Previous Chapter : XII. EVIDENCE AS TO MENDELIAN INHERITANCE IN MAN : PAGE 205.
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CHAPTER XIII
INTERMEDIATES BETWEEN VARIETIES AND THE "PURE LINES" OF JOHANNSEN.
MISUNDERSTANDING in regard to the physiological significance of intermediates has been a fertile
source of error, and more than any other perhaps has tended to obscure the true interpretation of genetic
phenomena. The existence of intermediates has been often alleged as a proof that segregation does not
occur in regard to the particular variations towards which the gradational forms seem to lead, and the
misuse of statistical method so frequent in biometrical attempts to investigate heredity has come about
chiefly through misinterpretation of the nature of such gradational forms.
These errors are of course a legacy from the period of the essayists, when evolution at large was the
chief object of study, and an examination of variation in detail as occurring in specific instances had not
been undertaken.
Biologists committed to the proposition that varieties arise through the transformation of masses of
individuals by the selective accumulation of minute differences saw that with each new case in which
discontinuous variation could be proved to occur, the scope for their views was reduced, and the existence
of intermediates constituted the most promising line of defence. When intermediate and gradational forms
could be found, the contention might always be hazarded that the definite and extreme forms had been
reached by transition through them.
Now the application of analytical methods to those
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cases where much gradation occurs is usually troublesome, but it is becoming clear that the difficulty is
often a practical one only. To analyse the intermediates and to determine the various elements that have
taken part in their production may be a very laborious process, still it can be shown in many examples that
the intermediate character is only a superficial or nett result of the interaction of factors which are
transmitted as units. The several classes under which such intermediate types present themselves are
worth enumerating.
1. Intermediates as heterozygous forms. Sometimes, as is the case of the blue Andalusian fowl, the
whole group of heterozygotes forms a recognizable, distinct class which might nevertheless be regarded
as intermediate between the two pure types black and white, with splashes of grey and black. Similar
intermediates occurring as heterozygous forms have now been often seen and described. In the case of
Primula Sinensis the heterozygote between the star-shaped flower of the modern stellata varieties and the
common type with imbricated petals is always an intermediate*.
In fowls though no perfectly distinct heterozygous type of comb exists, yet with experience it is
usually possible to distinguish the pure (DD) pea combs from the DR group, and, though with much less
confidence, the same distinction can sometimes be made among the rose-combs.
In Lychnis the crosses between white vespertina and red dioica are, so far as I know, always of an
intermediate shade of pink. In Pisum the crosses between the races I have called "half-dwarf," standing
about 3 to 4 feet high, and actual dwarfs, about 1 foot high, are usually intermediate in height. The
researches of Miss Wheldale on Antirrhinum have shown that in that species numerous intergrading forms
are definite heterozygotes, and Miss Marryat has proved that the same is true for Mirabilis
Such instances may be multiplied indefinitely. Dominance indeed is not often so pronounced that a
practised
* Whether any such intermediate flower shape exists as a gametic form which can be bred true I do not know for certain. Mr
Gregory and I have not met one among our derivatives from various crosses between stellata and Sinensis.
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observer cannot distinguish the DD types from the DR's with fair certainty by a thorough and minute
examination. As examples in which heterozygotes are indistinguishable from pure dominants, may be
mentioned tall and dwarf in Peas and Sweet Peas, coloured flowers and white flowers in Sweet Peas and
Stocks, hoary and glabrous in Stocks. In all these the one "dose" of a dominant factor is sufficient to
produce the full effect.
2. Intermediates due to subtraction-stages of dominant factors. This is also a large and important
class, one, too, which has been perhaps more especially a source of the confusion alluded to in the
introduction to this section. Of such intermediates the colour of the Dutch rabbit may be taken as a type.
