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.
Among the conifers many very good cases of reversions by buds are to be
found in gardens and glasshouses. They behave exactly like the whitish
currant. But as the varietal characters [169] are chiefly found in the
foliage and in the branches, these aberrations are to be seen on the
plants during the whole year. Moreover they are in some cases much more
numerous than in the first instance. The _Cryptomeria_ of Japan has a
variety with twigs resembling ropes. This is not caused by a twisting,
but only by a curvature of the needles in such a way that they seem to
grow in spiral lines around the twigs. This variety often reverts to the
type with widely spread, straight needles. And on many a specimen four,
five, or more reverted branches may be seen on different parts of the
same shrub. Still more widely cultivated is the shrub called
_Cephalotaxus pedunculata fastigiata_, and more commonly known under its
old name of _Podocarpus koraiana_. It is the broomlike variety of a
species, nearly allied to the common American and European species of
yew, (_Taxus minor_ and _T. baccata_). It is a low shrub, with broadly
linear leaves of a clear green. In the species the leaves are arranged
in two rows, one to the left and one to the right of the horizontally
growing and widely spreading branches. In the variety the branches are
erect and the leaves inserted on all sides. When sporting, it returns to
the bilateral prototype and flat wings of fan-shaped twigs are produced
laterally on its dense broom-like tufts.
[170] Wherever this variety is cultivated the same reversion may be
seen; it is produced abundantly, and even under seemingly normal
circumstances. But as in the case of the _Ribes_ all the specimens are
derived by buds from a single original plant. The variety was introduced
from Japan about the year 1860, but is probably much older. Nothing is
known as to its real origin. It never bears flowers or fruits. It is
curious to note that the analogous variety of the European yew, _Taxus
baccata fastigiata_, though much more commonly cultivated than the
_Cephalotaxus_, never reverts, at least as far as I have been able to
ascertain. This clearly corroborates the explanation given above.
After considering these rare instances of more widely known reversions,
we may now examine the question of atavism from a broader point of view.
But in doing so it should once more be remembered, that all cases of
hybridism and also all varieties sporting annually or frequently, are to
be wholly excluded. Only the very rare occurrence of instances of
atavism in varieties that are for the rest known to be absolutely
constant, is to be considered.
Atavism or reversion is the falling back to a prototype. But what is a
prototype? We may take the word in a physiologic or in a systematic
sense. Physiologically the signification is a [171] very narrowly
restricted one; and includes only those ancestors from which a form is
known to have been derived. But such evidence is of course historic. If
a variety has been observed to spring from a definite species, and if
the circumstances have been sufficiently ascertained not to leave the
slightest doubt as to its pure origin, and if moreover all the evidence
has been duly recorded, we may say that the origin of the variety is
historically known. In most cases we must be content with the testimony,
given somewhat later, and recorded after the new variety had the
opportunity of showing its greater merits.
If it now happens that such a variety of recorded origin should
occasionally revert to its parent-species, we have all we can wish for,
in the way of a thoroughly proved case of atavism. But such instances
are very rare, as the birth of most varieties has only been very
imperfectly controlled.
Next to this comes the systematic relation of a variety to its species.
The historic origin of the variety may be obscure, or may simply be
forgotten. But the distinguishing marks are of the order described in
our last lecture, either in the positive or in the negative direction,
and on this ground the rarer form is considered to be a variety of the
more wide-spread one. If [172] now the presumed variety sports and runs
over to the presumed type, the probability of the supposed relation is
evidently enhanced. But it is manifest that the explanation rests upon
the results of comparative studies, and not upon direct observations of
the phenomena themselves.
The nearer the relations between the two types in question, the less
exposed to doubt and criticism are the conclusions. But the domain of
atavism is not restricted to the cases described. Quite on the contrary
the facts that strike us most forcibly as being reversions are those
that are apt to give us an insight into the systematic affinity of a
higher degree. We are disposed to make use of them in our attempts to
perfect the natural system and to remould it in such a way as to become
a pedigree of the related groups. Such cases of atavism no doubt occur,
but the anomalies referred to them must be interpreted merely on the
ground of our assumptions as to the relative places in the system to be
assigned to the different forms.
