. Non-Mendelian phenomena, 33. False hybrids of Millardet, 34. Brief
historical notice, 36.
MENDEL’S EXPERIMENTS IN PLANT HYBRIDISATION, pp. 40–95.
Introductory Remarks, 40. Selection of Experimental Plants, 42.
Division and Arrangement of Experiments, 44. Characters selected, 45.
Number of first crosses, 47. Possible sources of error, 47. Forms of
the Hybrids, 49. Dominant and recessive, 49.
First generation bred from the Hybrids, 51. Numbers of each form in
offspring, 52. Second generation bred from the Hybrids, 55. Subsequent
generations bred from the Hybrids, 57.
Offspring of Hybrids in which several differentiating characters are
associated, 59. The reproductive cells of the Hybrids, 66. Statement
of Mendel’s essential deductions, 67. Experiments to determine
constitution of germ-cells, 68. Statement of purity of germ-cells, 72.
Experiments with _Phaseolus_, 76. Compound characters, 80. Concluding
Remarks, 84.
MENDEL’S EXPERIMENTS WITH HIERACIUM, 96–103.
A DEFENCE OF MENDEL’S PRINCIPLES OF HEREDITY, 104–208.
_Introductory_, 104.
I. The Mendelian Principle of Purity of Germ-cells and the Laws of
Heredity based on Ancestry, 108.
II. Mendel and the critic’s version of him.
The Law of Dominance, 117.
III. The facts in regard to Dominance of Characters in Peas, 119.
The normal characters: colours of cotyledons and seed-coats, 120.
Shape, 122. Stability and variability, 124. Results of crossing in
regard to seed-characters: normal and exceptional, 129. Analysis of
exceptions, 132. The “mule” or heterozygote, 133.
IV. Professor Weldon’s collection of “Other evidence concerning
Dominance in Peas.”
A. In regard to cotyledon colour: Preliminary, 137. Xenia, 139.
(1) Gärtner’s cases, 141. (2) Seton’s case, 143. (3) Tschermak’s
exceptions, 145. (3_a_) _Buchsbaum_ case, 145. (3_b_) _Telephone_
cases, 146. (3_c_) _Couturier_ cases, 147.
B. Seed-coats and Shapes. 1. Seed-coats, 148. 2. Seed-shapes: (_a_)
Rimpau’s cases, 150. (_b_) Tschermak’s cases, 152. 3. Other phenomena,
especially regarding seed-shapes, in the case of “grey” peas. Modern
evidence, 153.
C. Evidence of Knight and Laxton, 158.
D. Miscellaneous cases in other plants and animals:
1. Stocks (_Matthiola_). Hoariness, 169. Flower-colour, 170.
2. _Datura_, 172.
3. Colours of Rats and Mice, 173.
V. Professor Weldon’s quotations from Laxton, 178.
Illustration from _Primula sinensis_, 182.
VI. The Argument built on exceptions, 183.
Ancestry and Dominance, 185.
Ancestry and purity of germ-cells, 193.
The value of the appeal to Ancestry, 197.
VII. The question of absolute purity of germ-cells, 201.
Conclusion, 208.
ERRATA.
p. 22, par. 3, line 2, for “falls” read “fall.”
p. 63, line 12, for “_AabbC_” read “_AaBbc_.”
p. 66, in heading, for “OF HYBRIDS” read “OF THE HYBRIDS.”
_Note to_ p. 125. None of the yellow seeds produced by _Laxton’s Alpha_
germinated, though almost all the green seeds sown gave healthy plants.
The same was found in the case of _Express_, another variety which
bore some yellow seeds. In the case of _Blue Peter_, on the contrary,
the yellow seeds have grown as well as the green ones. Few however
were _wholly_ yellow. Of nine yellow seeds produced by crossing green
varieties together (p. 131), six did not germinate, and three which
did gave weak and very backward plants. Taken together, this evidence
makes it scarcely doubtful that the yellow colour in these cases was
pathological, and almost certainly due to exposure after ripening.
THE PROBLEMS OF HEREDITY AND THEIR SOLUTION[3].
[3] The first half of this paper is reprinted with additions and
modifications from the _Journal of the Royal Horticultural Society_,
1900, vol. XXV., parts 1 and 2. Written almost immediately after the
rediscovery of Mendel, it will be seen to be already in some measure
out of date, but it may thus serve to show the relation of the new
conceptions to the old.
An exact determination of the laws of heredity will probably work more
change in man’s outlook on the world, and in his power over nature,
than any other advance in natural knowledge that can be clearly
foreseen.
There is no doubt whatever that these laws can be determined. In
comparison with the labour that has been needed for other great
discoveries we may even expect that the necessary effort will be small.
It is rather remarkable that while in other branches of physiology such
great progress has of late been made, our knowledge of the phenomena
of heredity has increased but little; though that these phenomena
constitute the basis of all evolutionary science and the very central
problem of natural history is admitted by all. Nor is this due to the
special difficulty of such inquiries so much as to general neglect of
the subject.
It is in the hope of inducing others to follow these lines of
investigation that I take the problems of heredity as the subject of
this lecture to the Royal Horticultural Society.
No one has better opportunities of pursuing such work than
horticulturists and stock breeders. They are daily witnesses of
the phenomena of heredity. Their success also depends largely on a
knowledge of its laws, and obviously every increase in that knowledge
is of direct and special importance to them.
The want of systematic study of heredity is due chiefly to
misapprehension. It is supposed that such work requires a lifetime. But
though for adequate study of the complex phenomena of inheritance long
periods of time must be necessary, yet in our present state of deep
ignorance almost of the outline of the facts, observations carefully
planned and faithfully carried out for even a few years may produce
results of great value. In fact, by far the most appreciable and
definite additions to our knowledge of these matters have been thus
obtained.
There is besides some misapprehension as to the kind of knowledge which
is especially wanted at this time, and as to the modes by which we may
expect to obtain it. The present paper is written in the hope that it
may in some degree help to clear the ground of these difficulties by a
preliminary consideration of the question, How far have we got towards
an exact knowledge of heredity, and how can we get further?
Now this is pre-eminently a subject in which we must distinguish what
we _can_ do from what we want to do. We _want_ to know the whole
truth of the matter; we want to know the physical basis, the inward
and essential nature, “the causes,” as they are sometimes called,
of heredity: but we want also to know the laws which the outward and
visible phenomena obey.
Let us recognise from the outset that as to the essential nature of
these phenomena we still know absolutely nothing. We have no glimmering
of an idea as to what constitutes the essential process by which the
likeness of the parent is transmitted to the offspring. We can study
the processes of fertilisation and development in the finest detail
which the microscope manifests to us, and we may fairly say that we
have now a considerable grasp of the visible phenomena; but of the
nature of the physical basis of heredity we have no conception at all.
No one has yet any suggestion, working hypothesis, or mental picture
that has thus far helped in the slightest degree to penetrate beyond
what we see. The process is as utterly mysterious to us as a flash of
lightning is to a savage. We do not know what is the essential agent
in the transmission of parental characters, not even whether it is a
material agent or not. Not only is our ignorance complete, but no one
has the remotest idea how to set to work on that part of the problem.
We are in the state in which the students of physical science were,
in the period when it was open to anyone to believe that heat was a
material substance or not, as he chose.
But apart from any conception of the essential modes of transmission of
characters, we _can_ study the outward facts of the transmission. Here,
if our knowledge is still very vague, we are at least beginning to see
how we ought to go to work. Formerly naturalists were content with
the collection of numbers of isolated instances of transmission--more
especially, striking and peculiar cases--the sudden appearance of
highly prepotent forms, and the like. We are now passing out of that
stage. It is not that the interest of particular cases has in any way
diminished--for such records will always have their value--but it
has become likely that general expressions will be found capable of
sufficiently wide application to be justly called “laws” of heredity.
That this is so was till recently due almost entirely to the work of Mr
F. Galton, to whom we are indebted for the first systematic attempt to
enuntiate such a law.
All laws of heredity so far propounded are of a statistical character
and have been obtained by statistical methods. If we consider for a
moment what is actually meant by a “law of heredity” we shall see at
once why these investigations must follow statistical methods. For
a “law” of heredity is simply an attempt to declare the course of
heredity under given conditions. But if we attempt to predicate the
course of heredity we have to deal with conditions and groups of causes
wholly unknown to us, whose presence we cannot recognize, and whose
magnitude we cannot estimate in any particular case. The course of
heredity in particular cases therefore cannot be foreseen.
Of the many factors which determine the degree to which a given
character shall be present in a given individual only one is usually
known to us, namely, the degree to which that character is present
in the parents. It is common knowledge that there is not that close
correspondence between parent and offspring which would result were
this factor the only one operating; but that, on the contrary, the
resemblance between the two is only an uncertain one.
In dealing with phenomena of this class the study of single instances
reveals no regularity. It is only by collection of facts in great
numbers, and by statistical treatment of the mass, that any order or
law can be perceived. In the case of a chemical reaction, for instance,
by suitable means the conditions can be accurately reproduced, so that
in every individual case we can predict with certainty that the same
result will occur. But with heredity it is somewhat as it is in the
case of the rainfall. No one can say how much rain will fall to-morrow
in a given place, but we can predict with moderate accuracy how much
will fall next year, and for a period of years a prediction can be made
which accords very closely with the truth.
Similar predictions can from statistical data be made as to the
duration of life and a great variety of events, the conditioning causes
of which are very imperfectly understood. It is predictions of this
kind that the study of heredity is beginning to make possible, and in
that sense laws of heredity can be perceived.
We are as far as ever from knowing _why_ some characters are
transmitted, while others are not; nor can anyone yet foretell which
individual parent will transmit characters to the offspring, and which
will not; nevertheless the progress made is distinct.
As yet investigations of this kind have been made in only a few
instances, the most notable being those of Galton on human stature, and
on the transmission of colours in Basset hounds. In each of these cases
he has shown that the expectation of inheritance is such that a simple
arithmetical rule is approximately followed. The rule thus arrived at
is that of the whole heritage of the offspring the two parents together
on an average contribute one half, the four grandparents one-quarter,
the eight great-grandparents one-eighth, and so on, the remainder
being contributed by the remoter ancestors.
Such a law is obviously of practical importance. In any case to which
it applies we ought thus to be able to predict the degree with which
the purity of a strain may be increased by selection in each successive
generation.
To take a perhaps impossibly crude example, if a seedling show any
particular character which it is desired to fix, on the assumption that
successive self-fertilisations are possible, according to Galton’s
law the expectation of purity should be in the first generation of
self-fertilisation 1 in 2, in the second generation 3 in 4, in the
third 7 in 8, and so on[4].
[4] See later. Galton gave a simple diagrammatic representation of
his law in _Nature_, 1898, vol. LVII. p. 293.
But already many cases are known to which the rule in any simple
form will not apply. Galton points out that it takes no account of
individual prepotencies. There are, besides, numerous cases in which
on crossing two varieties the character of one variety almost always
appears in each member of the first cross-bred generation. Examples of
these will be familiar to those who have experience in such matters.
The offspring of the Polled Angus cow and the Shorthorn bull is almost
invariably polled or with very small loose “scurs.” Seedlings raised
by crossing _Atropa belladonna_ with the yellow-fruited variety have
without exception the blackish-purple fruits of the type. In several
hairy species when a cross with a glabrous variety is made, the first
cross-bred generation is altogether hairy[5].
[5] These we now recognize as examples of Mendelian ‘dominance.’
Still more numerous are examples in which the characters of one variety
very largely, though not exclusively, predominate in the offspring.
These large classes of exceptions--to go no further--indicate that,
as we might in any case expect, the principle is not of universal
application, and will need various modifications if it is to be
extended to more complex cases of inheritance of varietal characters.
No more useful work can be imagined than a systematic determination of
the precise “law of heredity” in numbers of particular cases.
Until lately the work which Galton accomplished stood almost alone in
this field, but quite recently remarkable additions to our knowledge
of these questions have been made. In the year 1900 Professor de Vries
published a brief account[6] of experiments which he has for several
years been carrying on, giving results of the highest value.
[6] _Comptes Rendus_, March 26, 1900, and _Ber. d. Deutsch. Bot.
Ges._ xviii. 1900, p. 83.
The description is very short, and there are several points as to which
more precise information is necessary both as to details of procedure
and as to statement of results. Nevertheless it is impossible to doubt
that the work as a whole constitutes a marked step forward, and the
full publication which is promised will be awaited with great interest.
The work relates to the course of heredity in cases where definite
varieties differing from each other in some _one_ definite character
are crossed together. The cases are all examples of discontinuous
variation: that is to say, cases in which actual intermediates between
the parent forms are not usually produced on crossing[7]. It is shown
that the subsequent posterity obtained by self-fertilising these
cross-breds or hybrids, or by breeding them with each other, break up
into the original parent forms according to fixed numerical rule.
[7] This conception of discontinuity is of course pre-Mendelian.
Professor de Vries begins by reference to a remarkable memoir by Gregor
Mendel[8], giving the results of his experiments in crossing varieties
of _Pisum sativum_. These experiments of Mendel’s were carried out on
a large scale, his account of them is excellent and complete, and the
principles which he was able to deduce from them will certainly play a
conspicuous part in all future discussions of evolutionary problems.
It is not a little remarkable that Mendel’s work should have escaped
notice, and been so long forgotten.
[8] ‘Versuche üb. Pflanzenhybriden’ in the _Verh. d. Naturf. Ver.
Brünn_, iv. 1865.
For the purposes of his experiments Mendel selected seven pairs of
characters as follows:--
1. Shape of ripe seed, whether round; or angular and wrinkled.
2. Colour of “endosperm” (cotyledons), whether some shade of yellow; or
a more or less intense green.
3. Colour of the seed-skin, whether various shades of grey and
grey-brown; or white.
4. Shape of seed-pod, whether simply inflated; or deeply constricted
between the seeds.
5. Colour of unripe pod, whether a shade of green; or bright yellow.
6. Nature of inflorescence, whether the flowers are arranged along the
axis of the plant; or are terminal and form a kind of umbel.
7. Length of stem, whether about 6 or 7 ft. long, or about 3/4 to
1-1/2 ft.
Large numbers of crosses were made between Peas differing in respect of
_one_ of each of these pairs of characters. It was found that in each
case the offspring of the cross exhibited the character of one of the
parents in almost undiminished intensity, and intermediates which could
not be at once referred to one or other of the parental forms were not
found.
In the case of each pair of characters there is thus one which in the
first cross prevails to the exclusion of the other. This prevailing
character Mendel calls the _dominant_ character, the other being the
_recessive_ character[9].
[9] Note that by these novel terms the complications involved by use
of the expression “prepotent” are avoided.
That the existence of such “dominant” and “recessive” characters is a
frequent phenomenon in cross-breeding, is well known to all who have
attended to these subjects.
By letting the cross-breds fertilise themselves Mendel next raised
another generation. In this generation were individuals which showed
the dominant character, but also individuals which presented the
recessive character. Such a fact also was known in a good many
instances. But Mendel discovered that in this generation the numerical
proportion of dominants to recessives is on an average of cases
approximately constant, being in fact _as three to one_. With very
considerable regularity these numbers were approached in the case of
each of his pairs of characters.
There are thus in the first generation raised from the cross-breds 75
per cent. dominants and 25 per cent. recessives.
These plants were again self-fertilised, and the offspring of each
plant separately sown. It next appeared that the offspring of the
recessives _remained pure recessive_, and in subsequent generations
never produced the dominant again.
But when the seeds obtained by self-fertilising the dominants were
examined and sown it was found that the dominants were not all alike,
but consisted of two classes, (1) those which gave rise to pure
dominants, and (2) others which gave a mixed offspring, composed partly
of recessives, partly of dominants. Here also it was found that the
average numerical proportions were constant, those with pure dominant
offspring being to those with mixed offspring as one to two. Hence
it is seen that the 75 per cent. dominants are not really of similar
constitution, but consist of twenty-five which are pure dominants and
fifty which are really cross-breds, though, like the cross-breds raised
by crossing the two original varieties, they only exhibit the dominant
character.
To resume, then, it was found that by self-fertilising the original
cross-breds the same proportion was always approached, namely--
25 dominants, 50 cross-breds, 25 recessives,
or 1_D_ : 2_DR_ : 1_R_.
Like the pure recessives, the pure dominants are thenceforth pure, and
only give rise to dominants in all succeeding generations studied.
On the contrary the fifty cross-breds, as stated above, have mixed
offspring. But these offspring, again, in their numerical proportions,
follow the same law, namely, that there are three dominants to one
recessive. The recessives are pure like those of the last generation,
but the dominants can, by further self-fertilisation, and examination
or cultivation of the seeds produced, be again shown to be made up of
pure dominants and cross-breds in the same proportion of one dominant
to two cross-breds.
The process of breaking up into the parent forms is thus continued in
each successive generation, the same numerical law being followed so
far as has yet been observed.
Mendel made further experiments with _Pisum sativum_, crossing pairs
of varieties which differed from each other in _two_ characters, and
the results, though necessarily much more complex, showed that the law
exhibited in the simpler case of pairs differing in respect of one
character operated here also.
In the case of the union of varieties _AB_ and _ab_ differing in two
distinct pairs of characters, _A_ and _a_, _B_ and _b_, of which _A_
and _B_ are dominant, _a_ and _b_ recessive, Mendel found that in the
first cross-bred generation there was only _one_ class of offspring,
really _AaBb_.
But by reason of the dominance of one character of each pair these
first crosses were hardly if at all distinguishable from _AB_.
By letting these _AaBb_’s fertilise themselves, only _four_ classes of
offspring seemed to be produced, namely,
_AB_ showing both dominant characters.
_Ab_ " dominant _A_ and recessive _b_.
_aB_ " recessive _a_ and dominant _B_.
_ab_ " both recessive characters _a_ and _b_.
The numerical ratio in which these classes appeared were also regular
and approached the ratio
9_AB_ : 3_Ab_ : 3_aB_ : 1_ab_.
But on cultivating these plants and allowing them to fertilise
themselves it was found that the members of the
RATIOS
1 _ab_ class produce only _ab_’s.
3 {1 _aB_ class may produce either all _aB_’s,
{2 _or_ both _aB_’s and _ab_’s.
RATIOS
3 { 1 _Ab_ class may produce either all _Ab_’s,
{ 2 _or_ both _Ab_’s and _ab_’s.
{ 1 _AB_ class may produce either all _AB_’s,
{ 2 _or_ both _AB_’s and _Ab_’s,
9 { 2 _or_ both _AB_’s and _aB_’s,
{ 4 _or_ all four possible classes again, namely,
{ _AB_’s, _Ab_’s, _aB_’s, and _ab_’s,
and the average number of members of each class will approach the ratio
1 : 3 : 3 : 9 as indicated above.
The details of these experiments and of others like them made with
_three_ pairs of differentiating characters are all set out in Mendel’s
memoir.
Professor de Vries has worked at the same problem in some dozen species
belonging to several genera, using pairs of varieties characterised by
a great number of characters: for instance, colour of flowers, stems,
or fruits, hairiness, length of style, and so forth. He states that in
all these cases Mendel’s principles are followed.
The numbers with which Mendel worked, though large, were not large
enough to give really smooth results[10]; but with a few rather
marked exceptions the observations are remarkably consistent, and
the approximation to the numbers demanded by the law is greatest in
those cases where the largest numbers were used. When we consider,
besides, that Tschermak and Correns announce definite confirmation in
the case of _Pisum_, and de Vries adds the evidence of his long series
of observations on other species and orders, there can be no doubt
that Mendel’s law is a substantial reality; though whether some of
the cases that depart most widely from it can be brought within the
terms of the same principle or not, can only be decided by further
experiments.
[10] Professor Weldon (p. 232) takes great exception to this
statement, which he considerately attributes to “some writers.”
After examining the conclusions he obtained by algebraical study of
Mendel’s figures I am disposed to think my statement not very far out.
One may naturally ask, How can these results be brought into harmony
with the facts of hybridisation hitherto known; and, if all this is
true, how is it that others who have carefully studied the phenomena
of hybridisation have not long ago perceived this law? The answer to
this question is given by Mendel at some length, and it is, I think,
satisfactory. He admits from the first that there are undoubtedly cases
of hybrids and cross-breds which maintain themselves pure and do not
break up. Such examples are plainly outside the scope of his law. Next
he points out, what to anyone who has rightly comprehended the nature
of discontinuity in variation is well known, that the variations in
_each_ character must be _separately_ regarded. In most experiments in
crossing, forms are taken which differ from each other in a multitude
of characters--some continuous, others discontinuous, some capable of
blending with their contraries, while others are not. The observer on
attempting to perceive any regularity is confused by the complications
thus introduced. Mendel’s law, as he fairly says, could only appear in
such cases by the use of overwhelming numbers, which are beyond the
possibilities of practical experiment. Lastly, no previous observer had
applied a strict statistical method.
Both these answers should be acceptable to those who have studied the
facts of variation and have appreciated the nature of Species in the
light of those facts. That different species should follow different
laws, and that the same law should not apply to all characters alike,
is exactly what we have every right to expect. It will also be
remembered that the principle is only explicitly declared to apply to
discontinuous characters[11]. As stated also it can only be true where
reciprocal crossings lead to the same result. Moreover, it can only be
tested when there is no sensible diminution in fertility on crossing.
[11] See later.
Upon the appearance of de Vries’ paper announcing the “rediscovery”
and confirmation of Mendel’s law and its extension to a great number
of cases two other observers came forward almost simultaneously and
independently described series of experiments fully confirming Mendel’s
work. Of these papers the first is that of Correns, who repeated
Mendel’s original experiment with Peas having seeds of different
colours. The second is a long and very valuable memoir of Tschermak,
which gives an account of elaborate researches into the results of
crossing a number of varieties of _Pisum sativum_. These experiments
were in many cases carried out on a large scale, and prove the main
fact enuntiated by Mendel beyond any possibility of contradiction.
The more exhaustive of these researches are those of Tschermak on
Peas and Correns on several varieties of Maize. Both these elaborate
investigations have abundantly proved the general applicability of
Mendel’s law to the character of the plants studied, though both
indicate some few exceptions. The details of de Vries’ experiments are
promised in the second volume of his most valuable _Mutationstheorie_.
Correns in regard to Maize and Tschermak in the case of _P. sativum_
have obtained further proof that Mendel’s law holds as well in the case
of varieties differing from each other in _two_ pairs of characters,
one of each pair being dominant, though of course a more complicated
expression is needed in such cases[12].
[12] Tschermak’s investigations were besides directed to a
re-examination of the question of the absence of beneficial
results on cross-fertilising _P. sativum_, a subject already much
investigated by Darwin, and upon this matter also important further
evidence is given in great detail.
That we are in the presence of a new principle of the highest
importance is manifest. To what further conclusions it may lead us
cannot yet be foretold. But both Mendel and the authors who have
followed him lay stress on one conclusion, which will at once suggest
itself to anyone who reflects on the facts. For it will be seen that
the results are such as we might expect if it be imagined that the
cross-bred plant produced pollen grains and egg-cells, each of which
bears only _one_ of the alternative varietal characters and not both.
If this were so, and if on an average the same number of pollen grains
and egg-cells transmit each of the two characters, it is clear that on
a random assortment of pollen grains and egg-cells Mendel’s law would
be obeyed. For 25 per cent. of “dominant” pollen grains would unite
with 25 per cent. “dominant” egg-cells; 25 per cent. “recessive” pollen
grains would similarly unite with 25 per cent. “recessive” egg-cells;
while the remaining 50 per cent. of each kind would unite together.
It is this consideration which leads both Mendel and those who have
followed him to assert that these facts of crossing prove that each
egg-cell and each pollen grain is pure in respect of each character
to which the law applies. It is highly desirable that varieties
differing in the form of their pollen should be made the subject of
these experiments, for it is quite possible that in such a case strong
confirmation of this deduction might be obtained. [Preliminary trials
made with reference to this point have so far given negative results.
Remembering that a pollen grain is not a germ-cell, but only a bearer
of a germ-cell, the hope of seeing pollen grains differentiated
according to the characters they bear is probably remote. Better hopes
may perhaps be entertained in regard to spermatozoa, or possibly female
cells.]
As an objection to the deduction of purity of germ-cells, however, it
is to be noted that though true intermediates did not generally occur,
yet the intensity in which the characters appeared did vary in degree,
and it is not easy to see how the hypothesis of _perfect_ purity in the
reproductive cells can be supported in such cases. Be this, however, as
it may, there is no doubt we are beginning to get new lights of a most
valuable kind on the nature of heredity and the laws which it obeys. It
is to be hoped that these indications will be at once followed up by
independent workers. Enough has been said to show how necessary it is
that the subjects of experiment should be chosen in such a way as to
bring the laws of heredity to a real test. For this purpose the first
essential is that the differentiating characters should be few, and
that all avoidable complications should be got rid of. Each experiment
should be reduced to its simplest possible limits. The results obtained
by Galton, and also the new ones especially described in this paper,
have each been reached by restricting the range of observation to one
character or group of characters, and it is certain that by similar
treatment our knowledge of heredity may be rapidly extended.
* * * * *
To the above popular presentation of the essential facts, made for
an audience not strictly scientific, some addition, however brief,
is called for. First, in regard to the law of Ancestry, spoken of on
p. 5. Those who are acquainted with Pearson’s _Grammar of Science_,
2nd ed. published early in 1900, the same author’s paper in _Proc.
R. S._ vol. 66, 1900, p. 140, or the extensive memoir (pubd. Oct.
1900), on the inheritance of coat-colour in horses and eye-colour in
man (_Phil. Trans._ 195, A, 1900, p. 79), will not need to be told that
the few words I have given above constitute a most imperfect diagram of
the operations of that law as now developed. Until the appearance of
these treatises it was, I believe, generally considered that the law
of Ancestral Heredity was to be taken as applying to phenomena like
these (coat-colour, eye-colour, &c.) where the inheritance is generally
_alternative_, as well as to the phenomena of _blended_ inheritance.
Pearson, in the writings referred to, besides withdrawing other large
categories of phenomena from the scope of its operations, points out
that the law of Ancestral Heredity does not satisfactorily express the
cases of alternative inheritance. He urges, and with reason, that these
classes of phenomena should be separately dealt with.
* * * * *
The whole issue as regards the various possibilities of heredity now
recognized will be made clearer by a very brief exposition of the
several conceptions involved.
If an organism producing germ-cells of a given constitution, uniform in
respect of the characters they bear, breeds with another organism[13]
bearing _precisely similar_ germ-cells, the offspring resulting will,
if the conditions are identical, be uniform.
[13] For simplicity the case of self-fertilisation is omitted from
this consideration.
In practice such a phenomenon is seen in _pure_-breeding. It is true
that we know no case in nature where all the germ-cells are thus
identical, and where no variation takes place beyond what we can
attribute to conditions, but we know many cases where such a result
is approached, and very many where all the essential features which we
regard as constituting the characters of the breed are reproduced with
approximate certainty in every member of the pure-bred race, which thus
closely approach to uniformity.
But if two germ-cells of dissimilar constitution unite in
fertilisation, what offspring are we to expect[14]? First let us
premise that the answer to this question is known experimentally to
differ for many organisms and for many classes of characters, and may
almost certainly be in part determined by external circumstances. But
omitting the last qualification, certain principles are now clearly
detected, though what principle will apply in any given case can only
be determined by direct experiment made with that case.
[14] In all the cases discussed it is assumed that the gametes are
similar except in regard to the “heritage” they bear, and that no
_original_ variation is taking place. The case of mosaics is also
left wholly out of account (see later).
This is the phenomenon of _cross_-breeding. As generally used, this
term means the union of members of dissimilar varieties, or species:
though when dissimilar gametes[15] produced by two individuals
of the same variety unite in fertilisation, we have essentially
_cross_-breeding in respect of the character or characters in which
those gametes differ. We will suppose, as before, that these two
gametes bearing properties unlike in respect of a given character, are
borne by different individuals.
[15] The term “gamete” is now generally used as the equivalent of
“germ-cell,” whether male or female, and the term “zygote” is here
used for brevity to denote the organism resulting from fertilisation.
In the simplest case, suppose a gamete from an individual presenting
any character in intensity _A_ unite in fertilisation with another
from an individual presenting the same character in intensity _a_. For
brevity’s sake we may call the parent individuals _A_ and _a_, and the
resulting zygote _Aa_. What will the structure of _Aa_ be in regard to
the character we are considering?
Up to Mendel no one proposed to answer this question in any other way
than by reference to the intensity of the character in the progenitors,
and _primarily_ in the parents, _A_ and _a_, in whose bodies the
gametes had been developed. It was well known that such a reference
gave a very poor indication of what _Aa_ would be. Both _A_ and _a_
may come from a population consisting of individuals manifesting the
same character in various intensities. In the pedigree of either _A_
or _a_ these various intensities may have occurred few or many times.
Common experience leads us to expect the probability in regard to _Aa_
to be influenced by this history. The next step is that which Galton
took. He extended the reference beyond the immediate parents of _Aa_,
to its grandparents, great-grandparents, and so on, and in the cases he
studied he found that from a knowledge of the intensity in which the
given character was manifested in each progenitor, even for some few
generations back, a fairly accurate prediction could be made, not as to
the character of any individual _Aa_, but as to the average character
of _Aa_’s of similar parentage, in general.
But suppose that instead of individuals presenting one character in
differing intensities, two individuals breed together distinguished by
characters which we know to be mutually exclusive, such as _A_ and _B_.
Here again we may speak of the individuals producing the gametes as _A_
and _B_, and the resulting zygote as _AB_. What will _AB_ be like? The
population here again may consist of many like _A_ and like _B_. These
two forms may have been breeding together indiscriminately, and there
may have been many or few of either type in the pedigree of either _A_
or _B_.
Here again Galton applied his method with remarkable success. Referring
to the progenitors of _A_ and _B_, determining how many of each type
there were in the direct pedigree of _A_ and of _B_, he arrived at the
same formula as before, with the simple difference that instead of
expressing the probable average intensity of one character in several
individuals, the prediction is given in terms of the probable number of
_A_’s and _B_’s that would result on an average when particular _A_’s
and _B_’s of known pedigree breed together.
The law as Galton gives it is as follows:--
“It is that the two parents contribute between them on the average
one-half, or (0·5) of the total heritage of the offspring; the four
grandparents, one-quarter, or (0·5)^2; the eight great-grandparents,
one-eighth, or (0·5)^3, and so on. Then the sum of the ancestral
contributions is expressed by the series
{(0·5) + (0·5)^2 + (0·5)^3, &c.},
which, being equal to 1, accounts for the whole heritage.”
In the former case where _A_ and _a_ are characters which can be
denoted by reference to a common scale, the law assumes of course that
the inheritance will be, to use Galton’s term, _blended_, namely that
the zygote resulting from the union of _A_ with _a_ will on the average
be more like _a_ than if _A_ had been united with _A_; and conversely
that an _Aa_ zygote will on the average _be more like A than an aa
zygote would be_.
But in the case of _A_’s and _B_’s, which are assumed to be mutually
exclusive characters, we cannot speak of blending, but rather, to use
Galton’s term, of _alternative_ inheritance.
Pearson, finding that the law whether formulated thus, or in the
modified form in which he restated it[16], did not express the
phenomena of alternative inheritance known to him with sufficient
accuracy to justify its strict application to them, and also on general
grounds, proposed that the phenomena of blended and alternative
inheritance should be treated apart--a suggestion[17] the wisdom of
which can scarcely be questioned.
[16] In Pearson’s modification the parents contribute 0·3, the
grandparents 0·15, the great-grandparents ·075.
[17] See the works referred to above.
Now the law thus imperfectly set forth and every modification of it is
incomplete in one respect. It deals only with the characters of the
resulting zygotes and predicates nothing in regard to the gametes which
go to form them. A good prediction may be made as to any given group of
zygotes, but the various possible constitutions of the gametes are not
explicitly treated.
