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Species and Varieties, Their Origin by Mutation

H >> Hugo DeVries >> Species and Varieties, Their Origin by Mutation




The most beautiful instances of such complex characters are offered by
the colors of some of the most prized garden-flowers. Rarely these are
of a single hue, often two or three shades contribute to the effect, and
in some cases special [149] spots or lines or tracings are to be seen on
a white or on a colored background. That such spots and lines are
separate units is obvious and is demonstrated by the fact that sometimes
spotless varieties occur, which in all other respects have kept the
colors of the species. The complexity of the color is equally evident,
whenever it is built up of constituents of the anthocyan and of the
yellow group. The anthocyan dye is limited to the sap-cavity of the
cells, while the yellow and pure orange colors are fixed in special
organs of the protoplasm. The observation under the microscope shows at
once the different units, which though lying in the same cell and in
almost immediate vicinity of each other are always wholly separated from
one another by the wall of the vacuole or sapfilled cell-cavity.

The combination of red and yellow gives a brown tinge, as in the
cultivated wall-flower, or those bright hues of a dark orange-red, which
are so much sought in tulips. By putting such flowers for a short time
in boiling water, the cells die and release the red pigment, which
becomes diffused in the surrounding fluids and the petals are left
behind with their yellow tinge. In this way it is easy to separate the
constituents, and demonstrate the compound nature of the original
colors.

[150] But the diversity of the color patterns is far from being
exhausted with these simple instances. Apart from them, or joined to
them, other complications are frequently seen, which it is impossible to
analyze in such an artificial way. Here we have to return to our former
principle, the comparison of different varieties. Assuming that single
units may be lost, irrespective of the others, we may expect to find
them segregated by variation, wherever a sufficiently wide range of
color-varieties is in cultivation. In fact, in most cases a high degree
of dissimilarity may be reached in the simplest way by such a separation
of the components, and by their combination into most diverse smaller
groups. A very nice instance of such an analysis of flower-colors is
afforded by the ordinary snapdragon. The beautiful brown red color of
this common garden-plant is composed on one side of yellow elements, on
the other of red units. Of the yellow there are two, one staining the
whole corolla with a light hue, as is to be seen in the pure yellow
variety called _luteum. This form has been produced by the loss of the
whole group of the red constituents. If the yellow tinge is also lost,
there arises a white variety, but this is not absolutely colorless, but
shows the other yellow constituent. This last stains only some small
parts [151] of the lips of the flower around the throat, brightening, as
it seems, the entrance for the visiting insects. In many of the red or
reddish varieties this one yellow patch remains, while the general
yellow hue fails. In the variety called "Brilliant" the yellow ground
makes the red color more shiny, and if it is absent the pure carmine
tinge predominates.

It is readily seen, that in the ordinary form the lips are of a darker
red than the tube. This evident dissimilarity indicates some complexity.
And in fact we have two varieties which exhibit the two causes of this
attribute separately. One of them is called "Delila," and has the red
color limited to the lips, whilst the tube is pure white. The other is
called "Fleshy," and is of a pale pink throughout the whole corolla.
Adding these two units to one another, we get the original dark red of
the wild type, and it may be briefly stated here, that the way of
effecting such an addition is given us in the crossing of the "Fleshy"
and the "Delila" variety, the hybrid showing the two colors and
returning thereby to the old prototype.

Other cases of compound flower colors or of color patterns might be
given as in the _Mimulus_ and the poppy, and in most of these cases some
varieties are to be seen in our gardens which show only the single
constituents of the group.

[152] Many dark flowers have an intermediate bright hued form besides
the white variety, as in the case of roses, asters, _Nicandra_ and so
on.

Intermediate forms with respect to stature may also be seen. The
opium-poppy, the snapdragon, peas, the _Nicandra_, and many other
garden-plants have not only dwarf varieties, but also some of
intermediate height. These, though they are intermediate between the
tall and dwarf types, cannot be considered as transitions, as between
them and the extremes, intermediates are, as a rule wholly lacking.
Instances of the same occurrence of three types may be seen in the seeds
of maize ("Cuzco," "Horse-dent" and "Gracillima") of beans and some
other plants. The _Xanthium Wootoni_, above referred to, with only part
of the prickles of Xanthium commune is also a very curious instance of
the demonstration of the compound nature of a character.