Though such instances cannot be considered as belonging strictly to the
subject we are dealing with, I think it may be as well to give an
example, especially as it affords an occasion for referring to the
highly important researches of Heinricher on the variability and
atavistic [173] tendencies of the pale blue flag or _Iris pallida_. The
flowers of the blue flags have a perianth of six segments united below
into a tube. The three outer parts are dilated and spreading, or
reflexed, while the three inner usually stand erect, but in most species
are broad and colored like the outer ones. Corresponding to the outer,
perianth-segments are the three stamens and the three, petal-like
divisions of the style, each bearing a transverse stigma immediately
above the anther. They are pollinated by bumble-bees, and in some
instances by flies of the genus _Rhingia_, which search for the honey,
brush the pollen out of the anthers and afterwards deposit it on the
stigma. According to systematic views of the monocotyledons the original
prototype of the genus _Iris_ must have had a whorl of six equal, or
nearly equal perianth-segments and six stamens, such as are now seen in
the more primitive types of the family of the lilies, as for instance in
the lilies themselves, the tulips, hyacinths and others. As to the
perianth this view is supported by the existence of one species, the
_Iris falcifolia_, the perianth of which consists of six equal parts.
But species with six stamens are wholly lacking. Heinricher however, in
cultivating some anomalous forms of _Iris pallida_, succeeded in filling
out this gap and in producing [174] flowers with a uniform perianth and
six stamens, recalling thereby the supposed ancestral type. The way in
which he got these was as follows: he started from some slight
deviations observed in the flowers of the pale species, sowed the seeds
in large numbers and selected from the seedlings only those which
clearly showed anomalies in the expected atavistic direction. By
repeating this during several generations he at last reached his goal
and was able to give reality to the prototype, which formerly was only a
hypothetical one. The _Iris kaempferi_, a large-flowered Japanese
species much cultivated in gardens, is very variable in the number of
the different parts of its flowers, and may in some instances be seen
even with six stamens. If studied in the same way as Heinricher's iris,
it no doubt will yield highly interesting and confirmatory results.
Many other instances of such systematic atavism could be given, and
every botanist can easily add some from memory. Many anomalies,
occurring spontaneously, are evidently due to the same principle, but it
would take too long to describe them.
Reversion may occur either by buds or by seeds. It is highly probable
that it occurs more readily by sexual than by asexual propagation. But
if we restrict the discussion to the limits [175] hitherto observed,
seed-reversions must be said to be extremely rare. Or rather cases which
are sufficiently certain to be relied upon, are very rare, and perhaps
wholly lacking. Most of the instances, recorded by various writers, are
open to question. Doubts exist as to the purity of the seeds and the
possibility of some unobserved cross disturbing the results.
In the next lecture we shall deal in general with the ordinary causes
and results of such crosses. We shall then see that they are so common
and occur so regularly under ordinary circumstances that we can never
rely on the absolute purity of any seeds, if the impossibility of an
occasional cross has not been wholly excluded, either by the
circumstances themselves, or by experimental precautions taken during
the flowering period.
For these reasons cases of atavism given without recording the
circumstances, or the precautions that guarantee the purity of the
fertilization, should always be disregarded. And moreover another proof
should always be demanded. The parent which yielded the seeds might be
itself a hybrid and liable to reversions by the ordinary laws of the
splitting up of hybrids. Such cases should likewise be discarded, since
they bring in confusing elements. If we review the long list of recorded
cases by these [176] strict methods of criticism very few instances will
be found that satisfy legitimate demands. On this ground it is by far
safer in the present state of our knowledge, to accept bud-variations
only as direct proofs of true atavism. And even these may not always be
relied on, as some hybrids are liable to split up in a vegetative way,
and in doing so to give rise to bud-variations that are in many respects
apparently similar to cases of atavism. But fortunately such instances
are as yet very rare.
After this discussion it would be bold indeed to give instances of
seed-atavism, and I believe that it will be better to refrain wholly
from doing so.
Many instances of so-called atavism are of purely morphologic nature.