Nevertheless a definite assumption is implicitly made regarding the
gametes. It is not in question that differences between these gametes
may occur in respect of the heritage they bear; yet it is assumed
that these differences will be distributed among the gametes of any
individual zygote in such a way that each gamete remains capable,
on fertilisation, of transmitting _all_ the characters (both of the
parent-zygote and of its progenitors) to the zygote which it then
contributes to form (and to the posterity of that zygote) in the
intensity indicated by the law. Hence the gametes of any individual
are taken as collectively a fair sample of all the racial characters
in their appropriate intensities, and this theory demands that there
shall have been no qualitative redistribution of characters among the
gametes of any zygote in such a way that some gametes shall be finally
excluded from partaking of and transmitting any specific part of
the heritage. The theory further demands--and by the analogy of what
we know otherwise not only of animals and plants, but of physical or
chemical laws, perhaps this is the most serious assumption of all--that
the structure of the gametes shall admit of their being capable of
transmitting any character in any intensity varying from zero to
totality with equal ease; and that gametes of each intensity are all
equally likely to occur, given a pedigree of appropriate arithmetical
composition.
Such an assumption appears so improbable that even in cases where
the facts seem as yet to point to this conclusion with exceptional
clearness, as in the case of human stature, I cannot but feel there is
still room for reserve of judgment.
However this may be, the Law of Ancestral Heredity, and all
modifications of it yet proposed, falls short in the respect specified
above, that _it does not directly attempt to give any account of the
distribution of the heritage among the gametes_ of any one individual.
Mendel’s conception differs fundamentally from that involved in the Law
of Ancestral Heredity. The relation of his hypothesis to the foregoing
may be most easily shown if we consider it first in application to the
phenomena resulting from the cross-breeding of two pure varieties.
Let us again consider the case of two varieties each displaying the
same character, but in the respective intensities _A_ and _a_. Each
gamete of the _A_ variety bears _A_, and each gamete of the _a_ variety
bears _a_. When they unite in fertilisation they form the zygote _Aa_.
What will be its characters? The Mendelian teaching would reply that
this can only be known by direct experiment with the two forms _A_ and
_a_, and that the characters _A_ and _a_ perceived in those two forms
or varieties need not give any indication as to the character of the
zygote _Aa_. It may display the character _A_, or _a_, or a character
half way between the two, or a character beyond _A_ or below _a_. The
character of _Aa_ is not regarded as a _heritage_ transmitted to it by
_A_ and by _a_, but as a character special and peculiar to _Aa_, just
as NaCl is not a body half way between sodium and chlorine, or such
that its properties can be predicted from or easily stated in terms of
theirs.
If a concrete case may help, a tall pea _A_ crossed with a dwarf _a_
often produces, not a plant having the height of either _A_ or _a_, but
something _taller_ than the pure tall variety _A_.
But if the case obeys the Mendelian principles--as does that here
quoted--then it can be declared _first_ that the gametes of _Aa_
will not be bearers of the character proper to _Aa_; but, generally
speaking, each gamete will either bear the pure _A_ character or the
pure _a_ character. There will in fact be a redistribution of the
characters brought in by the gametes which united to form the zygote
_Aa_, such that each gamete of _Aa_ is pure, as the parental gametes
were. _Secondly_ this redistribution will occur in such a way that, of
the gametes produced by such _Aa_’s, on an average there will be equal
numbers of _A_ gametes and of _a_ gametes.
Consequently if _Aa_’s breed together, the new _A_ gametes may meet
each other in fertilisation, forming a zygote _AA_, namely, the pure
_A_ variety again; similarly two _a_ gametes may meet and form _aa_,
or the pure _a_ variety again. But if an _A_ gamete meets an _a_ it
will once more form _Aa_, with its special character. This _Aa_ is
the hybrid, or “mule” form, or as I have elsewhere called it, the
_heterozygote_, as distinguished from _AA_ or _aa_ the _homozygotes_.
Similarly if the two gametes of two varieties distinguished by
characters, _A_ and _B_, which cannot be described in terms of any
common scale (such as for example the “rose” and “single” combs of
fowls) unite in fertilisation, again the character of the mule form
cannot be predicted. Before the experiment is made the “mule” may
present _any_ form. Its character or properties can as yet be no more
predicted than could those of the compounds of unknown elements before
the discovery of the periodic law.
But again--if the case be Mendelian--the gametes borne by _AB_ will be
either _A_’s or _B_’s[18], and the cross-bred _AB_’s breeding together
will form _AA_’s, _AB_’s and _ BB_’s. Moreover, if as in the normal
Mendelian case, _AB_’s bear on an average equal numbers of _A_ gametes
and _B_ gametes, the numerical ratio of these resulting zygotes to each
other will be
1 _AA_ : 2 _AB_ : 1 _BB_.
[18] This conception was clearly formed by Naudin simultaneously
with Mendel, but it was not worked out by him and remained a mere
suggestion. In one place also Focke came very near to the same idea
(see Bibliography).
We have seen that Mendel makes no prediction as to the outward and
visible characters of _AB_, but only as to the essential constitution
and statistical condition of its gametes in regard to the characters
_A_ and _B_. Nevertheless in a large number of cases the character of
_AB_ is known to fall into one of three categories (omitting mosaics).
(1) The cross-bred may almost always resemble one of its pure
parents so closely as to be practically indistinguishable from that
pure form, as in the case of the yellow cotyledon-colour of certain
varieties of peas when crossed with green-cotyledoned varieties; in
which case the parental character, yellow, thus manifested by the
cross-bred is called “dominant” and the parental character, green,
not manifested, is called recessive.
(2) The cross-bred may present some condition intermediate between
the two parental forms, in which case we may still retain the term
“blend” as applied to the zygote.
Such an “intermediate” may be the apparent mean between the two
parental forms or be nearer to one or other in any degree. Such
a case is that of a cross between a rich crimson Magenta Chinese
Primrose and a clear White, giving a flower of a colour appropriately
described as a “washy” magenta.
(3) The cross-bred may present some form quite different from that
of either pure parent. Though, as has been stated, nothing can be
predicted of an unknown case, we already know a considerable number
of examples of this nature in which the mule-form _approaches
sometimes with great accuracy to that of a putative ancestor, near
or remote_. It is scarcely possible to doubt that several--though
perhaps not all--of Darwin’s “reversions on crossing” were of this
nature.
Such a case is that of the “wild grey mouse” produced by the union
of an albino tame mouse and a piebald Japanese mouse[19]. These
“reversionary” mice bred together produce the parental tame types,
some other types, and “reversionary” mice again.
[19] See von Guaita, _Ber. naturf. Ges. Freiburg_ X. 1898 and XI.
1899, quoted by Professor Weldon (see later).
From what has been said it will now be clear that the applicability of
the Mendelian hypothesis has, intrinsically, nothing whatever to do
with the question of the inheritance being _blended_ or _alternative_.
In fact, as soon as the relation of zygote characters to gamete
characters is appreciated, it is difficult to see any reason for
supposing that the manifestation of characters seen in the zygotes
should give any indication as to their mode of allotment among the
gametes.
On a previous occasion I pointed out that the terms “Heredity” and
“Inheritance” are founded on a misapplication of metaphor, and in the
light of our present knowledge it is becoming clearer that the ideas of
“transmission” of a character by parent to offspring, or of there being
any “contribution” made by an ancestor to its posterity, must only be
admitted under the strictest reserve, and merely as descriptive terms.
* * * * *
We are now presented with some entirely new conceptions:--
(1) The purity of the gametes in regard to certain characters.
(2) The distinction of all zygotes according as they are or are not
formed by the union of like or unlike gametes. In the former case,
apart from Variation, they breed true when mated with their like; in
the latter case their offspring, collectively, will be heterogeneous.
(3) If the zygote be formed by the union of dissimilar gametes, we
may meet the phenomenon of (_a_) dominant and recessive characters;
(_b_) a blend form; (_c_) a form distinct from either parent, often
reversionary[20].
[20] This fact sufficiently indicates the difficulties involved in a
superficial treatment of the phenomenon of reversion. To call such
reversions as those named above “returns to ancestral type” would be,
if more than a descriptive phrase were intended, quite misleading. It
is not the ancestral _type_ that has come back, but something else
has come in its guise, as the offspring presently prove. For the
first time we thus begin to get a rationale of “reversion.”
But there are additional and even more significant deductions from the
facts. We have seen that the gametes are differentiated in respect
of pure characters. Of these pure characters there may _conceivably_
be any number associated together in one organism. In the pea Mendel
detected at least seven--not all seen by him combined in the same
plant, but there is every likelihood that they are all capable of being
thus combined.
Each such character, which is capable of being dissociated or replaced
by its contrary, must henceforth be conceived of as a distinct
_unit-character_; and as we know that the several unit-characters are
of such a nature that any one of them is capable of independently
displacing or being displaced by one or more alternative characters
taken singly, we may recognize this fact by naming such unit-characters
_allelomorphs_. So far, we know very little of any allelomorphs
existing otherwise than as _pairs_ of contraries, but this is probably
merely due to experimental limitations and the rudimentary state of our
knowledge.
In one case (combs of fowls) we know three characters, _pea_ comb,
_rose_ comb and _single_ comb; of which _pea_ and _single_, or _rose_
and _single_, behave towards each other as a pair of allelomorphs, but
of the behaviour of _pea_ and _rose_ towards each other we know as yet
nothing.
We have no reason as yet for affirming that any phenomenon properly
described as _displacement_ of one allelomorph by another occurs,
though the metaphor may be a useful one. In all cases where _dominance_
has been perceived, we can affirm that the members of the allelomorphic
pair stand to each other in a relation the nature of which we are as
yet wholly unable to apprehend or illustrate.
To the new conceptions already enumerated we may therefore add
(4) _Unit-characters_ of which some, _when once arisen by Variation_,
are alternative to each other in the constitution of the gametes,
according to a definite system.
From the relations subsisting between these characters, it follows that
as each zygotic union of allelomorphs is _resolved_ on the formation
of the gametes, no zygote can give rise to gametes collectively
representing more than _two_ characters allelomorphic to each other,
apart from new variation.
From the fact of the existence of the interchangeable characters we
must, for purposes of treatment, and to complete the possibilities,
necessarily form the conception of an _irresoluble base_, though
whether such a conception has any objective reality we have no means as
yet of determining.
We have now seen that when the varieties _A_ and _B_ are crossed
together, the heterozygote, _AB_, produces gametes bearing the pure
_A_ character and the pure _B_ character. In such a case we speak of
such characters as _simple_ allelomorphs. In many cases however a more
complex phenomenon happens. The character brought in on fertilisation
by one or other parent may be of such a nature that when the zygote,
_AB_, forms its gametes, these are not individually bearers merely
of _A_ and _B_, _but of a number of characters themselves again
integral_, which in, say _A_, behaved as one character so long as its
gametes united in fertilisation with others like themselves, but on
cross-fertilisation are resolved and redistributed among the gametes
produced by the cross-bred zygote.
In such a case we call the character _A_ a _compound_ allelomorph,
and we can speak of the integral characters which constitute it as
_hypallelomorphs_. We ought to write the heterozygote (_A A′ A″_ ...)
_B_ and the gametes produced by it may be of the form _A_, _A′_, _A″_,
_A‴_,... _B_. Or the resolution may be incomplete in various degrees,
as we already suspect from certain instances; in which case we may have
gametes _A_, _A′ A″_, _A‴ A″″_, _A′ A″ A^v_,... _B_, and so on. Each
of these may meet a similar or a dissimilar gamete in fertilisation,
forming either a homozygote, or a heterozygote with its distinct
properties.
In the case of compound allelomorphs we know as yet nothing of the
statistical relations of the several gametes.
Thus we have the conception
(5) _of a Compound character_, borne by one gamete, transmitted
entire as a single character so long as fertilisation only occurs
between like gametes, or is, in other words, “symmetrical,” but if
fertilisation take place with a dissimilar gamete (or possibly by
other causes), resolved into integral constituent characters, each
separately transmissible.
Next, as, by the union of the gametes bearing the various
hypallelomorphs with other such gametes, or with gametes bearing
simple allelomorphs, in fertilisation, a number of new zygotes will
be formed, such as may not have been seen before in the breed: these
will inevitably be spoken of as _varieties_; and it is difficult not to
extend the idea of variation to them. To distinguish these from other
variations--which there must surely be--we may call them
(6) _Analytical_ variations in contradistinction to
(7) _Synthetical_ variations, occurring not by the separation of
pre-existing constituent-characters but by the addition of new
characters.
Lastly, it is impossible to be presented with the fact that in
Mendelian cases the cross-bred produces on an average _equal_ numbers
of gametes of each kind, that is to say, a symmetrical result, without
suspecting that this fact must correspond with some symmetrical figure
of distribution of those gametes in the cell-divisions by which they
are produced.
* * * * *
At the present time these are the main conceptions--though by no means
all--arising directly from Mendel’s work. The first six are all more
or less clearly embodied by him, though not in every case developed
in accordance with modern knowledge. The seventh is not a Mendelian
conception, but the facts before us justify its inclusion in the above
list though for the present it is little more than a mere surmise.
* * * * *
In Mendelian cases it will now be perceived that all the zygotes
composing the population consist of a limited number of possible types,
each of definite constitution, bearing gametes also of a limited and
definite number of types, and definite constitution in respect of
pre-existing characters. It is now evident that in such cases each
several progenitor need not be brought to account in reckoning the
probable characters of each descendant; for the gametes of cross-breds
are differentiated at each successive generation, some parental
(Mendelian) characters being left out in the composition of each gamete
produced by a zygote arising by the union of bearers of opposite
allelomorphs.
When from these considerations we return to the phenomena comprised in
the Law of Ancestral Heredity, what certainty have we that the same
conceptions are not applicable there also?
It has now been shown that the question whether in the cross-bred
zygotes in general the characters blend or are mutually exclusive is an
entirely subordinate one, and distinctions with regard to the essential
nature of heredity based on these circumstances become irrelevant.
In the case of a population presenting continuous variation in
regard to say, stature, it is easy to see how purity of the gametes
in respect of any intensities of that character might not in
ordinary circumstances be capable of detection. There are doubtless
more than two pure gametic forms of this character, but there may
quite conceivably be six or eight. When it is remembered that each
heterozygous combination of any two may have its own appropriate
stature, and that such a character is distinctly dependent on external
conditions, the mere fact that the observed curves of stature give
“chance distributions” is not surprising and may still be compatible
with purity of gametes in respect of certain pure types. In peas (_P.
sativum_), for example, from Mendel’s work we know that the tall forms
and the extreme dwarf forms exhibit gametic purity. I have seen at
Messrs Sutton’s strong evidence of the same nature in the case of
the tall Sweet Pea (_Lathyrus odoratus_) and the dwarf or procumbent
“Cupid” form.
But in the case of the Sweet Pea we know at least one pure form of
definitely intermediate height, and in the case of _P. sativum_ there
are many. When the _extreme_ types breed together it will be remembered
the heterozygote commonly exceeds the taller in height. In the next
generation, since there is, in the case of extremes, so much margin
between the types of the two pure forms, the return of the offspring
to the three forms of which two are homozygous and one heterozygous is
clearly perceptible.
If however instead of pure extreme varieties we were to take a pair of
varieties differing normally by only a foot or two, we might, owing
to the masking effects of conditions, &c., have great difficulty in
distinguishing the three forms in the second generation. There would
besides be twice as many heterozygous individuals as homozygous
individuals of each kind, giving a symmetrical distribution of
heights, and who might not--in pre-Mendelian days--have accepted such
evidence--made still less clear by influence of conditions--as proof of
Continuous Variation both of zygotes and gametes?
Suppose, then, that instead of two pure types, we had six or eight
breeding together, each pair forming their own heterozygote, there
would be a very remote chance of such purity or fixity of type whether
of gamete or zygote being detected.
_Dominance_, as we have seen, is merely a phenomenon incidental to
specific cases, between which no other common property has yet been
perceived. In the phenomena of _blended_ inheritance we clearly have no
dominance. In the cases of _alternative_ inheritance studied by Galton
and Pearson there is evidently no _universal_ dominance. From the
tables of Basset hound pedigrees there is clearly no definite dominance
of either of the coat-colours. In the case of eye-colour the published
tables do not, so far as I have discovered, furnish the material for a
decision, though it is scarcely possible the phenomenon, even if only
occasional, could have been overlooked. We must take it, then, there is
no sensible dominance in these cases; but whether there is or is not
sensible gametic purity is an altogether different question, which,
so far as I can judge, is as yet untouched. It may perfectly well be
that we shall be compelled to recognize that in many cases there is no
such purity, and that the characters may be carried by the gametes
in any proportion from zero to totality, just as some substances may
be carried in a solution in any proportion from zero to saturation
without discontinuous change of properties. That this will be found
true in _some_ cases is, on any hypothesis, certain; but to prove the
fact for any given case will be an exceedingly difficult operation, and
I scarcely think it has been yet carried through in such a way as to
leave no room for doubt.
Conversely, the _absolute_ and _universal_ purity of the gametes has
certainly not yet been determined for any case; not even in those
cases where it looks most likely that such universal purity exists.
Impairment of such purity we may conceive either to occur in the form
of mosaic gametes, or of gametes with blended properties. On analogy
and from direct evidence we have every right to believe that gametes
of both these classes may occur in rare and exceptional cases, of as
yet unexplored nature[21], but such a phenomenon will not diminish the
significance of observed purity.
[21] It will be understood from what follows, that the existence of
mosaic zygotes is no _proof_ that either component gamete was mosaic.
* * * * *
We have now seen the essential nature of the Mendelian principles and
are able to appreciate the exact relation in which they stand to the
group of cases included in the Law of Ancestral Heredity. In seeking
any general indication as to the common properties of the phenomena
which are already known to obey Mendelian principles we can as yet
point to none, and whether some such common features exist or not is
unknown.
* * * * *
There is however one group of cases, definite though as yet not
numerous, where we know that the Mendelian principles do not apply.
These are the phenomena upon which Mendel touches in his brief paper
on _Hieracium_. As he there states, the hybrids, if they are fertile
at all, produce offspring like themselves, not like their parents. In
further illustration of this phenomenon he cites Wichura’s _Salix_
hybrids. Perhaps some dozen other such illustrations could be given
which rest on good evidence. To these cases the Mendelian principle
will in nowise apply, nor is it easy to conceive any modification of
the law of ancestral heredity which can express them. There the matter
at present rests. Among these cases, however, we perceive several more
or less common features. They are often, though not always, hybrids
between forms differing in many characters. The first cross frequently
is not the exact intermediate between the two parental types, but
may as in the few _Hieracium_ cases be irregular in this respect.
There is often some degree of sterility. In the absence of fuller and
statistical knowledge of such cases further discussion is impossible.
* * * * *
Another class of cases, untouched by any hypothesis of heredity yet
propounded, is that of the false hybrids of Millardet, where we
have fertilisation without transmission of one or several parental
characters. In these not only does the first cross show, in some
respect, the character or characters of _one parent only_, but in
its posterity _no reappearance of the lost character or characters
is observed_. The nature of such cases is still quite obscure, but
we have to suppose that the allelomorph of one gamete only developes
after fertilisation to the exclusion of the corresponding allelomorph
of the other gamete, much--if the crudity of the comparison may be
pardoned--as occurs on the female side in parthenogenesis without
fertilisation at all.
To these as yet altogether unconformable cases we can scarcely doubt
that further experiment will add many more. Indeed we already have
tolerably clear evidence that many phenomena of inheritance are of a
much higher order of complexity. When the paper on _Pisum_ was written
Mendel apparently inclined to the view that with modifications his
law might be found to include all the phenomena of hybridisation, but
in the brief subsequent paper on _Hieracium_ he clearly recognized
the existence of cases of a different nature. Those who read that
contribution will be interested to see that he lays down a principle
which may be extended from hybridisation to heredity in general, that
the laws of each new case must be determined by separate experiment.
* * * * *
As regards the Mendelian principles, which it is the chief aim of
this introduction to present clearly before the reader, a professed
student of variation will easily be able to fill in the outline now
indicated, and to illustrate the various conceptions from phenomena
already familiar. To do this is beyond the scope of this short sketch.
But enough perhaps has now been said to show that by the application of
those principles we are enabled to reach and deal in a comprehensive
manner with phenomena of a fundamental nature, lying at the very root
of all conceptions not merely of the physiology of reproduction and
heredity, but even of the essential nature of living organisms; and I
think that I used no extravagant words when, in introducing Mendel’s
work to the notice of readers of the Royal Horticultural Society’s
Journal, I ventured to declare that his experiments are worthy to rank
with those which laid the foundation of the Atomic laws of Chemistry.
As some biographical particulars of this remarkable investigator will
be welcome, I give the following brief notice, first published by Dr
Correns on the authority of Dr von Schanz: Gregor Johann Mendel was
born on July 22, 1822, at Heinzendorf bei Odrau, in Austrian Silesia.
He was the son of well-to-do peasants. In 1843 he entered as a novice
the “Königinkloster,” an Augustinian foundation in Altbrünn. In 1847 he
was ordained priest. From 1851 to 1853 he studied physics and natural
science at Vienna. Thence he returned to his cloister and became a
teacher in the Realschule at Brünn. Subsequently he was made Abbot,
and died January 6, 1884. The experiments described in his papers were
carried out in the garden of his Cloister. Besides the two papers on
hybridisation, dealing respectively with _Pisum_ and _Hieracium_,
Mendel contributed two brief notes to the _Verh. Zool. bot. Verein_,
Wien, on _Scopolia margaritalis_ (1853, III., p. 116) and on _Bruchus
pisi_ (_ibid._ 1854, IV., p. 27). In these papers he speaks of himself
as a pupil of Kollar.
Mendel published in the Brünn journal statistical observations of a
meteorological character, but, so far as I am aware, no others relating
to natural history. Dr Correns tells me that in the latter part of his
life he engaged in the Ultramontane Controversy. He was for a time
President of the Brünn Society[22].
[22] A few additional particulars are given in Tschermak’s edition.
For the photograph of Mendel which forms the frontispiece to this work,
I am indebted to the Very Rev. Dr Janeischek, the present Abbot of
Brünn, who most kindly supplied it for this purpose.
So far as I have discovered there was, up to 1900, only one reference
to Mendel’s observations in scientific literature, namely that of
Focke, _Pflanzenmischlinge_, 1881, p. 109, where it is simply stated
that Mendel’s numerous experiments on _Pisum_ gave results similar to
those obtained by Knight, but that he believed he had found constant
numerical ratios among the types produced by hybridisation. In the same
work a similar brief reference is made to the paper on _Hieracium_.
It may seem surprising that a work of such importance should so long
have failed to find recognition and to become current in the world of
science. It is true that the journal in which it appeared is scarce,
but this circumstance has seldom long delayed general recognition. The
cause is unquestionably to be found in that neglect of the experimental
study of the problem of Species which supervened on the general
acceptance of the Darwinian doctrines. The problem of Species, as
Kölreuter, Gärtner, Naudin, Wichura, and the other hybridists of the
middle of the nineteenth century conceived it, attracted thenceforth
no workers. The question, it was imagined, had been answered and the
debate ended. No one felt much interest in the matter. A host of other
lines of work were suddenly opened up, and in 1865 the more original
investigators naturally found those new methods of research more
attractive than the tedious observations of the hybridisers, whose
inquiries were supposed, moreover, to have led to no definite result.
Nevertheless the total neglect of such a discovery is not easy to
account for. Those who are acquainted with the literature of this
branch of inquiry will know that the French Academy offered a prize
in 1861 to be awarded in 1862 on the subject “_Étudier les Hybrides
végétaux au point de vue de leur fécondité et de la perpétuité de
leurs caractères_.” This subject was doubtless chosen with reference
to the experiments of Godron of Nancy and Naudin, then of Paris. Both
these naturalists competed, and the accounts of the work of Godron on
_Datura_ and of Naudin on a number of species were published in the
years 1864 and 1865 respectively. Both, especially the latter, are
works of high consequence in the history of the science of heredity.
In the latter paper Naudin clearly enuntiated what we shall henceforth
know as the Mendelian conception of the dissociation of characters of
cross-breds in the formation of the germ-cells, though apparently he
never developed this conception.
In the year 1864, George Bentham, then President of the Linnean
Society, took these treatises as the subject of his address to the
Anniversary meeting on the 24 May, Naudin’s work being known to him
from an abstract, the full paper having not yet appeared. Referring
to the hypothesis of dissociation which he fully described, he said
that it appeared to be new and well supported, but required much more
confirmation before it could be held as proven. (_J. Linn. Soc., Bot._,
VIII., _Proc._, p. XIV.)
In 1865, the year of Mendel’s communication to the Brünn Society,
appeared Wichura’s famous treatise on his experiments with _Salix_
to which Mendel refers. There are passages in this memoir which come
very near Mendel’s principles, but it is evident from the plan of his
experiments that Mendel had conceived the whole of his ideas before
that date.
In 1868 appeared the first edition of Darwin’s _Animals and Plants_,
marking the very zenith of these studies, and thenceforth the decline
in the experimental investigation of Evolution and the problem of
Species has been steady. With the rediscovery and confirmation of
Mendel’s work by de Vries, Correns and Tschermak in 1900 a new era
begins.
That Mendel’s work, appearing as it did, at a moment when several
naturalists of the first rank were still occupied with these problems,
should have passed wholly unnoticed, will always remain inexplicable,
the more so as the Brünn Society exchanged its publications with most
of the Academies of Europe, including both the Royal and Linnean
Societies.
Naudin’s views were well known to Darwin and are discussed in _Animals
and Plants_ (ed. 1885, II., p. 23); but, put forward as they were
without full proof, they could not command universal credence. Gärtner,
too, had adopted opposite views; and Wichura, working with cases of
another order, had proved the fact that some hybrids breed true.
Consequently it is not to be wondered at that Darwin was sceptical.
Moreover, the Mendelian idea of the “hybrid-character,” or heterozygous
form, was unknown to him, a conception without which the hypothesis of
dissociation of characters is quite imperfect.
Had Mendel’s work come into the hands of Darwin, it is not too much
to say that the history of the development of evolutionary philosophy
would have been very different from that which we have witnessed.
EXPERIMENTS IN PLANT-HYBRIDISATION[23].
By Gregor Mendel.
(_Read at the Meetings of the 8th February and 8th March, 1865._)
[23] [This translation was made by the Royal Horticultural Society,
and is reprinted with modifications and corrections, by permission.
The original paper was published in the _Verh. naturf. Ver. in Brünn,
Abhandlungen_, IV. 1865, which appeared in 1866.]
INTRODUCTORY REMARKS.
Experience of artificial fertilisation, such as is effected with
ornamental plants in order to obtain new variations in colour, has
led to the experiments which will here be discussed. The striking
regularity with which the same hybrid forms always reappeared whenever
fertilisation took place between the same species induced further
experiments to be undertaken, the object of which was to follow up the
developments of the hybrids in their progeny.
To this object numerous careful observers, such as Kölreuter, Gärtner,
Herbert, Lecoq, Wichura and others, have devoted a part of their lives
with inexhaustible perseverance. Gärtner especially, in his work “Die
Bastarderzeugung im Pflanzenreiche” (The Production of Hybrids in the
Vegetable Kingdom), has recorded very valuable observations; and quite
recently Wichura published the results of some profound investigations
into the hybrids of the Willow. That, so far, no generally applicable
law governing the formation and development of hybrids has been
successfully formulated can hardly be wondered at by anyone who
is acquainted with the extent of the task, and can appreciate the
difficulties with which experiments of this class have to contend. A
final decision can only be arrived at when we shall have before us the
results of detailed experiments made on plants belonging to the most
diverse orders.
Those who survey the work done in this department will arrive at the
conviction that among all the numerous experiments made, not one has
been carried out to such an extent and in such a way as to make it
possible to determine the number of different forms under which the
offspring of hybrids appear, or to arrange these forms with certainty
according to their separate generations, or to definitely ascertain
their statistical relations[24].
[24] [It is to the clear conception of these three primary
necessities that the whole success of Mendel’s work is due. So far as
I know this conception was absolutely new in his day.]
It requires indeed some courage to undertake a labour of such
far-reaching extent; it appears, however, to be the only right way by
which we can finally reach the solution of a question the importance of
which cannot be over-estimated in connection with the history of the
evolution of organic forms.
The paper now presented records the results of such a detailed
experiment. This experiment was practically confined to a small plant
group, and is now, after eight years’ pursuit, concluded in all
essentials. Whether the plan upon which the separate experiments were
conducted and carried out was the best suited to attain the desired end
is left to the friendly decision of the reader.
SELECTION OF THE EXPERIMENTAL PLANTS.
The value and utility of any experiment are determined by the fitness
of the material to the purpose for which it is used, and thus in the
case before us it cannot be immaterial what plants are subjected to
experiment and in what manner such experiments are conducted.
The selection of the plant group which shall serve for experiments of
this kind must be made with all possible care if it be desired to avoid
from the outset every risk of questionable results.
The experimental plants must necessarily--
1. Possess constant differentiating characters.
2. The hybrids of such plants must, during the flowering period, be
protected from the influence of all foreign pollen, or be easily
capable of such protection.
The hybrids and their offspring should suffer no marked disturbance in
their fertility in the successive generations.
Accidental impregnation by foreign pollen, if it occurred during the
experiments and were not recognized, would lead to entirely erroneous
conclusions. Reduced fertility or entire sterility of certain forms,
such as occurs in the offspring of many hybrids, would render the
experiments very difficult or entirely frustrate them. In order to
discover the relations in which the hybrid forms stand towards each
other and also towards their progenitors it appears to be necessary
that all members of the series developed in each successive generation
should be, _without exception_, subjected to observation.
At the very outset special attention was devoted to the _Leguminosæ_
on account of their peculiar floral structure. Experiments which were
made with several members of this family led to the result that the
genus _Pisum_ was found to possess the necessary conditions.
Some thoroughly distinct forms of this genus possess characters which
are constant, and easily and certainly recognisable, and when their
hybrids are mutually crossed they yield perfectly fertile progeny.
Furthermore, a disturbance through foreign pollen cannot easily occur,
since the fertilising organs are closely packed inside the keel and
the anther bursts within the bud, so that the stigma becomes covered
with pollen even before the flower opens. This circumstance is of
especial importance. As additional advantages worth mentioning, there
may be cited the easy culture of these plants in the open ground and
in pots, and also their relatively short period of growth. Artificial
fertilisation is certainly a somewhat elaborate process, but nearly
always succeeds. For this purpose the bud is opened before it is
perfectly developed, the keel is removed, and each stamen carefully
extracted by means of forceps, after which the stigma can at once be
dusted over with the foreign pollen.
In all, thirty-four more or less distinct varieties of Peas were
obtained from several seedsmen and subjected to a two years’ trial. In
the case of one variety there were remarked, among a larger number of
plants all alike, a few forms which were markedly different. These,
however, did not vary in the following year, and agreed entirely
with another variety obtained from the same seedsmen; the seeds
were therefore doubtless merely accidentally mixed. All the other
varieties yielded perfectly constant and similar offspring; at any
rate, no essential difference was observed during two trial years. For
fertilisation twenty-two of these were selected and cultivated during
the whole period of the experiments. They remained constant without
any exception.
Their systematic classification is difficult and uncertain. If we
adopt the strictest definition of a species, according to which only
those individuals belong to a species which under precisely the same
circumstances display precisely similar characters, no two of these
varieties could be referred to one species. According to the opinion of
experts, however, the majority belong to the species _Pisum sativum_;
while the rest are regarded and classed, some as sub-species of _P.
sativum_, and some as independent species, such as _P. quadratum_, _P.
saccharatum_, and _P. umbellatum_. The positions, however, which may be
assigned to them in a classificatory system are quite immaterial for
the purposes of the experiments in question. It has so far been found
to be just as impossible to draw a sharp line between the hybrids of
species and varieties as between species and varieties themselves.