Summarizing the conclusions that may be drawn from the evidence given in
this lecture, we have seen that varieties differ from elementary species
in that they do not possess anything really new. They originate for the
greater part in a negative way, by the apparent loss of some quality,
and rarely in a positive manner by acquiring a character, already seen
in allied species. These characters are not of the nature of [153]
morphologic entities, but are to be considered as physiologic units,
present in all parts of the organisms, and manifesting themselves where
ever occasion is afforded. They are units in the sense that they may
appear and disappear singly. But very often they are combined to yield
compound characters, which are capable of analysis. Opportunities for
such an analysis are afforded by these groups of cultivated varieties,
of which some members show a single distinguishing quality, or a number
of them.


[154]

LECTURE VI

STABILITY AND REAL ATAVISM

It is generally believed that varieties are principally distinguished
from species by their inconstancy. This conception is derived from some
special cases and transferred to others, and in its common form this
belief must have originated from the confusion which exists as to the
meaning of the term variety. It is true that vegetative varieties as a
rule run back, when propagated by seeds; they are an obvious instance of
inconstancy. In the second place we have considered the group of
inconstant or sporting varieties, which of course we must exclude when
studying the stability of other types. However, even these sporting
varieties are unstable only to a certain degree, and in a broader sense
will prove to be as true to their character as the most constant types.

Having separated these two groups, which include also the wide range of
hybrid forms, we may next consider only those varieties of pure origin,
and ordinarily propagated by seeds, [155] which have been discussed in
former chapters. Their general character lies in their fidelity to type,
and in the fact that this is single, and not double, as in the sporting
varieties.

But the current belief is, that they are only true to their
peculiarities to a certain degree, and that from time to time, and not
rarely, they revert to the type from which they have arisen. Such
reversion is supposed to prove that they are mere varieties, and at the
same time to indicate empirically the species from which they have
sprung.

In the next lecture we shall examine critically the evidence on which
this assumption rests. Before doing so however, it will be necessary to
collate the cases in which there is no reversion at all, or in which the
reversion is absent at least in experimental and pure sowings.

In the present state of our knowledge it is very difficult to decide,
whether or not true reversion occurs in constant varieties. If it does
occur, it surely does so very rarely and only under unusual
circumstances, or in particular individuals. However when such
individuals are multiplied by buds and especially when they are the only
representatives of their type, the reversion, though theoretically rare,
will be shown by nearly every specimen of the variety. Examples of this
will be given below.

[156] They are generally called atavists or reversionists, but even
these terms are sometimes used in a different sense.

Lastly it is to be said that the empirical and experimental evidence as
to the question of constancy is not as extensive as it should be. The
experimental conditions are seldom described, and it is only recently
that an interest in the matter has been awakened. Much remains to be
done. Among other things the innumerable varieties of trees, shrubs and
perennial herbs should be tested as to their constancy when grown from
purely fertilized seeds. Many of them may be included among the number
that sport constantly.

Leaving aside the doubtful or insufficiently studied cases, we may now
turn our attention to the facts that prove the absolute stability of a
large number of varieties, at least as far as such completeness can be
attained by experiment or observation.

The best proof is afforded by the varieties which grow wild in
localities where they are quite isolated from the species, and where for
this reason, no possibility of crossing disturbs the significance of the
proof. As one instance the rayless form of the wild camomile, or the
_Matricaria Chamomilla discoidea_ may be mentioned. Many systematists
have been so strongly [157] impressed with its absolute constancy and
its behavior as an ordinary species, that they have elevated it, as it
is called, to the rank of a species. As such it is described under the
name of _Matricaria discoidea_ DC. It is remarkable for its rapid and
widespread distribution, as of late years it has become naturalized in
different parts of America and of Europe, where it is to be seen
especially in France and in Norway. Experimentally I raised in
succeeding years between 1000 and 2000 seedlings, but observed no trace
of reversion, either in the strongest or in the numerous very small and
weak individuals which appeared in the cultures.