The most interesting cases are those furnished by the forms which some
plants bear only while young, and which evidently connect them with
allied species, in which the same features may be seen in the adult
state. Some species of the genus _Acacia_ bear bipinnate leaves, while
others have no leaves at all, but bear broadened and flattened petioles
instead. The second type is presumed to be descended from the first by
the loss of the leaflets and the modification of the stalks into flat
and simple phyllodes. But many of them are liable to recall this
primitive form [177] when very young, in the first two or three, or
sometimes in eight or ten primary leaves. These leaves are small because
of the weakness of the young plant and therefore often more or less
reduced in structure. But they are usually strictly bipinnate and
thereby give testimony as to their descent from species which bear such
leaves throughout their life.
Other similar cases could be given, but this will suffice. They once
more show how necessary it is to separate the different cases, thrown
together until now, under this general name of atavism. It would be far
better to give them all special names, and as long as these are not
available we must be cautious not to be misguided by the name, and
especially not to confuse different phenomena with one another, because
at the present time they bear the same names.
Taking into consideration the relatively numerous restrictions resulting
from this discussion, we will now make a hasty survey of some of the
more notable and generally acknowledged cases of atavism by
bud-propagation. But it should be repeated once more that most of the
highly cultivated plants, grown as vegetables, or for their fruit or
flowers, have so many crosses in their ancestry, that it seems better to
exclude them from all considerations, in which purity of [178] descent
is a requisite. By so doing, we exclude most of the facts which were
until now generally relied upon. For the roses, the hyacinths, the
tulips, the chrysanthemums always have furnished the largest
contributions to the demonstrations of bud-variation. But they have been
crossed so often, that doubt as to the purity of the descent of any
single form may recur, and may destroy the usefulness of their many
recorded cases of bud-variation for the demonstration of real atavism.
The same assertion holds good in many other cases, as with _Azalea_ and
_Camellia_. And the striped varieties of these genera belong to the
group of ever-sporting forms, and therefore will be considered later on.
So it is with carnations and pinks, which occasionally vary by layering,
and of which some kinds are so uncertain in character that they are
called by floriculturists "catch-flowers." On the other hand there is a
larger group of cases of reversion by buds, which is probably not of
hybrid nature, nor due to innate inconstancy of the variety, but must be
considered as pure atavism. I refer to the bud-variations of so many of
our cultivated varieties of shrubs and trees. Many of them are
cultivated because of their foliage. They are propagated by grafting,
and in most cases it is probable that all the numerous specimens [179]
of the same variety have been derived in this way from one primitive,
aberrant individual. We may disregard variegated leaves, spotted or
marked with white or yellow, because they are too inconstant types.
We may next turn our attention to the varieties of trees with cut
leaves, as the oakleaved _Laburnum_, the parsley-leaved vine and the
fern-leaved birch. Here the margin of the leaves is deeply cut and
divided by many incisions, which sometimes change only the outer parts
of the blade, but in other cases may go farther and reach, or nearly
reach, the midvein, and change the simple leaf into a seemingly compound
structure. The anomaly may even lead to the almost complete loss of all
the chorophyll-tissue and the greater part of the lateral veins, as in
the case of the cut-leaved beech or _Fagus sylvatica pectinata_.
Such varieties are often apt to revert by buds to the common forms. The
cut-leaved beech sometimes reverts partially only, and the branches
often display the different forms of cut-leaved, fern-like, oak-leaved
and other variously shaped leaves on the same twigs. But this is merely
due to the wide variability of the degree of fissure and is to be
considered only as a fluctuation between somewhat widely distant
extremes, which may even apparently include [180] the form of the common
beech-leaves. It is not a bud-variation at all, and it is to be met with
quite commonly while the true reversions by buds are very rare and are
of the nature of sports appearing suddenly and remaining constant on the
same twig. Analogous phenomena of wide variability with true reversion
may be seen in the variety of the European hornbeam called _Carpinus
Betulus heterophylla_. The leaves of this tree generally show the
greatest diversity in form. Some other cases have been brought together
by Darwin. In the first place a subvariety of the weeping-willow with
leaves rolled up into a spiral coil. A tree of this kind kept true for
twenty-five years and then threw out a single upright shoot bearing flat
leaves. The barberry (_Berberis_) offers another case; it has a well
known variety with seedless fruit, which can be propagated by cuttings
or layers, but its runners are said always to revert to the common form,
and to produce ordinary berries with seeds. Most of the cases referred
to by Darwin, however, seem to be doubtful and cannot be considered as
true proofs of atavism until more is known about the circumstances under
which they were produced.