DIVISION AND ARRANGEMENT OF THE EXPERIMENTS.
If two plants which differ constantly in one or several characters
be crossed, numerous experiments have demonstrated that the common
characters are transmitted unchanged to the hybrids and their progeny;
but each pair of differentiating characters, on the other hand,
unite in the hybrid to form a new character, which in the progeny of
the hybrid is usually variable. The object of the experiment was to
observe these variations in the case of each pair of differentiating
characters, and to deduce the law according to which they appear in
the successive generations. The experiment resolves itself therefore
into just as many separate experiments as there are constantly
differentiating characters presented in the experimental plants.
The various forms of Peas selected for crossing showed differences in
the length and colour of the stem; in the size and form of the leaves;
in the position, colour, and size of the flowers; in the length of
the flower stalk; in the colour, form, and size of the pods; in the
form and size of the seeds; and in the colour of the seed-coats and
the albumen [cotyledons]. Some of the characters noted do not permit
of a sharp and certain separation, since the difference is of a “more
or less” nature, which is often difficult to define. Such characters
could not be utilised for the separate experiments; these could only be
confined to characters which stand out clearly and definitely in the
plants. Lastly, the result must show whether they, in their entirety,
observe a regular behaviour in their hybrid unions, and whether from
these facts any conclusion can be come to regarding those characters
which possess a subordinate significance in the type.
The characters which were selected for experiment relate:
1. To the _difference in the form of the ripe seeds_. These are either
round or roundish, the wrinkling, when such occurs on the surface,
being always only shallow; or they are irregularly angular and deeply
wrinkled (_P. quadratum_).
2. To the _difference in the colour of the seed albumen_
(endosperm)[25]. The albumen of the ripe seeds is either pale yellow,
bright yellow and orange coloured, or it possesses a more or less
intense green tint. This difference of colour is easily seen in the
seeds as their coats are transparent.
[25] [Mendel uses the terms “albumen” and “endosperm” somewhat
loosely to denote the cotyledons, containing food-material, within
the seed.]
3. To the _difference in the colour of the seed-coat_. This is either
white, with which character white flowers are constantly correlated; or
it is grey, grey-brown, leather-brown, with or without violet spotting,
in which case the colour of the standards is violet, that of the wings
purple, and the stem in the axils of the leaves is of a reddish tint.
The grey seed-coats become dark brown in boiling water.
4. To the _difference in the form of the ripe pods_. These are
either simply inflated, never contracted in places; or they are
deeply constricted between the seeds and more or less wrinkled (_P.
saccharatum_).
5. To the _difference in the colour of the unripe pods_. They are
either light to dark green, or vividly yellow, in which colouring the
stalks, leaf-veins, and calyx participate[26].
[26] One species possesses a beautifully brownish-red coloured pod,
which when ripening turns to violet and blue. Trials with this
character were only begun last year. [Of these further experiments it
seems no account was published. Correns has since worked with such a
variety.]
6. To the _difference in the position of the flowers_. They are either
axial, that is, distributed along the main stem; or they are terminal,
that is, bunched at the top of the stem and arranged almost in a false
umbel; in this case the upper part of the stem is more or less widened
in section (_P. umbellatum_)[27].
[27] [This is often called the Mummy Pea. It shows slight fasciation.
The form I know has white standard and salmon-red wings.]
7. To the _difference in the length of the stem_. The length of the
stem[28] is very various in some forms; it is, however, a constant
character for each, in so far that healthy plants, grown in the same
soil, are only subject to unimportant variations in this character.
[28] [In my account of these experiments (_R.H.S. Journal_, vol. XXV.
p. 54) I misunderstood this paragraph and took “axis” to mean the
_floral_ axis, instead of the main axis of the plant. The unit
of measurement, being indicated in the original by a dash (′), I
carelessly took to have been an _inch_, but the translation here
given is evidently correct.]
In experiments with this character, in order to be able to discriminate
with certainty, the long axis of 6–7 ft. was always crossed with the
short one of 3/4 ft. to 1-1/2 ft.
Each two of the differentiating characters enumerated above were united
by cross-fertilisation. There were made for the
1st trial 60 fertilisations on 15 plants.
2nd " 58 " " 10 "
3rd " 35 " " 10 "
4th " 40 " " 10 "
5th " 23 " " 5 "
6th " 34 " " 10 "
7th " 37 " " 10 "
From a larger number of plants of the same variety only the most
vigorous were chosen for fertilisation. Weakly plants always afford
uncertain results, because even in the first generation of hybrids,
and still more so in the subsequent ones, many of the offspring either
entirely fail to flower or only form a few and inferior seeds.
Furthermore, in all the experiments reciprocal crossings were effected
in such a way that each of the two varieties which in one set of
fertilisations served as seed-bearers in the other set were used as
pollen plants.
The plants were grown in garden beds, a few also in pots, and were
maintained in their naturally upright position by means of sticks,
branches of trees, and strings stretched between. For each experiment
a number of pot plants were placed during the blooming period in a
greenhouse, to serve as control plants for the main experiment in the
open as regards possible disturbance by insects. Among the insects[29]
which visit Peas the beetle _Bruchus pisi_ might be detrimental to the
experiments should it appear in numbers. The female of this species is
known to lay the eggs in the flower, and in so doing opens the keel;
upon the tarsi of one specimen, which was caught in a flower, some
pollen grains could clearly be seen under a lens. Mention must also be
made of a circumstance which possibly might lead to the introduction
of foreign pollen. It occurs, for instance, in some rare cases that
certain parts of an otherwise quite normally developed flower wither,
resulting in a partial exposure of the fertilising organs. A defective
development of the keel has also been observed, owing to which the
stigma and anthers remained partially uncovered[30]. It also sometimes
happens that the pollen does not reach full perfection. In this event
there occurs a gradual lengthening of the pistil during the blooming
period, until the stigmatic tip protrudes at the point of the keel.
This remarkable appearance has also been observed in hybrids of
_Phaseolus_ and _Lathyrus_.
[29] [It is somewhat surprising that no mention is made of Thrips,
which swarm in Pea flowers. I had come to the conclusion that this is
a real source of error and I see Laxton held the same opinion.]
[30] [This also happens in Sweet Peas.]
The risk of false impregnation by foreign pollen is, however, a very
slight one with _Pisum_, and is quite incapable of disturbing the
general result. Among more than 10,000 plants which were carefully
examined there were only a very few cases where an indubitable false
impregnation had occurred. Since in the greenhouse such a case was
never remarked, it may well be supposed that _Bruchus pisi_, and
possibly also the described abnormalities in the floral structure, were
to blame.
THE FORMS OF THE HYBRIDS.[31]
[31] [Mendel throughout speaks of his cross-bred Peas as “hybrids,” a
term which many restrict to the offspring of two distinct _species_.
He, as he explains, held this to be only a question of degree.]
Experiments which in previous years were made with ornamental plants
have already afforded evidence that the hybrids, as a rule, are not
exactly intermediate between the parental species. With some of the
more striking characters, those, for instance, which relate to the form
and size of the leaves, the pubescence of the several parts, &c., the
intermediate, indeed, was nearly always to be seen; in other cases,
however, one of the two parental characters was so preponderant that it
was difficult, or quite impossible, to detect the other in the hybrid.
This is precisely the case with the Pea hybrids. In the case of each
of the seven crosses the hybrid-character resembles[32] that of one of
the parental forms so closely that the other either escapes observation
completely or cannot be detected with certainty. This circumstance is
of great importance in the determination and classification of the
forms under which the offspring of the hybrids appear. Henceforth in
this paper those characters which are transmitted entire, or almost
unchanged in the hybridisation, and therefore in themselves constitute
the characters of the hybrid, are termed the _dominant_, and those
which become latent in the process _recessive_. The expression
“recessive” has been chosen because the characters thereby designated
withdraw or entirely disappear in the hybrids, but nevertheless
reappear unchanged in their progeny, as will be demonstrated later on.
[32] [Note that Mendel, with true penetration, avoids speaking of the
hybrid-character as “transmitted” by either parent, thus escaping the
error pervading modern views of heredity.]
It was furthermore shown by the whole of the experiments that it is
perfectly immaterial whether the dominant character belong to the
seed-bearer or to the pollen parent; the form of the hybrid remains
identical in both cases. This interesting fact was also emphasised by
Gärtner, with the remark that even the most practised expert is not in
a position to determine in a hybrid which of the two parental species
was the seed or the pollen plant[33].
[33] [Gärtner, p. 223.]
Of the differentiating characters which were used in the experiments
the following are dominant:
1. The round or roundish form of the seed with or without shallow
depressions.
2. The yellow colouring of the seed albumen [cotyledons].
3. The grey, grey-brown, or leather-brown colour of the seed-coat, in
connection with violet-red blossoms and reddish spots in the leaf axils.
4. The simply inflated form of the pod.
5. The green colouring of the unripe pod in connection with the same
colour in the stems, the leaf-veins and the calyx.
6. The distribution of the flowers along the stem.
7. The greater length of stem.
With regard to this last character it must be stated that the longer
of the two parental stems is usually exceeded by the hybrid, which is
possibly only attributable to the greater luxuriance which appears in
all parts of plants when stems of very different length are crossed.
Thus, for instance, in repeated experiments, stems of 1 ft. and 6 ft.
in length yielded without exception hybrids which varied in length
between 6 ft. and 7-1/2 ft.
The hybrid seeds in the experiments with seed-coat are often more
spotted, and the spots sometimes coalesce into small bluish-violet
patches. The spotting also frequently appears even when it is absent as
a parental character.
The hybrid forms of the seed-shape and of the albumen are developed
immediately after the artificial fertilisation by the mere influence of
the foreign pollen. They can, therefore, be observed even in the first
year of experiment, whilst all the other characters naturally only
appear in the following year in such plants as have been raised from
the crossed seed.
THE FIRST GENERATION [BRED] FROM THE HYBRIDS.
In this generation there reappear, together with the dominant
characters, also the recessive ones with their full peculiarities,
and this occurs in the definitely expressed average proportion of
three to one, so that among each four plants of this generation three
display the dominant character and one the recessive. This relates
without exception to all the characters which were embraced in the
experiments. The angular wrinkled form of the seed, the green colour of
the albumen, the white colour of the seed-coats and the flowers, the
constrictions of the pods, the yellow colour of the unripe pod, of the
stalk of the calyx, and of the leaf venation, the umbel-like form of
the inflorescence, and the dwarfed stem, all reappear in the numerical
proportion given without any essential alteration. _Transitional forms
were not observed in any experiment._
Once the hybrids resulting from reciprocal crosses are fully
formed, they present no appreciable difference in their subsequent
development, and consequently the results [of the reciprocal crosses]
can be reckoned together in each experiment. The relative numbers
which were obtained for each pair of differentiating characters are as
follows:
Expt. 1. Form of seed.--From 253 hybrids 7,324 seeds were obtained in
the second trial year. Among them were 5,474 round or roundish ones
and 1,850 angular wrinkled ones. Therefrom the ratio 2·96 to 1 is
deduced.
Expt. 2. Colour of albumen.--258 plants yielded 8,023 seeds, 6,022
yellow, and 2,001 green; their ratio, therefore, is as 3·01 to 1.
In these two experiments each pod yielded usually both kinds of seed.
In well-developed pods which contained on the average six to nine
seeds, it often occurred that all the seeds were round (Expt. 1) or
all yellow (Expt. 2); on the other hand there were never observed more
than five angular or five green ones in one pod. It appears to make no
difference whether the pods are developed early or later in the hybrid
or whether they spring from the main axis or from a lateral one. In
some few plants only a few seeds developed in the first formed pods,
and these possessed exclusively one of the two characters, but in the
subsequently developed pods the normal proportions were maintained
nevertheless.
As in separate pods, so did the distribution of the characters vary in
separate plants. By way of illustration the first ten individuals from
both series of experiments may serve[34].
[34] [It is much to be regretted that Mendel does not give the
complete series individually. No one who repeats such experiments
should fail to record the _individual_ numbers, which on seriation
are sure to be full of interest.]
Experiment 1. Experiment 2.
Form of Seed. Colour of Albumen.
Plants. Round. Angular. Yellow. Green.
1 45 12 25 11
2 27 8 32 7
3 24 7 14 5
4 19 10 70 27
5 32 11 24 13
6 26 6 20 6
7 88 24 32 13
8 22 10 44 9
9 28 6 50 14
10 25 7 44 18
As extremes in the distribution of the two seed characters in one
plant, there were observed in Expt. 1 an instance of 43 round and only
2 angular, and another of 14 round and 15 angular seeds. In Expt. 2
there was a case of 32 yellow and only 1 green seed, but also one of 20
yellow and 19 green.
These two experiments are important for the determination of the
average ratios, because with a smaller number of experimental plants
they show that very considerable fluctuations may occur. In counting
the seeds, also, especially in Expt. 2, some care is requisite, since
in some of the seeds of many plants the green colour of the albumen is
less developed, and at first may be easily overlooked. The cause of the
partial disappearance of the green colouring has no connection with the
hybrid-character of the plants, as it likewise occurs in the parental
variety. This peculiarity is also confined to the individual and is
not inherited by the offspring. In luxuriant plants this appearance
was frequently noted. Seeds which are damaged by insects during their
development often vary in colour and form, but, with a little practice
in sorting, errors are easily avoided. It is almost superfluous
to mention that the pods must remain on the plants until they are
thoroughly ripened and have become dried, since it is only then that
the shape and colour of the seed are fully developed.
Expt. 3. Colour of the seed-coats.--Among 929 plants 705 bore
violet-red flowers and grey-brown seed-coats; 224 had white flowers
and white seed-coats, giving the proportion 3·15 to 1.
Expt. 4. Form of pods.--Of 1,181 plants 882 had them simply inflated,
and in 299 they were constricted. Resulting ratio, 2·95 to 1.
Expt. 5. Colour of the unripe pods.--The number of trial plants was
580, of which 428 had green pods and 152 yellow ones. Consequently
these stand in the ratio 2·82 to 1.
Expt. 6. Position of flowers.--Among 858 cases 651 blossoms were
axial and 207 terminal. Ratio, 3·14 to 1.
Expt. 7. Length of stem.--Out of 1,064 plants, in 787 cases the
stem was long, and in 277 short. Hence a mutual ratio of 2·84 to
1. In this experiment the dwarfed plants were carefully lifted and
transferred to a special bed. This precaution was necessary, as
otherwise they would have perished through being overgrown by their
tall relatives. Even in their quite young state they can be easily
picked out by their compact growth and thick dark-green foliage.
If now the results of the whole of the experiments be brought together,
there is found, as between the number of forms with the dominant and
recessive characters, an average ratio of 2·98 to 1, or 3 to 1.
The dominant character can have here a _double signification_--viz.
that of a parental-character, or a hybrid-character[35]. In which
of the two significations it appears in each separate case can only
be determined by the following generation. As a parental character
it must pass over unchanged to the whole of the offspring; as a
hybrid-character, on the other hand, it must observe the same behaviour
as in the first generation.
[35] [This paragraph presents the view of the hybrid-character as
something incidental to the hybrid, and not “transmitted” to it--a
true and fundamental conception here expressed probably for the first
time.]
THE SECOND GENERATION [BRED] FROM THE HYBRIDS.
Those forms which in the first generation maintain the recessive
character do not further vary in the second generation as regards this
character; they remain constant in their offspring.
It is otherwise with those which possess the dominant character in
the first generation [bred from the hybrids]. Of these _two_-thirds
yield offspring which display the dominant and recessive characters
in the proportion of 3 to 1, and thereby show exactly the same ratio
as the hybrid forms, while only _one_-third remains with the dominant
character constant.
The separate experiments yielded the following results:--
Expt. 1.--Among 565 plants which were raised from round seeds of
the first generation, 193 yielded round seeds only, and remained
therefore constant in this character; 372, however, gave both round
and angular seeds, in the proportion of 3 to 1. The number of the
hybrids, therefore, as compared with the constants is 1·93 to 1.
Expt. 2.--Of 519 plants which were raised from seeds whose albumen
was of yellow colour in the first generation, 166 yielded exclusively
yellow, while 353 yielded yellow and green seeds in the proportion
of 3 to 1. There resulted, therefore, a division into hybrid and
constant forms in the proportion of 2·13 to 1.
For each separate trial in the following experiments 100 plants
were selected which displayed the dominant character in the first
generation, and in order to ascertain the significance of this, ten
seeds of each were cultivated.
Expt. 3.--The offspring of 36 plants yielded exclusively grey-brown
seed-coats, while of the offspring of 64 plants some had grey-brown
and some had white.
Expt. 4.--The offspring of 29 plants had only simply inflated pods;
of the offspring of 71, on the other hand, some had inflated and some
constricted.
Expt. 5.--The offspring of 40 plants had only green pods; of the
offspring of 60 plants some had green, some yellow ones.
Expt. 6.--The offspring of 33 plants had only axial flowers; of the
offspring of 67, on the other hand, some had axial and some terminal
flowers.
Expt. 7.--The offspring of 28 plants inherited the long axis, and
those of 72 plants some the long and some the short axis.
In each of these experiments a certain number of the plants came
constant with the dominant character. For the determination of the
proportion in which the separation of the forms with the constantly
persistent character results, the two first experiments are of especial
importance, since in these a larger number of plants can be compared.
The ratios 1·93 to 1 and 2·13 to 1 gave together almost exactly the
average ratio of 2 to 1. The sixth experiment has a quite concordant
result; in the others the ratio varies more or less, as was only to be
expected in view of the smaller number of 100 trial plants. Experiment
5, which shows the greatest departure, was repeated, and then in lieu
of the ratio of 60 and 40 that of 65 and 35 resulted. _The average
ratio of 2 to 1 appears, therefore, as fixed with certainty._ It is
therefore demonstrated that, of those forms which possess the dominant
character in the first generation, in two-thirds the hybrid character
is embodied, while one-third remains constant with the dominant
character.
The ratio of 3 to 1, in accordance with which the distribution of the
dominant and recessive characters results in the first generation,
resolves itself therefore in all experiments into the ratio of 2 :
1 : 1 if the dominant character be differentiated according to its
significance as a hybrid character or a parental one. Since the members
of the first generation spring directly from the seed of the hybrids,
_it is now clear that the hybrids form seeds having one or other of the
two differentiating characters, and of these one-half develop again the
hybrid form, while the other half yield plants which remain constant
and receive the dominant or recessive characters [respectively] in
equal numbers_.
THE SUBSEQUENT GENERATIONS [BRED] FROM THE HYBRIDS.
The proportions in which the descendants of the hybrids develop and
split up in the first and second generations presumably hold good for
all subsequent progeny. Experiments 1 and 2 have already been carried
through six generations, 3 and 7 through five, and 4, 5, and 6 through
four, these experiments being continued from the third generation with
a small number of plants, and no departure from the rule has been
perceptible. The offspring of the hybrids separated in each generation
in the ratio of 2 : 1 : 1 into hybrids and constant forms.
If _A_ be taken as denoting one of the two constant characters, for
instance the dominant, _a_, the recessive, and _Aa_ the hybrid form in
which both are conjoined, the expression
_A_ + 2_Aa_ + _a_
shows the terms in the series for the progeny of the hybrids of two
differentiating characters.
The observation made by Gärtner, Kölreuter, and others, that hybrids
are inclined to revert to the parental forms, is also confirmed by the
experiments described. It is seen that the number of the hybrids which
arise from one fertilisation, as compared with the number of forms
which become constant, and their progeny from generation to generation,
is continually diminishing, but that nevertheless they could not
entirely disappear. If an average equality of fertility in all plants
in all generations be assumed, and if, furthermore, each hybrid forms
seed of which one-half yields hybrids again, while the other half is
constant to both characters in equal proportions, the ratio of numbers
for the offspring in each generation is seen by the following summary,
in which _A_ and _a_ denote again the two parental characters, and _Aa_
the hybrid forms. For brevity’s sake it may be assumed that each plant
in each generation furnishes only 4 seeds.
Ratios.
Generation _A_ _Aa_ _a_ _A_ :_Aa_ : _a_
1 1 2 1 1 : 2 : 1
2 6 4 6 3 : 2 : 3
3 28 8 28 7 : 2 : 7
4 120 16 120 15 : 2 : 15
5 496 32 496 31 : 2 : 31
_n_ 2^{_n_}-1 : 2 : 2^{_n_}-1
In the tenth generation, for instance, 2^{_n_}-1 = 1023. There result,
therefore, in each 2,048 plants which arise in this generation 1,023
with the constant dominant character, 1,023 with the recessive
character, and only two hybrids.
THE OFFSPRING OF HYBRIDS IN WHICH SEVERAL DIFFERENTIATING CHARACTERS
ARE ASSOCIATED.
In the experiments above described plants were used which differed only
in one essential character[36]. The next task consisted in ascertaining
whether the law of development discovered in these applied to each
pair of differentiating characters when several diverse characters are
united in the hybrid by crossing. As regards the form of the hybrids
in these cases, the experiments showed throughout that this invariably
more nearly approaches to that one of the two parental plants which
possesses the greater number of dominant characters. If, for instance,
the seed plant has a short stem, terminal white flowers, and simply
inflated pods; the pollen plant, on the other hand, a long stem,
violet-red flowers distributed along the stem, and constricted pods;
the hybrid resembles the seed parent only in the form of the pod; in
the other characters it agrees with the pollen parent. Should one of
the two parental types possess only dominant characters, then the
hybrid is scarcely or not at all distinguishable from it.
[36] [This statement of Mendel’s in the light of present knowledge
is open to some misconception. Though his work makes it evident that
such varieties may exist, it is very unlikely that Mendel could
have had seven pairs of varieties such that the members of each
pair differed from each other in _only_ one considerable character
(_wesentliches Merkmal_). The point is probably of little theoretical
or practical consequence, but a rather heavy stress is thrown on
“_wesentlich_.”]
Two experiments were made with a larger number of plants. In the first
experiment the parental plants differed in the form of the seed and
in the colour of the albumen; in the second in the form of the seed,
in the colour of the albumen, and in the colour of the seed-coats.
Experiments with seed characters give the result in the simplest and
most certain way.
In order to facilitate study of the data in these experiments, the
different characters of the seed plant will be indicated by _A_, _B_,
_C_, those of the pollen plant by _a_, _b_, _c_, and the hybrid forms
of the characters by _Aa_, _Bb_, and _Cc_.
Expt. 1.--_AB_, seed parents; _ab_, pollen parents;
_A_, form round; _a_, form angular;
_B_, albumen yellow. _b_, albumen green.
The fertilised seeds appeared round and yellow like those of the seed
parents. The plants raised therefrom yielded seeds of four sorts, which
frequently presented themselves in one pod. In all 556 seeds were
yielded by 15 plants, and of these there were:--
315 round and yellow,
101 angular and yellow,
108 round and green,
32 angular and green.
All were sown the following year. Eleven of the round yellow seeds did
not yield plants, and three plants did not form seeds. Among the rest:
38 had round yellow seeds _AB_
65 round yellow and green seeds _ABb_
60 round yellow and angular yellow seeds _AaB_
138 round yellow and green, angular yellow
and green seeds _AaBb_.
From the angular yellow seeds 96 resulting plants bore seed, of which:
28 had only angular yellow seeds _aB_
68 angular yellow and green seeds _aBb_.
From 108 round green seeds 102 resulting plants fruited, of which:
35 had only round green seeds _Ab_
67 round and angular green seeds _Aab_.
The angular green seeds yielded 30 plants which bore seeds all of like
character; they remained constant _ab_.
The offspring of the hybrids appeared therefore under nine different
forms, some of them in very unequal numbers. When these are collected
and co-ordinated we find:
38 plants with the sign _AB_
35 " " " _Ab_
28 " " " _aB_
30 " " " _ab_
65 " " " _ABb_
68 " " " _aBb_
60 " " " _AaB_
67 " " " _Aab_
138 " " " _AaBb_.
The whole of the forms may be classed into three essentially different
groups. The first embraces those with the signs _AB_, _Ab_, _aB_, and
_ab_ : they possess only constant characters and do not vary again
in the next generation. Each of these forms is represented on the
average thirty-three times. The second group embraces the signs _ABb_,
_aBb_, _AaB_, _Aab_ : these are constant in one character and hybrid
in another, and vary in the next generation only as regards the hybrid
character. Each of these appears on an average sixty-five times. The
form _AaBb_ occurs 138 times: it is hybrid in both characters, and
behaves exactly as do the hybrids from which it is derived.
If the numbers in which the forms belonging to these classes appear be
compared, the ratios of 1, 2, 4 are unmistakably evident. The numbers
32, 65, 138 present very fair approximations to the ratio numbers of
33, 66, 132.
The developmental series consists, therefore, of nine classes, of which
four appear therein always once and are constant in both characters;
the forms _AB_, _ab_, resemble the parental forms, the two others
present combinations between the conjoined characters _A_, _a_, _B_,
_b_, which combinations are likewise possibly constant. Four classes
appear always twice, and are constant in one character and hybrid
in the other. One class appears four times, and is hybrid in both
characters. Consequently the offspring of the hybrids, if two kinds of
differentiating characters are combined therein, are represented by the
expression
_AB_ + _Ab_ + _aB_ + _ab_ + 2_ABb_ + 2_aBb_ + 2_AaB_ + 2_Aab_ + 4_AaBb_.
This expression is indisputably a combination series in which the two
expressions for the characters _A_ and _a_, _B_ and _b_, are combined.
We arrive at the full number of the classes of the series by the
combination of the expressions:
_A_ + 2_Aa_ + _a_
_B_ + 2_Bb_ + _b_.
Second Expt.
_ABC_, seed parents; _abc_, pollen parents;
_A_, form round; _a_, form angular;
_B_, albumen yellow; _b_, albumen green;
_C_, seed-coat grey-brown. _c_, seed-coat white.
This experiment was made in precisely the same way as the previous
one. Among all the experiments it demanded the most time and trouble.
From 24 hybrids 687 seeds were obtained in all: these were all either
spotted, grey-brown or grey-green, round or angular[37]. From these in
the following year 639 plants fruited, and, as further investigation
showed, there were among them:
8 plants _ABC_. 22 plants _ABCc_. 45 plants _ABbCc_.
14 " _ABc_. 17 " _AbCc_. 36 " _aBbCc_.
9 " _AbC_. 25 " _aBCc_. 38 " _AaBCc_.
11 " _Abc_. 20 " _abCc_. 40 " _AabCc_.
8 " _aBC_. 15 " _ABbC_. 49 " _AabbC_.
10 " _aBc_. 18 " _ABbc_. 48 " _AaBbc_.
10 " _abC_. 19 " _aBbC_.
7 " _abc_. 24 " _aBbc_.
14 " _AaBC_. 78 " _AaBbCc_.
18 " _AaBc_.
20 " _AabC_.
16 " _Aabc_.
[37] [Note that Mendel does not state the cotyledon-colour of the
first crosses in this case; for as the coats were thick, it could not
have been seen without opening or peeling the seeds.]
The whole expression contains 27 terms. Of these 8 are constant in all
characters, and each appears on the average 10 times; 12 are constant
in two characters, and hybrid in the third; each appears on the average
19 times; 6 are constant in one character and hybrid in the other two;
each appears on the average 43 times. One form appears 78 times and is
hybrid in all of the characters. The ratios 10, 19, 43, 78 agree so
closely with the ratios 10, 20, 40, 80, or 1, 2, 4, 8, that this last
undoubtedly represents the true value.
The development of the hybrids when the original parents differ
in three characters results therefore according to the following
expression:
_ABC_ + _ABc_ + _AbC_ + _Abc_ + _aBC_ + _aBc_ + _abC_ + _abc_ +
2 _ABCc_ + 2 _AbCc_ + 2 _aBCc_ + 2 _abCc_ + 2 _ABbC_ + 2 _ABbc_ +
2 _aBbC_ + 2 _aBbc_ + 2 _AaBC_ + 2 _AaBc_ + 2 _AabC_ + 2 _Aabc_ +
4 _ABbCc_ + 4 _aBbCc_ + 4 _AaBCc_ + 4 _AabCc_ + 4 _AaBbC_ +
4 _AaBbc_ + 8 _AaBbCc_.
Here also is involved a combination series in which the expressions for
the characters _A_ and _a_, _B_ and _b_, _C_ and _c_, are united. The
expressions
_A_ + 2 _Aa_ + _a_
_B_ + 2 _Bb_ + _b_
_C_ + 2 _Cc_ + _c_
give all the classes of the series. The constant combinations which
occur therein agree with all combinations which are possible between
the characters _A_, _B_, _C_, _a_, _b_, _c_; two thereof, _ABC_ and
_abc_, resemble the two original parental stocks.
In addition, further experiments were made with a smaller number
of experimental plants in which the remaining characters by twos
and threes were united as hybrids: all yielded approximately the
same results. There is therefore no doubt that for the whole of
the characters involved in the experiments the principle applies
that _the offspring of the hybrids in which several essentially
different characters are combined represent the terms of a series
of combinations, in which the developmental series for each pair of
differentiating characters are associated_. It is demonstrated at the
same time that _the relation of each pair of different characters
in hybrid union is independent of the other differences in the two
original parental stocks_.
If _n_ represent the number of the differentiating characters in
the two original stocks, 3^{_n_} gives the number of terms of the
combination series, 4^{_n_} the number of individuals which belong to
the series, and 2^{_n_} the number of unions which remain constant.
The series therefore embraces, if the original stocks differ in four
characters, 3^4 = 81 of classes, 4^4 = 256 individuals, and 2^4 = 16
constant forms; or, which is the same, among each 256 offspring of the
hybrids there are 81 different combinations, 16 of which are constant.
All constant combinations which in Peas are possible by the combination
of the said seven differentiating characters were actually obtained
by repeated crossing. Their number is given by 2^7 = 128. Thereby is
simultaneously given the practical proof _that the constant characters
which appear in the several varieties of a group of plants may be
obtained in all the associations which are possible according to the
[mathematical] laws of combination, by means of repeated artificial
fertilisation_.
As regards the flowering time of the hybrids, the experiments are
not yet concluded. It can, however, already be stated that the
period stands almost exactly between those of the seed and pollen
parents, and that the constitution of the hybrids with respect to
this character probably happens in the same way as in the case of the
other characters. The forms which are selected for experiments of this
class must have a difference of at least twenty days from the middle
flowering period of one to that of the other; furthermore, the seeds
when sown must all be placed at the same depth in the earth, so that
they may germinate simultaneously. Also, during the whole flowering
period, the more important variations in temperature must be taken into
account, and the partial hastening or delaying of the flowering which
may result therefrom. It is clear that this experiment presents many
difficulties to be overcome and necessitates great attention.
If we endeavour to collate in a brief form the results arrived at, we
find that those differentiating characters which admit of easy and
certain recognition in the experimental plants, all behave exactly
alike in their hybrid associations. The offspring of the hybrids of
each pair of differentiating characters are, one-half, hybrid again,
while the other half are constant in equal proportions having the
characters of the seed and pollen parents respectively. If several
differentiating characters are combined by cross-fertilisation in a
hybrid, the resulting offspring form the terms of a combination series
in which the permutation series for each pair of differentiating
characters are united.
The uniformity of behaviour shown by the whole of the characters
submitted to experiment permits, and fully justifies, the acceptance of
the principle that a similar relation exists in the other characters
which appear less sharply defined in plants, and therefore could not
be included in the separate experiments. An experiment with peduncles
of different lengths gave on the whole a fairly satisfactory result,
although the differentiation and serial arrangement of the forms could
not be effected with that certainty which is indispensable for correct
experiment.
THE REPRODUCTIVE CELLS OF HYBRIDS.