The Dutch rabbit has the posterior half of the body coloured, while the anterior half is white except for
patches including the eyes and ears. Such a rabbit may be described as in a sense intermediate between the
self-coloured rabbit and the albino. Its particoloured nature is due to the restriction of one (or conceivably
more) of its pigment-factors to certain areas. In the self-coloured animal this factor has a more extended
distribution. But in speaking of the Dutch pattern as an intermediate, an essential point, the definiteness of
the Dutch pattern, is missed. It is intermediate only in a strictly limited sense. It is not a transition-stage
between a self-colour and an albino, nor would there be any better chance of breeding albinos from
genetically pure Dutch than there would be from genetically pure self coloured rabbits. The Dutch pattern
in fact constitutes a definite type and depends for its appearance on the presence of a certain factor in a
fairly definite stage, and it does not appear when self-colours are crossed with albinos unless the particular
necessary factor in its appropriate stage is introduced in one of the parents.
Within the Dutch class there are subordinate types again, the interrelations of which have not yet been
worked out. Some of these are very probably gametic types which could be bred pure, while others are
probably caused by fluctuations of an irregular kind or by the interactions between the factors. It will need
very elaborate research
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to determine the interrelations of these finer differences. As one of the few points that are clear may be
mentioned the impossibility of fixing a form with the sharply-cut division between the white and the
colour which is the fancier's ideal for Dutch. From this negative evidence it may be inferred that that
particular distribution is not one of those represented as a gametic type.
Intermediates of this sort are commonly met with in breeding. The "half-dwarf" peas are an instance.
In Stocks Miss Saunders has described a "half-hoary" race (19, p. 33) in which the lower surfaces of the
leaves were hoary while the upper surfaces were glabrous or nearly so. No such "intermediates" have
occurred among the thousands of plants raised by Miss Saunders except when the definite "half-hoary"
type was originally introduced as a parent. In the Sweet Pea one of the most definite and unchanging types
is the nearly white flower with a pink streak, that comes next to the pure white if judged by the criterion
of amount of deficiency of colour. In the absence of breeding experiments an ignorant person having
raised such a plant as offspring of a purple or a red might consider that he had made an advance towards
pure white. Nevertheless he would be no more likely to raise true whites from such a tinged white than if
he began with a wild Sicilian purple pea. Indeed if I required to raise a white Sweet Pea de novo, I should
think the chance of getting it from the wild pea much better than from the tinged white, for again and
again wild types have been found to throw off albinos soon after their introduction into cultivation-by the
occurrence, of course, of a new genetic variation.
3. Intermediates produced by the interference of other factors. There are many intermediates of this
kind also. In both animals (Fowl, Rabbit, Mouse) and plants (Primula) we have seen that a dominant
factor may exist which has the power, for example, of suppressing the development of colour, leaving
those parts white which in the absence of that factor would otherwise be coloured. This suppressing or
inhibiting effect may be total, but when it is not total some puzzling intermediate types may be
constituted. Only most careful breeding experiments can reveal the true
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nature of these cases. To the ordinary observer the "Painted Lady" Sweet Pea with its whitish wings and
red standard might appear intermediate between a white and a totally red type. But as experiment shows,
the bicolour type with its whitish wings is dominant to the self-coloured forms, and its wings are white,
not as the Dutch rabbit is, owing to the omission of something, but on account of the addition of another
distinct element which suppresses the pink colour in that particular part more or less completely. The real
white is of course due to the absence of one or more of the colour-factors, an altogether different cause,
and it is therefore evident that to speak of the bicolour form as in any way intermediate between selfcolour and white would be a complete misrepresentation of the facts.
Similarly in the Rabbit, Hurst has shown that the curious type known as "English" pattern - something
like a Dalmatian dog but for the more or less regular disposition of the spots - is a dominant to self colour,
though a careless observer might guess it to be a transition-stage towards albinism.
In such cases again there is every likelihood that careful selection might succeed in isolating
subordinate types in which the suppression attains particular degrees of completeness ranging within well
defined limits, but the evidence distinctly negatives any attempt to treat these several forms as a
continuous series in which any member is capable of reproducing any other among its offspring.
4. Intermediates as fluctuational forms. Lastly there are intermediates due to the disturbing effects of
many small causes not of genetic but presumably of environmental origin. Such are the fluctuations in the
weight of individual seeds which Johannsen has studied with success in beans (Phaseolus).