The tansy-ragwort or _Senecio Jacobaea_ may be chosen as a second
instance. It is a perennial herb with short rootstocks and stout stems
bearing numerous short-peduncled heads in large compact corymb; it
multiplies itself abundantly by seeds and is very common on the sand
dunes of Holland. It has two forms, differing only in the occurrence or
the lack of the ray florets. But these two varieties occupy different
localities and are even limited to different provinces. As far as I have
been able to ascertain on numerous excursions during a series of years,
they never sport, and are only intermingled on the outskirts of their
habitats. The rayless form is generally considered as the [158] variety
but it is quite as stable as the radiate species.

The radiate varieties of marigold, quoted in a former lecture, seem to
be equally constant, when growing far away from their prototypes. I
sowed the seeds of a single plant of the radiate form of _Bidens
cernua_, and found all of the seedlings came true, and in the next year
I had from their seed between 2,000 and 3,000 flowering individuals, all
equally radiate. Many species of composites have been tried, and they
are all constant. On the other hand rare sports of this kind have been
observed by Murr and other authors.

Many kinds of vegetables and of fruits give instances of stability.
White strawberries, green grapes, white currants, crisped lettuce,
crisped parsley and some other crisped forms may be cited. The spinage
without prickles is a widely known instance. White-flowered flax never
reverts to the blue prototype, if kept pure. Sugar-peas and sugar-corn
afford further instances. Strawberries without runners have come true
from seed ever since their first appearance, over a hundred years ago.

Many garden-varieties, the stability of which under ordinary
circumstances is doubtful, because of their being sown too close to
other varieties of the same species, have been tested in [159] respect
to their stability by different writers and at different times. In doing
this it is plain that it is very essential to be sure of the purity of
the seed. Specimens must be grown in positions isolated from their
allies, and if possible be pollinated artificially with the exclusion of
the visits of insects. This may be done in different ways. If it is a
rare species, not cultivated in the neighborhood, it is often sufficient
to make sure of this fact. Pollen may be conveyed by bees from distances
of some ten or twenty meters, or in rare cases from some hundred meters
and more, but a greater distance is ordinarily sufficient for isolation.
If the flowers fertilize themselves, as is more often the case than is
generally supposed, or if it is easy to pollinate them artificially,
with their own pollen or in small groups of similar individuals, the
best way is to isolate them by means of close coverings. When flowering,
the plants are as a rule too large to be put under bell-glasses, and
moreover such coverings would keep the air moist, and cause the
flower-buds to be thrown off. The best coverings are of netting, or of
canvas of sufficiently wide mesh, although after a long experience I
greatly prefer cages of fine iron-wire, which are put around and over
the whole plant or group of plants, and fastened securely and tightly to
the ground.