The results of the previously described experiments induced further
experiments, the results of which appear fitted to afford some
conclusions as regards the composition of the egg and pollen cells of
hybrids. An important matter for consideration is afforded in _Pisum_
by the circumstance that among the progeny of the hybrids constant
forms appear, and that this occurs, too, in all combinations of the
associated characters. So far as experience goes, we find it in every
case confirmed that constant progeny can only be formed when the egg
cells and the fertilising pollen are of like character, so that both
are provided with the material for creating quite similar individuals,
as is the case with the normal fertilisation of pure species[38]. We
must therefore regard it as essential that exactly similar factors are
at work also in the production of the constant forms in the hybrid
plants. Since the various constant forms are produced in _one_ plant,
or even in _one_ flower of a plant, the conclusion appears logical
that in the ovaries of the hybrids there are formed as many sorts of
egg cells, and in the anthers as many sorts of pollen cells, as there
are possible constant combination forms, and that these egg and pollen
cells agree in their internal composition with those of the separate
forms.
[38] [“False hybridism” was of course unknown to Mendel.]
In point of fact it is possible to demonstrate theoretically that
this hypothesis would fully suffice to account for the development of
the hybrids in the separate generations, if we might at the same time
assume that the various kinds of egg and pollen cells were formed in
the hybrids on the average in equal numbers[39].
[39] [This and the preceding paragraph contain the essence of the
Mendelian principles of heredity.]
In order to bring these assumptions to an experimental proof, the
following experiments were designed. Two forms which were constantly
different in the form of the seed and the colour of the albumen were
united by fertilisation.
If the differentiating characters are again indicated as _A_, _B_, _a_,
_b_, we have:
_AB_, seed parent; _ab_, pollen parent;
_A_, form round; _a_, form angular;
_B_, albumen yellow. _b_, albumen green.
The artificially fertilised seeds were sown together with several seeds
of both original stocks, and the most vigorous examples were chosen for
the reciprocal crossing. There were fertilised:
1. The hybrids with the pollen of _AB_.
2. The hybrids " " _ab_.
3. _AB_ " " the hybrids.
4. _ab_ " " the hybrids.
For each of these four experiments the whole of the flowers on three
plants were fertilised. If the above theory be correct, there must be
developed on the hybrids egg and pollen cells of the forms _AB_, _Ab_,
_aB_, _ab_, and there would be combined:--
1. The egg cells _AB_, _Ab_, _aB_, _ab_ with the pollen cells _AB_.
2. The egg cells _AB_, _Ab_, _aB_, _ab_ with the pollen cells _ab_.
3. The egg cells _AB_ with the pollen cells _AB_, _Ab_, _aB_, _ab_.
4. The egg cells _ab_ with the pollen cells _AB_, _Ab_, _aB_, _ab_.
From each of these experiments there could then result only the
following forms:--
1. _AB_, _ABb_, _AaB_, _AaBb_.
2. _AaBb_, _Aab_, _aBb_, _ab_.
3. _AB_, _ABb_, _AaB_, _AaBb_.
4. _AaBb_, _Aab_, _aBb_, _ab_.
If, furthermore, the several forms of the egg and pollen cells of the
hybrids were produced on an average in equal numbers, then in each
experiment the said four combinations should stand in the same ratio
to each other. A perfect agreement in the numerical relations was,
however, not to be expected, since in each fertilisation, even in
normal cases, some egg cells remain undeveloped or subsequently die,
and many even of the well-formed seeds fail to germinate when sown. The
above assumption is also limited in so far that, while it demands the
formation of an equal number of the various sorts of egg and pollen
cells, it does not require that this should apply to each separate
hybrid with mathematical exactness.
The first and second experiments had primarily the object of proving
the composition of the hybrid egg cells, while the third and fourth
experiments were to decide that of the pollen cells[40]. As is shown by
the above demonstration the first and second experiments and the third
and fourth experiments should produce precisely the same combinations,
and even in the second year the result should be partially visible in
the form and colour of the artificially fertilised seed. In the first
and third experiments the dominant characters of form and colour, _A_
and _B_, appear in each union, and are also partly constant and partly
in hybrid union with the recessive characters _a_ and _b_, for which
reason they must impress their peculiarity upon the whole of the seeds.
All seeds should therefore appear round and yellow, if the theory be
justified. In the second and fourth experiments, on the other hand,
one union is hybrid in form and in colour, and consequently the seeds
are round and yellow; another is hybrid in form, but constant in the
recessive character of colour, whence the seeds are round and green;
the third is constant in the recessive character of form but hybrid in
colour, consequently the seeds are angular and yellow; the fourth is
constant in both recessive characters, so that the seeds are angular
and green. In both these experiments there were consequently four sorts
of seed to be expected--viz. round and yellow, round and green, angular
and yellow, angular and green.
[40] [To prove, namely, that both were similarly differentiated, and
not one or other only.]
The crop fulfilled these expectations perfectly. There were obtained in
the
1st Experiment, 98 exclusively round yellow seeds;
3rd " 94 " " " "
In the 2nd Experiment, 31 round and yellow, 26 round and green, 27
angular and yellow, 26 angular and green seeds.
In the 4th Experiment, 24 round and yellow, 25 round and green, 22
angular and yellow, 27 angular and green seeds.
A favourable result could now scarcely be doubted; the next generation
must afford the final proof. From the seed sown there resulted for the
first experiment 90 plants, and for the third 87 plants which fruited:
these yielded for the--
1st Exp. 3rd Exp.
20 25 round yellow seeds _AB_
23 19 round yellow and green seeds _ABb_
25 22 round and angular yellow seeds _AaB_
22 21 round and angular green and yellow seeds _AaBb_
In the second and fourth experiments the round and yellow seeds yielded
plants with round and angular yellow and green seeds, _AaBb_.
From the round green seeds plants resulted with round and angular green
seeds, _Aab_.
The angular yellow seeds gave plants with angular yellow and green
seeds, _aBb_.
From the angular green seeds plants were raised which yielded again
only angular and green seeds, _ab_.
Although in these two experiments likewise some seeds did not
germinate, the figures arrived at already in the previous year were not
affected thereby, since each kind of seed gave plants which, as regards
their seed, were like each other and different from the others. There
resulted therefore from the
2nd Exp. 4th Exp.
31 24 plants of the form _AaBb_
26 25 " " _Aab_
27 22 " " _aBb_
26 27 " " _ab_
In all the experiments, therefore, there appeared all the forms which
the proposed theory demands, and also in nearly equal numbers.
In a further experiment the characters of floral colour and length of
stem were experimented upon, and selection so made that in the third
year of the experiment each character ought to appear in half of all
the plants if the above theory were correct. _A_, _B_, _a_, _b_ serve
again as indicating the various characters.
_A_, violet-red flowers. _a_, white flowers.
_B_, axis long. _b_, axis short.
The form _Ab_ was fertilised with _ab_, which produced the hybrid
_Aab_. Furthermore, _aB_ was also fertilised with _ab_, whence the
hybrid _aBb_. In the second year, for further fertilisation, the hybrid
_Aab_ was used as seed parent, and hybrid _aBb_ as pollen parent.
Seed parent, _Aab_. Pollen parent, _aBb_.
Possible egg cells, _Abab_. Pollen cells, _aBab_.
From the fertilisation between the possible egg and pollen cells four
combinations should result, viz.:--
_AaBb_ + _aBb_ + _Aab_ + _ab_.
From this it is perceived that, according to the above theory, in the
third year of the experiment out of all the plants
Half should have violet-red flowers (_Aa_), Classes 1, 3
" " " white flowers (_a_) " 2, 4
" " " a long axis (_Bb_) " 1, 2
" " " a short axis (_b_) " 3, 4
From 45 fertilisations of the second year 187 seeds resulted, of which
only 166 reached the flowering stage in the third year. Among these the
separate classes appeared in the numbers following:--
Class. Colour of flower. Stem.
1 violet-red long 47 times
2 white long 40 "
3 violet-red short 38 "
4 white short 41 "
There consequently appeared--
The violet-red flower colour (_Aa_) in 85 plants.
" white " " (_a_) in 81 "
" long stem (_Bb_) in 87 "
" short " (_b_) in 79 "
The theory adduced is therefore satisfactorily confirmed in this
experiment also.
For the characters of form of pod, colour of pod, and position of
flowers experiments were also made on a small scale, and results
obtained in perfect agreement. All combinations which were possible
through the union of the differentiating characters duly appeared, and
in nearly equal numbers.
Experimentally, therefore, the theory is justified _that the pea
hybrids form egg and pollen cells which, in their constitution,
represent in equal numbers all constant forms which result from the
combination of the characters when united in fertilisation_.
The difference of the forms among the progeny of the hybrids, as well
as the respective ratios of the numbers in which they are observed,
find a sufficient explanation in the principle above deduced. The
simplest case is afforded by the developmental series of each pair
of differentiating characters. This series is represented by the
expression _A_ + 2_Aa_ + _a_, in which _A_ and _a_ signify the forms
with constant differentiating characters, and _Aa_ the hybrid form
of both. It includes in three different classes four individuals. In
the formation of these, pollen and egg cells of the form _A_ and _a_
take part on the average equally in the fertilisation; hence each form
[occurs] twice, since four individuals are formed. There participate
consequently in the fertilisation--
The pollen cells _A_ + _A_ + _a_ + _a_
The egg cells _A_ + _A_ + _a_ + _a_.
It remains, therefore, purely a matter of chance which of the two sorts
of pollen will become united with each separate egg cell. According,
however, to the law of probability, it will always happen, on the
average of many cases, that each pollen form _A_ and _a_ will unite
equally often with each egg cell form _A_ and _a_, consequently one of
the two pollen cells _A_ in the fertilisation will meet with the egg
cell _A_ and the other with an egg cell _a_, and so likewise one pollen
cell _a_ will unite with an egg cell _A_, and the other with egg cell
_a_.
Pollen cells _A_ _A_ _a_ _a_
| \ / |
| \ / |
| x |
| / \ |
| / \ |
\|/ \/ \/ \|/
Egg cells _A_ _A_ _a_ _a_
The result of the fertilisation may be made clear by putting the signs
for the conjoined egg and pollen cells in the form of fractions, those
for the pollen cells above and those for the egg cells below the line.
We then have
_A_/_A_ + _A_/_a_ + _a_/_A_ + _a_/_a_.
In the first and fourth term the egg and pollen cells are of like kind,
consequently the product of their union must be constant, viz. _A_ and
_a_; in the second and third, on the other hand, there again results a
union of the two differentiating characters of the stocks, consequently
the forms resulting from these fertilisations are identical with
those of the hybrid from which they sprang. _There occurs accordingly
a repeated hybridisation._ This explains the striking fact that the
hybrids are able to produce, besides the two parental forms, offspring
which are like themselves; _A_/_a_ and _a_/_A_ both give the same union
_Aa_, since, as already remarked above, it makes no difference in the
result of fertilisation to which of the two characters the pollen or
egg cells belong. We may write then--
_A_/_A_ + _A_/_a_ + _a_/_A_ + _a_/_a_ = _A_ + 2_Aa_ + _a_.
This represents the average result of the self-fertilisation of the
hybrids when two differentiating characters are united in them. In
solitary flowers and in solitary plants, however, the ratios in which
the forms of the series are produced may suffer not inconsiderable
fluctuations[41]. Apart from the fact that the numbers in which both
sorts of egg cells occur in the seed vessels can only be regarded as
equal on the average, it remains purely a matter of chance which of
the two sorts of pollen may fertilise each separate egg cell. For this
reason the separate values must necessarily be subject to fluctuations,
and there are even extreme cases possible, as were described earlier
in connection with the experiments on the form of the seed and the
colour of the albumen. The true ratios of the numbers can only be
ascertained by an average deduced from the sum of as many single values
as possible; the greater the number the more are merely chance elements
eliminated.
[41] [Whether segregation by such units is more than purely
fortuitous could probably be determined by seriation.]
The developmental series for hybrids in which two kinds of
differentiating characters are united contains among sixteen
individuals nine different forms, viz., _AB_ + _Ab_ + _aB_ +
_ab_ + 2_ABb_ + 2_aBb_ + 2_AaB_ + 2_Aab_ + 4_AaBb_. Between the
differentiating characters of the original stocks _Aa_ and _Bb_ four
constant combinations are possible, and consequently the hybrids
produce the corresponding four forms of egg and pollen cells _AB_,
_Ab_, _aB_, _ab_, and each of these will on the average figure four
times in the fertilisation, since sixteen individuals are included in
the series. Therefore the participators in the fertilisation are--
Pollen cells _AB_ + _AB_ + _AB_ + _AB_ + _Ab_ + _Ab_ + _Ab_ + _Ab_ +
_aB_ + _aB_ + _aB_ + _aB_ + _ab_ + _ab_ + _ab_ + _ab_.
Egg cells _AB_ + _AB_ + _AB_ + _AB_ + _Ab_ + _Ab_ + _Ab_ + _Ab_ +
_aB_ + _aB_ + _aB_ + _aB_ + _ab_ + _ab_ + _ab_ + _ab_.
In the process of fertilisation each pollen form unites on an average
equally often with each egg cell form, so that each of the four pollen
cells _AB_ unites once with one of the forms of egg cell _AB_, _Ab_,
_aB_, _ab_. In precisely the same way the rest of the pollen cells
of the forms _Ab_, _aB_, _ab_ unite with all the other egg cells. We
obtain therefore--
_AB_/_AB_ + _AB_/_Ab_ + _AB_/_aB_ + _AB_/_ab_ + _Ab_/_AB_ + _Ab_/_Ab_ +
_Ab_/_aB_ + _Ab_/_ab_ + _aB_/_AB_ + _aB_/_Ab_ + _aB_/_aB_ + _aB_/_ab_ +
_ab_/_AB_ + _ab_/_Ab_ + _ab_/_aB_ + _ab_/_ab_,
or
_AB_ + _ABb_ + _AaB_ + _AaBb_ + _ABb_ + _Ab_ + _AaBb_ + _Aab_ + _AaB_ +
_AaBb_ + _aB_ + _aBb_ + _AaBb_ + _Aab_ + _aBb_ + _ab_ = _AB_ + _Ab_ +
_aB_ + _ab_ + 2_ABb_ + 2_aBb_ + 2_AaB_ + 2_Aab_ + 4_AaBb_[42].
[42] [In the original the sign of equality (=) is here represented by
+, evidently a misprint.]
In precisely similar fashion is the developmental series of hybrids
exhibited when three kinds of differentiating characters are conjoined
in them. The hybrids form eight various kinds of egg and pollen
cells--_ABC_, _ABc_, _AbC_, _Abc_, _aBC_, _aBc_, _abC_, _abc_--and each
pollen form unites itself again on the average once with each form of
egg cell.
The law of combination of different characters which governs the
development of the hybrids finds therefore its foundation and
explanation in the principle enunciated, that the hybrids produce egg
cells and pollen cells which in equal numbers represent all constant
forms which result from the combinations of the characters brought
together in fertilisation.
EXPERIMENTS WITH HYBRIDS OF OTHER SPECIES OF PLANTS.
It must be the object of further experiments to ascertain whether
the law of development discovered for _Pisum_ applies also to the
hybrids of other plants. To this end several experiments were recently
commenced. Two minor experiments with species of _Phaseolus_ have been
completed, and may be here mentioned.
An experiment with _Phaseolus vulgaris_ and _Phaseolus nanus_ gave
results in perfect agreement. _Ph. nanus_ had together with the dwarf
axis simply inflated green pods. _Ph. vulgaris_ had, on the other hand,
an axis 10 feet to 12 feet high, and yellow coloured pods, constricted
when ripe. The ratios of the numbers in which the different forms
appeared in the separate generations were the same as with _Pisum_.
Also the development of the constant combinations resulted according to
the law of simple combination of characters, exactly as in the case of
_Pisum_. There were obtained--
Constant Axis Colour of Form of
combinations the unripe pods. the ripe pods.
1 long green inflated
2 " " constricted
3 " yellow inflated
4 " " constricted
5 short green inflated
6 " " constricted
7 " yellow inflated
8 " " constricted
The green colour of the pod, the inflated forms, and the long axis
were, as in _Pisum_, dominant characters.
Another experiment with two very different species of _Phaseolus_ had
only a partial result. _Phaseolus nanus_, L., served as seed parent,
a perfectly constant species, with white flowers in short racemes and
small white seeds in straight, inflated, smooth pods; as pollen parent
was used _Ph. multiflorus_, W., with tall winding stem, purple-red
flowers in very long racemes, rough, sickle-shaped crooked pods, and
large seeds which bore black flecks and splashes on a peach-blood-red
ground.
The hybrids had the greatest similarity to the pollen parent, but the
flowers appeared less intensely coloured. Their fertility was very
limited; from seventeen plants, which together developed many hundreds
of flowers, only forty-nine seeds in all were obtained. These were of
medium size, and were flecked and splashed similarly to those of _Ph.
multiflorus_, while the ground colour was not materially different. The
next year forty-four plants were raised from these seeds, of which only
thirty-one reached the flowering stage. The characters of _Ph. nanus_,
which had been altogether latent in the hybrids, reappeared in various
combinations; their ratio, however, with relation to the dominant
characters was necessarily very fluctuating owing to the small number
of trial plants. With certain characters, as in those of the axis and
the form of pod, it was, however, as in the case of _Pisum_, almost
exactly 1 : 3.
Insignificant as the results of this experiment may be as regards
the determination of the relative numbers in which the various
forms appeared, it presents, on the other hand, the phenomenon of a
remarkable change of colour in the flowers and seed of the hybrids. In
_Pisum_ it is known that the characters of the flower- and seed-colour
present themselves unchanged in the first and further generations, and
that the offspring of the hybrids display exclusively the one or the
other of the characters of the original stocks[43]. It is otherwise
in the experiment we are considering. The white flowers and the
seed-colour of _Ph. nanus_ appeared, it is true, at once in the first
generation [_from_ the hybrids] in one fairly fertile example, but the
remaining thirty plants developed flower colours which were of various
grades of purple-red to pale violet. The colouring of the seed-coat was
no less varied than that of the flowers. No plant could rank as fully
fertile; many produced no fruit at all; others only yielded fruits from
the flowers last produced, which did not ripen. From fifteen plants
only were well-developed seeds obtained. The greatest disposition to
infertility was seen in the forms with preponderantly red flowers,
since out of sixteen of these only four yielded ripe seed. Three of
these had a similar seed pattern to _Ph. multiflorus_, but with a more
or less pale ground colour; the fourth plant yielded only one seed of
plain brown tint. The forms with preponderantly violet coloured flowers
had dark brown, black-brown, and quite black seeds.
[43] [This is the only passage where Mendel can be construed as
asserting universal dominance for _Pisum_; and even here, having
regard to the rest of the paper, it is clearly unfair to represent
him as predicating more than he had seen in his own experiments.
Moreover in flower and seed-coat colour (which is here meant), using
his characters dominance must be almost universal, if not quite.]
The experiment was continued through two more generations under
similar unfavourable circumstances, since even among the offspring of
fairly fertile plants there were still some which were less fertile
or even quite sterile. Other flower- and seed-colours than those
cited did not subsequently present themselves. The forms which in the
first generation [bred from the hybrids] contained one or more of the
recessive characters remained, as regards these, constant without
exception. Also of those plants which possessed violet flowers and
brown or black seed, some did not vary again in these respects in
the next generation; the majority, however, yielded, together with
offspring exactly like themselves, some which displayed white flowers
and white seed-coats. The red flowering plants remained so slightly
fertile that nothing can be said with certainty as regards their
further development.
Despite the many disturbing factors with which the observations had
to contend, it is nevertheless seen by this experiment that the
development of the hybrids, with regard to those characters which
concern the form of the plants, follows the same laws as does _Pisum_.
With regard to the colour characters, it certainly appears difficult
to perceive a substantial agreement. Apart from the fact that from the
union of a white and a purple-red colouring a whole series of colours
results, from purple to pale violet and white, the circumstance is a
striking one that among thirty-one flowering plants only one received
the recessive character of the white colour, while in _Pisum_ this
occurs on the average in every fourth plant.
Even these enigmatical results, however, might probably be explained
by the law governing _Pisum_ if we might assume that the colour of
the flowers and seeds of _Ph. multiflorus_ is a combination of two
or more entirely independent colours, which individually act like
any other constant character in the plant. If the flower colour A
were a combination of the individual characters _A_{1} + _A_{2} +
... which produce the total impression of a purple colouration, then
by fertilisation with the differentiating character, white colour,
_a_, there would be produced the hybrid unions _A_{1}_a_ + _A_{2}_a_
+ ... and so would it be with the corresponding colouring of the
seed-coats[44]. According to the above assumption, each of these hybrid
colour unions would be independent, and would consequently develop
quite independently from the others. It is then easily seen that
from the combination of the separate developmental series a perfect
colour-series must result. If, for instance, _A_ = _A_{1} + _A_{2},
then the hybrids _A_{1}_a_ and _A_{2}_a_ form the developmental series--
_A_{1} + 2_A_{1}_a_ + _a_
_A_{2} + 2_A_{2}_a_ + _a_.
[44] [It appears to me clear that this expression is incorrectly
given, and the argument regarding compound characters is consequently
not legitimately developed. The original compound character should
be represented as _A_{1}_A_{2}_A_{3} ... which when fertilised by
_a_{1} gives _A_{1}_A_{2}_A_{3} ... a as the hybrid of the first
generation. Mendel practically tells us these were all alike,
and there is nothing to suggest that they were diverse. When on
self-fertilisation, they break up, they will produce the gametes he
specifies; but they may also produce _A_{1}_A_{1} and _A_{2}_A_{2},
_A_{1}_A_{2}_a_, &c., thereby introducing terms of a nature different
from any indicated by him. That this point is one of the highest
significance, both practical and theoretical, is evident at once.]
The members of this series can enter into nine different combinations,
and each of these denotes another colour[45]--
1 _A_{1}A_{2}_ 2 _A_{1}aA_{2}_ 1 _A_{2}a_
2 _A_{1}A_{2}a_ 4 _A_{1}aA_{2}a_ 2 _A_{2}aa_
1 _A_{1}a_ 2 _A_{1}aa_ 1 _aa_.
[45] [It seems very doubtful if the zygotes are correctly represented
by the terms _A_{1}aA_{2}a_, _A_{2}aa_, _A_{1}aa_; for in the hybrids
_A_{1}a_, &c. the allelomorphs _A_{1}_ and _a_, &c. should by
hypothesis be separated in the gametes.]
The figures prescribed for the separate combinations also indicate how
many plants with the corresponding colouring belong to the series.
Since the total is sixteen, the whole of the colours are on the average
distributed over each sixteen plants, but, as the series itself
indicates, in unequal proportions.
Should the colour development really happen in this way, we could offer
an explanation of the case above described, viz. that the white flowers
and seed-coat colour only appeared once among thirty-one plants of the
first generation. This colouring appears only once in the series, and
could therefore also only be developed once in the average in each
sixteen, and with three colour characters only once even in sixty-four
plants.
It must, however, not be forgotten that the explanation here attempted
is based on a mere hypothesis, only supported by the very imperfect
result of the experiment just described. It would, however, be well
worth while to follow up the development of colour in hybrids by
similar experiments, since it is probable that in this way we might
learn the significance of the extraordinary variety in the colouring of
our ornamental flowers.
So far, little at present is known with certainty beyond the fact that
the colour of the flowers in most ornamental plants is an extremely
variable character. The opinion has often been expressed that the
stability of the species is greatly disturbed or entirely upset by
cultivation, and consequently there is an inclination to regard the
development of cultivated forms as a matter of chance devoid of rules;
the colouring of ornamental plants is indeed usually cited as an
example of great instability. It is, however, not clear why the simple
transference into garden soil should result in such a thorough and
persistent revolution in the plant organism. No one will seriously
maintain that in the open country the development of plants is ruled
by other laws than in the garden bed. Here, as there, changes of type
must take place if the conditions of life be altered, and the species
possesses the capacity of fitting itself to its new environment. It is
willingly granted that by cultivation the origination of new varieties
is favoured, and that by man’s labour many varieties are acquired
which, under natural conditions, would be lost; but nothing justifies
the assumption that the tendency to the formation of varieties is so
extraordinarily increased that the species speedily lose all stability,
and their offspring diverge into an endless series of extremely
variable forms. Were the change in the conditions of vegetation the
sole cause of variability we might expect that those cultivated plants
which are grown for centuries under almost identical conditions would
again attain constancy. That, as is well known, is not the case,
since it is precisely under such circumstances that not only the
most varied but also the most variable forms are found. It is only
the _Leguminosæ_, like _Pisum_, _Phaseolus_, _Lens_, whose organs of
fertilisation are protected by the keel, which constitute a noteworthy
exception. Even here there have arisen numerous varieties during a
cultural period of more than 1000 years; these maintain, however, under
unchanging environments a stability as great as that of species growing
wild.
It is more than probable that as regards the variability of cultivated
plants there exists a factor which so far has received little
attention. Various experiments force us to the conclusion that our
cultivated plants, with few exceptions, are _members of various hybrid
series_, whose further development in conformity with law is changed
and hindered by frequent crossings _inter se_. The circumstance must
not be overlooked that cultivated plants are mostly grown in great
numbers and close together, affording the most favourable conditions
for reciprocal fertilisation between the varieties present and the
species itself. The probability of this is supported by the fact
that among the great array of variable forms solitary examples are
always found, which in one character or another remain constant, if
only foreign influence be carefully excluded. These forms develop
precisely as do those which are known to be members of the compound
hybrid series. Also with the most susceptible of all characters, that
of colour, it cannot escape the careful observer that in the separate
forms the inclination to vary is displayed in very different degrees.
Among plants which arise from _one_ spontaneous fertilisation there
are often some whose offspring vary widely in the constitution and
arrangement of the colours, while others furnish forms of little
deviation, and among a greater number solitary examples occur which
transmit the colour of the flowers unchanged to their offspring. The
cultivated species of _Dianthus_ afford an instructive example of
this. A white-flowered example of _Dianthus caryophyllus_, which itself
was derived from a white-flowered variety, was shut up during its
blooming period in a greenhouse; the numerous seeds obtained therefrom
yielded plants entirely white-flowered like itself. A similar result
was obtained from a subspecies, with red flowers somewhat flushed with
violet, and one with flowers white, striped with red. Many others, on
the other hand, which were similarly protected, yielded progeny which
were more or less variously coloured and marked.
Whoever studies the colouration which results in ornamental plants
from similar fertilisation can hardly escape the conviction that here
also the development follows a definite law which possibly finds
its expression _in the combination of several independent colour
characters_.
CONCLUDING REMARKS.
It can hardly fail to be of interest to compare the observations made
regarding _Pisum_ with the results arrived at by the two authorities
in this branch of knowledge, Kölreuter and Gärtner, in their
investigations. According to the opinion of both, the hybrids in outer
appearance present either a form intermediate between the original
species, or they closely resemble either the one or the other type, and
sometimes can hardly be discriminated from it. From their seeds usually
arise, if the fertilisation was effected by their own pollen, various
forms which differ from the normal type. As a rule, the majority of
individuals obtained by one fertilisation maintain the hybrid form,
while some few others come more like the seed parent, and one or other
individual approaches the pollen parent. This, however, is not the case
with all hybrids without exception. With some the offspring have more
nearly approached, some the one and some the other, original stock,
or they all incline more to one or the other side; while with others
_they remain perfectly like the hybrid_ and continue constant in their
offspring. The hybrids of varieties behave like hybrids of species, but
they possess greater variability of form and a more pronounced tendency
to revert to the original type.
With regard to the form of the hybrids and their development, as a rule
an agreement with the observations made in _Pisum_ is unmistakable. It
is otherwise with the exceptional cases cited. Gärtner confesses even
that the exact determination whether a form bears a greater resemblance
to one or to the other of the two original species often involved
great difficulty, so much depending upon the subjective point of view
of the observer. Another circumstance could, however, contribute to
render the results fluctuating and uncertain, despite the most careful
observation and differentiation; for the experiments plants were mostly
used which rank as good species and are differentiated by a large
number of characters. In addition to the sharply defined characters,
where it is a question of greater or less similarity, those characters
must also be taken into account which are often difficult to define
in words, but yet suffice, as every plant specialist knows, to give
the forms a strange appearance. If it be accepted that the development
of hybrids follows the law which is valid for _Pisum_, the series
in each separate experiment must embrace very many forms, since the
number of the components, as is known, increases with the number of
the differentiating characters in _cubic ratio_. With a relatively
small number of experimental-plants the result therefore could only be
approximately right, and in single cases might fluctuate considerably.
If, for instance, the two original stocks differ in seven characters,
and 100 and 200 plants were raised from the seeds of their hybrids to
determine the grade of relationship of the offspring, we can easily see
how uncertain the decision must become, since for seven differentiating
characters the combination series contains 16,384 individuals under
2187 various forms; now one and then another relationship could assert
its predominance, just according as chance presented this or that form
to the observer in a majority of cases.
If, furthermore, there appear among the differentiating characters at
the same time dominant characters, which are transferred entire or
nearly unchanged to the hybrids, then in the terms of the developmental
series that one of the two original stocks which possesses the
majority of dominant characters must always be predominant. In the
experiment described relative to _Pisum_, in which three kinds of
differentiating characters were concerned, all the dominant characters
belonged to the seed parent. Although the terms of the series in their
internal composition approach both original stock plants equally,
in this experiment the type of the seed parent obtained so great
a preponderance that out of each sixty-four plants of the first
generation fifty-four exactly resembled it, or only differed in one
character. It is seen how rash it may be under such circumstances to
draw from the external resemblances of hybrids conclusions as to their
internal nature.
Gärtner mentions that in those cases where the development was regular
among the offspring of the hybrids the two original species were not
reproduced, but only a few closely approximating individuals. With
very extended developmental series it could not in fact be otherwise.
For seven differentiating characters, for instance, among more than
16,000 individuals--offspring of the hybrids--each of the two original
species would occur only once. It is therefore hardly possible that
these should appear at all among a small number of experimental plants;
with some probability, however, we might reckon upon the appearance in
the series of a few forms which approach them.
We meet with an _essential difference_ in those hybrids which remain
constant in their progeny and propagate themselves as truly as the pure
species. According to Gärtner, to this class belong the _remarkably
fertile hybrids_ _Aquilegia atropurpurea canadensis_, _Lavatera
pseudolbia thuringiaca_, _Geum urbano-rivale_, and some _Dianthus_
hybrids; and, according to Wichura, the hybrids of the Willow species.
For the history of the evolution of plants this circumstance is of
special importance, since constant hybrids acquire the status of
new species. The correctness of this is evidenced by most excellent
observers, and cannot be doubted. Gärtner had opportunity to follow
up _Dianthus Armeria deltoides_ to the tenth generation, since it
regularly propagated itself in the garden.
With _Pisum_ it was shown by experiment that the hybrids form egg and
pollen cells of _different_ kinds, and that herein lies the reason
of the variability of their offspring. In other hybrids, likewise,
whose offspring behave similarly we may assume a like cause; for
those, on the other hand, which remain constant the assumption appears
justifiable that their fertilising cells are all alike and agree with
the foundation-cell [fertilised ovum] of the hybrid. In the opinion of
renowned physiologists, for the purpose of propagation one pollen cell
and one egg cell unite in Phanerogams[46] into a single cell, which
is capable by assimilation and formation of new cells to become an
independent organism. This development follows a constant law, which
is founded on the material composition and arrangement of the elements
which meet in the cell in a vivifying union. If the reproductive cells
be of the same kind and agree with the foundation cell [fertilised
ovum] of the mother plant, then the development of the new individual
will follow the same law which rules the mother plant. If it chance
that an egg cell unites with a _dissimilar_ pollen cell, we must then
assume that between those elements of both cells, which determine
the mutual differences, some sort of compromise is effected. The
resulting compound cell becomes the foundation of the hybrid organism,
the development of which necessarily follows a different scheme from
that obtaining in each of the two original species. If the compromise
be taken to be a complete one, in the sense, namely, that the hybrid
embryo is formed from cells of like kind, in which the differences are
_entirely and permanently accommodated_ together, the further result
follows that the hybrids, like any other stable plant species, remain
true to themselves in their offspring. The reproductive cells which are
formed in their seed vessels and anthers are of one kind, and agree
with the fundamental compound cell [fertilised ovum].
