No organism exhibits all of its qualities at any one time. Many of them
are generally dormant and await a period of activity. For some of them
this period comes regularly, while in others the awakening depends upon
external influences, and consequently occurs very irregularly. Those of
the first group correspond to the differences in age; the second
constitute the responses of the plant to stimuli including
wound-injuries.
Some illustrative examples may be quoted in order to give a precise idea
of this general conception of dormant or latent characters. Seed leaves
are only developed in the seed and the seedling; afterwards, during the
entire lifetime of the plant, the faculty of producing them is not made
use of. Every new generation of seeds however, bears the same kind of
seed leaves, and hence it is manifest that it is the same quality, which
shows itself from time to time.
The primary leaves, following the seed-leaves, are different in many
species, from the later ones, and the difference is extremely pronounced
in some cases of reduction. Often, when leaves are lacking in the adult
plant, being replaced by flattened stalks as in the case of the acacias,
or by thorns, or green stems and twigs as in the prickly broom or _Ulex
europaeus_, the first leaves of the young plant may be more highly
differentiated, being pinnate in the first case and bearing three
leaflets in the second instance. This curious behavior which is very
common, brings the plants, when young, nearer to their allies than in
the adult state, and manifestly implies that the more perfect state of
the leaves is latent throughout the life of the plant, with the
exception of the early juvenile period.
_Eucalyptus Globulus_, the Australian gum tree, has opposite and broadly
sessile leaves during the first years of its life. Later these disappear
and are replaced by long sickle-shaped foliage organs, which seem to be
scattered irregularly along the branches. The juvenile characters
manifestly lie dormant during the adult period, and that this is so, may
be shown artificially by cutting off the whole crown of the tree, when
the stem responds by producing numerous new branches, which assume the
[218] shape proper to the young trees, bearing sessile and opposite
leaves.
It seems quite unnecessary to give further instances. They are familiar
to every student. It is almost safe to say that every character has its
periods of activity and of inactivity, and numbers of flowers and fruits
can be mentioned as illustrations. One fact may be added to show that
nearly every part of the plant must have the power of producing all or
nearly all the characters of the individual to which it belongs. This
proof is given by the formation of adventitious buds. These, when once
formed, may grow out into twigs, with leaves and flowers and roots. They
may even be separated from the plants and used as cuttings to reproduce
the whole. Hence we may conclude that all tissues, which possess the
power of producing adventitious buds, must conceal in a latent state,
all the numerous characters required for the full development of the
whole individual.
Adventitious buds may proceed from specialized cells, as on the margin
of the leaves of _Bryophyllum calycinum_; or from the cells of special
tissues, as in the epidermis of the begonias; or they may be provoked by
wounds in nearly every part of the plant, provided it be able to heal
the wound by swelling tissues or [219] callus. The best instance is
afforded by elms and by the horse-chestnut. If the whole tree is hewn
down the trunk tries to repair the injury by producing small
granulations of tissue between the wood and the bark, which gradually
coalesce while becoming larger. From this new ring of living matter
innumerable buds arise, that expand into leafy branches, showing clearly
that the old trunk possesses, in a latent state, all the qualities of
the whole crown. Indeed, such injured stumps may be used for the
production of copses and hedges.
All the hitherto recorded cases of latency have this in common, that
they may become active during the life-time of any given individual
once, or oftener. This may be called the ordinary type of latency.
Besides this there is another form of latent characters, in which this
awakening power is extremely limited, or wholly absent. It is the
systematic latency, which may be said to belong to species and varieties
in the same way as the ordinary latency belongs to individuals. As this
individual latency may show itself from time to time during the life of
a given plant, the first may only become active from time to time during
the whole existence of the variety or the species. It has no regular
period of activity, nor may it be incited by artificial stimulation.
[220] It emerges from concealment only very rarely and only on its own
initiative. Such instances of atavism have been described in previous
lectures, and their existence has been proved beyond doubt.
