Hugo de Vries.

Species and varieties; their origin by mutation. Lectures delivered at the University of California. Edited by Daniel Trembly MacDougal online

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Online LibraryHugo de VriesSpecies and varieties; their origin by mutation. Lectures delivered at the University of California. Edited by Daniel Trembly MacDougal → online text (page 15 of 48)
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be slight individual differences, but each unit-
character will become opposed to, and united
with, the same unit-character in the other par-
ent. In the offspring the units will thus
be paired, each pair consisting of two equivalent
units. As to their character the units of each
single pair are the same, only they may exhibit
slight differences as to the degree of develop-
ment of this character.

Now we may apply this conception to the sex-
ual combination of two different elementary
species, assuming one to be the derivative of
the other. The differentiating mark is only
present in one of the parents and wanting in the
other. While all other units are paired in the
hybrid, this one is not. It meets with no mate,
and must therefore remain unpaired. The
hybrid of two such elementary species is in
some way incomplete and unnatural. In the
ordinary course of things all individuals derive



254 Retrograde Varieties

their qualities from both parents ; for each sin-
gle mark they possess at least two units. Prac-
tically but not absolutely equal, these two op-
ponents always work together and give to the
offspring a likeness to both parents. No un-
paired qualities occur in normal offspring ; these
constitute the essential features of the hybrids
of species and are at the same time the cause of
their wide deviations from the ordinary rules.

Turning now to the varieties, we likewise
need discuss their differentiating marks only.
In the negative types, these consist of the ap-
parent loss of some quality which was active in
the species. But it was pointed out in our last
lecture that such a change is an apparent loss.
On a closer inquiry we are led to the assumption
of a latent or dormant state. The presumably
lost characters have not absolutely, or at least
not permanently disappeared. They show
their presence by some slight indication of the
quality they represent, or by occasional revers-
ions. They are not wanting, but only latent.

Basing our discussion concerning the process
of crossing on this conception, and still limiting
the discussion to one differentiating mark, we
come to the inference, that this mark is present
and active in the species, and present but dor-
mant in the variety. Thus it is present in both,
and as all other characters not differentiating



Unbalanced Crosses 255

find their mates in the cross, so these two will
also meet one another. They will unite just as
well as though they were both active or both
dormant. For essentially they are the same,
only differing in their degree of activity. From
this we can infer, that in the crossing of varie-
ties, no unpaired remainder is left, all units
combining in pairs exactly as in ordinary fertil-
ization.

Setting aside the contrast between activity
and latency in this single pair, the procedure in
the inter-crossing of varieties is the same as in
ordinary normal fertilization.

Summarizing this discussion we may con-
clude that in normal fertilization and in the
inter-crossing of varieties all characters are
paired, while in crosses between elementary
species the differentiating marks are not mated.

In order to distinguish these two great types
of fertilization we will use the term bisexual for
the one and unisexual for the other. The term
balanced crosses then conveys the idea of com-
plete bisexuality, all unit-characters combining
in pairs. Unbalanced crosses are those in which
one or more units do not find their mates and
therefore remain unpaired. This distinction
was proposed by Macfarlane when studying
the minute structure of plant-hybrids in com-
parison with that of their parents (1892).



256 Retrograde Varieties

In the first place it shows that a species-
hybrid may inherit the distinguishing marks
of both parents. In this way it may become in-
termediate between them, having some charac-
ters in common with the pollen-parent and others
with the pistil-parent. As far as these charac-
ters do not interfere with each other, they may
be fully developed side by side, and in the main
this is the way in which hybrid characters are
evolved. But in most cases our existing knowl-
edge of the units is far too slender to give a
complete analysis, even of these distinguishing
marks alone. We recognize the parental marks
more or less clearly, but are not prepared for
exact delimitations. Leaving these theoretical
considerations, we will pass to the description
of some illustrative examples.

In the first place I will describe a hybrid
between two species of Oenothera, which I
made some years ago. The parents were the
common evening-primrose or Oenothera bien-
nis and of its small-flowered congener, Oeno-
thera muricata. These two forms were distin-
guished by Linnaeus as different species, but
have been considered by subsequent writers as
elementary species or so-called systematic va-
rieties of one species designated with the name
of the presumably older type, the 0. biennis.
Varietal differences in a physiologic sense they



Unbalanced Crosses 257

do not possess, and for this reason afford a
pure instance of unbalanced union, though dif-
fering in more than one point.