[46] In _Pisum_ it is placed beyond doubt that for the formation of
the new embryo a perfect union of the elements of both fertilising
cells must take place. How could we otherwise explain that among
the offspring of the hybrids both original types reappear in equal
numbers and with all their peculiarities? If the influence of the
egg cell upon the pollen cell were only external, if it fulfilled
the _rôle_ of a nurse only, then the result of each artificial
fertilisation could be no other than that the developed hybrid
should exactly resemble the pollen parent, or at any rate do so very
closely. This the experiments so far have in no wise confirmed. An
evident proof of the complete union of the contents of both cells is
afforded by the experience gained on all sides that it is immaterial,
as regards the form of the hybrid, which of the original species is
the seed parent or which the pollen parent.
With regard to those hybrids whose progeny is _variable_ we may perhaps
assume that between the differentiating elements of the egg and pollen
cells there also occurs a compromise, in so far that the formation of a
cell as foundation of the hybrid becomes possible; but, nevertheless,
the arrangement between the conflicting elements is only temporary and
does not endure throughout the life of the hybrid plant. Since in the
habit of the plant no changes are perceptible during the whole period
of vegetation, we must further assume that it is only possible for
the differentiating elements to liberate themselves from the enforced
union when the fertilising cells are developed. In the formation of
these cells all existing elements participate in an entirely free and
equal arrangement, in which it is only the differentiating ones which
mutually separate themselves. In this way the production would be
rendered possible of as many sorts of egg and pollen cells as there are
combinations possible of the formative elements.
The attribution attempted here of the essential difference in the
development of hybrids to _a permanent or temporary union_ of the
differing cell elements can, of course, only claim the value of an
hypothesis for which the lack of definite data offers a wide field.
Some justification of the opinion expressed lies in the evidence
afforded by _Pisum_ that the behaviour of each pair of differentiating
characters in hybrid union is independent of the other differences
between the two original plants, and, further, that the hybrid
produces just so many kinds of egg and pollen cells as there are
possible constant combination forms. The differentiating characters
of two plants can finally, however, only depend upon differences in
the composition and grouping of the elements which exist in the
foundation-cells [fertilised ova] of the same in vital interaction[47].
[47] “_Welche in den Grundzellen derselben in lebendiger
Wechselwirkung stehen._”
Even the validity of the law formulated for _Pisum_ requires still to
be confirmed, and a repetition of the more important experiments is
consequently much to be desired, that, for instance, relating to the
composition of the hybrid fertilising cells. A differential [element]
may easily escape the single observer[48], which although at the outset
may appear to be unimportant, may yet accumulate to such an extent
that it must not be ignored in the total result. Whether the variable
hybrids of other plant species observe an entire agreement must also
be first decided experimentally. In the meantime we may assume that in
material points a difference in principle can scarcely occur, since the
unity in the developmental plan of organic life is beyond question.
[48] “_Dem einzelnen Beobachter kann leicht ein Differenziale
entgehen._”
In conclusion, the experiments carried out by Kölreuter, Gärtner,
and others with respect to _the transformation of one species into
another by artificial fertilisation_ merit special mention. A special
importance has been attached to these experiments, and Gärtner reckons
them among “the most difficult of all in hybridisation.”
If a species _A_ is to be transformed into a species _B_, both must be
united by fertilisation and the resulting hybrids then be fertilised
with the pollen of _B_; then, out of the various offspring resulting,
that form would be selected which stood in nearest relation to _B_ and
once more be fertilised with _B_ pollen, and so continuously until
finally a form is arrived at which is like _B_ and constant in its
progeny. By this process the species _A_ would change into the species
_B_. Gärtner alone has effected thirty such experiments with plants of
genera _Aquilegia_, _Dianthus_, _Geum_, _Lavatera_, _Lychnis_, _Malva_,
_Nicotiana_, and _Œnothera_. The period of transformation was not alike
for all species. While with some a triple fertilisation sufficed,
with others this had to be repeated five or six times, and even in
the same species fluctuations were observed in various experiments.
Gärtner ascribes this difference to the circumstance that “the specific
[_typische_] force by which a species, during reproduction, effects
the change and transformation of the maternal type varies considerably
in different plants, and that, consequently, the periods within which
the one species is changed into the other must also vary, as also the
number of generations, so that the transformation in some species is
perfected in more, and in others in fewer generations.” Further, the
same observer remarks “that in these transformation experiments a good
deal depends upon which type and which individual be chosen for further
transformation.”
If it may be assumed that in these experiments the constitution of
the forms resulted in a similar way to that of _Pisum_, the entire
process of transformation would find a fairly simple explanation.
The hybrid forms as many kinds of egg cells as there are constant
combinations possible of the characters conjoined therein, and one
of these is always of the same kind as the fertilising pollen cells.
Consequently there always exists the possibility with all such
experiments that even from the second fertilisation there may result a
constant form identical with that of the pollen parent. Whether this
really be obtained depends in each separate case upon the number of
the experimental plants, as well as upon the number of differentiating
characters which are united by the fertilisation. Let us, for
instance, assume that the plants selected for experiment differed in
three characters, and the species _ABC_ is to be transformed into the
other species _abc_ by repeated fertilisation with the pollen of the
latter; the hybrids resulting from the first cross form eight different
kinds of egg cells, viz.:
_ABC_, _ABc_, _AbC_, _aBC_, _Abc_, _aBc_, _abC_, _abc_.
These in the second year of experiment are united again with the pollen
cells _abc_, and we obtain the series
_AaBbCc_ + _AaBbc_ + _AabCc_ + _aBbCc_ + _Aabc_ + _aBbc_ + _abCc_ +
_abc_.
Since the form _abc_ occurs once in the series of eight components,
it is consequently little likely that it would be missing among the
experimental plants, even were these raised in a smaller number,
and the transformation would be perfected already by a second
fertilisation. If by chance it did not appear, then the fertilisation
must be repeated with one of those forms nearest akin, _Aabc_, _aBbc_,
_abCc_. It is perceived that such an experiment must extend the farther
_the smaller the number of experimental plants and the larger the
number of differentiating characters_ in the two original species;
and that, furthermore, in the same species there can easily occur a
delay of one or even of two generations such as Gärtner observed.
The transformation of widely divergent species could generally only
be completed in five or six years of experiment, since the number of
different egg cells which are formed in the hybrid increases in square
ratio with the number of differentiating characters.
Gärtner found by repeated experiments that the respective period of
transformation varies in many species, so that frequently a species
_A_ can be transformed into a species _B_ a generation sooner
than can species _B_ into species _A_. He deduces therefrom that
Kölreuter’s opinion can hardly be maintained that “the two natures
in hybrids are perfectly in equilibrium.” It appears, however, that
Kölreuter does not merit this criticism, but that Gärtner rather has
overlooked a material point, to which he himself elsewhere draws
attention, viz. that “it depends which individual is chosen for further
transformation.” Experiments which in this connection were carried out
with two species of _Pisum_ demonstrated that as regards the choice of
the fittest individuals for the purpose of further fertilisation it
may make a great difference which of two species is transformed into
the other. The two experimental plants differed in five characters,
while at the same time those of species _A_ were all dominant and
those of species _B_ all recessive. For mutual transformation _A_
was fertilised with pollen of _B_, and _B_ with pollen of _A_, and
this was repeated with both hybrids the following year. With the
first experiment _B_/_A_ there were eighty-seven plants available in
the third year of experiment for the selections of individuals for
further crossing, and these were of the possible thirty-two forms;
with the second experiment _A_/_B_ seventy-three plants resulted,
which _agreed throughout perfectly in habit with the pollen parent_;
in their internal composition, however, they must have been just as
varied as the forms of the other experiment. A definite selection was
consequently only possible with the first experiment; with the second
some plants selected at random had to be excluded. Of the latter only
a portion of the flowers were crossed with the _A_ pollen, the others
were left to fertilise themselves. Among each five plants which were
selected in both experiments for fertilisation there agreed, as the
following year’s culture showed, with the pollen parent:--
1st Experiment. 2nd Experiment.
2 plants -- in all characters
3 " -- " 4 "
-- 2 plants " 3 "
-- 2 " " 2 "
-- 1 plant " 1 character
In the first experiment, therefore, the transformation was completed;
in the second, which was not continued further, two more fertilisations
would probably have been required.
Although the case may not frequently occur that the dominant characters
belong exclusively to one or the other of the original parent plants,
it will always make a difference which of the two possesses the
majority. If the pollen parent shows the majority, then the selection
of forms for further crossing will afford a less degree of security
than in the reverse case, which must imply a delay in the period of
transformation, provided that the experiment is only considered as
completed when a form is arrived at which not only exactly resembles
the pollen plant in form, but also remains as constant in its progeny.
Gärtner, by the results of these transformation experiments, was led to
oppose the opinion of those naturalists who dispute the stability of
plant species and believe in a continuous evolution of vegetation. He
perceives in the complete transformation of one species into another
an indubitable proof that species are fixed within limits beyond which
they cannot change. Although this opinion cannot be unconditionally
accepted we find on the other hand in Gärtner’s experiments a
noteworthy confirmation of that supposition regarding variability of
cultivated plants which has already been expressed.
Among the experimental species there were cultivated plants, such as
_Aquilegia atropurpurea_ and _canadensis_, _Dianthus caryophyllus_,
_chinensis_, and _japonicus_, _Nicotiana rustica_ and _paniculata_, and
hybrids between these species lost none of their stability after four
or five generations[49].
[49] [The argument of these two last paragraphs appears to be that
though the general mutability of natural species might be doubtful,
yet among cultivated plants the transference of characters may be
accomplished, and may occur by integral steps until one species is
definitely “transformed” into the other.]
ON HIERACIUM-HYBRIDS OBTAINED BY ARTIFICIAL FERTILISATION
By G. Mendel.
(_Communicated to the Meeting 9 June, 1869[50]._)
[50] [Published in _Verh. naturf. Ver. Brünn, Abhandlungen_, VIII.
1869, p. 26, which appeared in 1870.]
Although I have already undertaken many experiments in fertilisation
between species of _Hieracium_, I have only succeeded in obtaining the
following 6 hybrids, and only from one to three specimens of them.
_H. Auricula_ ♀ × _H. aurantiacum_ ♂
_H. Auricula_ ♀ × _H. Pilosella_ ♂
_H. Auricula_ ♀ × _H. pratense_ ♂
_H. echioides_[51] ♀ × _H. aurantiacum_ ♂
_H. præaltum_ ♀ × _H. flagellare_ Rchb. ♂
_H. præaltum_ ♀ × _H. aurantiacum_ ♂
[51] The plant used in this experiment is not exactly the typical _H.
echioides_. It appears to belong to the series transitional to _H.
præaltum_, but approaches more nearly to _H. echioides_ and for this
reason was reckoned as belonging to the latter.
The difficulty of obtaining a larger number of hybrids is due to the
minuteness of the flowers and their peculiar structure. On account of
this circumstance it was seldom possible to remove the anthers from
the flowers chosen for fertilisation without either letting pollen
get on to the stigma or injuring the pistil so that it withered away.
As is well known, the anthers are united to form a tube, which closely
embraces the pistil. As soon as the flower opens, the stigma, already
covered with pollen, protrudes. In order to prevent self-fertilisation
the anther-tube must be taken out before the flower opens, and for this
purpose the bud must be slit up with a fine needle. If this operation
is attempted at a time when the pollen is mature, which is the case two
or three days before the flower opens, it is seldom possible to prevent
self-fertilisation; for with every care it is not easily possible to
prevent a few pollen grains getting scattered and communicated to
the stigma. No better result has been obtained hitherto by removing
the anthers at an earlier stage of development. Before the approach
of maturity the tender pistil and stigma are exceedingly sensitive
to injury, and even if they are not actually injured, they generally
wither and dry up after a little time if deprived of their protecting
investments. I hope to obviate this last misfortune by placing the
plants after the operation for two or three days in the damp atmosphere
of a greenhouse. An experiment lately made with _H. Auricula_ treated
in this way gave a good result.
To indicate the object with which these fertilisation experiments were
undertaken, I venture to make some preliminary remarks respecting the
genus _Hieracium_. This genus possesses such an extraordinary profusion
of distinct forms that no other genus of plants can compare with it.
Some of these forms are distinguished by special peculiarities and
may be taken as type-forms of species, while all the rest represent
intermediate and transitional forms by which the type-forms are
connected together. The difficulty in the separation and delimitation
of these forms has demanded the close attention of the experts.
Regarding no other genus has so much been written or have so many and
such fierce controversies arisen, without as yet coming to a definite
conclusion. It is obvious that no general understanding can be arrived
at, so long as the value and significance of the intermediate and
transitional forms is unknown.
Regarding the question whether and to what extent hybridisation plays
a part in the production of this wealth of forms, we find very various
and conflicting views held by leading botanists. While some of them
maintain that this phenomenon has a far-reaching influence, others, for
example, Fries, will have nothing to do with hybrids in _Hieracia_.
Others take up an intermediate position; and while granting that
hybrids are not rarely formed between the species in a wild state,
still maintain that no great importance is to be attached to the fact,
on the ground that they are only of short duration. The [suggested]
causes of this are partly their restricted fertility or complete
sterility; partly also the knowledge, obtained by experiment, that in
hybrids self-fertilisation is always prevented if pollen of one of
the parent-forms reaches the stigma. On these grounds it is regarded
as inconceivable that _Hieracium_ hybrids can constitute and maintain
themselves as fully fertile and constant forms when growing near their
progenitors.
The question of the origin of the numerous and constant intermediate
forms has recently acquired no small interest since a famous
_Hieracium_ specialist has, in the spirit of the Darwinian teaching,
defended the view that these forms are to be regarded as [arising] from
the transmutation of lost or still existing species.
From the nature of the subject it is clear that without an exact
knowledge of the structure and fertility of the hybrids and the
condition of their offspring through several generations no one
can undertake to determine the possible influence exercised by
hybridisation over the multiplicity of intermediate forms in
_Hieracium_. The condition of the _Hieracium_ hybrids in the range
we are concerned with must necessarily be determined by experiments;
for we do not possess a complete theory of hybridisation, and we
may be led into erroneous conclusions if we take rules deduced from
observation of certain other hybrids to be Laws of hybridisation, and
try to apply them to _Hieracium_ without further consideration. If by
the experimental method we can obtain a sufficient insight into the
phenomenon of hybridisation in _Hieracium_, then by the help of the
experience which has been collected respecting the structural relations
of the wild forms, a satisfactory judgment in regard to this question
may become possible.
Thus we may express the object which was sought after in these
experiments. I venture now to relate the very slight results which I
have as yet obtained with reference to this object.
1. Respecting the structure of the hybrids, we have to record the
striking phenomenon that the forms hitherto obtained by similar
fertilisation are not identical. The hybrids _H. præaltum_ ♀ x _H.
aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ are each
represented by two, and _H. Auricula_ ♀ x _H. pratense_ ♂ by three
individuals, while as to the remainder only one of each has been
obtained.
If we compare the individual characters of the hybrids with the
corresponding characters of the two parent types, we find that they
sometimes present intermediate structures, but are sometimes so near
to one of the parent characters that the [corresponding] character
of the other has receded considerably or almost evades observation.
So, for instance, we see in one of the two forms of _H. Auricula_ ♀ x
_H. aurantiacum_ ♂ pure yellow disc-florets; only the petals of the
marginal florets are on the outside tinged with red to a scarcely
noticeable degree: in the other on the contrary the colour of these
florets comes very near to _H. aurantiacum_, only in the centre of the
disc the orange red passes into a deep golden-yellow. This difference
is noteworthy, for the flower-colour in _Hieracium_ has the value of a
constant character. Other similar cases are to be found in the leaves,
the peduncles, &c.
If the hybrids are compared with the parent types as regards the sum
total of their characters, then the two forms of _H. præaltum_ ♀ x _H.
aurantiacum_ ♂ constitute approximately intermediate forms which do
not agree in certain characters. On the contrary in _H. Auricula_ ♀
x _H. aurantiacum_ ♂ and in _H. Auricula_ ♀ x _H. pratense_ ♂ we see
the forms widely divergent, so that one of them is nearer to the one
and the other to the other parental type, while in the case of the
last-named hybrid there is still a third which is almost precisely
intermediate between them.
The conviction is then forced on us that we have here only single terms
in an unknown series which may be formed by the direct action of the
pollen of one species on the egg-cells of another.
2. With a single exception the hybrids in question form seeds capable
of germination. _H. echioides_ ♀ x _H. aurantiacum_ ♂ may be described
as fully fertile; _H. præaltum_ ♀ x _H. flagellare_ ♂ as fertile; _H.
præaltum_ ♀ x _H. aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. pratense_ ♂
as partially fertile; _H. Auricula_ ♀ x _H. Pilosella_ ♂ as slightly
fertile, and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ as unfertile. Of the
two forms of the last named hybrid, the red-flowered one was completely
sterile, but from the yellow-flowered one a single well-formed seed
was obtained. Moreover it must not pass unmentioned that among the
seedlings of the partially fertile hybrid _H. præaltum_ ♀ x _H.
aurantiacum_ ♂ there was one plant which possessed full fertility.
[3.] As yet the offspring produced by self-fertilisation of the hybrids
have not varied, but agree in their characters both with each other and
with the hybrid plant from which they were derived.
From _H. præaltum_ ♀ x _H. flagellare_ ♂ two generations have flowered;
from _H. echioides_ ♀ x _H. aurantiacum_ ♂, _H. præaltum_ ♀ x _H.
aurantiacum_ ♂, _H. Auricula_ ♀ x _H. Pilosella_ ♂ one generation in
each case has flowered.
4. The fact must be declared that in the case of the fully fertile
hybrid _H. echioides_ ♀ x _H. aurantiacum_ ♂ the pollen of the parent
types was not able to prevent self-fertilisation, though it was applied
in great quantity to the stigmas protruding through the anther-tubes
when the flowers opened.
From two flower-heads treated in this way seedlings were produced
resembling this hybrid plant. A very similar experiment, carried
out this summer with the partially fertile _H. præaltum_ ♀ x _H.
aurantiacum_ ♂ led to the conclusion that those flower-heads in which
pollen of the parent type or of some other species had been applied
to the stigmas, developed a notably larger number of seeds than those
which had been left to self-fertilisation alone. The explanation of
this result must only be sought in the circumstance that as a large
part of the pollen-grains of the hybrid, examined microscopically,
show a defective structure, a number of egg-cells capable of
fertilisation do not become fertilised by their own pollen in the
ordinary course of self-fertilisation.
It not rarely happens that in fully fertile species in the wild state
the formation of the pollen fails, and in many anthers not a single
good grain is developed. If in these cases seeds are nevertheless
formed, such fertilisation must have been effected by foreign pollen.
In this way hybrids may easily arise by reason of the fact that many
forms of insects, notably the industrial Hymenoptera, visit the flowers
of _Hieracia_ with great zeal and are responsible for the pollen
which easily sticks to their hairy bodies reaching the stigmas of
neighbouring plants.
From the few facts that I am able to contribute it will be evident
the work scarcely extends beyond its first inception. I must express
some scruple in describing in this place an account of experiments
just begun. But the conviction that the prosecution of the proposed
experiments will demand a whole series of years, and the uncertainty
whether it will be granted to me to bring the same to a conclusion have
determined me to make the present communication. By the kindness of Dr
Nägeli, the Munich Director, who was good enough to send me species
which were wanting, especially from the Alps, I am in a position to
include a larger number of forms in my experiments. I venture to hope
even next year to be able to contribute something more by way of
extension and confirmation of the present account.
If finally we compare the described result, still very uncertain,
with those obtained by crosses made between forms of _Pisum_, which
I had the honour of communicating in the year 1865, we find a very
real distinction. In _Pisum_ the hybrids, obtained from the immediate
crossing of two forms, have in all cases the same type, but their
posterity, on the contrary, are variable and follow a definite law in
their variations. In _Hieracium_ according to the present experiments
the exactly opposite phenomenon seems to be exhibited. Already in
describing the _Pisum_ experiments it was remarked that there are also
hybrids whose posterity do not vary, and that, for example, according
to Wichura the hybrids of _Salix_ reproduce themselves like pure
species. In _Hieracium_ we may take it we have a similar case. Whether
from this circumstance we may venture to draw the conclusion that the
polymorphism of the genera _Salix_ and _Hieracium_ is connected with
the special condition of their hybrids is still an open question, which
may well be raised but not as yet answered.
A DEFENCE OF MENDEL’S PRINCIPLES OF HEREDITY.
“_The most fertile men of science have made blunders, and their
consciousness of such slips has been retribution enough; it is only
their more sterile critics who delight to dwell too often and too
long on such mistakes._” BIOMETRIKA, 1901.
INTRODUCTORY.
On the rediscovery and confirmation of Mendel’s Law by de Vries,
Correns, and Tschermak two years ago, it became clear to many
naturalists, as it certainly is to me, that we had found a principle
which is destined to play a part in the Study of Evolution comparable
only with the achievement of Darwin--that after the weary halt of forty
years we have at last begun to march.
If we look back on the post-Darwinian period we recognize one notable
effort to advance. This effort--fruitful as it proved, memorable as it
must ever be--was that made by Galton when he enuntiated his Law of
Ancestral Heredity, subsequently modified and restated by Karl Pearson.
Formulated after long and laborious inquiry, this principle beyond
question gives us an expression including and denoting many phenomena
in which previously no regularity had been detected. But to practical
naturalists it was evident from the first that there are great groups
of facts which could not on any interpretation be brought within the
scope of Galton’s Law, and that by no emendation could that Law be
extended to reach them. The existence of these phenomena pointed to a
different physiological conception of heredity. Now it is precisely
this conception that Mendel’s Law enables us to form. Whether the
Mendelian principle can be extended so as to include some apparently
Galtonian cases is another question, respecting which we have as yet no
facts to guide us, but we have certainly no warrant for declaring such
an extension to be impossible.
Whatever answer the future may give to that question, it is clear from
this moment that every case which obeys the Mendelian principle is
removed finally and irretrievably from the operations of the Law of
Ancestral Heredity.
At this juncture Professor Weldon intervenes as a professed exponent
of Mendel’s work. It is not perhaps to a devoted partisan of the Law
of Ancestral Heredity that we should look for the most appreciative
exposition of Mendel, but some bare measure of care and accuracy in
representation is demanded no less in justice to fine work, than by the
gravity of the issue.
Professor Weldon’s article appears in the current number of
_Biometrika_, Vol. I. Pt. II. which reached me on Saturday, Feb. 8.
The paper opens with what purports to be a restatement of Mendel’s
experiments and results. In this “restatement” a large part of Mendel’s
experiments--perhaps the most significant--are not referred to at
all. The perfect simplicity and precision of Mendel’s own account are
destroyed; with the result that the reader of Professor Weldon’s paper,
unfamiliar with Mendel’s own memoir, can scarcely be blamed if he fail
to learn the essence of the discovery. Of Mendel’s conception of the
hybrid as a distinct entity with characters proper to itself, apart
from inheritance--the most novel thing in the whole paper--Professor
Weldon gives no word. Upon this is poured an undigested mass of
miscellaneous “facts” and statements from which the reader is asked
to conclude, first, that a proposition attributed to Mendel regarding
dominance of one character is not of “general”[52] application, and
finally that “all work based on Mendel’s method” is “vitiated” by a
“fundamental mistake,” namely “the neglect of ancestry[53].”
[52] The words “general” and “universal” appear to be used by
Professor Weldon as interchangeable. Cp. Weldon, p. 235 and
elsewhere, with Abstract given below.
[53] These words occur p. 252: “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, &c.” As a matter of fact
the view indicated in these last words is especially repugnant to the
Mendelian principle, as will be seen.
To find a parallel for such treatment of a great theme in biology we
must go back to those writings of the orthodox which followed the
appearance of the “Origin of Species.”
On 17th December 1900 I delivered a Report to the Evolution Committee
of the Royal Society on the experiments in Heredity undertaken by Miss
E. R. Saunders and myself. This report has been offered to the Society
for publication and will I understand shortly appear. In it we have
attempted to show the extraordinary significance of Mendel’s principle,
to point out what in his results is essential and what subordinate, the
ways in which the principle can be extended to apply to a diversity
of more complex phenomena--of which some are incautiously cited by
Professor Weldon as conflicting facts--and lastly to suggest a few
simple terms without which (or some equivalents) the discussion of such
phenomena is difficult. Though it is impossible here to give an outline
of facts and reasoning there set out at length, I feel that his article
needs an immediate reply. Professor Weldon is credited with exceptional
familiarity with these topics, and his paper is likely to be accepted
as a sufficient statement of the case. Its value will only be known to
those who have either worked in these fields themselves or have been at
the trouble of thoughtfully studying the original materials.
The nature of Professor Weldon’s article may be most readily indicated
if I quote the summary of it issued in a paper of abstracts sent out
with Review copies of the Part. This paper was most courteously sent to
me by an editor of _Biometrika_ in order to call my attention to the
article on Mendel, a subject in which he knew me to be interested. The
abstract is as follows.
“Few subjects have excited so much interest in the last year or two
as the laws of inheritance in hybrids. Professor W. F. R. Weldon
describes the results obtained by Mendel by crossing races of Peas
which differed in one or more of seven characters. From a study of
the work of other observers, and from examination of the ‘Telephone’
group of hybrids, the conclusion is drawn that Mendel’s results
do not justify any general statement concerning inheritance in
cross-bred Peas. A few striking cases of other cross-bred plants and
animals are quoted to show that the results of crossing cannot, as
Mendel and his followers suggest, be predicted from a knowledge of
the characters of the two parents crossed without knowledge of the
more remote ancestry.”
Such is the judgment a fellow-student passes on this mind
“_Voyaging through strange seas of thought alone._”
The only conclusion which most readers could draw from this abstract
and indeed from the article it epitomizes, is that Mendel’s discovery
so far from being of paramount importance, rests on a basis which
Professor Weldon has shown to be insecure, and that an error has come
in through disregard of the law of Ancestral Heredity. On examining the
paper it is perfectly true that Professor Weldon is careful nowhere
directly to question Mendel’s facts or his interpretation of them,
for which indeed in some places he even expresses a mild enthusiasm,
but there is no mistaking the general purpose of the paper. It must
inevitably produce the impression that the importance of the work
has been greatly exaggerated and that supporters of current views on
Ancestry may reassure themselves. That this is Professor Weldon’s own
conclusion in the matter is obvious. After close study of his article
it is evident to me that Professor Weldon’s criticism is baseless and
for the most part irrelevant, and I am strong in the conviction that
the cause which will sustain damage from this debate is not that of
Mendel.
I. THE MENDELIAN PRINCIPLE OF PURITY OF GERM-CELLS AND THE LAWS OF
HEREDITY BASED ON ANCESTRY.
Professor Weldon’s article is entitled “Mendel’s Laws of Alternative
Inheritance in Peas.” This title expresses the scope of Mendel’s work
and discovery none too precisely and even exposes him to distinct
misconception.
To begin with, it says both too little and too much. Mendel did
certainly determine Laws of Inheritance in peas--not precisely the
laws Professor Weldon has been at the pains of drafting, but of that
anon. Having done so, he knew what his discovery was worth. He saw,
and rightly, that he had found a principle which _must_ govern a wide
area of phenomena. He entitles his paper therefore “_Versuche über
Pflanzen-Hybriden_,” or, Experiments in Plant-Hybridisation.
Nor did Mendel start at first with any particular intention respecting
Peas. He tells us himself that he wanted to find the laws of
inheritance in _hybrids_, which he suspected were definite, and that
after casting about for a suitable subject, he found one in peas, for
the reasons he sets out.
In another respect the question of title is much more important. By
the introduction of the word “Alternative” the suggestion is made that
the Mendelian principle applies peculiarly to cases of “alternative”
inheritance. Mendel himself makes no such limitation in his earlier
paper, though perhaps by rather remote implication in the second, to
which the reader should have been referred. On the contrary, he wisely
abstains from prejudicial consideration of unexplored phenomena.
* * * * *
To understand the significance of the word “alternative” as introduced
by Professor Weldon we must go back a little in the history of these
studies. In the year 1897 Galton formally announced the Law of
Ancestral Heredity referred to in the _Introduction_, having previously
“stated it briefly and with hesitation” in _Natural Inheritance_,
p. 134. In 1898 Professor Pearson published his modification and
generalisation of Galton’s Law, introducing a correction of admitted
theoretical importance, though it is not in question that the principle
thus restated is fundamentally not very different from Galton’s[54].
_It is an essential part of the Galton-Pearson Law of Ancestral
Heredity that in calculating the probable structure of each descendant
the structure of each several ancestor must be brought to account._
[54] I greatly regret that I have not a precise understanding of the
basis of the modification proposed by Pearson. His treatment is in
algebraical form and beyond me. Nevertheless I have every confidence
that the arguments are good and the conclusion sound. I trust it may
not be impossible for him to provide the non-mathematical reader with
a paraphrase of his memoir. The arithmetical differences between the
original and the modified law are of course clear.
Professor Weldon now tells us that these two papers of Galton and of
Professor Pearson have “given us an expression for the effects of
_blended_ inheritance which seems likely to prove generally applicable,
though the constants of the equations which express the relation
between divergence from the mean in one generation, and that in
another, may require modification in special cases. Our knowledge of
_particulate_ or mosaic inheritance, and of _alternative_ inheritance,
is however still rudimentary, and there is so much contradiction
between the results obtained by different observers, that the evidence
available is difficult to appreciate.”
But Galton stated (p. 401) in 1897 that his statistical law of heredity
“appears to be universally applicable to bi-sexual descent.” Pearson
in re-formulating the principle in 1898 made no reservation in regard
to “alternative” inheritance. On the contrary he writes (p. 393) that
“if Mr Galton’s law can be firmly established, _it is a complete
solution, at any rate to a first approximation, of the whole problem
of heredity_,” and again (p. 412) that “it is highly probable that
it [this law] is the simple descriptive statement which brings into
a single focus all the complex lines of hereditary influence. If
Darwinian evolution be natural selection combined with _heredity_,
then the single statement which embraces the whole field of heredity
must prove almost as epoch-making as the law of gravitation to the
astronomer[55].”
[55] I have searched Professor Pearson’s paper in vain for any
considerable reservation regarding or modification of this general
statement. Professor Pearson enuntiates the law as “only correct
on certain limiting hypotheses,” but he declares that of these the
most important is “the absence of reproductive selection, i.e. the
negligible correlation of fertility with the inherited character, and
the absence of sexual selection.” The case of in-and-in breeding is
also reserved.
As I read there comes into my mind that other fine passage where
Professor Pearson warns us
“There is an insatiable desire in the human breast to resume in some
short formula, some brief statement, the facts of human experience.
It leads the savage to ‘account’ for all natural phenomena by
deifying the wind and the stream and the tree. It leads civilized
man, on the other hand, to express his emotional experience in works
of art, and his physical and mental experience in the formulae or
so-called laws of science[56].”
[56] K. Pearson, _Grammar of Science_, 2nd ed. 1900, p. 36.
No naturalist who had read Galton’s paper and had tried to apply it to
the facts he knew could fail to see that here was a definite advance.
We could all perceive phenomena that were in accord with it and there
was no reasonable doubt that closer study would prove that accord to
be close. It was indeed an occasion for enthusiasm, though no one
acquainted with the facts of experimental breeding could consider the
suggestion of universal application for an instant.
But two years have gone by, and in 1900 Pearson writes[57] that the
values obtained from the Law of Ancestral Heredity
[57] _Grammar of Science_, 2nd ed. 1900, p. 480.
