INTERMEDIATES
So far as we have gone we have found it possible to express the various
characters of animals and plants in terms of definite factors which are
carried by the gametes, and are distributed according to a definite scheme.
Whatever may be the nature of these factors it is possible for purposes of
analysis to treat them as indivisible entities which may or may not be
present in any given gamete. When the factor is present it is present as a
whole. The visible properties developed by a zygote in the course of its
growth depend upon the nature and variety of the factors carried in by the
two gametes which went to its making, and to a less degree upon whether
each factor was brought in by both gametes or by one only. If the given
factor is brought in by one gamete only, the resulting heterozygote may be
more or less intermediate between the homozygous form with a double dose of
the factor and the homozygous form which is entirely destitute of the
factor. Cases in point are those of the primula flowers and the Andalusian
fowls. Nevertheless these intermediates produce only pure gametes, as is
{126} shown by the fact that the pure parental types appear in a certain
proportion of their offspring. In such cases as these there is but a single
type of intermediate, and the simple ratio in which this and the two
homozygous forms appear renders the interpretation obvious. But the nature
of the F_2 generation may be much more complex, and, where we are dealing
with factors which interact upon one another, may even present the
appearance of a series of intermediate forms grading from the condition
found in one of the original parents to that which occurred in the other.
As an illustration we may consider the cross between the Brown Leghorn and
Silky fowls which we have already dealt with in connection with the
inheritance of sex. The offspring of a Silky hen mated with a Brown Leghorn
are in both sexes birds with but a trace of the Silky pigmentation. But
when such birds are bred together they produce a generation consisting of
chicks as deeply pigmented as the original Silky parent, chicks devoid of
pigment like the Brown Leghorn, and chicks in which the pigmentation shows
itself in a variety of intermediate stages. Indeed from a hundred chicks
bred in this way it would be possible to pick out a number of individuals
and arrange them in an apparently continuous series of gradually increasing
pigmentation, with the completely unpigmented at one end and the most
deeply pigmented at the other. Nevertheless, the case is one in which
complete segregation of the different factors takes {127}
[Illustration]place, place, and the apparently continuous series of
intermediates is the result of the interaction of the different factors
upon one another. The constitution of the F_1 [male] is a ffPpIi, and such
a bird produces in equal numbers the four sorts of gametes fPI, fPi, fpI,
fpi. The constitution of the F_1 [female] in this case is FfPpIi. Owing to
the repulsion between F and I she produces the four kinds of gametes FPi,
Fpi, fPI, fpi, and produces them in equal numbers. The result of bringing
two such series of gametes together is shown in Fig. 28. Out of the sixteen
types of zygote formed one (FfPPii) is homozygous for the pigmentation
factor, and does not contain the inhibitor factor. Such a bird is as deeply
pigmented as the pure Silky parent. Two, again, contain a single dose of P
in the absence of I. These are nearly as dark as the pure Silky. Four
zygotes are destitute of P, though they may or may not contain I. These
birds are completely devoid of pigment like the Brown Leghorn. The
remaining nine zygotes show {128} various combinations of the two factors P
and I, being either PPIi, PPII, PpII, or PpIi, and in each of these cases
the pigment is more or less intense according to the constitution of the
bird. Thus a bird of the constitution PPIi approaches in pigmentation a
bird of the constitution Ppii, while a bird of the constitution PpII has
but little more pigment than the unpigmented bird. In this way we have
seven distinct grades of pigmentation, and the series is further
complicated by the fact that these various grades exhibit a rather
different amount of pigmentation according as they occur in a male or a
female bird, for, generally speaking, the female of a given grade exhibits
rather more pigment than the corresponding male. The examination of a
number of birds bred in this way might quite well suggest that in this case
we were dealing with a character which could break up, as it were, to give
a continuous series of intergrading forms between the two extremes. With
the constant handling of large numbers it becomes possible to recognise
most of the different grades, though even so it is possible to make
mistakes. Nevertheless, as breeding tests have amply shown, we are dealing
with but two interacting factors which segregate cleanly from one another
according to the strict Mendelian rule. The approach to continuity in
variation exhibited by the F_2 generation depends upon the fact that these
two factors interact upon one another, and to different degrees according
as the zygote is for one {129} or other or both of them in a homozygous or
a heterozygous state. Moreover, certain of these intermediates will breed
true to an intermediate condition of the pigmentation. A male of the
constitution ffPPII when bred with females of the constitution FfPPIi will
produce only males like itself and females like the maternal parent. We
have dealt with this case in some detail, because the existence of families
showing a series of intermediate stages between two characters has
sometimes been brought forward in opposition to the view that the
characters of organisms depend upon specific factors which are transmitted
according to the Mendelian rule. But, as this case from poultry shows
clearly, neither the existence of such a continuous series of
intermediates, nor the fact that some of them may breed true to the
intermediate condition, are incompatible with the Mendelian principle of
segregation.
[Illustration: FIG. 28.
Diagram to illustrate the nature and composition of the F_2 generations
arising from the cross of Silky hen with Brown Leghorn cock.]