I have made reciprocal crosses, taking at one
time the small-flowered and at the other the
common species as pistillate parent. These
crosses do not lead to the same hybrid as is
ordinarily observed in analogous cases ; quite on
the contrary, the two types are different in most
features, both resembling the pollen-parent
far more than the pistil-parent. The same
curious result was reached in sundry other re-
ciprocal crosses between species of this genus.
But I will limit myself here to one of the two
hybrids.

In the summer of 1895 I castrated some flow-
ers of 0. muricata, and pollinated them with 0.
biennis, surrounding the flowers with paper
bags so as to exclude the visits of insects. I
sowed the seeds in 1896 and the hybrids were
biennial and flowered abundantly the next year
and were artificially fertilized with their own
pollen, but gave only a very small harvest.
Many capsules failed, and the remaining con-
tained only some few ripe seeds.

From these I had in the following year the
second hybrid generation, and in the same way
I cultivated also the third and fourth. These
were as imperfectly fertile as the first, and in



258 Retrograde Varieties

some years did not give any seed at all, so that
the operation had to be repeated in order to
continue the experiment. Last summer (1903)
I had a nice lot of some 25 biennial specimens
blooming abundantly. All in all I have grown
some 500 hybrids, and of these about 150 speci-
mens flowered.

These plants were all of the same type, re-
sembling in most points the pollen-parent, and
in some others the pistil-parent of the original
cross. The most obvious characteristic marks
are afforded by the flowers, which in 0. muri-
cata are not half so large as in biennis,
though borne by a calyx-tube of the same
length. In this respect the hybrid is like the
biennis bearing the larger flowers. These may
at times seem to deviate a little in the direction
of the other parent, being somewhat smaller
and of a slightly paler color. But it is very
difficult to distinguish between them, and if
biennis and hybrid flowers were separated from
the plants and thrown together, it is very doubt-
ful whether one would succeed in separating
them.

The next point is offered by the foliage. The
leaves of 0. biennis are broad, those of 0. muri-
cata narrow. The hybrid has the broad leaves
of 0. biennis during most of its life and at the
time of flowering. Yet small deviations in the



Unbalanced Crosses 259

direction of the other parent are not wanting,
and in winter the leaves of the hybrid rosettes
are often much narrower than those of 0. bien-
nis, and easily distinguishable from both par-
ents. A third distinction consists in the den-
sity of the spike. The distance between the in-
sertion of the flowers of 0. biennis is great when
compared with that of 0. muricata. Hence the
flowers of the latter species are more crowded
and those of 0. biennis more dispersed, the
spikes of the first being densely crowned with
flowers and flower-buds while those of 0. biennis
are more elongated and slender. As a further
consequence the 0. biennis opens on the same
evening only one, two or three flowers on the
same spike, whereas 0. muricata bears often
eight or ten or more flowers at a time. In this
respect the hybrid is similar to the pistil-parent,
and the crowding of the broad flowers at the
top of the spikes causes the hybrids to be much
more showy than either of the parent types.

Other distinguishing marks are not recorded
by the systematists, or are not so sharply sepa-
rated as to allow of the corresponding qualities
of the hybrids being compared with them.

This hybrid remains true to the description
given. In some years I cultivated two gener-



260 Retrograde Varieties

ations so as to be able to compare them with
one another, but did not find any difference.
The most interesting point however, is the like-
ness between the first generation, which ob-
viously must combine in its internal structure
the units of both parents, and the second and
later generations which are only of a derivative
nature. Next to this stands the fact that in
each generation all individuals are alike. No
reversion to the parental forms either in the
whole type or in the single characteristics has
ever been observed, though the leaves of some
hundreds, and the spikes and flowers of some
150 individual plants have been carefully ex-
amined. No segregation or splitting up takes
place.

Here we have a clear, undoubted and rela-
tively simple, case of a true and pure species-
hybrid. No occurrence of possible varietal
characteristics obscures the result, and in this
respect this hybrid stands out much more
clearly than all those between garden-plants,
where varietal marks nearly always play a most
important part.