“seem to fit the observed facts fairly well in the case of _blended_
inheritance. In other words we have a certain amount of evidence in
favour of the conclusion: _That whenever the sexes are equipotent,
blend their characters and mate pangamously, all characters will be
inherited at the same rate_,”
or, again in other words, that the Law of Ancestral Heredity after
the glorious launch in 1898 has been home for a complete refit. The
top-hamper is cut down and the vessel altogether more manageable;
indeed she looks trimmed for most weathers. Each of the qualifications
now introduced wards off whole classes of dangers. Later on (pp. 487–8)
Pearson recites a further list of cases regarded as exceptional. “All
characters will be inherited at the same rate” might indeed almost be
taken to cover the results in Mendelian cases, though the mode by which
those results are arrived at is of course wholly different.
Clearly we cannot speak of the Law of Gravitation now. Our Tycho Brahe
and our Kepler, with the yet more distant Newton, are appropriately
named as yet to come[58].
[58] _Phil. Trans._ 1900, vol. 195, A, p. 121.
But the truth is that even in 1898 such a comparison was scarcely
happy. Not to mention moderns, these high hopes had been finally
disposed of by the work of the experimental breeders such as Kölreuter,
Knight, Herbert, Gärtner, Wichura, Godron, Naudin, and many more. To
have treated as non-existent the work of this group of naturalists,
who alone have attempted to solve the problems of heredity and
species--Evolution, as we should now say--by the only sound
method--_experimental breeding_--to leave out of consideration almost
the whole block of evidence collected in _Animals and Plants_--Darwin’s
finest legacy as I venture to declare--was unfortunate on the part of
any exponent of Heredity, and in the writings of a professed naturalist
would have been unpardonable. But even as modified in 1900 the Law
of Ancestral Heredity is heavily over-sparred, and any experimental
breeder could have increased Pearson’s list of unconformable cases by
as many again.
But to return to Professor Weldon. He now repeats that the Law of
Ancestral Heredity seems likely to prove generally applicable to
_blended_ inheritance, but that the case of _alternative_ inheritance
is for the present reserved. We should feel more confidence in
Professor Weldon’s exposition if he had here reminded us that the
special case which fitted Galton’s Law so well that it emboldened
him to announce that principle as apparently “universally applicable
to bi-sexual descent” was one of _alternative_ inheritance--namely
the coat-colour of Basset-hounds. Such a fact is, to say the least,
ominous. Pearson, in speaking (1900) of this famous case of Galton’s,
says that these phenomena of alternative inheritance must be treated
separately (from those of blended inheritance)[59], and for them he
deduces a proposed “_law of reversion_,” based of course on ancestry.
He writes, “In both cases we may speak of a law of ancestral heredity,
but the first predicts the probable character of the individual
produced by a given ancestry, while the second tells us the
percentages of the total offspring which on the average revert to each
ancestral type[60].”
[59] “If this be done, we shall, I venture to think, keep not only
our minds, but our points for observation, clearer; and further, the
failure of Mr Galton’s statement in the one case will not in the
least affect its validity in the other.” Pearson (32), p. 143.
[60] _Grammar of Science_, 1900, p. 494. See also Pearson, _Proc.
Roy. Soc._ 1900, LXVI. pp. 142–3.
With the distinctions between the original Law of Ancestral Heredity,
the modified form of the same law, and the Law of Reversion, important
as all these considerations are, we are not at present concerned.
For the Mendelian principle of heredity asserts a proposition
absolutely at variance with all the laws of ancestral heredity, however
formulated. In those cases to which it applies strictly, this principle
declares that the cross-breeding of parents _need_ not diminish
the purity of their germ-cells or consequently the purity of their
offspring. When in such cases individuals bearing opposite characters,
_A_ and _B_, are crossed, the germ-cells of the resulting cross-bred,
_AB_, are each to be bearers either of character A or of character _B_,
not both.
Consequently when the cross-breds breed either together or with the
pure forms, individuals will result of the forms _AA_, _AB_, _BA_,
_BB_[61]. Of these the forms _AA_ and _BB_, formed by the union of
similar germs, are stated to be as pure as if they had had no cross in
their pedigree, and henceforth their offspring will be no more likely
to depart from the _A_ type or the _B_ type respectively, than those of
any other originally pure specimens of these types.
[61] On an average of cases, in equal numbers, as Mendel found.
Consequently in such examples it is _not_ the fact that each ancestor
must be brought to account as the Galton-Pearson Law asserts, and we
are clearly dealing with a physiological phenomenon not contemplated by
that Law at all.
Every case therefore which obeys the Mendelian principle is in direct
contradiction to the proposition to which Professor Weldon’s school is
committed, and it is natural that he should be disposed to consider
the Mendelian principle as applying especially to “alternative”
inheritance, while the law of Galton and Pearson is to include the
phenomenon of blended inheritance. The latter, he tells us, is “the
most usual case,” a view which, if supported by evidence, might not be
without value.
It is difficult to blame those who on first acquaintance concluded
Mendel’s principle can have no strict application save to alternative
inheritance. Whatever blame there is in this I share with Professor
Weldon and those whom he follows. Mendel’s own cases were almost all
alternative; also the fact of dominance is very dazzling at first. But
that was two years ago, and when one begins to see clearly again, it
does not look so certain that the real essence of Mendel’s discovery,
the purity of germ-cells in respect of certain characters, may not
apply also to some phenomena of blended inheritance. The analysis of
this possibility would take us to too great length, but I commend to
those who are more familiar with statistical method, the consideration
of this question: whether dominance being absent, indefinite, or
suppressed, the phenomena of heritages completely blended in the
zygote, may not be produced by gametes presenting Mendelian purity of
characters. A brief discussion of this possibility is given in the
Introduction, p. 31.
Very careful inquiry would be needed before such a possibility could
be negatived. For example, we know that the Laws based on Ancestry
can apply to _alternative_ inheritance; witness the case of the
Basset-hounds. Here there is no simple Mendelian dominance; but are we
sure there is no purity of germ-cells? The new conception goes a long
way and it may well reach to such facts as these.
But for the present we will assume that Mendel’s principle applies only
to _certain phenomena of alternative inheritance_, which is as far as
our warrant yet runs.
No close student of the recent history of evolutionary thought needs
to be told what the attitude of Professor Weldon and his followers
has been towards these same disquieting and unwelcome phenomena of
alternative inheritance and discontinuity in variation. Holding at
first each such fact for suspect, then treating them as rare and
negligible occurrences, he and his followers have of late come slowly
to accede to the facts of discontinuity a bare and grudging recognition
in their scheme of evolution[62].
[62] Read in this connexion Pearson, K., _Grammar of Science_, 2nd
ed. 1900, pp. 390–2.
Professor Weldon even now opens his essay with the statement--or
perhaps reminiscence--that “it is perfectly possible and indeed
probable that the difference between these forms of inheritance
[blended, mosaic, and alternative] is only one of degree.” This may
be true; but reasoning favourable to this proposition could equally
be used to prove the difference between mechanical mixture and
chemical combination to be a difference of degree.
Therefore on the announcement of that discovery which once and for all
ratifies and consolidates the conception of discontinuous variation,
and goes far to define that of alternative inheritance, giving a finite
body to what before was vague and tentative, it is small wonder if
Professor Weldon is disposed to criticism rather than to cordiality.
* * * * *
We have now seen what is the essence of Mendel’s discovery based on a
series of experiments of unequalled simplicity which Professor Weldon
does not venture to dispute.
II. MENDEL AND THE CRITIC’S VERSION OF HIM.
_The “Law of Dominance.”_
I proceed to the question of dominance which Professor Weldon treats as
a prime issue, almost to the virtual concealment of the great fact of
gametic purity.
Cross-breds in general, _AB_ and _BA_, named above, may present many
appearances. They may all be indistinguishable from _A_, or from _B_;
some may appear _A_’s and some _B_’s; they may be patchworks of both;
they may be blends presenting one or many grades between the two; and
lastly they _may have an appearance special to themselves_ (_being in
the latter case, as it often happens, “reversionary”_), a possibility
which Professor Weldon does not stop to consider, though it is the clue
that may unravel many of the facts which mystify him now.
Mendel’s discovery became possible because he worked with regular
cases of the first category, in which he was able to recognize that
_one_ of each of the pairs of characters he studied _did_ thus prevail
and _was_ “dominant” in the cross-bred to the exclusion of the other
character. This fact, which is still an accident of particular cases,
Professor Weldon, following some of Mendel’s interpreters, dignifies by
the name of the “Law of Dominance,” though he omits to warn his reader
that Mendel states no “Law of Dominance” whatever. The whole question
whether one or other character of the antagonistic pair is dominant
though of great importance is logically a subordinate one. It depends
on the specific nature of the varieties and individuals used, sometimes
probably on the influence of external conditions and on other factors
we cannot now discuss. There is as yet no universal law here perceived
or declared.
Professor Weldon passes over the proof of the purity of the germ-cells
lightly enough, but this proposition of dominance, suspecting its
weakness, he puts prominently forward. Briefest equipment will
suffice. Facing, as he supposes, some new pretender--some local
Theudas--offering the last crazy prophecy,--any argument will do
for such an one. An eager gathering in an unfamiliar literature, a
scrutiny of samples, and he will prove to us with small difficulty that
dominance of yellow over green, and round over wrinkled, is irregular
even in peas after all; that in the sharpness of the discontinuity
exhibited by the variations of peas there are many grades; that many
of these grades co-exist in the same variety; that some varieties may
perhaps be normally intermediate. All these propositions are supported
by the production of a collection of evidence, the quality of which we
shall hereafter consider. “Enough has been said,” he writes (p. 240),
“to show the grave discrepancy between the evidence afforded by
Mendel’s own experiments and that obtained by other observers, equally
competent and trustworthy.”
We are asked to believe that Professor Weldon has thus discovered “a
fundamental mistake” vitiating all that work, the importance of which,
he elsewhere tells us, he has “no wish to belittle.”
III. THE FACTS IN REGARD TO DOMINANCE OF CHARACTERS IN PEAS.
Professor Weldon refers to no experiments of his own and presumably
has made none. Had he done so he would have learnt many things about
dominance in peas, whether of the yellow cotyledon-colour or of the
round form, that might have pointed him to caution.
In the year 1900 Messrs Vilmorin-Andrieux & Co. were kind enough to
send to the Cambridge Botanic Garden on my behalf a set of samples
of the varieties of _Pisum_ and _Phaseolus_, an exhibit of which
had greatly interested me at the Paris Exhibition of that year. In
the past summer I grew a number of these and made some preliminary
cross-fertilizations among them (about 80 being available for these
deductions) with a view to a future study of certain problems,
Mendelian and others. In this work I had the benefit of the assistance
of Miss Killby of Newnham College. Her cultivations and crosses were
made independently of my own, but our results are almost identical. The
experience showed me, what a naturalist would expect and practical men
know already, that _a great deal turns on the variety used_; that some
varieties are very sensitive to conditions while others maintain their
type sturdily; that in using certain varieties Mendel’s experience
as to dominance is regularly fulfilled, while in the case of other
varieties irregularities and even some contradictions occur. That the
dominance of yellow cotyledon-colour over green, and the dominance of
the smooth form over the wrinkled, is a _general_ truth for _Pisum
sativum_ appears at once; that it is a universal truth I cannot believe
any competent naturalist would imagine, still less assert. Mendel
certainly never did. When he speaks of the “law” or “laws” that he
has established for _Pisum_ he is referring to his own discovery of
the purity of the germ-cells, that of the statistical distribution of
characters among them, and the statistical grouping of the different
germ-cells in fertilization, and not to the “Law of Dominance” which he
never drafted and does not propound.
The issue will be clearer if I here state briefly what, as far
as my experience goes, are the facts in regard to the characters
_cotyledon-colour_ and _seed-shapes_ in peas. I have not opportunity
for more than a passing consideration of the _seed-coats_ of pure
forms[63]; that is a maternal character, a fact I am not sure Professor
Weldon fully appreciates. Though that may be incredible, it is evident
from many passages that he has not, in quoting authorities, considered
the consequences of this circumstance.
[63] The whole question as to seed-coat colour is most complex.
Conditions of growth and ripening have a great effect on it. Mr
Arthur Sutton has shown me samples of _Ne Plus Ultra_ grown in
England and abroad. This pea has yellow cotyledons with seed-coats
either yellow or “blue.” The foreign sample contained a much greater
proportion of the former. He told me that generally speaking this is
the case with samples ripened in a hot, dry climate.
Unquestionable Xenia appears occasionally, and will be spoken of
later. Moreover to experiment with such a _plant_-character an extra
generation has to be sown and cultivated. Consequently the evidence
is meagre.
_The normal characters: colour of cotyledons and seed-coats._
Culinary peas (_P. sativum_, omitting purple sorts) can primarily be
classified on colour into two groups, yellow and green. In the green
certain pigmentary matters persist in the ripe seed which disappear
or are decomposed in the yellow as the seed ripens. But it may be
observed that the “green” class itself is treated as of two divisions,
_green_ and _blue_. In the seedsmen’s lists the classification is made
on the _external appearance_ of the seed, without regard to whether
the colour is due to the seed-coat, the cotyledons, or both. As a rule
perhaps yellow coats contain yellow cotyledons, and green coats green
cotyledons, though yellow cotyledons in green coats are common, e.g.
_Gradus_, of which the cotyledons are yellow while the seed-coats
are about as often green as yellow (or “white,” as it is called
technically). Those called “blue” consist mostly of seeds which have
green cotyledons seen through transparent skins, or yellow cotyledons
combined with green skins. The skins may be roughly classified into
thin and transparent, or thick and generally at some stage pigmented.
In numerous varieties the colour of the cotyledon is wholly yellow,
or wholly green. Next there are many varieties which are constant in
habit and other properties but have seeds belonging to these two colour
categories in various proportions. How far these proportions are known
to be constant I cannot ascertain.
Of such varieties showing mixture of _cotyledon_-colours nearly all can
be described as dimorphic in colour. For example in Sutton’s _Nonpareil
Marrowfat_ the cotyledons are almost always _either_ yellow _or_ green,
with some piebalds, and the colours of the seed-coats are scarcely
less distinctly dimorphic. In some varieties which exist in both
colours intermediates are so common that one cannot assert any regular
dimorphism[64].
[64] Knowing my interest in this subject Professor Weldon was so good
as to forward to me a series of his peas arranged to form a scale of
colours and shapes, as represented in his Plate I. I have no doubt
that the use of such colour-scales will much facilitate future study
of these problems.
There are some varieties which have cotyledons green and intermediate
shading to greenish yellow, like _Stratagem_ quoted by Professor
Weldon. Others have yellow and intermediate shading to yellowish green,
such as McLean’s _Best of all_[65]. I am quite disposed to think
there may be truly monomorphic varieties with cotyledons permanently
of intermediate colour only, but so far I have not seen one[66]. The
variety with greatest _irregularity_ (apart from regular dimorphism)
in cotyledon-colour I have seen is a sample of “_mange-tout à rames, à
grain vert_,” but it was a good deal injured by weevils (_Bruchus_),
which always cause irregularity or change of colour.
[65] I notice that Vilmorin in the well-known _Plantes Potagères_,
1883, classifies the intermediate-coloured peas with the _green_.
[66] Similarly though _tall_ and _dwarf_ are Mendelian characters,
peas occur of all heights and are usually classified as tall,
half-dwarfs, and dwarfs.
Lastly in some varieties there are many piebalds or mosaics.
From what has been said it will be evident that the description of a
pea in an old book as having been green, blue, white, and so forth,
unless the cotyledon-colour is distinguished from seed-coat colour,
needs careful consideration before inferences are drawn from it.
_Shape._
In regard to shape, if we keep to ordinary shelling peas, the facts are
somewhat similar, but as shape is probably more sensitive to conditions
than cotyledon-colour (not than _seed-coat_ colour) there are
irregularities to be perhaps ascribed to this cause. Broadly, however,
there are two main divisions, round and wrinkled. It is unquestioned
that between these two types every intermediate occurs. Here again
a vast number of varieties can be at once classified into round and
wrinkled (the classification commonly used), others are intermediate
normally. Here also I suspect some fairly clear sub-divisions might be
made in the wrinkled group and in the round group too, but I would not
assert this as a fact.
I cannot ascertain from botanists what is the nature of the difference
between round and wrinkled peas, though no doubt it will be easily
discovered. In maize the round seeds contain much unconverted starch,
while in the wrinkled or sugar-maize this seems to be converted in
great measure as the seed ripens; with the result that, on drying, the
walls collapse. In such seeds we may perhaps suppose that the process
of conversion, which in round seeds takes place on germination, is
begun earlier, and perhaps the variation essentially consists in the
premature appearance of the converting ferment. It would be most rash
to suggest that such a process may be operating in the pea, for the
phenomenon may have many causes; but however that may be, there is
evidently a difference of such a nature that when the water dries out
of the seed on ripening, its walls collapse[67]; and this collapse may
occur in varying degrees.
[67] Wrinkling must of course be distinguished further from the
squaring due to the peas pressing against each other in the pod.
In connexion with these considerations I may mention that Vilmorin
makes the interesting statement that most peas retain their vitality
three years, dying as a rule rapidly after that time is passed,
though occasionally seeds seven or eight years old are alive; but
that _wrinkled_ peas germinate as a rule less well than round, and do
not retain their vitality so long as the round. Vilmorin-Andrieux,
_Plantes Potagères_, 1883, p. 423. Similar statements regarding the
behaviour of wrinkled peas in India are made by Firminger, _Gardening
for India_, 3rd ed. 1874, p. 146.
In respect of _shape_ the seeds of a variety otherwise stable are
as a rule fairly uniform, the co-existence of both shapes and of
intermediates between them in the same variety is not infrequent. As
Professor Weldon has said, _Telephone_ is a good example of an extreme
case of mixture of both colours and shapes. _William I._ is another.
It may be mentioned that regular dimorphism in respect of shape is
not so common as dimorphism in respect of colour. Of great numbers of
varieties seen at Messrs Suttons’ I saw none so distinctly dimorphic in
shape as _William I._ which nevertheless contains all grades commonly.
So far I have spoken of the shapes of ordinary English culinary peas.
But if we extend our observations to the shapes of _large-seeded_ peas,
which occur for the most part among the sugar-peas (_mange-touts_),
of the “grey” peas with coloured flowers, etc., there are fresh
complications to be considered.
Professor Weldon does not wholly avoid these (as Mendel did in regard
to shape) and we will follow him through his difficulties hereafter.
For the present let me say that the classes _round_ and _wrinkled_ are
not readily applicable to those other varieties and are not so applied
either by Mendel or other practical writers on these subjects. To use
the terms indicated in the Introduction, _seed-shape_ depends on more
than one pair of allelomorphs--possibly on several.
_Stability and Variability._
Generally speaking peas which when seen in bulk are monomorphic in
colour and shape, will give fairly true and uniform offspring (but
such strict monomorphism is rather exceptional). Instances to the
contrary occur, and in my own brief experience I have seen some.
In a row of _Fill-basket_ grown from selected seed there were two
plants of different habit, seed-shape, etc. Each bore pods with seeds
few though large and round. Again _Blue Peter_ (blue and round) and
_Laxton’s Alpha_ (blue and wrinkled), grown in my garden and left to
nature uncovered, have each given a considerable proportion of seeds
with _yellow_ cotyledons, about 20% in the case of _Laxton’s Alpha_.
The distribution of these on the plants I cannot state. The plants
bearing them in each case sprang from green-cotyledoned seeds taken
from samples containing presumably unselected green seeds only. A part
of this exceptional result may be due to crossing, but heterogeneity of
conditions[68] especially in or after ripening is a more likely cause,
hypotheses I hope to investigate next season. Hitherto I had supposed
the crossing, if any, to be done by _Bruchus_ or Thrips, but Tschermak
also suspects _Megachile_, the leaf-cutter bee, which abounds in my
garden.
[68] Cotyledon-colour is not nearly so sensitive to ordinary changes
in conditions as coat-colour, provided the coat be uninjured. But
even in monomorphic _green_ varieties, a seed which for any cause has
burst on ripening, has the exposed parts of its cotyledons _yellow_.
The same may be the case in seeds of green varieties injured by
_Bruchus_ or birds. These facts make one hesitate before denying
the effects of conditions on the cotyledon-colour even of uninjured
seeds, and the variation described above may have been simply
weathering. The seeds were gathered very late and many were burst in
_Laxton’s Alpha_. I do not yet know they are alive.
Whatever the cause, these irregularities may undoubtedly occur; and if
they be proved to be largely independent of crossing and conditions,
this will in nowise vitiate the truth of the Mendelian principle.
For in that case it may simply be variability. Such true variation,
or sporting, in the pea is referred to by many observers. Upon this
subject I have received most valuable facts from Mr Arthur Sutton,
who has very kindly interested himself in these inquiries. He tells
me that several highly bred varieties, selected with every possible
care, commonly throw a small but constant proportion of poor and almost
vetch-like plants, with short pods and small round seeds, which are
hoed out by experienced men each year before ripening. Other high-class
varieties always, wherever grown, and when far from other sorts,
produce a small percentage of some one or more definite “sports.” Of
these peculiar sports he has sent me a collection of twelve, taken from
as many standard varieties, each “sport” being represented by eight
seeds, which though quite distinct from the type agree with each other
in almost all cases.
In two cases, he tells me, these seed-sports sown separately have
been found to give plants identical with the standard type and must
therefore be regarded as sports in _seed characters_ only; in other
cases change of plant-type is associated with the change of seed-type.
In most standard varieties these definite sports are not very common,
but in a few they are common enough to require continual removal by
selection[69].
[69] It is interesting to see that in at least one case the same--or
practically the same--variety has been independently produced by
different raisers, as we now perceive, by the fortuitous combination
of similar allelomorphs. _Sutton’s Ringleader_ and _Carter’s First
Crop_ (and two others) are cases in point, and it is peculiarly
instructive to see that in the discussion of these varieties when
they were new, one of the points indicating their identity was taken
to be the fact that they produced _the same “rogues.”_ See _Gard.
Chron._ 1865, pp. 482 and 603; 1866, p. 221; 1867, pp. 546 and 712.
Rimpau quotes Blomeyer (_Kultur der Landw. Nutzpflanzen_, Leipzig,
1889, pp. 357 and 380) to the effect that _purple_-flowered plants
with _wrinkled_ seeds may spring as direct sports from peas with
_white_ flowers and _round_ seeds. I have not seen a copy of
Blomeyer’s work. Probably this “wrinkling” was “indentation.”
I hope before long to be able to give statistical details and
experiments relating to this extraordinarily interesting subject. As
de Vries writes in his fine work _Die Mutationstheorie_ (I. p. 580),
“a study of the seed-differences of inconstant, or as they are
called, ‘still’ unfixed varieties, is a perfect treasure-house of new
discoveries.”
Let us consider briefly the possible significance of these facts in the
light of Mendelian teaching. First, then, it is clear that as regards
most of such cases the hypothesis is not excluded that these recurring
sports may be due to the fortuitous concurrence of certain scarcer
hypallelomorphs, which may either have been free in the original parent
varieties from which the modern standard forms were raised, or may have
been freed in the crossing to which the latter owe their origin (see
p. 28). This possibility raises the question whether, if we could make
“_pure_ cultures” of the gametes, any variations of this nature would
ever occur. This may be regarded as an unwarrantable speculation, but
it is not wholly unamenable to the test of experiments.
But variability, in the sense of division of gonads into heterogeneous
gametes, may surely be due to causes other than crossing. This we
cannot doubt. Cross-fertilization of the zygote producing those gametes
is _one_ of the causes of such heterogeneity among them. We cannot
suppose it to be the sole cause of this phenomenon.
When Mendel asserts the purity of the germ-cells of cross-breds he
cannot be understood to mean that they are _more pure_ than those of
the original parental races. These must have varied in the past. The
wrinkled seed arose from the round, the green from the yellow (or _vice
versâ_, if preferred), and probably numerous intermediate forms from
both.
The variations, or as I provisionally conceive it, that differentiant
division among the gametes of which variation (neglecting environment)
is the visible expression, has arisen and can arise at one or more
points of time, and we have no difficulty in believing it to occur now.
In many cases we have clear evidence that it does. Crossing,--dare
we call it asymmetrical fertilization?--is _one_ of the causes of
the production of heterogeneous gametes--the result of divisions
qualitatively differentiant and perhaps asymmetrical[70].
[70] The asymmetries here conceived may of course be combined in
an inclusive symmetry. Till the differentiation can be optically
recognized in the gametes we shall probably get no further with this
part of the problem.
There are other causes and we have to find them. Some years ago I
wrote that consideration of the causes of variation was in my judgment
premature[71]. Now that through Mendel’s work we are clearing our
minds as to the fundamental nature of “gametic” variation, the time is
approaching when an investigation of such causes may be not unfruitful.
[71] _Materials for the Study of Variation_, 1894, p. 78.
Of _variation_ as distinct from _transmission_ why does Professor
Weldon take no heed? He writes (p. 244):
“If Mendel’s statements were universally valid, even among Peas, the
characters of the seeds in the numerous hybrid races now existing
should fall into one or other of a few definite categories, which
should not be connected by intermediate forms.”
Now, as I have already pointed out, Mendel made no pretence of
universal statement: but had he done so, the conclusion, which
Professor Weldon here suggests should follow from such a universal
statement, is incorrectly drawn. Mendel is concerned with the laws of
_transmission of existing characters_, not with _variation_, which he
does not discuss.
Nevertheless Professor Weldon has some acquaintance with the general
fact of variability in certain peas, which he mentions (p. 236), but
the bearing of this fact on the difficulty he enuntiates escapes him.
_Results of crossing in regard to seed characters: normal and
exceptional._
The conditions being the same, the question of the characters of the
cross-bred zygotes which we will call _AB_’s depends primarily on the
specific nature of the varieties which are crossed to produce them. It
is unnecessary to point out that if all _AB_’s are to look alike, both
the varieties _A_ and _B_ must be _pure_--not in the common sense of
descended, as far as can be traced, through individuals identical with
themselves, but pure in the Mendelian sense, that is to say that each
must be at that moment producing only homogeneous gametes bearing the
same characters _A_ and _B_ respectively. Purity of pedigree in the
breeder’s sense is a distinct matter altogether. The length of time--or
if preferred--the number of generations through which a character of a
variety has remained pure, alters the probability of its _dominance_,
i.e. its appearance when a gamete bearing it meets another bearing an
antagonistic character, no more, so far as we are yet aware, than the
length of time a stable element has been isolated alters the properties
of the chemical compound which may be prepared from it.
Now when individuals (bearing contrary characters), pure in the sense
indicated, are crossed together, the question arises, What will be the
appearance of the first cross individuals? Here again, _generally
speaking_, when thoroughly green cotyledons are crossed with thoroughly
yellow cotyledons, the first-cross seeds will have yellow cotyledons;
when fully round peas are crossed with fully wrinkled the first result
will _generally speaking_ be _round_, often with slight pitting as
Mendel has stated. This has been the usual experience of Correns,
Tschermak, Mendel, and myself[72] and, as we shall see, the amount of
clear and substantial evidence to the contrary is still exceedingly
small. But as any experienced naturalist would venture to predict,
there is no _universal_ rule in the matter. As Professor Weldon himself
declares, had there been such a universal rule it would surely have
been notorious. He might further have reflected that in Mendel’s day,
when hybridisation was not the _terra incognita_ it has since become,
the assertion of such universal propositions would have been peculiarly
foolish. Mendel does not make it; but Professor Weldon perceiving the
inherent improbability of the assertion conceives at once that Mendel
_must_ have made it, and if Mendel doesn’t say so in words then he must
have implied it. As a matter of fact Mendel never treats dominance as
more than an incident in his results, merely using it as a means to an
end, and I see no reason to suppose he troubled to consider to what
extent the phenomenon is or is not universal--a matter with which he
had no concern.
[72] The varieties used were _Express_, _Laxton’s Alpha_,
_Fillbasket_, _McLean’s Blue Peter_, _Serpette nain blanc_, _British
Queen_, _très nain de Bretagne_, Sabre, _mange-tout_ Debarbieux, and
a large “grey” sugar-pea, _pois sans parchemin géant à très large
cosse_. Not counting the last two, five are round and three are
wrinkled. As to cotyledons, six have yellow and four have green. In
about 80 crosses I saw no exception to dominance of yellow; but one
apparently clear case of dominance of wrinkled and some doubtful ones.
Of course there may be exceptions. As yet we cannot detect the causes
which control them, though injury, impurity, accidental crossing,
mistakes of various kinds, account for many. Mendel himself says, for
instance, that unhealthy or badly grown plants give uncertain results.
Nevertheless there seems to be a true residuum of exceptions not to be
explained away. I will recite some that I have seen. In my own crosses
I have seen green × green give yellow four times. This I incline
to attribute to conditions or other disturbance, for the natural
pods of these plants gave several yellows. At Messrs Suttons’ I saw
second-generation seeds got by allowing a cross of _Sutton’s Centenary_
(gr. wr.) × _Eclipse_ (gr. rd.) to go to seed; the resulting seeds were
both green and _yellow_, wrinkled and round. But in looking at a sample
of _Eclipse_ I found a few _yellow_ seeds, say two per cent., which may
perhaps be the explanation. Green wrinkled × green round _may_ give all
wrinkled, and again wrinkled × wrinkled may give _round_[73]. Of this
I saw a clear case--supposing no mistake to have occurred--at Messrs
Suttons’. Lastly we have the fact that in exceptional cases crossing
two forms--apparently pure in the strict sense--may give a mixture in
the _first_ generation. There are doubtless examples also of unlikeness
between reciprocals, and of this too I have seen one putative case[74].
[73] Professor Weldon may take this as a famous blow for Mendel, till
he realizes what is meant by Mendel’s “Hybrid-character.”
[74] In addition to those spoken of later, where the great difference
between reciprocals is due to the _maternal_ characters of the seeds.
Such facts thus set out for the first cross-bred generation may without
doubt be predicated for subsequent generations.
What then is the significance of the facts?
_Analysis of exceptions._
Assuming that all these “contradictory” phenomena happened truly as
alleged, and were not pathological or due to error--an explanation
which seems quite inadequate--there are at least four possible accounts
of such diverse results--each valid, without any appeal to ancestry.
1. That dominance may exceptionally fail--or in other words
be created on the side which is elsewhere recessive. For this
exceptional failure we have to seek exceptional causes. The
artificial _creation_ of dominance (in a character usually recessive)
has not yet to my knowledge been demonstrated experimentally, but
experiments are begun by which such evidence may conceivably be
obtained.
2. There may be what is known to practical students of evolution as
the _false hybridism of Millardet_, or in other words, fertilisation
with--from unknown causes--transmission of none or of only some
of the characters of one pure parent. The applicability of this
hypothesis to the colours and shapes of peas is perhaps remote,
but we may notice that it is one possible account of those rare
cases where two pure forms give a _mixed_ result in the first
generation, even assuming the gametes of each pure parent to be truly
monomorphic as regards the character they bear. The applicability of
this suggestion can of course be tested by study of the subsequent
generations, self-fertilised or fertilised by similar forms produced
in the same way. In the case of a _genuine_ false-hybrid the lost
characters will not reappear in the posterity.
3. The result may not be a case of transmission at all as it is at
present conceived, but of the creation on crossing of something
_new_. Our _AB_’s may have one or more characters _peculiar
to themselves_. We may in fact have made a distinct “mule” or
heterozygote form. Where this is the case, there are several
subordinate possibilities we need not at present pursue.
4. There may be definite _variation_ (distinct from that proper to
the “mule”) consequent on causes we cannot yet surmise (see pp. 125
and 128).
The above possibilities are I believe at the present time the only
ones that need to be considered in connexion with these exceptional
cases[75]. They are all of them capable of experimental test and in
certain instances we are beginning to expect the conclusion.
[75] I have not here considered the case in which male and female
elements of a pure variety are not homologous and the variety is a
_permanent_ monomorphic “mule.” Such a phenomenon, when present, will
prove itself in reciprocal crossing. I know no such case in peas for
certain.