[160] Paper bags also may be made use of. They are slipped over the
flowering branches, and bound together around the twigs, thus enclosing
the flowers. It is necessary to use prepared papers, in order that they
may resist rain and wind. The best sort, and the one that I use almost
exclusively in my fertilization-experiments, is made of parchment-paper.
This is a wood-pulp preparation, freed artificially from the so-called
wood-substance or lignin. Having covered the flowers with care, and
having gathered the seeds free from intermixtures and if possible
separately for each single individual, it only remains to sow them in
quantities that will yield the greatest possible number of individuals.
Reversions are supposed to be rare and small groups of seedlings of
course would not suffice to bring them to light. Only sowings of many
hundreds or thousands of individuals are decisive. Such sowings can be
made in one year, or can be extended over a series of years and of
generations. Hildebrand and Hoffman have preferred the last method, and
so did Hofmeister and many others. Hildebrand sowed the white hyacinth,
and the white varieties of the larkspur, the stock and the sweet pea.
Hoffman cultivated the white flax and many other varieties and
Hofmeister extended his sowings [161] over thirty years with the white
variety of the yellow foxglove (_Digitalis parviflora_). White-flowered
varieties of perennial garden plants were used in my own experiments. I
bought the plants, flowered them under isolation in the way described
above, gathered the seeds from each individual separately and sowed them
in isolated groups, keeping many hundreds and in some cases above a
thousand plants up to the time of flowering. Among them I found only one
inconstant variety, the white form of the yellow columbine, _Aquilegia
chrysantha_. It evidently belonged to the group of sporting varieties
already referred to. All others came absolutely true to type without any
exception. The species experimented with, were _Campanula persicifolia_,
_Hyssopus officinalis_, _Lobelia syphilitica_, _Lychnis chalcedonica_,
_Polemonium dissectum_, _Salvia sylvestris_ and some others. Tested in
the same way I found the white varieties of the following annual plants
also quite true: _Chrysanthemum coronarium_, _Godetia amoena_, _Linum
usitatissimum_, _Phlox drummondi_, and _Silene Armeria_. To these may be
added the white hemlock stork's-bill (_Erodium cicutarium album_) which
grows very abundantly in some parts of my fatherland, and is easily
recognizable by its pure green leaves and stems, even when not
flowering. I cultivated it, in large numbers [162] during five
succeeding generations, but was never able to find even the slightest
indication of a reversion to the red prototype. The scarlet pimpernel or
_Anagallis arvensis_ has a blue variety which is absolutely constant.
Even in Britton and Brown's "Flora," which rarely enumerates varieties,
it is mentioned as being probably a distinct species. Eight hundred
blooming seedlings were obtained from isolated parents, all of the same
blue color. The New Zealand spinage (_Tetragonia expansa_) has a
greenish and a brownish variety, the red color extending over the whole
foliage, including the stems and the branches. I have tried both of them
during several years, and they never sported into each other. I raised
more than 5,000 seedlings, from the different seeds of one lot of the
green variety in succeeding years, but neither those germinating in the
first year, nor the others coming into activity after two, three or four
years of repose gave any sign of the red color of the original species.

It is an old custom to designate intermediate forms as hybrids,
especially when both the types are widely known and the intermediates
rare. Many persons believe that in doing so, they are giving an
explanation of the rarer forms. But since the laws of hybridism are
coming to be known we shall have to break with [163] all such usages. So
for instance there are numerous flowers which are of a dark red or a
dark blue color, and which, besides a white variety, have a pink or a
pale blue form. Such pale varieties are of exactly the same value as
others, and on testing they are found to be equally stable. So for
instance the pink variety of the Sweet William (_Silene Armeria rosea_),
the _Clarkia pulchella carnea_ and the pale variety of the corn-cockle,
called usually _Agrostemma Githago nicaeensis_ or even simply _A.
nicaeensis_. The latter variety I found pure during ten succeeding
generations. Another notable stable intermediate form is the poppy
bearing the Danish flag (_Papaver somniferum Danebrog_). It is an old
variety, and absolutely pure when cultivated separately. A long list of
other instances might easily be given.

Many garden-varieties, that are still universally prized and cultivated
are very old. It is curious to note how often such forms have been
introduced as novelties. The common foxglove is one of the best
examples. It has a monstrous variety, which is very showy because it
bears on the summit of its raceme and branches, large erect cup-shaped
flowers, which have quite a different aspect from the normal
thimbleshaped side-blossoms. These flowers are ordinarily described as
belonging to the anomaly [164] known as "peloria," or regular form of a
normally symmetric type; they are large and irregular on the stems and
the vigorous branches but slender and quinate on the weaker twigs. Their
beauty and highly interesting anomalous character has been the cause of
their being described many times, and nearly always as a novelty; they
have been recently re-introduced into horticulture as such, though they
were already cultivated before the middle of the last century. About
that time very good descriptions with plates were published in the
journal "Flora" by Vrolik, but afterwards they seem to have been
forgotten. The peloric variety of the foxglove always comes true from
seed, though in the strict sense of the word which we have chosen for
our discussion, it does not seem to be a constant and pure variety.