From the breeder's point of view our hybrid
Oenothera would be a distinct gain, were it not
for the difficulty of its propagation. But to en-
large the range of the varieties this simple and
stable form would need to be treated anew, by



Unbalanced Crosses 261

crossing it with the parent-types. Such experi-
ments however, have miscarried owing to the
too stable nature of the unit-characters.

This stability and this absence of the split-
ting shown by varietal marks in the offspring
of hybrids is one of the best proofs of unisex-
ual unions. It is often obscured by the accom-
panying varietal marks, or overlooked for this
reason. Only in rare cases it is to be met with
in a pure state and some examples are given of
this below.

Before doing so, I must call your attention
to another feature of the unbalanced unions.
This is the diminution of the fertility, a phe-
nomenon universally known as occurring in
hybridizations. It has two phases. First, the
diminished chance of the crosses themselves of
giving full crops of seed, as compared with the
pure fertilization of either parent. And, sec-
ondly, the fertility of the hybrids themselves.
Seemingly, all grades of diminished fertility
occur and the oldest authors on hybrids have
pointed out that a very definite relation exists
between the differences of the parents and the
degree of sterility, both of the cross and of the
hybrid offspring. In a broad sense these two
factors are proportionate to each other, the
sterility being the greater, the lesser the affin-
ity between the parents. Many writers have



262 Retrograde Varieties

tried to trace this rule in the single cases, but
have met with nearly unsurmountable difficul-
ties, owing chiefly to our ignorance of the units
which form the differences between the parents
in the observed cases.

In. the case of Oenothera muricata x bien-
nis the differentiating units reduce the fertility
to a low degree, threatening the offspring with
almost complete infertility and extinction.
But then we do not know whether these charac-
ters are really units, or perhaps only seemingly
so and are in reality composed of smaller en-
tities which as yet we are not able to segregate.
And as long as we are devoid of empirical means
of deciding such questions, it seems useless to go
farther into the details of the question of the
sterility. It should be stated here however,
that pure varietal crosses, when not accompan-
ied by unbalanced characters, have never showed
any tendency to diminished fertility. Hence
there can be little doubt that the unpaired units
are the cause of this decrease in reproductive
power.

The genus Oenothera is to a large degree de-
void of varietal characteristics, especially in
the subgenus Onagra, to which biennis, mur-
icata, lamarckiana and some others belong.
On the other hand it seems to be rich in
elementary species, but an adequate study of



Unbalanced Crosses 263

them has as yet not been made. Unfortunately
many of the better systematists are in the habit
of throwing all these interesting forms to-
gether, and of omitting their descriptive study.
I have made a large number of crosses be-
tween such undescribed types and as a rule
got constant hybrid races. Only one or two
exceptions could be quoted, as for instance the
Oenothera brevistylis, which in its crosses al-
ways behaves as a pure retrogressive variety.
Instead of giving an exhaustive survey of
hybrids, I simply cite my crosses between
lamarckiana and biennis, as having nearly the
aspect of the last named species, and remaining
true to this in the second generation without
any sign of reversion or of splitting. I have
crossed another elementary species, the Oeno-
thera hirtella with some of my new and with
some older Linnean species, and got several
constant hybrid races. Among these the off-
spring of a cross between muricata and hirtella
is still in cultivation. The cross was made in
the summer of 1897 and last year (1903) I grew
the fourth generation of the hybrids. These
had the characters of the muricata in their nar-
row leaves, but the elongated spikes and rela-
tively large flowers of the hirtella parent, and
remained true to this type, showing only slight
fluctuations and never reverting or segregating



264 Retrograde Varieties

the mixed characters. Both parents bear large
capsules with an abundance of seed, but in the
hybrids the capsules remain narrow and weak,
ripening not more than one-tenth the usual
quantity of seed. Both parents are easily
cultivated in annual generations and the same
holds good for the hybrid. But whereas the
hybrid of muricata and biennis is a stout plant,
this type is weak with badly developed foliage,
and very long strict spikes. Perhaps it was
not able to withstand the bad weather of the
last few years.

A goodly number of constant hybrids are de-
scribed in literature, or cultivated in fields and
gardens. In such cases the essential question
is not whether they are now constant, but
whether they have been so from the beginning,
or whether they prove to be constant whenever
the original cross is repeated. For constant
hybrids may also be the issue of incipient split-
tings, as we shall soon see.