_The “mule” or heterozygote._
There can be little doubt that in many cases it is to the third
category that the phenomena belong. An indication of the applicability
of this reasoning will generally be found in the fact that in such
“mule” forms the colour or the shape of the seeds will be recognizably
peculiar and proper to the specimens themselves, as distinct from their
parents, and we may safely anticipate that when those seeds are grown
the plants will show some character which is recognizable as novel. The
_proof_ that the reasoning may apply can as yet only be got by finding
that the forms in question cannot breed true even after successive
selections, but constantly break up into the same series of forms[76].
[76] It will be understood that a “mule” form is quite distinct from
what is generally described as a “blend.” One certain criterion of
the “mule” form is the fact that it cannot be fixed, see p. 25. There
is little doubt that Laxton had such a “mule” form when he speaks
of “the remarkably fine but unfixable pea, Evolution.” _J. R. Hort.
Soc._ XII. 1890, p. 37 (_v. infra_).
This conception of the “mule” form, or “hybrid-character” as Mendel
called it, though undeveloped, is perfectly clear in his work. He
says that the dominant character may have two significations, it may
be either a parental character or a hybrid-character, and it must be
differentiated according as it appears in the one capacity or the
other. He does not regard the character displayed by the hybrid,
whether dominant or other, _as a thing inherited from or transmitted by
the pure parent at all, but as the peculiar function or property of the
hybrid_. When this conception has been fully understood and appreciated
in all its bearings it will be found to be hardly less fruitful than
that of the purity of the germ-cells.
The two parents are two--let us say--substances[77] represented
by corresponding gametes. These gametes unite to form a new
“substance”--the cross-bred zygote. This has its own properties and
structure, just as a chemical compound has, and the properties of this
new “substance” are _not more strictly_ traceable to, or “inherited”
from, those of the two parents than are those of a new chemical
compound “inherited” from those of the component elements. If the
case be one in which the gametes are pure, the new “substance” is not
represented by them, but the compound is again dissociated into its
components, each of which is separately represented by gametes.
[77] Using the word metaphorically.
The character of the cross-bred zygote may be anything. It may be
something we have seen before in one or other of the parents, it may
be intermediate between the two, or it may be something new. All
these possibilities were known to Mendel and he is perfectly aware
that his principle is equally applicable to all. The first case is
his “dominance.” That he is ready for the second is sufficiently
shown by his brief reference to time of flowering considered as a
character (p. 65). The hybrids, he says, flower at a time _almost
exactly intermediate_ between the flowering times of the parents,
and he remarks that the development of the hybrids in this case
probably happens in the same way as it does in the case of the other
characters[78].
[78] “_Ueber die Blüthezeit der Hybriden sind die Versuche noch
nicht abgeschlossen. So viel kann indessen schon angegeben werden,
dass dieselbe fast genau in der Mitte zwischen jener der Samen- und
Pollenpflanze steht, und die Entwicklung der Hybriden bezüglich
dieses Merkmales wahrscheinlich in der nämlichen Weise erfolgt, wie
es für die übrigen Merkmale der Fall ist._” Mendel, p. 23.
That he was thoroughly prepared for the third possibility appears
constantly through the paper, notably in the argument based on the
_Phaseolus_ hybrids, and in the statement that the hybrid between talls
and dwarfs is generally taller than the tall parent, having increased
height as its “hybrid-character.”
All this Professor Weldon has missed. In place of it he offers us the
_sententia_ that no one can expect to understand these phenomena if he
neglect ancestry. This is the idle gloss of the scribe, which, if we
erase it not thoroughly, may pass into the text.
Enough has been said to show how greatly Mendel’s conception of
heredity was in advance of those which pass current at the present
day; I have here attempted the barest outline of the nature of the
“hybrid-character,” and I have not sought to indicate the conclusions
that we reach when the reasoning so clear in the case of the hybrid is
applied to the pure forms and their own characters.
In these considerations we reach the very base on which all conceptions
of heredity and variation must henceforth rest, and that it is now
possible for us to attempt any such analysis is one of the most
far-reaching consequences of Mendel’s principle. Till two years ago no
one had made more than random soundings of this abyss.
I have briefly discussed these possibilities to assist the reader in
getting an insight into Mendel’s conceptions. But in dealing with
Professor Weldon we need not make this excursion; for his objection
arising from the absence of uniform regularity in dominance is not in
point.
The soundness of Mendel’s work and conclusions would be just as
complete if dominance be found to fail often instead of rarely. For
it is perfectly certain that varieties _can_ be chosen in such a way
that the dominance of one character over its antagonist is so regular a
phenomenon that it _can_ be used in the way Mendel indicates. He chose
varieties, in fact, in which a known character _was_ regularly dominant
and it is because he did so that he made his discovery[79]. When
Professor Weldon speaks of the existence of fluctuation and diversity
in regard to dominance as proof of a “grave discrepancy” between
Mendel’s facts and those of other observers[80], he merely indicates
the point at which his own misconceptions began.
[79] As has been already shown the discovery could have been made
equally well and possibly with greater rapidity in a case in which
the hybrid had a character distinct from either parent. The cases
that would _not_ have given a clear result are those where there is
irregular dominance of one or other parent.
[80] Weldon, p. 240.
From Mendel’s style it may be inferred that if he had meant to state
universal dominance in peas he would have done so in unequivocal
language. Let me point out further that of the 34 varieties he
collected for study, he discarded 12 as not amenable to his
purposes[81]. He tells us he would have nothing to do with characters
which were not sharp, but of a “more or less” description. As the
34 varieties are said to have all come true from seed, we may
fairly suppose that the reason he discarded twelve was that they
were unsuitable for his calculations, having either ill-defined and
intermediate characters, or possibly defective and irregular dominance.
[81] See p. 43.
IV. PROFESSOR WELDON’S COLLECTION OF “OTHER EVIDENCE CONCERNING
DOMINANCE IN PEAS.”
_A. In regard to cotyledon colour: Preliminary._
I have been at some pains to show how the contradictory results, no
doubt sometimes occurring, on which Professor Weldon lays such stress,
may be comprehended without any injury to Mendel’s main conclusions.
This excursion was made to save trouble with future discoverers of
exceptions, though the existence of such facts need scarcely disturb
many minds. As regards the dominance of yellow cotyledon-colour over
green the whole number of genuine unconformable cases is likely to
prove very small indeed, though in regard to the dominance of round
shape over wrinkled we may be prepared for more discrepancies. Indeed
my own crosses alone are sufficient to show that in using some
varieties irregularities are to be expected. Considering also that
the shapes of peas depend unquestionably on more than one pair of
allelomorphs I fully expect regular blending in some cases.
As however it may be more satisfactory to the reader and to Professor
Weldon if I follow him through his “contradictory” evidence I will
endeavour to do so. Those who have even a slight practical acquaintance
with the phenomena of heredity will sympathize with me in the
difficulty I feel in treating this section of his arguments with that
gravity he conceives the occasion to demand.
In following the path of the critic it will be necessary for me to
trouble the reader with a number of details of a humble order, but the
journey will not prove devoid of entertainment.
Now exceptions are always interesting and suggestive things, and
sometimes hold a key to great mysteries. Still when a few exceptions
are found disobeying rules elsewhere conformed to by large classes of
phenomena it is not an unsafe course to consider, with such care as
the case permits, whether the exceptions may not be due to exceptional
causes, or failing such causes whether there may be any possibility of
error. But to Professor Weldon, an exception is an exception--and as
such may prove a very serviceable missile; so he gathers them as they
were “smooth stones from the brook.”
Before examining the quality of this rather miscellaneous ammunition I
would wish to draw the non-botanical reader’s attention to one or two
facts of a general nature.
For our present purpose the seed of a pea may be considered as
consisting of two parts, the _embryo with its cotyledons_, enclosed in
a _seed-coat_. It has been known for about a century that this coat or
skin is a _maternal_ structure, being part of the mother plant just
as much as the pods are, and consequently not belonging to the next
generation at all. If then any changes take place in it consequent
on fertilisation, they are to be regarded not as in any sense a
transmission of character by heredity, but rather as of the nature
of an “infection.” If on the other hand it is desired to study the
influence of hereditary transmission on seed-coat characters, then the
crossed seeds must be sown and the seed-coats of their seeds studied.
Such infective changes in maternal tissues have been known from early
times, a notable collection of them having been made especially by
Darwin; and for these cases Focke suggested the convenient word
_Xenia_. With this familiar fact I would not for a moment suppose
Professor Weldon unacquainted, though it was with some surprise that I
found in his paper no reference to the phenomenon.
For as it happens, xenia is not at all a rare occurrence with _certain
varieties_ of peas; though in them, as I believe is generally the case
with this phenomenon, it is highly irregular in its manifestations,
being doubtless dependent on slight differences of conditions during
ripening.
The coats of peas differ greatly in different varieties, being
sometimes thick and white or yellow, sometimes thick and highly
pigmented with green or other colours, in both of which cases it
may be impossible to judge the cotyledon-colour without peeling
off the opaque coat; or the coats may be very thin, colourless and
transparent, so that the cotyledon-colour is seen at once. It was
such a transparent form that Mendel says he used for his experiments
with cotyledon-colour. In order to see xenia a pea with a _pigmented_
seed-coat should be taken as seed-parent, and crossed with a variety
having a different cotyledon-colour. There is then a fair chance of
seeing this phenomenon, but much still depends on the variety. For
example, _Fillbasket_ has green cotyledons and seed-coat green except
near the hilar surface. Crossed with _Serpette nain blanc_ (yellow
cotyledons and yellow coat) this variety gave three pods with 17 seeds
in which the seed-coats were almost full yellow (xenia). Three other
pods (25 seeds), similarly produced, showed slight xenia, and one pod
with eight seeds showed little or none.
On the other hand _Fillbasket_ fertilised with _nain de Bretagne_
(yellow cotyledons, seed-coats yellow to yellowish green) gave six pods
with 39 seeds showing slight xenia, distinct in a few seeds but absent
in most.
Examples of xenia produced by the contrary proceeding, namely
fertilising a yellow pea with a green, may indubitably occur and I
have seen doubtful cases; but as by the nature of the case these are
_negative_ phenomena, i.e. the seed-coat remaining greenish and _not_
going through its normal maturation changes, they must always be
equivocal, and would require special confirmation before other causes
were excluded.
Lastly, the special change (xenia) Mendel saw in “grey” peas,
appearance or increase of purple pigment in the thick coats, following
crossing, is common but also irregular.
If a _transparent_ coated form be taken as seed-parent there is no
appreciable xenia, so far as I know, and such a phenomenon would
certainly be paradoxical[82].
[82] In some transparent coats there is pigment, but so little as a
rule that xenia would be scarcely noticeable.
In this connection it is interesting to observe that Giltay, whom
Professor Weldon quotes as having obtained purely Mendelian results,
got no xenia though searching for it. If the reader goes carefully
through Giltay’s numerous cases, he will find, _almost_ without doubt,
that none of them were such as produce it. _Reading Giant_, as Giltay
states, has a _transparent_ skin, and the only xenia likely to occur
in the other cases would be of the peculiar and uncertain kind seen in
using “grey” peas. Professor Weldon notes that Giltay, who evidently
worked with extreme care, _peeled_ his seeds before describing them,
a course which Professor Weldon, not recognizing the distinction
between the varieties with opaque and transparent coats, himself wisely
recommends. The coincidence of the peeled seeds giving simple Mendelian
results is one which might have alarmed a critic less intrepid than
Professor Weldon.
Bearing in mind, then, that the coats of peas may be transparent or
opaque; and in the latter case may be variously pigmented, green, grey,
reddish, purplish, etc.; that in any of the latter cases there may or
may not be xenia; the reader will perceive that to use the statements
of an author, whether scientific or lay, to the effect that on crossing
varieties he obtained peas of such and such colours _without specifying
at all whether the coats were transparent or whether the colours he saw
were coat- or cotyledon-colours_ is a proceeding fraught with peculiar
and special risks.
(1) _Gärtner’s cases._ Professor Weldon gives, as exceptions, a series
of Gärtner’s observations. Using several varieties, amongst them
_Pisum sativum macrospermum_, a “grey” pea, with coloured flowers and
seed-coats[83], he obtained results partly Mendelian and partly, as now
alleged, contradictory. The latter consist of seeds “dirty yellow” and
“yellowish green,” whereas it is suggested they should have been simply
yellow.
[83] Usually correlated characters, as Mendel knew.
Now students of this department of natural history will know that
these same observations of Gärtner’s, whether rightly or wrongly, have
been doing duty for more than half a century as stock illustrations
of xenia. In this capacity they have served two generations of
naturalists. The ground nowadays may be unfamiliar, but others have
travelled it before and recorded their impressions. Darwin, for
example, has the following passage[84]:
[84] _Animals and Plants_, 2nd ed. 1885, p. 428.
“These statements led Gärtner, who was highly sceptical on the
subject, carefully to try a long series of experiments; he selected
the most constant varieties, and the results conclusively showed
_that the colour of the skin of the pea_ is modified when pollen of a
differently coloured variety is used.” (The italics are mine.)
In the true spirit of inquiry Professor Weldon doubtless reflected,
“’Tis not _Antiquity_ nor _Author_,
That makes _Truth Truth_, altho’ _Time’s Daughter_”;
but perhaps a word of caution to the reader that another interpretation
exists would have been in place. It cannot be without amazement
therefore that we find him appropriating these examples as referring to
cotyledon-colour, with never a hint that the point is doubtful.
Giltay, without going into details, points out the ambiguity[85]. As
Professor Weldon refers to the writings both of Darwin and Giltay,
it is still more remarkable that he should regard the phenomenon as
clearly one of cotyledon-colour and not coat-colour as Darwin and many
other writers have supposed.
[85] “_Eine andere Frage ist jedoch, ob der Einfluss des Pollens auf
den Keim schon äusserlich an diesen letzteren sichtbar sein kann.
Darwin führt mehrere hierher gehörige Fälle an, und wahrscheinlich
sind auch die Resultate der von Gärtner über diesen Gegenstand
ausgeführten Experimente hier zu erwähnen, wenn es auch nicht ganz
deutlich ist, ob der von Gärtner erwähnte directe Einfluss des
Pollens sich nur innerhalb der Grenzen des Keimes merklich macht oder
nicht._” p. 490.
Without going further it would be highly improbable that Gärtner
is speaking solely or even chiefly of the cotyledons, from the
circumstance that these observations are given as evidence of “_the
influence of foreign pollen on the female organs_”; and that Gärtner
was perfectly aware of the fact that the coat of the seed was a
maternal structure is evident from his statement to that effect on
p. 80.
To go into the whole question in detail would require considerable
space; but indeed it is unnecessary to labour the point. The reader who
examines Gärtner’s account with care, especially the peculiar phenomena
obtained in the case of the “grey” pea (_macrospermum_), with specimens
before him, will have no difficulty in recognizing that Gärtner is
simply describing the seeds _as they looked in their coats_, and is not
attempting to distinguish cotyledon-characters and coat-characters.
If he had peeled them, which in the case of “grey” peas would be
_absolutely necessary_ to see cotyledon-colour, he must surely have
said so.
Had he done so, he would have found the cotyledons full yellow in every
ripe seed; for I venture to assert that anyone who tries, as we have,
crosses between a yellow-cotyledoned “grey” pea, such as Gärtner’s was,
with any pure green variety will see that there is no question whatever
as to absolute dominance of the yellow cotyledon-character here, more
striking than in any other case. If exceptions are to be looked for,
they will not be found _there_; and, except in so far as they show
simple dominance of yellow, Gärtner’s observations cannot be cited in
this connection at all.
(2) _Seton’s case._ Another exception given by Professor Weldon is much
more interesting and instructive. It is the curious case of Seton[86].
Told in the words of the critic it is as follows:--
“Mr Alexander Seton crossed the flowers of _Dwarf Imperial_, ‘a
well-known green variety of the Pea,’ with the pollen of ‘a white
free-growing variety.’ Four hybrid seeds were obtained, ‘which did
not differ in appearance from the others of the female parent.’
These seeds therefore did _not_ obey the law of dominance, or if the
statement be preferred, greenness became dominant in this case. The
seeds were sown, and produced plants bearing ‘green’ and ‘white’
seeds side by side in the same pod. An excellent coloured figure of
one of these pods is given (_loc. cit._ Plate 9, Fig. 1), and is the
only figure I have found which illustrates segregation of colours in
hybrid Peas of the second generation.”
[86] Appendix to paper of Goss, _Trans. Hort. Soc._ v. 1822, pub.
1824 (_not_ 1848, as given by Professor Weldon), p. 236.
Now if Professor Weldon had applied to this case the same independence
of judgment he evinced in dismissing Darwin’s interpretation of
Gärtner’s observations, he might have reached a valuable result.
Knowing how difficult it is to give all the points in a brief citation,
I turned up the original passage, where I find it stated that the mixed
seeds of the second generation “were all completely either of one
colour or the other, none of them having an intermediate tint, as Mr
Seton had expected.” The utility of this observation of the absence of
intermediates, is that it goes some way to dispose of the suggestion of
xenia as a cause contributing to the result.
Moreover, feeling perfectly clear, from the fact of the absence of
intermediates, that the case must be one of simple dominance in
spite of first appearances, I suggest the following account with
every confidence that it is the true one. There have been several
“_Imperials_,” though _Dwarf Imperial_, in a form which I can feel
sure is Seton’s form, I have not succeeded in seeing; but from
Vilmorin’s description that the peas when ripe are “_franchement
verts_” I feel no doubt it was a green pea _with a green skin_. If it
had had a transparent skin this description would be inapplicable.
Having then a green skin, which may be assumed with every probability
of truth, the seeds, even though the cotyledons were yellow, might,
especially if examined fresh, be indistinguishable from those of
the maternal type. Next from the fact of the mixture in the second
generation we learn that the _semi-transparent seed-coat of the
paternal form was dominant_ as a plant-character, and indeed the
coloured plate makes this fairly evident. It will be understood that
this explanation is as yet suggestive, but from the facts of the
second generation, any supposition that there was real irregularity in
dominance in this case is out of the question[87].
[87] Since the above passage was written I find the “_Imperials_”
described in “Report of Chiswick Trials,” _Proc. R. Hort. Soc._ 1860,
I. p. 340, as “skin thick”; and on p. 360 “skin thick, blue”; which
finally disposes of this “exception.”
(3) _Tschermak’s exceptions._ These are a much more acceptable lot
than those we have been considering. Tschermak was thoroughly alive
to the seed-coat question and consequently any exception stated as an
unqualified fact on his authority must be accepted. The nature of these
cases we shall see. Among the many varieties he used, some being _not_
monomorphic, it would have been surprising if he had not found true
irregularities in dominance.
(3 _a_) _Buchsbaum case._ This variety, growing in the open, gave
once a pod in which _every seed but one was green_. In stating this
case Professor Weldon refers to _Buchsbaum_ as “a yellow-seeded
variety.” Tschermak[88], however, describes it as having “_gelbes,
öfters gelblich-grünes Speichergewebe_” (cotyledons); and again
says the cotyledon-colour is “_allerdings gerade bei Buchsbaum zur
Spontanvariation nach gelb-grün neigend!_” The (!) is Tschermak’s.
Therefore Professor Weldon can hardly claim _Buchsbaum_ as
“yellow-seeded” without qualification.
[88] (36), p. 502 and (37), p. 663.
_Buchsbaum_ in fact is in all probability a blend-form and certainly
not a true, stable yellow. One of the green seeds mentioned above grew
and gave 15 _yellows_ and three _greens_, and the result showed pretty
clearly, as Tschermak says, that there had been an accidental cross
with a tall green.
On another occasion _Telephone_ ♀ (another impure green) × _Buchsbaum_
gave four _yellow smooth and_ two _green wrinkled_, but one [? both:
the grammar is obscure] of the greens did not germinate[89].
[89] Professor Weldon should have alluded to this. _Dead_ seeds have
no bearing on these questions, seeing that their characters may be
pathological. The same seeds are later described as “_wie Telephone
selbst_,” so, apart from the possibility of death, they may also have
been self-fertilised.
(3 _b_) _Telephone cases._ _Telephone_, crossed with at least one
yellow variety (_Auvergne_) gave all or some green or greenish. These
I have no doubt are good cases of “defective dominance” of yellow.
But it must be noted that _Telephone is an impure green_. Nominally a
green, it is as Professor Weldon has satisfied himself, very irregular
in colour, having many intermediates shading to pure yellow and many
piebalds. It is the variety from which alone Professor Weldon made
his colour-scale. _I desire therefore to call special attention to
the fact that Telephone, though not a pure green, Tschermak’s sample
being as he says “gelblichweiss grün,” a yellowish-white-green in
cotyledon-colour, is the variety which has so far contributed the
clearest evidence of the green colour dominating in its crosses with a
yellow_; and that _Buchsbaum_ is probably a similar case. To this point
we shall return. It may not be superfluous to mention also that one
cross between _Fillbasket_ (a thorough _green_) and _Telephone_ gave
three _yellowish_ green seeds (Tschermak, (36), p. 501).
(3 _c_) _Couturier cases._ This fully yellow variety in crosses with
two fully green sorts gave seeds either yellow or greenish yellow. In
one case _Fillbasket_ ♀ fertilised by _Couturier_ gave mixed seeds,
green and yellow. For any evidence to the contrary, the green in this
case may have been self-fertilised. Nevertheless, taking the evidence
together, I think it is most likely that _Couturier_ is a genuine
case of imperfect dominance of yellow. If so, it is the only true
“exception” in crosses between stable forms.
* * * * *
We have now narrowed down Professor Weldon’s exceptions to dominance of
cotyledon-colour to two varieties, one yellow (_Couturier_), and one
yellow “tending to green” (_Buchsbaum_), which show imperfect dominance
of yellow; and one variety, _Telephone_, an impure and irregular green,
which shows occasional but uncertain dominance of _green_.
What may be the meaning of the phenomenon shown by the unstable or
mosaic varieties we cannot tell; but I venture to suggest that when we
more fully appreciate the nature and genesis of the gametes, it will be
found that the peculiarities of heredity seen in these cases have more
in common with those of “false hybridism” (see p. 34) than with any
true failure of dominance.
Before, however, feeling quite satisfied in regard even to this
residuum of exceptions, one would wish to learn the subsequent fate
of these aberrant seeds and how their offspring differed from that of
their sisters. One only of them can I yet trace, viz. the green seed
from _Telephone_ ♀ × _Buchsbaum_ ♂, which proved a veritable “green
dominant.” As for the remainder, Tschermak promises in his first
paper to watch them. But in his second paper the only passage I can
find relating to them declares that perhaps some of the questionable
cases he mentioned in his first paper “_are attributable to similar
isolated anomalies in dominance; some proved themselves by subsequent
cultivation to be cases of accidental self-fertilisation; others failed
to germinate_[90].” I may warn those interested in these questions,
that in estimating changes due to ripening, _dead_ seeds are not
available.
[90] “_Vielleicht sind einige der l.c. 507 bis 508 erwähnten
fraglichen Fälle auf ähnliche vereinzelte Anomalien der
Merkmalswerthigkeit zu beziehen; einige erwiesen sich allerdings beim
Anbau als Producte ungewollter Selbstbefruchtung, andere keimten
nicht._”
_B. Seed-coats and shapes._
1. _Seed-coats._ Professor Weldon lays some stress on the results
obtained by Correns[91] in crossing a pea having green cotyledons and a
thin almost colourless coat (_grüne späte Erfurter Folger-erbse_) with
two purple-flowered varieties. The latter are what are known in England
as “grey” peas, though the term grey is not generally appropriate.
[91] Regarding this case I have to thank Professor Correns for a
good deal of information which he kindly sent me in response to my
inquiry. I am thus able to supplement the published account in some
particulars.
In these varieties the cotyledon-colour is yellow and the coats are
usually highly coloured or orange-brown. In reciprocal crosses Correns
found no change from the maternal seed-coat-colour or seed-shape.
On sowing these peas he obtained plants bearing peas which, using
the terminology of Mendel and others, he speaks of as the “first
generation.”
These peas varied in the colour of their seed-coats from an almost
colourless form slightly tinged with green like the one parent to the
orange-brown of the other parent. The seeds varied in this respect not
only from plant to plant, but from pod to pod, and from seed to seed,
as Professor Correns has informed me.
The peas with more highly-coloured coats were sown and gave rise to
plants with seeds showing the whole range of seed-coat-colours again.
Professor Weldon states that in this case neither the law of
dominance nor the law of segregation was observed; and the same is
the opinion of Correns, who, as I understand, inclines to regard the
colour-distribution as indicating a “mosaic” formation. This is perhaps
conceivable; and in that case the statement that there was no dominance
would be true, and it would also be true that the unit of segregation,
if any, was smaller than the individual plant and may in fact be the
individual seed.
A final decision of this question is as yet impossible. Nevertheless
from Professor Correns I have learnt one point of importance, namely,
that the coats of all these seeds were _thick_, like that of the
coloured and as usual dominant form. There is no “mosaic” of coats
like one parent and coats like the other, though there may be a mosaic
of colours. In regard to the distribution of _colour_ however the
possibility does not seem to me excluded that we are here dealing
with changes influenced by conditions. I have grown a “grey” pea and
noticed that the seed-coats ripened in my garden differ considerably
and not quite uniformly from those received from and probably
ripened in France, mine being mostly pale and greyish, instead of
reddish-brown. We have elsewhere seen (p. 120) that pigments of the
seed-coat-colour may be very sensitive to conditions, and slight
differences of moisture, for example, may in some measure account
for the differences in colour. Among my crosses I have a pod of such
“grey” peas fertilised by _Laxton’s Alpha_ (green cotyledons, coat
transparent). It contained five seeds, of which four were _red-brown
on one side_ and grey with purple specks on the other. The fifth was
of the grey colour on both sides. I regard this difference not as
indicating segregation of character but merely as comparable with the
difference between the two sides of a ripe apple, and I have little
doubt that Correns’ case may be of the same nature[92]. Phenomena
somewhat similar to these will be met with in Laxton’s case of the
“maple” seeded peas (see p. 161).
[92] Mr Hurst, of Burbage, tells me that in varieties having coats
green or white, e.g. _American Wonder_, the white coats are mostly
from early, the green from later pods, the tints depending on
conditions and exposure.
2. _Seed-shapes._ Here Professor Weldon has three sets of alleged
exceptions to the rule of dominance of round shape over wrinkled. The
first are Rimpau’s cases, the second are Tschermak’s cases, the third
group are cases of “grey” peas, which we will treat in a separate
section (see pp. 153 and 158).
(_a_) _Rimpau’s cases._ Professor Weldon quotes Rimpau as having
crossed wrinkled and round peas[93] and found the second hybrid
generation dimorphic as usual. The wrinkled peas were selected and
sown and gave wrinkled peas _and round_ peas, becoming “true” to the
wrinkled character in one case only in the fifth year, while in the
second case--that of a _Telephone_ cross--there was a mixture of round
and wrinkled similarly resulting from _wrinkled_ seed for two years,
but the experiment was not continued.
[93] In the first case _Knight’s Marrow_ with _Victoria_, both ways;
in the second _Victoria_ with _Telephone_, both ways.
These at first sight look like genuine exceptions. In reality, however,
they are capable of a simple explanation. It must be remembered that
Rimpau was working in ignorance of Mendel’s results, was not testing
any rule, and was not on the look out for irregularities. Now all who
have crossed wrinkled and round peas on even a moderate scale will have
met with the fact that there is frequently _some_ wrinkling in the
cross-bred seeds. Though round when compared with the true wrinkled,
these are often somewhat more wrinkled than the round type, and in
irregular degrees. For my own part I fully anticipate that we may find
rare cases of complete blending in this respect though I do not as yet
know one.
Rimpau gives a photograph of eight peas (Fig. 146) which he says
represent the wrinkled form derived from this cross. It is evident
that these are not from _one pod_ but a miscellaneous selection. On
close inspection it will be seen that while the remainder are shown
with their _cotyledon_-surfaces upwards, the two peas at the lower
end of the row are represented with their _hilar_-surfaces upwards.
Remembering this it will be recognized that these two lower peas are in
fact _not_ fully wrinkled peas but almost certainly _round_ “hybrids,”
and the depression is merely that which is often seen in round peas
(such as _Fillbasket_), squared by mutual pressure. Such peas, when
sown, might of course give some round.
As Tschermak writes ((37), p. 658), experience has shown him that
cross-bred seeds with character transitional between “round” and
“wrinkled” behave as hybrids, and have both wrinkled and round
offspring, and he now reckons them accordingly with the round dominants.
Note further the fact that Rimpau found the wrinkled form came true
in the _fifth_ year, while the round gave at first more, later fewer,
wrinkleds, not coming true till the _ninth_ year. This makes it quite
clear that there _was_ dominance of the round form, but that the
heterozygotes were not so sharply distinguishable from the two pure
forms as to be separated at once by a person not on the look-out for
the distinctions. Nevertheless there _was_ sufficient difference to
lead to a practical distinction of the cross-breds both from the pure
dominants and from the pure recessives.
The _Telephone_ case may have been of the same nature; though, as we
have seen above, this pea is peculiar in its colour-heredity and may
quite well have followed a different rule in shape also. As stated
before, the wrinkled offspring were not cultivated after the third
year, but the _round_ seeds are said to have still given some wrinkleds
in the eighth year after the cross, as would be expected in a simple
Mendelian case.
(_b_) _Tschermak’s cases._ The cases Professor Weldon quotes from
Tschermak all relate to crosses with _Telephone_ again, and this fact
taken with the certainty that the colour-heredity of _Telephone_ is
abnormal makes it fairly clear that there is here something of a really
exceptional character. What the real nature of the exception is, and
how far it is to be taken as contradicting the “law of dominance,” is
quite another matter.
3. _Other phenomena, especially regarding seed-shapes, in the case of
“grey” peas. Modern evidence._ Professor Weldon quotes from Tschermak
the interesting facts about the “grey” pea, _Graue Riesen_, but does
not attempt to elucidate them. He is not on very safe ground in
adducing these phenomena as conflicting with the “law of dominance.”
Let us see whither we are led if we consider these cases. On p. 124
I mentioned that the classes round and wrinkled do not properly hold
if we try to extend them to large-seeded sorts, and that these cases
require separate consideration. In many of such peas, which usually
belong either to the classes of sugar-peas (_mange-touts_) or “grey”
peas (with coloured flowers), the seeds would be rather described as
irregularly indented, lumpy or stony[94], than by any use of the terms
round or wrinkled. One sugar-pea (_Debarbieux_) which I have used
has large flattish, smooth, yellow seeds with white skins, and this
also in its crossings follows the rules about to be described for the
large-seeded “grey” peas.
[94] Gärtner’s _macrospermum_ was evidently one of these, though
from the further account (p. 498) it was probably more wrinkled.
There are of course _mange-touts_ which have perfectly round seeds.
Mendel himself showed that the _mange-tout_ character, the soft
constricted pod, was transferable. There are also _mange-touts_
with fully wrinkled seeds and “grey” peas with small seeds (see
Vilmorin-Andrieux, _Plantes Potagères_, 1883).
In the large “grey” peas the most conspicuous feature is the seed-coat,
which is grey, brownish, or of a bright reddish colour. Such seed-coats
are often speckled with purple, and on boiling these seed-coats turn
dark brown. They are in fact the very peas used by Mendel in making up
his third pair of characters. Regarding them Professor Weldon, stating
they may be considered separately, writes as follows:--
“Tschermak has crossed _Graue Riesen_ with five races of _P.
sativum_, and he finds that the form of the first hybrid seeds
_follows the female parent_, so that if races of _P. sativum_
with round smooth seeds be crossed with _Graue Riesen_ (which has
flattened, feebly wrinkled seeds) the hybrids will be round and
smooth or flattened and wrinkled, as the _P. sativum_ or the _Graue
Riesen_ is used as female parent[95]. There is here a more complex
phenomenon than at first sight appears; because if the flowers of the
first hybrid generation are self-fertilised, the resulting seeds of
the second generation invariably resemble those of the _Graue Riesen_
in shape, although in colour they follow Mendel’s law of segregation!”