In connection with intermediates a more cogent objection to the Mendelian
view is the case of the first cross between two definite varieties
thenceforward breeding true. The case that will naturally occur to the
reader is that of the mulatto, which results from the cross between the
negro and the white. According to general opinion, these mulattos, of
intermediate pigmentation, continue to produce mulattos. Unfortunately this
interesting case has never been critically investigated, and the statement
that the mulatto breeds true rests almost entirely upon {130} information
that is general and often vague. It may be that the inheritance of skin
pigmentation in this instance is a genuine exception to the normal rule,
but at the same time it must not be forgotten that it may be one in which
several interacting factors are concerned, and that the pure white and the
pure black are the result of combinations which from their rarity are apt
to be overlooked. But until we are in possession of accurate information it
is impossible to pronounce definitely upon the nature of the inheritance in
this case.
[Illustration: FIG. 29.
Pedigree of a family which originated from a cross between a Hindu and a
European. Black signs denote individuals as dark as average Hindus. Plain
signs denote quite-fair members, while those with a dot in the centre are
intermediate.]
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On the other hand, from the cross between the darkly pigmented Eastern
races and the white segregation seems to occur in subsequent generations.
Families are to be found in which one parent is a pure white, while the
other has arisen from the cross between the dark and light in the first or
some subsequent generation. Such families may contain children
indistinguishable from pure blonds as well as children of very dark and of
intermediate shades. As an example, I may give the following pedigree,
which was kindly communicated to me by an Anglo-Indian friend (Fig. 29).
The family had resided in England for several generations, so that in this
case there was no question of a further admixture of black. Most noticeable
is the family produced by a very dark lady who had married a white man.
Some of the children were intermediate in colour, but two were fair whites
and two were dark as dark Hindus. This sharp segregation or splitting out
of blacks and whites in addition to intermediates strongly suggests that
the nature of the inheritance is Mendelian, though it may be complicated by
the existence of several factors which may also react upon one another. Nor
must it be forgotten that in so far as these different factors are
concerned the whites themselves may differ in constitution without showing
any trace of it in their appearance. Before the case can be regarded as
settled all these different possibilities will have to be definitely
tested. With the dark Eastern races as with the negro we cannot {132} hope
to come to any conclusion until we have evidence collected by critical and
competent observers.
Though for the present we must regard the case of the negro as not proven,
there are nevertheless two others in which the heredity would appear not to
follow the Mendelian rule. Castle in America crossed the lop-eared rabbit
with the normal form, and found that the F_1 animals were intermediate with
respect to their ears. And subsequent experiment showed that, on the whole,
they bred true to this intermediate condition. The other case relates to
Lepidoptera. The speckled wood butterfly (_Pararge egeria_) has a southern
form which differs from the northern one in the greater brightness and
depth of its yellow-brown markings. The northern form is generally
distinguished as var. _egeriades_. Bateson crossed the southern form from
the south of France with the paler British form, and found that the
offspring were more or less intermediate in colour, and that in subsequent
generations the parental types did not recur. These cases at present stand
alone. It is possible that further research may reveal complications which
mask or interfere with an underlying process of segregation. Or it may be
that segregation does not occur owing to some definite physiological reason
which at present we do not understand.
And here it is impossible not to recall Mendel's own experiences with the
Hawkweeds (_Hieracium_). This {133} genus of plants exhibits an
extraordinary profusion of forms differing from one another sometimes in a
single feature, sometimes in several. The question as to how far these
numerous forms were to be classified as distinct species, how far as
varieties, and how far as products of chance hybridisation, was even at
that time a source of keen controversy among botanists. There is little
doubt that Mendel undertook his experiments on the Hawkweeds in the hope
that the conception of unit-characters so brilliantly demonstrated for the
pea would serve to explain the great profusion of forms among the
Hieraciums. Owing to the minute size of their florets, these plants offer
very considerable technical difficulties in the way of cross fertilisation.
By dint of great perseverance and labour, however, Mendel succeeded in
obtaining a few crosses between different forms. These hybrids were reared
and a further generation produced from them, and, no doubt somewhat to
Mendel's chagrin, every one of them proved to breed true. There was a
complete absence of that segregation of characters which he had shown to
exist in peas and beans, and had probably looked forward with some
confidence to finding in _Hieracium_. More than thirty years passed before
the matter was cleared up. To-day we know that the peculiar behaviour of
the hybrid Hieraciums is due to the fact that they normally produce seed by
a peculiar process of parthenogenesis. It is possible to take an unopened
flower and to shear off with a {134} razor all the male organs together
with the stigmata through which the pollen reaches the ovules. The flower,
nevertheless, sets perfectly good seed. But the cells from which the seeds
develop are not of the same nature as the normal ovules of a plant. They
are not gametes but retain the double structure of the maternal cells. They
are rather to be regarded as of the nature of buds which early become
detached from the parent stock to lead an independent existence, and, like
buds, they reproduce exactly the maternal characteristics. The discovery of
the true nature of this case was only rendered possible by the development
of the study of cytology, and it was not given to Mendel to live long
enough to learn why his hybrid Hieraciums all bred true.
* * * * *
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