Among other examples we may begin with
the hybrid alfalfa or hybrid lucerne (Medicago
media}. It often originates spontaneously be-
tween the common purple lucerne or alfalfa
and its wild ally with yellow flowers and pro-
cumbent stems, the Medicago falcata. This
hybrid is cultivated in some parts of Germany
on a large scale, as it is more productive than



Unbalanced Crosses 265

the ordinary lucerne. It always comes true
from seed and may be seen in a wild state in
parks and on lawns. It is one of the oldest
hybrids with a pure and known lineage. The
original cross has been repeated by Urban, who
found the hybrid race to be constant from the
beginning.

Another very notorious constant hybrid race
is the Aegilops speltaeformis. It has been
cultivated in botanic gardens for more than
half a century, mostly in annual or biennial
generations. It is sufficiently fertile and al-
ways comes true. Numerous records have
been made of it, since formerly it was believed
by Fabre and others to be a spontaneous transi-
tion from some wild species of grass to the ordi-
nary wheat, not a cross. Godron, however,
showed that it can be produced artificially, and
how it has probably sprung into existence
wherever it is found wild. The hybrid between
Aegilops ovata, a small weed, and the common
wheat is of itself sterile, producing no good pol-
len. But it may be fertilized by the pollen of
wheat and then gives rise to a secondary
hybrid, which is no other than the Aegilops
speltaeformis. This remained constant in God-
ron 's experiments during a number of genera-
tions, and has been constant up to the present
time.



266 Retrograde Varieties

Constant hybrids have been raised by Mil-
lardet between several species of strawberries.
He combined the old cultivated forms with newly
discovered types from American localities.
They ordinarily showed only the characteristics
of one of their parents and did not exhibit any
new combination of qualities, but they came
true to this type in the second and later gener-
ations.

In the genus Anemone, Janczewski obtained
the same results. Some characters of course
may split, but others remain constant, and
when only such are present, hybrid races result
with new combinations of characters, which are
as constant as the best species of the same ge-
nus. The hybrids of Janczewski were quite fer-
tile, and he points out that there is no good
reason why they should not be considered as
good new species. If they had not been pro-
duced artificially, but found in the wild state,
their origin would have been unknown, and there
can be no doubt that they would have been de-
scribed by the best systematists as species of the
same value as their parents. Such is especially
the case with a hybrid between Anemone magel-
lanica and the common Anemone sylvestris.

Starting from similar considerations Kerner
von Marilaun pointed out the fact long ago that
many so-called species, of rare occurrence,



Unbalanced Crosses



267



standing between two allied types, may be con-
sidered to have originated by a cross. Surely
a wide field for abuse is opened by such an as-
sertion, and it is quite a common habit to con-
sider intermediate forms as hybrids, on the
grounds afforded by their external characters
alone, and without any exact knowledge of their
real origin and often without knowing anything
as to their constancy from seed. All such ap-
parent explanations are now slowly becoming
antiquated and obsolete, but the cases adduced
by Kerner seem to stand this test.

Kerner designates a willow, Salix ehrhart-
iana as a constant hybrid between Salix alba
and S. pentandra. Rhododendron intermed-
ium is an intermediate form between the hairy
and the rusty species from the Swiss Alps, R.
hirsutum and R. ferrugineum, the former grow-
ing on chalky, and the other on silicious soils.
Wherever both these types of soil occur in the
same valley and these two species approach
one another, the hybrid R. intermedium is pro-
duced, and is often seen to be propagating itself
abundantly. As is indicated by the name, it
combines the essential characters of both par-
ents.

Linaria italica is a hybrid toad-flax between
L. genistifolia and L. vulgaris, a cross which I
have repeated in my garden. Drosera obovata



268 Retrograde Varieties

is a hybrid sundew between D. anglica and D.
rotundifolia. Primula variabilis is a hybrid
between the two common primroses, P. offici-
nalis and P. grandiflora. The willow-herb
(Epilobium), the self-heal (Brunella) and the
yellow pond-lilies (Nuphar) afford other in-
stances of constant wild hybrids.