[95] Correns found a similar result.
From this account who would not infer that we have here some mystery
which does not accord with the Mendelian principles? As a matter of
fact the case is dominance in a perfectly obvious if distinct form.
_Graue Riesen_, a large grey sugar-pea, the _pois sans parchemin
géant_ of the French seedsmen, has full-yellow cotyledons and a highly
coloured seed-coat of varying tints. In shape the seed is somewhat
flattened with irregular slight indentations, lightly wrinkled if
the term be preferred. Tschermak speaks of it in his first paper as
“_Same flach, zusammengedrückt_”--a flat, compressed seed; in his
second paper as “_flache, oft schwach gerunzelte Cotyledonen-form_,”
or cotyledon-shape, flat, often feebly wrinkled, as Professor Weldon
translates.
First-crosses made from this variety, each with a different form
of _P. sativum_, are stated on the authority of Tschermak’s five
cases, to follow exclusively the maternal seed-shape. From “_schwach
gerunzelte_,” “feebly wrinkled,” Professor Weldon easily passes to
“wrinkled,” and tells us that according as a round _sativum_ or the
_Graue Riesen_ is used as mother, the first-cross seeds “will be round
and smooth or flattened and wrinkled.”
As a matter of fact, however, the seeds of _Graue Riesen_ though
_slightly_ wrinkled do not belong to the “wrinkled” class; but if the
classification “wrinkled” and “round” is to be extended to such peas at
all, they belong to the _round_. Mendel is careful to state that his
_round_ class are “either spherical or roundish, the depressions on the
surface, when there are any, always slight”; while the “wrinkled” class
are “irregularly angular, deeply wrinkled[96].”
[96] “_Entweder kugelrund oder rundlich, die Einsenkungen, wenn
welche an der Oberfläche vorkommen, immer nur seicht, oder sie sind
unregelmässig kantig, tief runzlig_ (_P. quadratum_).”
On this description alone it would be very likely that _Graue Riesen_
should fall into the _round_ class, and as such it behaves in its
crosses, _being dominant over wrinkled_ (see Nos. 3 and 6, below). I
can see that in this case Professor Weldon has been partly misled by
expressions of Tschermak’s, but the facts of the second generation
should have aroused suspicion. Neither author notices that as all
five varieties crossed by Tschermak with _Graue Riesen_ were _round_,
the possibilities are not exhausted. Had Tschermak tried a really
wrinkled _sativum_ with _Graue Riesen_ he would have seen this obvious
explanation.
As some of my own few observations of first-crosses bear on this point
I may quote them, imperfect though they are.
I grew the purple-flowered sugar-pea “_Pois sans parchemin géant à très
large cosse_,” a soft-podded “_mange-tout_” pea, flowers and seed-coats
coloured, from Vilmorin’s, probably identical with _Graue Riesen_.
1. One flower of this variety fertilised with _Pois très nain de
Bretagne_ (very small seed; yellow cotyledons; very round) gave
seven seeds indistinguishable (in their coats) from those of the
mother, save for a doubtful increase in purple pigmentation of coats.
2. Fertilised by _Laxton’s Alpha_ (green; wrinkled; coats
transparent), two flowers gave 11 seeds exactly as above, the purple
being in this case clearly increased.
In the following the purple sugar-pea was _father_.
3. _Laxton’s Alpha_ (green; wrinkled; coats transparent) fertilised
by the purple sugar-pea gave one pod of four seeds with yellow
cotyledons and _round_ form.
4. _Fillbasket_ (green; smooth but squared; coats green) fertilised
by the _purple_ sugar-pea gave one pod with six seeds, yellow
cotyledons[97]; _Fillbasket_ size and shape; but the normally green
coat yellowed near _the hilum_ by xenia.
[97] The colour is the peculiarly deep yellow of the “grey”
_mange-tout_.
5. _Express_ (“blue”-green cotyledons and transparent skins; round)
fertilised with _purple sugar-pea_ gave one pod with four seeds, yellow
cotyledons, shape round, much as in _Fillbasket_.
6. _British Queen_ (yellow cotyledons, wrinkled, white coats) ♀ ×
purple sugar-pea gave two pods with seven seeds, cotyledons yellow,
coats _tinged greenish_ (xenia?), all _round_.
So much for the “_Purple_” sugar-pea.
I got similar results with _Mange-tout Debarbieux_. This is a
soft-podded _Mange-tout_ or sugar-pea, with white flowers, large,
flattish, smooth seeds, scarcely dimpled; yellow cotyledons.
7. _Debarbieux_ fertilised by _Serpette nain blanc_ (yellow cotyledons;
wrinkled; white skin; dwarf) gave one pod with six seeds, size and
shape of _Debarbieux_, with slight dimpling.
8. _Debarbieux_ by _nain de Bretagne_ (very small; yellow cotyledons;
very round) gave three pods, 12 seeds, all yellow cotyledons, of which
two pods had eight seeds identical in shape with _Debarbieux_, while
the third had four seeds like _Debarbieux_ but more dimpled. The
reciprocal cross gave two seeds exactly like _nain de Bretagne_.
But it may be objected that the shape of this large grey pea is very
peculiar[98]; and that it maintains its type remarkably when fertilised
by many distinct varieties though its pollen effects little or no
change in them; for, so long as round varieties of _sativum_ are
used as mothers, this is true as we have seen. But when once it is
understood that in _Graue Riesen_ there is no question of wrinkling,
seeing that the variety behaves as a _round_ variety, the shape and
especially the size of the seed must be treated as a maternal property.
[98] It is certainly subject to considerable changes according
to conditions. Those ripened in my garden are without exception
much larger and flatter than Vilmorin’s seeds (now two years old)
from which they grew. The colour of the coats is also much duller.
These changes are just what is to be expected from the English
climate--taken with the fact that my sample of this variety was late
sown.
_Why_ the distinction between the shape of _Graue Riesen_ and that of
ordinary round peas should be a matter of maternal physiology we do
not know. The question is one for the botanical chemist. But there
is evidently very considerable regularity, the seeds borne by the
_cross-breds_ exhibiting the form of the “grey” pea, which is then
a dominant character as much as the seed-coat characters are. And
that is what Tschermak’s _Graue Riesen_ crosses actually did, thereby
exhibiting dominance in a very clear form. To interject these cases as
a mystery without pointing out how easily they can be reconciled with
the “law of dominance” may throw an unskilled reader into gratuitous
doubt.
Finally, since _the wrinkled peas_, _Laxton’s Alpha_ and _British
Queen_, _pollinated by a large flat mange-tout, witness Nos. 3 and 6
above_, became round in both cases where this experiment was made, we
here merely see the usual dominance of the non-wrinkled character;
though of course if a _round_-seeded mother be used there can be no
departure from the maternal shape, as far as roundness is concerned.
Correns’ observations on the shapes of a “grey” pea crossed with a
round shelling pea, also quoted by Professor Weldon as showing no
dominance of roundness, are of course of the same nature as those just
discussed.
_C. Evidence of Knight and Laxton._
In the last two sections we have seen that in using peas of the “grey”
class, i.e. with brown, red, or purplish coats, special phenomena are
to be looked for, and also that in the case of large “indented” peas,
the phenomena of size and shape may show some divergence from that
simple form of the phenomenon of dominance seen when ordinary round and
wrinkled are crossed. Here the fuller discussion of these phenomena
must have been left to await further experiment, were it not that we
have other evidence bearing on the same questions.
The first is that of Knight’s well-known experiments, long familiar but
until now hopelessly mysterious. I have not space to quote the various
interpretations which Knight and others have put upon them, but as the
Mendelian principle at once gives a complete account of the whole,
this is scarcely necessary, though the matter is full of historical
interest.
Crossing a white pea with a very large grey purple-flowered form
Knight (21) found that the peas so produced “were not in any sensible
degree different from those afforded by other plants of the same
[white] variety; owing, I imagine, to the external covering of the
seed (as I have found in other plants) being furnished entirely by
the female[99].” All grew very tall[100], and had colours of male
parent[101]. The seeds they produced were dark grey[102].
[99] Thus avoiding the error of Seton, see p. 144. There is no xenia
perhaps because the seed-coat of mother was a transparent coat.
[100] As heterozygotes often do.
[101] Dominance of the purple form.
[102] Dominance of the grey coat as a maternal character.
“I had frequent occasion to observe, in this plant [the hybrid], a
stronger tendency to produce purple blossoms, and coloured seeds,
than white ones; for when I introduced the farina of a purple blossom
into a white one, the whole of the seeds in the succeeding year
became coloured [viz. _DR_ × _D_ giving _DD_ and _DR_]; but, when I
endeavoured to discharge this colour, by reversing the process, a part
only of them afforded plants with white blossoms; this part sometimes
occupying one end of the pod, and being at times irregularly intermixed
with those which, when sown, retained their colour” [viz. _DR_ × _R_
giving _DR_ and _RR_] (draws conclusions, now obviously erroneous[103]).
[103] Sherwood’s view (_J. R. Hort. Soc._ XXII. p. 252) that this was
the origin of the “Wrinkled” pea, seems very dubious.
In this account we have nothing not readily intelligible in the light
of Mendel’s hypothesis.
The next evidence is supplied by an exceptionally complete record of
a most valuable experiment made by Laxton[104]. The whole story is
replete with interest, and as it not only carries us on somewhat beyond
the point reached by Mendel, but furnishes an excellent illustration of
how his principles may be applied, I give the whole account in Laxton’s
words, only altering the paragraphing for clearness, and adding a
commentary. The paper appears in _Jour. Hort. Soc._ N.S. III. 1872,
p. 10, and very slightly abbreviated in _Jour. of Hort._ XVIII. 1870,
p. 86. Some points in the same article do not specially relate to this
section, but for simplicity I treat the whole together.
[104] It will be well known to all practical horticulturalists that Laxton,
originally of Stamford, made and brought out a large number of the best
known modern peas. The firm is now in Bedford.
It is not too much to say that two years ago the whole of this story
would have been a maze of bewildering confusion. There are still some
points in it that we cannot fully comprehend, for the case is one of
far more than ordinary complexity, but the general outlines are now
clear. In attempting to elucidate the phenomena it will be remembered
that there are no statistics (those given being inapplicable), and the
several offspring are only imperfectly referred to the several classes
of seeds. This being so, our rationale cannot hope to be complete.
Laxton states that as the seeds of peas are liable to change colour
with keeping, for this and other reasons he sent to the Society a part
of the seeds resulting from his experiment before it was brought to a
conclusion.
“The seeds exhibited were derived from a single experiment. Amongst
these seeds will be observed some of several remarkable colours,
including black, violet, purple-streaked and spotted, maple, grey,
greenish, white, and almost every intermediate tint, the varied
colours being apparently produced on the outer coat or envelope of
the cotyledons only.
The peas were selected for their colours, &c., from the third year’s
sowing in 1869 of the produce of a cross in 1866 of the early round
white-seeded and white-flowered garden variety “Ringleader,” which
is about 2-1/2 ft. in height, fertilised by the pollen of the common
purple-flowered “maple” pea, which is taller than “Ringleader,” and
has slightly indented seeds. I effected impregnation by removing
the anthers of the seed-bearer, and applying the pollen at an early
stage. This cross produced a pod containing five round white peas,
exactly like the ordinary “Ringleader” seeds[105].
[105] A round white ♀ × grey ♂ giving the usual result, round,
“white” (yellow) seeds.
In 1867 I sowed these seeds, and all five produced tall purple-flowered
purplish-stemmed plants[106], and the seeds, with few exceptions,
had all maple or brownish-streaked envelopes of various shades; the
remainder had entirely violet or deep purple-coloured envelopes[107]:
in shape the peas were partly indented; but a few were round[108].
Some of the plants ripened off earlier than the “maple,” which, in
comparison with “Ringleader,” is a late variety; and although the
pods were in many instances partially abortive, the produce was very
large[109].
[106] Tall heterozygotes, with normal dominance of purple flowers.
[107] Here we see dominance of the _pigmented_ seed-coat as a
maternal character over _white_ seed-coat. The colours of the
seed-coats are described as essentially two: maple or brown-streaked,
and violet, the latter being a small minority. As the sequel shows,
the latter are heterozygotes, not breeding true. Now Mendel found,
and the fact has been confirmed both by Correns and myself, that
crossing a grey pea which is capable of producing purple leads to
such production as a form of xenia.
We have here therefore in the purple seeds the union of dissimilar
gametes, with production of xenia. But as the brown-streaked
seeds are also in part heterozygous, the splitting of a compound
allelomorph has probably taken place, though without precise
statistics and allotment of offspring among the several seeds the
point is uncertain. The colour of seed-coats in “grey” peas and
probably “maples” also is, as was stated on p. 150, sensitive to
conditions, but the whole difference between “maples” and purple is
too much to attribute safely to such irregularity. “Maple” is the
word used to describe certain seed-coats which are pigmented with
intricate brown mottlings on a paler buff ground. In French they are
_perdrix_.
[108] This is not, as it stands, explicable. It seems from this point
and also from what follows that if the account is truly given, some
of the plants may have been mosaic with segregation of characters in
particular flowers; but see subsequent note.
[109] As, commonly, in heterozygotes when fertile.
In 1868 I sowed the peas of the preceding year’s growth, and selected
various plants for earliness, productiveness, &c. Some of the plants
had light-coloured stems and leaves; these all showed white flowers,
and produced round white seeds[110]. Others had purple flowers, showed
the purple on the stems and at the axils of the stipules, and produced
seeds with maple, grey, purple-streaked, or mottled, and a few only,
again, with violet-coloured envelopes. Some of the seeds were round,
some partially indented[111]. The pods on each plant, in the majority
of instances, contained peas of like characters; but in a few cases
the peas in the same pod varied slightly, and in some instances a
pod or two on the same plant contained seeds all distinct from the
remainder[112]. The white-flowered plants were generally dwarfish,
of about the height of “Ringleader”; but the coloured-flowered sorts
varied altogether as to height, period of ripening, and colour and
shape of seed[113]. Those seeds with violet-coloured envelopes produced
nearly all maple- or parti-coloured seeds, and only here and there one
with a violet-coloured envelope; that colour, again, appeared only
incidentally, and in a like degree in the produce of the maple-coloured
seeds[114].
[110] Recessive in flower-colour, seed-coat colour, and in seed-shape
as a maternal character: pure recessives as the sequel proved.
[111] These are then a mixture of pure dominants and cross-bred
dominants, and are now inextricably confused. This time the round
seeds may have been all on particular plants--showing recessive
seed-shape as a maternal character. It seems just possible that
this fact suggested the idea of “round” seeds on the _coloured_
plants in the last generation. Till that result is confirmed it
should be regarded as very doubtful on the evidence. But we cannot
at the present time be sure how much difference there was between
these round seeds and the _normal_ maples in point of shape; and
on the whole it seems most probable that the roundness was a mere
fluctuation, such as commonly occurs among the peas with large
indented seeds.
[112] Is this really evidence of segregation of characters, the
flower being the unit? In any case the possibility makes the
experiment well worth repeating, especially as Correns has seen a
phenomenon conceivably similar.
[113] Being a mixture of heterozygotes (probably involving several
pairs of allelomorphs) and homozygotes.
[114] This looks as if the violet colour was merely due to
irregularity of xenia.
In 1869 the seeds of various selections of the previous year were again
sown separately; and the white-seeded peas again produced only plants
with white flowers and round white seeds[115]. Some of the coloured
seeds, which I had expected would produce purple-flowered plants,
produced plants with white flowers and round white seeds only[116]; the
majority, however, brought plants with purple flowers and with seeds
principally marked with purple or grey, the maple- or brown-streaked
being in the minority[117]. On some of the purple-flowered plants
were again a few pods with peas differing entirely from the remainder
on the same plant. In some pods the seeds were all white, in others
all black, and in a few, again, all violet[118]; but those plants
which bore maple-coloured seeds seemed the most constant and fixed
in character of the purple-flowered seedlings[119], and the purplish
and grey peas, being of intermediate characters, appeared to vary
most[120]. The violet-coloured seeds again produced almost invariably
purplish, grey, or maple peas, the clear violet colour only now and
then appearing, either wholly in one pod or on a single pea or two in
a pod. All the seeds of the purple-flowered plants were again either
round or only partially indented; and the plants varied as to height
and earliness. In no case, however, does there seem to have been an
intermediate-coloured flower; for although in some flowers I thought
I found the purple of a lighter shade, I believe this was owing to
light, temperature, or other circumstances, and applied equally to the
parent maple. I have never noticed a single tinted white flower nor an
indented white seed in either of the three years’ produce. The whole
produce of the third sowing consisted of seeds of the colours and in
the approximate quantities in order as follows,--viz.: 1st, white,
about half; 2nd, purplish, grey, and violet (intermediate colours),
about three-eighths; and, 3rd, maple, about one-eighth.
[115] Pure recessives.
[116] Pure recessives in coats showing maternal dominant character.
[117] Now recognized as pure homozygotes.
[118] This seems almost certainly segregation by flower-units, and is
as yet inexplicable on any other hypothesis. Especially paradoxical
is the presence of “white” seeds on these plants. The impression is
scarcely resistible that some remarkable phenomenon of segregation
was really seen here.
[119] Being now homozygotes.
[120] Being heterozygotes exclusively.
From the above I gather that the white-flowered white-seeded pea is
(if I may use the term) an original variety well fixed and distinct
entirely from the maple, that the two do not thoroughly intermingle
(for whenever the white flower crops out, the plant and its parts all
appear to follow exactly the characters of the white pea), and that the
maple is a cross-bred variety which has become somewhat permanent and
would seem to include amongst its ancestors one or more bearing seeds
either altogether or partly violet- or purple-coloured; for although
this colour does not appear on the seed of the “maple,” it is very
potent in the variety, and appears in many parts of the plant and its
offspring from cross-fertilised flowers, sometimes on the external
surface or at the sutures of the pods of the latter, at others on
the seeds and stems, and very frequently on the seeds; and whenever
it shows itself on any part of the plant, the flowers are invariably
purple. My deductions have been confirmed by intercrosses effected
between the various white-, blue-, some singularly bright green-seeded
peas which I have selected, and the maple- and purple-podded and the
purple-flowered sugar peas, and by reversing those crosses.
I have also deduced from my experiments, in accordance with the
conclusions of the late Mr Knight and others, that the colours of the
envelopes of the seeds of peas immediately resulting from a cross are
never changed[121]. I find, however, that the colour and probably the
substance of the cotyledons are sometimes, but not always, changed
by the cross fertilisation of two different varieties; and I do not
agree with Mr Knight that the form and size of the seeds produced are
unaltered[122]; for I have on more than one occasion observed that
the cotyledons in the seeds directly resulting from a cross of a blue
wrinkled pea fertilised by the pollen of a white round variety have
been of a greenish-white colour[123], and the seeds nearly round[124]
and larger or smaller according as there may have been a difference in
the size of the seeds of the two varieties[125].
[121] The nature of this mistake is now clear; for as stated above
xenia is only likely to occur when the maternal seed-coat is
pigmented. The violet coats in this experiment are themselves cases
of xenia.
[122] Knight, it was seen, crossed round ♀ × indented ♂ and
consequently got no change of form.
[123] Cotyledons seen through coat.
[124] Ordinary dominance of round.
[125] This is an extraordinary statement to be given as a general
truth. There are sometimes indications of this kind, but certainly
the facts are not usually as here stated.
I have also noticed that a cross between a round white and a blue
wrinkled pea will in the third and fourth generations (second and third
years’ produce) at times bring forth blue round, blue wrinkled, white
round and white wrinkled peas in the same pods, that the white round
seeds, when again sown, will produce only white round seeds, that
the white wrinkled seeds will, up to the fourth or fifth generation,
produce both blue and white wrinkled and round peas, that the blue
round peas will produce blue wrinkled and round peas, but that the
blue wrinkled peas will bear only blue wrinkled seeds[126]. This
would seem to indicate that the white round and the blue wrinkled peas
are distinct varieties derived from ancestors respectively possessing
one only of those marked qualities; and, in my opinion, the white
round peas trace their origin to a dwarfish pea having white flowers
and round white seeds, and the blue wrinkled varieties to a tall
variety, having also white flowers but blue wrinkled seeds. It is
also noticeable, that from a single cross between two different peas
many hundreds of varieties, not only like one or both parents and
intermediate, but apparently differing from either, may be produced
in the course of three or four years (the shortest time which I have
ascertained it takes to attain the climax of variation in the produce
of cross-fertilised peas, and until which time it would seem useless
to expect a fixed seedling variety to be produced[127]), although a
reversion to the characters of either parent, or of any one of the
ancestors, may take place at an earlier period.
[126] If we were obliged to suppose that this is a matured conclusion
based on detailed observation it would of course constitute the most
serious “exception” yet recorded. But it is clear that the five
statements are not mutually consistent. We have dominance of round
white in first cross.
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.”
V. PROFESSOR WELDON’S QUOTATIONS FROM LAXTON.
In support of his conclusions Professor Weldon adduces two passages
from Laxton, some of whose testimony we have just considered. This
further evidence of Laxton is so important that I reproduce it in full.
The first passage, published in 1866, is as follows:--
“The results of experiments in crossing the Pea tend to show that
the colour of the immediate offspring or second generation sometimes
follows that of the female parent, is sometimes intermediate between
that and the male parent, and is sometimes distinct from both; and
although at times it partakes of the colour of the male, it has
not been ascertained by the experimenter ever to follow the exact
colour of the male parent[139]. In shape, the seed frequently has an
intermediate character, but as often follows that of either parent.
In the second generation, in a single pod, the result of a cross
of Peas different in shape and colour, the seeds are sometimes all
intermediate, sometimes represent either or both parents in shape
or colour, and sometimes both colours and characters, with their
intermediates, appear. The results also seem to show that the third
generation or the immediate offspring of a cross, frequently varies
from its parents in a limited manner--usually in one direction
only, but that the fourth generation produces numerous and wider
variations[140]; the seed often reverting partly to the colour and
character of its ancestors of the first generation, partly partaking
of the various intermediate colours and characters, and partly
sporting quite away from any of its ancestry.”
[139] This is of course on account of the maternal seed characters.
Unless the coat-characters are treated separately from the
cotyledon-characters Laxton’s description is very accurate. Both
this and the statements respecting the “shape” of the seeds, a term
which as used by Laxton means much more than merely “wrinkled” and
“smooth,” are recognizably true as general statements.
[140] Separation of hypallelomorphs.
Here Professor Weldon’s quotation ceases. It is unfortunate he did
not read on into the very next sentence with which the paragraph
concludes:--
“These sports appear to become fixed and permanent in the next
and succeeding generations; and the tendency to revert and sport
thenceforth seems to become checked if not absolutely stopped[141].”
[141] The combinations being exhausted. Perhaps Professor Weldon
thought his authority was here lapsing into palpable nonsense!
Now if Professor Weldon instead of leaving off on the word “ancestry”
had noticed this passage, I think his article would never have been
written.
Laxton proceeds:--
“The experiments also tend to show that the height of the plant
is singularly influenced by crossing; a cross between two dwarf
peas, commonly producing some dwarf and some tall [? in the second
generation]; but on the other hand, a cross between two tall peas
does not exhibit a tendency to diminution in height.
“No perceptible difference appears to result from reversing the
parents; the influence of the pollen of each parent at the climax or
fourth generation producing similar results[142].”
[142] Laxton constantly refers to this conception of the “climax”
of--as we now perceive--analytical variation and recombination. Many
citations could be given respecting his views on this “climax” (cp.
p. 167).
The significance of this latter testimony I will presently discuss.
Professor Weldon next appeals to a later paper of Laxton’s published in
1890. From it he quotes this passage:
“By means, however, of cross-fertilisation alone, and unless it be
followed by careful and continuous selection, the labours of the
cross-breeder, instead of benefiting the gardener, may lead to utter
confusion,”
Here again the reader would have gained had Professor Weldon, instead
of leaving off at the comma, gone on to the end of the paragraph, which
proceeds thus:--
“because, as I have previously stated, the Pea under ordinary
conditions is much given to sporting and reversion, for when two
dissimilar old or fixed varieties have been cross-fertilised,
three or four generations at least must, under the most favourable
circumstances, elapse before the progeny will become fixed or
settled; and from one such cross I have no doubt that, by sowing
every individual Pea produced during the three or four generations,
hundreds of different varieties may be obtained; but as might be
expected, I have found that where the two varieties desired to be
intercrossed are unfixed, confusion will become confounded[143],
and the variations continue through many generations, the number at
length being utterly incalculable.”
[143] Further subdivision and recombination of hypallelomorphs.
Professor Weldon declares that Laxton’s “experience was altogether
different from that of Mendel.” The reader will bear in mind that when
Laxton speaks of fixing a variety he is not thinking particularly of
seed-characters, but of all the complex characters, fertility, size,
flavour, season of maturity, hardiness, etc., which go to make a
serviceable pea. Considered carefully, Laxton’s testimony is so closely
in accord with Mendelian expectation that I can imagine no chance
description in non-Mendelian language more accurately stating the
phenomena.
Here we are told in unmistakable terms the breaking up of the original
combination of characters on crossing, their re-arrangement, that
at the fourth or fifth generation the possibilities of sporting
[sub-division of compound allelomorphs and re-combinations of them?]
are exhausted, that there are then definite forms which if selected
are thenceforth fixed [produced by union of similar gametes?]
that it takes longer to select some forms [dominants?] than others
[recessives?], that there may be “mule” forms[144] or forms which
cannot be fixed at all[145] [produced by union of dissimilar gametes?].
[144] For instance the _talls_ produced by crossing _dwarfs_ are such
“mules.” Tschermak found in certain cases distinct increase in height
in such a case, though not always (p. 531).
[145] “The remarkably fine but unfixable pea _Evolution_.” Laxton, p.
37.
But Laxton tells us more than this. He shows us that numbers of
varieties may be obtained--hundreds--“incalculable numbers.” Here
too if Professor Weldon had followed Mendel with even moderate care
he would have found the secret. For in dealing with the crosses of
_Phaseolus_ Mendel clearly forecasts the conception of _compound
characters themselves again consisting of definite units_, all of which
may be separated and re-combined in the possible combinations, laying
for us the foundation of the new science of Analytical Biology.
How did Professor Weldon, after reading Mendel, fail to perceive these
principles permeating Laxton’s facts? Laxton must have seen the very
things that Mendel saw, and had he with his other gifts combined that
penetration which detects a great principle hidden in the thin mist of
“exceptions,” we should have been able to claim for him that honour
which must ever be Mendel’s in the history of discovery.
When Laxton speaks of selection and the need for it, he means, what
the raiser of new varieties almost always means, the selection of
_definite_ forms, not impalpable fluctuations. When he says that
without selection there will be utter confusion, he means--to use
Mendelian terms--that the plant which shows the desired combination of
characters must be chosen and bred from, and that if this be not done
the grower will have endless combinations mixed together in his stock.
If however such a selection be made in the fourth or fifth generation
the breeder may very possibly have got a fixed form--namely, one that
will breed true[146]. On the other hand he may light on one that does
not breed true, and in the latter case it may be that the particular
type he has chosen is not represented in the gametes and will _never_
breed true, though selected to the end of time. Of all this Mendel has
given us the simple and final account.
[146] Apart from fresh original variations, and perhaps in some cases
imperfect homozygosis of some hypallelomorphs.
At Messrs Sutton and Sons, to whom I am most grateful for unlimited
opportunities of study, I have seen exactly such a case as this. For
many years Messrs Sutton have been engaged in developing new strains
of the Chinese Primrose (_Primula sinensis_, hort.). Some thirty
thoroughly distinct and striking varieties (not counting the _Stellata_
or “Star” section) have already been produced which breed true or
very nearly so. In 1899 Messrs Sutton called my attention to a strain
known as “Giant Lavender,” a particularly fine form with pale magenta
or lavender flowers, telling me that it had never become fixed. On
examination it appeared that self-fertilised seed saved from this
variety gave some magenta-reds, some lavenders, and some which are
white on opening but tinge with very faint pink as the flower matures.
On counting these three forms in two successive years the following
figures appeared. Two separately bred batches raised from “Giant
Lavender” were counted in each year.
Magenta Lavender White
red faintly tinged
1901 1st batch 19 27 14
" 2nd " 9 20 9
1902 1st " 12 23 11
" 2nd " 14 26 11
-- -- --
54 96 45
The numbers 54 : 96 : 45 approach the ratio 1 : 2 : 1 so nearly that
there can be no doubt we have here a simple case of Mendelian laws,
operating without definite dominance, but rather with blending.
When Laxton speaks of the “remarkably fine but unfixable pea
_Evolution_” we now know for the first time exactly what the phenomenon
meant. It, like the “Giant Lavender,” was a “mule” form, not
represented by germ-cells, and in each year arose by “self-crossing.”
This is only one case among many similar ones seen in the Chinese
Primrose. In others there is no doubt that more complex factors are at
work, the subdivision of compound characters, and so on. The history
of the “Giant Lavender” goes back many years and is not known with
sufficient precision for our purposes, but like all these forms it
originated from crossings among the old simple colour varieties of
_sinensis_.
VI. THE ARGUMENT BUILT ON EXCEPTIONS.
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
1 _DD_ : 2 _DR_ : 1 _RR_,
but he fertilised those cross-breds (_DR_’s) both with the pure
dominant (_D_) and with the pure recessive (_R_) varieties
reciprocally, obtaining in the former case the ratio
1 _DD_ : 1 _DR_
and in the latter the ratio
1 _DR_ : 1 _RR_.
The _DD_ group and the _RR_ group thus produced giving on
self-fertilisation pure _D_ offspring and pure _R_ offspring
respectively, while the _DR_ groups gave again
1 _DD_ : 2 _DR_ : 1 _RR_.
How does Professor Weldon propose to deal with these results, and by
what reasoning can he suggest that considerations of ancestry are to be
applied to them? If I may venture to suggest what was in Mendel’s mind
when he applied this further test to his principles it was perhaps some
such considerations as the following. Knowing that the cross-breds on
self-fertilisation give
1 _DD_ : 2 _DR_ : 1 _RR_
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.
VII. THE QUESTION OF ABSOLUTE PURITY OF GERM-CELLS.
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.
CONCLUSION.
I trust what I have written has convinced the reader that we are,
as was said in opening, at last beginning to move. Professor Weldon
declares he has “no wish to belittle the importance of Mendel’s
achievement”; he desires “simply to call attention to a series of
facts which seem to him to suggest fruitful lines of inquiry.” In
this purpose I venture to assist him, for I am disposed to think that
unaided he is--to borrow Horace Walpole’s phrase--about as likely to
light a fire with a wet dish-clout as to kindle interest in Mendel’s
discoveries by his tempered appreciation. If I have helped a little in
this cause my time has not been wasted.
In these pages I have only touched the edge of that new country which
is stretching out before us, whence in ten years’ time we shall
look back on the present days of our captivity. Soon every science
that deals with animals and plants will be teeming with discovery,
made possible by Mendel’s work. The breeder, whether of plants or
of animals, no longer trudging in the old paths of tradition, will
be second only to the chemist in resource and in foresight. Each
conception of life in which heredity bears a part--and which of them is
exempt?--must change before the coming rush of facts.
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_Received as this sheet goes to press:--_
CORRENS, C. Die Ergebnisse der neuesten Bastardforschungen
für die Vererbungslehre, _Ber. deut. bot. Ges._, XIX.,
Generalversammlungs-Heft 1.
---- Ueber den Modus und den Zeitpunkt der Spaltung der Anlagen bei
den Bastarden vom Erbsen-Typus, _Bot. Ztg._, 1902, p. 65.
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