When the weights of the seeds of a single variety of bean are marshalled statistically they arrange
themselves in a normal curve round the weight of greatest frequency. Similarly when the seeds are sown
and the seeds from the self-fertilised individual plants which grow from them are harvested separately, the
crops from each individual again can be grouped according to their weights in normal curves
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round the most frequent or modal weight characteristic of each individual. There will then be seen that
some rough correspondence exists between the modes for the individual plants and the weight of the
individual seeds from which they sprang. The heavier families on the whole come from the heavier seeds,
and the lighter from the lighter seeds. Such correspondence is nevertheless very imperfect, and the weight
of the seed chosen from the original mass gives only a vague indication of what the modal weight of that
plant will be.
But when any one family raised from a single seed was taken, and the heavier and the lighter of its
self-fertilised seeds chosen and separately sown, Johannsen found that the modal weights were
approximately the same for the produce of both the heavier and the lighter seeds. Selection inside the
family raised from a single seed did not alter the modal weight, which went back or regressed to that of
the individual common parent. Such a family thus raised from the single seed constitutes what Johannsen
has called a pure line. Within the genetically pure line there are fluctuations in weight, but these
fluctuations are due to interference which is external, or environmental in they wide sense, and selection
of those extremes which are due to such interference produces no effect on the result, for the differences
in the weights of the seeds of a pure plant do not indicate differences between the germ-cells which
produced them (164).
It is to variation of this type that the name fluctuation is given.
Johannsen has made a similar series of experiments in regard to the proportion of defective grains
present in certain barleys and has reached a similar conclusion.
Whether the distinctions characteristic of the pure lines segregate or not cannot yet be said. By the
nature of such cases the distinctions themselves are fine, and even if segregation occurred the proof of its
existence would involve many and serious practical difficulties.
When the complex diversity of intermediates is thus appreciated it will be understood that extreme
caution is needed before the statement is made that in any specific
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case segregation does not occur. Even a sensible continuity between varieties is no proof that there is not
segregation which with careful work could be demonstrated. To take for instance the case of piebald mice,
it would be possible with industry to collect from the results of miscellaneous breeding specimens ranging
from a self-colour to a black eyed white. From such series any number of false conclusions could be
drawn respecting the continuous variation of mice. By appealing to the "ruby" eyes of certain chocolates
an uncritical observer might even argue that a series of intermediates connected the black and the pink
eyes.
Analytical breeding immediately shows the fallacy of such deductions. For it would be found that the
so-called intermediates consisted of numbers of genetically distinct types with distinct genetic properties
depending on the factors which constituted them. Some would carry the colour-factors for the Dutch
pattern, others those for more complete or less complete pigmentation, while others would owe their
partial whiteness to the presence of the dominant factor which can suppress pigmentation in several stages
of completeness.
In general it is to be concluded that when the interbreeding of recognizably distinct types produces
only those types again without intermediates or with only few individuals that can be so regarded, the fact
points strongly to the existence of gametic segregation ; but on the other hand the appearance of even a
complete series of intergrading forms is not to be accepted as proof that gametic segregation does not
occur.
Misconception of the nature and significance of intermediates has deprived the work of the biometrical
school of scientific value as a contribution to the study of heredity. This is well seen in the case of the
colours of Shirley Poppies, one of the subjects with reference to which copious statistics have been
amassed and published*. The colours are red, pink, and white, of many shades, and mixtures of these tints
variously disposed and graded. The problem was to discover the laws of inheritance of the colours. The
* Biometrika, II. I, 1902, and IV. 4, 1906.
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Mendelian method would be to institute breeding experiments with individual plants, self-fertilising and
crossing these individuals together, and recording the offspring produced by each plant and by each
combination. True that by sufficient search flowers can be found which when arranged in order provide a
series of intergrades from the darkest red to the whites so nearly complete as to persuade an observer of
moderate experience that there is full continuity. Nevertheless, from what we know of every such a case
to which Mendelian analysis has been extensively applied, there cannot be the smallest doubt that, as in
Antirrhinum, the Mouse, and many other examples, the apparent continuity is misleading. It will, when
analysis is undertaken, unquestionably be shown that there are definite, pure homozygous types,
containing factors which display the usual phenomena of dominance, and segregate of many others are a
full green in an orderly fashion ; and that though, between these, intermediates may occur, yet they again
will be found for the most part to be various consequences of heterozygosis. There may, not improbably,
be a residuum of intermediates which cannot be thus represented, but it will be surprising if these are
more than insignificant fractions of the whole, owing their peculiarities to various fluctuating influences.