Red or brown-leaved varieties of trees and shrubs also occasionally
produce green-leaved branches, and in this way revert to the type [181]
from which they must evidently have arisen. Instances are on record of
the hazel, _Corylus Avellana_, of the allied _Corylus tubulosa_, of the
red beech, the brown birch and of some other purple varieties. Even the
red bananas, which bear fruits without seeds and therefore have no other
way of being propagated than by buds, have produced a green variety with
yellow fruits. The _Hortensia_ of our gardens is another instance of a
sterile form which has been observed to throw out a branch with cymes
bearing in their center the usual small staminate and pistillate flowers
instead of the large radiate and neutral corollas of the variety,
thereby returning to the original wild type. Crisped weeping-willows,
crisped parsley and others have reverted in a similar manner.
All such cases are badly in need of a closer investigation. And as they
occur only occasionally, or as it is commonly stated, by accident, the
student of nature should be prepared to examine carefully any case which
might present itself to him. Many phases of this difficult problem could
no doubt be solved in this way. First of all the question arises as to
whether the case is one of real atavism, or is only seemingly so, being
due to hybrid or otherwise impure descent of the varying individual, and
secondly whether it may be only an instance of the regularly [182]
occurring so-called atavism of the sporting varieties with which we
shall deal in a later lecture. If it proves to be real atavism and rare,
the case should be accurately described and figured, or photographed if
possible; and the exact position of the reverting bud should be
ascertained. Very likely the so-called dormant or resting buds are more
liable to reversions than the primary ones in the arils of the leaves of
young twigs. Then the characters of the atavistic branches should be
minutely compared with those of the presumed ancestor; they may be quite
identical with them or slightly divergent, as has been asserted in some
instances. The atavism may be complete in one case, but more or less
incomplete in others. By far the most interesting point is the question,
as to what is to be expected from the seeds of such an atavistic branch.
Will they keep true to the reverted character, or return to the
characters of the plant which bears the retrograde branch? Will all of
them do so, or only part of them, and how large a part? It is very
astonishing that this question should still be unsolved where so many
individual trees bear atavistic branches that remain on them through
long series of years. But then many such branches do not flower at all,
or if they flower and bear seed, no care is taken to prevent [183]
cross-fertilization with the other flowers of the same plant, and the
results have no scientific value. For anyone who cares to work with the
precautions prescribed by science, a wide field is here open for
investigation, because old reverted branches may be met with much less
rarely than new ones.
Finally the possibility is always to be considered that the tendency to
bud-reversions may be a special feature of some individuals, and may not
be met with in others of the same variety. I have spoken of this before.
For the practical student it indicates that a specimen, once observed to
produce atavistic buds, may be expected to do the same thing again. And
then there is a very good chance that by combining this view with the
idea that dormant buds are more apt to revert than young ones, we may
get at a method for further investigation, if we recur to the practice
of pruning. By cutting away the young twigs in the vicinity of dormant
buds, we may incite these to action. Evidently we are not to expect that
in so doing they will all become atavistic. For this result is not at
all assured; on the contrary, all that we might hope to attain would be
the possibility of some of them being induced to sport in the desired
direction.
Many questions in scientific research can only [184] be answered by long
and arduous work in well equipped laboratories; they are not to be
attempted by every one. But there are other problems which the most
complete of institutions are not able to study if opportunity is not
offered them, and such opportunities are apt to occur more often in
fields, gardens, parks, woods and plains, than in the relatively small
experimental gardens of even the largest institution. Therefore,
whosoever has the good fortune to find such sports, should never allow
the occasion to pass without making an investigation that may bring
results of very great importance to science.
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