Macfarlane has discovered a natural hybrid
between two species of sundew in the swamps
near Atco, N. J. The parents, D. intermedia
and D. filiformis, were growing abundantly all
around, but of the hybrid only a group of eleven
plants was found. A detailed comparison of
the hybrid with its parents demonstrated a
minute blending of the anatomical peculiarities
of the parental species.

Luther Burbank of Santa Eosa, California,
has produced a great many hybrid brambles,
the qualities of which in many respects surpass
those of the wild species. Most of them are
only propagated by cuttings and layers, not
being stable from seed. But some crosses be-
tween the blackberry and the raspberry (R.
fruticosus and R. idaeus) which bear good
fruit and have become quite popular, are so
fixed in their type as to reproduce their com-
posite characters from seed with as much regu-
larity as the species of Rubus found in nature.
Among them are the " Phenomenal " and the



Unbalanced Crosses



269



" Primus. " The latter is a cross between the
Californian dewberry and the Siberian rasp-
berry and is certainly to be regarded as a good
stable species, artificially produced. Bell
Salter crossed the willow-herbs Epilobium tet-
ragonum and E. montanum, and secured inter-
mediate hybrids which remained true to their
type during four successive generations.

Other instances might be given. Many of
them are to be found in horticultural and bo-
tanical journals which describe their system-
atic and anatomical details. The question of
stability is generally dealt with in an inci-
dental manner, and in many cases it is diffi-
cult to reach conclusions from the facts given.
Especially disturbing is the circumstance that
from a horticultural point of view it is quite
sufficient that a new type should repeat itself in
some of its offspring to be called stable, and
that for this reason absolute constancy is rarely
proved.

The range of constant hybrids would be
larger by far were it not for two facts. The
first is the absolute sterility of so many beauti-
ful hybrids, and the second is the common occur-
rence of retrogressive characters among culti-
vated plants. To describe the importance of
both these groups of facts would take too much



270 Retrograde Varieties

time, and therefore it seems best to give some
illustrative examples instead.

Among the species of Ribes or currant, which
are cultivated in our gardens, the most beauti-
ful are without doubt the Californian and the
Missouri currant, or Ribes sanguineum and R.
aureum. A third form, often met with, is ' l Gor-
don 's currant/' which is considered to be a
hybrid between the two. It has some peculiar-
ities of both parents. The leaves have the gen-
eral form of the Californian parent, but are as
smooth as the Missouri species. The racemes
or flower-spikes are densely flowered as in the
red species, but the flowers themselves are of a
yellow tinge, with only a flesh-red hue on the
outer side of the calyx. It grows vigorously
and is easily multiplied by cuttings, but it never
bears any fruit. Whether it would be constant,
if fertile, is therefore impossible to decide.

Berberis ilicifolia is considered as a hybrid
between the European barberry (B. vulgaris)
and the cultivated shrub Mahonia aquifolia.
The latter has pinnate leaves, the former undi-
vided ones. The hybrid has undivided leaves
which are more spiny than those of the Euro-
pean parent, and which are not deciduous like
them, but persist during the winter, a peculiar-
ity inherited from the Mahonia. As far as I



Unbalanced Crosses 271

have been able to ascertain, this hybrid never
produces seed.

Another instance of an absolutely sterile
hybrid is the often quoted Cytisus adami. It
is a cross between the common laburnum
(Cytisus Labiirnum) and another species of the
same genus, C. purpureus, and has some traits
of both. But since the number of differentiat-
ing marks is very great in this case, most of the
organs have become intermediate. It is abso-
lutely sterile. But it has the curious peculiar-
ity of splitting in a vegetative way. It has been
multiplied on a large scale by grafting, and was
widely found in the parks and gardens of
Europe during the last century. Nearly all
these specimens reverted from time to time to
the presumable parents. Not rarely a bud of
Adam's laburnum assumed all the qualities of
the common laburnum, its larger leaves, richer
flowered racemes, large and brightly yellow
flowers and its complete fertility. Other buds
on the same tree reverted to the purple parent,
with its solitary small flowers, its dense shrub-
like branches and very small leaves. These too
are fertile, though not producing their seeds as



Online LibraryHugo de VriesSpecies and varieties; their origin by mutation. Lectures delivered at the University of California. Edited by Daniel Trembly MacDougal → online text (page 15 of 48)