The ground will then be approximately mapped, and the laws of colour-inheritance approximately
established for the Shirley Poppy.
The method of Professor Pearson and his assistants dispenses with all analysis. No attempt is made to
discover the factors concerned, to distinguish the pure types, or the properties of dominance which they
possess. No artificial crosses are undertaken*. The flowers are left uncovered, open to the insects which
may introduce the pollen of any other flower which they happen to be carrying. The records show –
probably with much accuracy – the colours of the offspring derived from mothers of recorded colours, and
no more. In this attempt to study the law of heredity of colour, the fathers of the progeny examined are in
every case unknown.
* Self-fertilisation was subsequently tried, but, owing to self-sterility, gave no results.
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According to the biometrical system, if the characters of one parent and of the offspring are known,
the material can, after mathematical treatment by means of the correlation-table, be used to deduce a law
of inheritance. The theoretical conceptions upon which this method is based were formed and elaborated
first in ignorance and subsequently in disregard of Mendelian facts ; but improbable as their validity was
in pre-Mendelian times, it must now be obvious that these methods can merely obscure the essential
phenomena they were devised to discover.
There are moreover abundant examples of other fluctuations more obviously depending on
environmental influences. In studying the inheritance of the cotyledon-colours in the seeds of peas such
cases are met with. The cotyledons of many varieties are in all ripe seeds a full yellow, while those of
many others are full green. The contrast between the yellow and the green seeds in F2 after segregation
may be perfectly sharp and clear. Some varieties, especially the more modern sorts, have many seeds
which are in various degrees partly yellow and partly green, and occasionally such particoloured seeds are
the commonest type of the variety. In his criticism of Mendel's work the late Professor Weldon called
attention to these particoloured seeds, contending that they showed failure of segregation. Before looking
into the facts I was disposed to admit the justice of this contention. After experimenting with such kinds
however I found that the particoloured appearance was not caused by any admixture of the yellow
character, but by exposure to sun and weather. All those seeds which tinge are by nature green and breed
true to greenness. If in a pod several are found yellowed, this bleaching affects those sides of the seeds
which face the same way and is obviously due to effects of light just like the reddening of the exposed
sides of many apples.
The hereditary property characteristic of these tinging sorts is the power of bleaching readily if
exposed, but the bleaching is checked and can generally be prevented if the pods are gathered as soon as
they are ripe. It may be thought that the power of bleaching to which the ordinary varieties owe their
colour is the same faculty intensified. It may be so, but there are very marked distinctions ;
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for seeds of yellow varieties turn yellow before they harden, while the particoloured sorts bleach after they
have become ripe.
In some yellow varieties greenish seeds occur. This phenomenon, the converse of the last, occurs in
wet years with many kinds, some of course being specially liable.
The fact that such differences are fluctuational and not genetic appears at once when breeding is
begun ; for if in a green which bleaches (e.g. American Wonder) the yellowest and the greenest are taken
from the same plant and sown, there will be no corresponding difference between their respective
produce. By selecting the greenest and yellowest from a large mass of commercial seeds of a variety, not
harvested plant by plant, some correspondence may be obtained occasionally (e.g. Telephone) but the
occurrence merely means that the mass, though called one variety, had not really been selected down to a
"pure line," but was in fact a mixture of sorts. In such a type as "Nonpareil," for instance, where the
difference between yellows and green is actually genetic, the yellows and the greens will give distinct
results. "Nonpareil," in fact, is a form which consists of true yellows and true greens in about equal
numbers, an it is simply a mixture of two varieties not sensibly different except in seed-colour. No doubt
it arose from a plan saved in one of the later generations derived from a cross which plant was
homozygous in other respects but heterozygous in seed-colour.
Next Chapter
XIV. MISCELLANEOUS EXCEPTIONAL AND UNCONFORMABLE PHENOMENA
PAGE 245.
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