It is very interesting to compare old botanical books, or even old
drawings and engravings containing figures of anomalous plants. The
celebrated Pinacothec of Munich contains an old picture by Holbein
(1495-1543) representing St. Sebastian in a flower-garden. Of the plants
many are clearly recognizable, and among others there is one of the
"one-leaved" variety of the strawberry, which may still be met with in
botanical gardens. In the year 1671 a Dutch botanist, Abraham Munting
published [165] a large volume on garden-plants, containing a great
number of very good engravings. Most of them of course show normal
plants, but intermixed with these are varieties, that are still in
cultivation and therefore must be at least two centuries old. Others,
though not figured, are easily recognized by their names and
descriptions. The cockscomb is the most widely known, but many white or
double flowered varieties were already cultivated at that time. The
striped Jalappa, the crested Sedum, the fasciated crown-imperial, white
strawberries, red gooseberries and many others were known to Munting.

Some varieties are as old as culture itself, and it is generally known
that the Romans cultivated the white form of the opium-poppy and used
the foliage of the red variety of the sugarbeet as a vegetable.

In our time flowers and fruits are changing nearly as rapidly as the
fancies and tastes of men. Every year new forms are introduced and usurp
the place of older ones. Many are soon forgotten. But if we look at old
country gardens, a goodly number of fine and valued old sorts are still
to be found. It would be worth while to make special collections of
living plants of old varieties, which surely would be a good and
interesting work and bring about a conviction [166] of the stability of
pure strains. Coming now to the other side of the question, we may
consider those cases of reversion which have been recorded from time to
time, and which always have been considered as direct proofs of the
varietal character of the reverting form. Reversion means the falling
back or returning to another type, and the word itself expresses the
idea that this latter type is the form from which the variety has
arisen.

Some instances of atavism of this kind are well known, as they are often
repeated by individuals that are multiplied by buds or by grafting.
Before looking attentively into the different features of the many cases
of rare reversions it will be advisable to quote a few examples.

The flowering-currant of the Pacific Coast or North American scarlet
ribes (_Ribes sanguineum_), a very popular ornamental shrub, will serve
as a good example. It is prized because of its brilliant red racemes of
flowers which blossom early in the spring, before the appearance of the
leaves. From this species a white form has arisen, which is an old and
widely cultivated one, but not so highly prized because of its pale
flowers. These are not of a pure white, but have retained a faint
reddish hue. The young twigs and the stalks of the [167] leaves afford
an instance of correlated variability since in the species the red color
shows itself clearly mixed with the green, while in the variety this
tinge is wholly wanting.

Occasionally this white-flowered currant reverts back to the original
red type and the reversion takes place in the bud. One or two buds on a
shrub bearing perhaps a thousand bunches of white flowers produce twigs
and leaves in which the red pigment is noticeable and the flowers of
which become brightly colored. If such a twig is left on the shrub, it
may grow further, ramify and evolve into a larger group of branches. All
of them keep true to the old type. Once reverted, the branches remain
forever atavistic. It is a very curious sight, these small groups of red
branches among the many white ones. And for this reason attention is
often called to it, and more than once I myself have had the opportunity
of noting its peculiarities. It seems quite certain that by planting
such shrubs in a garden, we may rely upon seeing sooner or later some
new buds reverting to the prototype.

Very little attention seems hitherto to have been given to this curious
phenomenon, though in many respects it deserves a closer investigation.
The variety is said to have originated from seed in Scotland, many years
ago, and [168] seems to be propagated only by cuttings or by grafting.
If this is true, all specimens must be considered as constituting
together only one individual, notwithstanding their wide distribution in
the gardens and parks of so many countries. This induces me to suppose,
that the tendency to reversion is not a character of the variety as
such, but rather a peculiarity of this one individual. In other words it
seems probable that when the whitish variety arises a second time from
the red species, it is not at all necessary that it should exhibit this
same tendency to revert. Or to put it still in another way, I think that
we may suppose that a variety, which might be produced repeatedly from
the same original stock, would only in rare individuals have a tendency
to revert, and in most cases would be as absolutely constant as the
species itself.

Such a conception would give us a distinct insight into the cause of the
rarity of these reversions. Many varieties of shrubs and trees have
originated but once or twice. Most of them must therefore, if our
supposition is correct, be expected to be stable and only a few may be
expected to be liable to reversions.

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