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.