Systematic latency explains the innumerable instances in which species
are seen to lack definite characteristics which ordinarily do not fail,
either in plants at large, or in the group or family to which the plant
belongs. If we take for instance the broom-rape or _Orobanche_, or some
other pale parasite, we explain their occurrence in families of plants
with green leaves, by the loss of the leaves and of the green color. But
evidently this loss is not a true one, but only the latency of those
characters. And even this latency is not a complete one, as little
scales remind us of the leaves, and traces of chlorophyll still exist in
the tissues. Numerous other cases will present themselves to every
practical botanist.
Taking for granted that characters, having once been acquired, may
become latent, and that this process is of universal occurrence
throughout the whole vegetable and animal kingdom, we may now come to a
more precise and clear conception of the existing differences between
species and varieties.
For this purpose we must take a somewhat [221] broader view of the whole
evolution of the vegetable kingdom. It is manifest that highly developed
plants have a larger number of characters than the lower groups. These
must have been acquired in some way, during preceding times. Such
evolution must evidently be called a process of improvement, or a
progressive evolution. Contrasted to this is the loss, or the latency of
characters, and this may be designated retrogressive or retrograde
evolution. But there is still a third possibility. For a latent
character may reassume its activity, return to the active state, and
become once more an important part of the whole organization. This
process may be designated as degressive evolution; it obviously
completes the series of the general types of evolution.
Advancement in general in living nature depends on progressive
evolution. In different parts of the vegetable kingdom, and even in
different families this progression takes place on different lines. By
this means it results in an ever increasing divergency between the
several groups. Every step is an advance, and many a step must have been
taken to produce flowering plants from the simplest unicellular algae.
But related to, and very intimately connected with this advancement is
the retrogressive [222] evolution. It is equally universal, perhaps
never failing. No great changes have been attained, without acquiring
new qualities on one side, and reducing others to latency. Everywhere
such retrogressions may be seen. The polypetalous genera _Pyrola_,
_Ledum_, and _Monotropa_ among the sympetalous heaths, are a remarkable
instance of this. The whole evolution of the monocotyledons from the
lowest orders of dicotyledons implies the seeming loss of cambial growth
and many other qualities. In the order of aroids, from the calamus-root
or sweet flag, with its small but complete flowers, up to the reduced
duckweeds (_Lemna_), almost an unbroken line of intermediate steps may
be traced showing everywhere the concurrence of progressive and
retrogressive evolution.
Degressive evolution is not so common by far, and is not so easy to
recognize, but no doubt it occurs very frequently. It is generally
called atavism, or better, systematic atavism, and the clearest cases
are those in which a quality which is latent in the greater part of a
family or group, becomes manifest in one of its members. Bracts in the
inflorescence of crucifers are ordinarily wanting, but may be seen in
some genera, _Erucastrum pollichii_ being perhaps the [223] most widely
known instance, although other cases might easily be cited.
For our special purpose we may take up only the more simple cases that
may be available for experimental work. The great lines of evolution of
whole families and even of genera and of many larger species obviously
lie outside the limits of experimental observation. They are the outcome
of the history of the ancestors of the present types, and a repetition
of their history is far beyond human powers. We must limit ourselves to
the most recent steps, to the consideration of the smallest differences.
But it is obvious that these may be included under the same heads as the
larger and older ones. For the larger movements are manifestly to be
considered only as groups of smaller steps, going in the same direction.
Hence we conclude, that even the smallest steps in the evolution of
plants which we are able to observe, may be divided into progressive,
retrogressive and degressive ones. The acquisition of a single new
quality is the most simple step in the progressive line, the becoming
latent and the reactivating of this same quality are the prototypes of
the two other classes.
Having taken this theoretical point of view, it remains to inquire, how
it concurs with the [224] various facts, given in former lectures and
how it may be of use in our further discussions.
It is obvious that the differences between elementary species and
varieties on the one hand, and between the positive and negative
varieties as distinguished above, are quite comparable with our
theoretical views. For we have seen that varieties can always be
considered as having originated by an apparent loss of some quality of
the species, or by the resumption of a quality which in allied species
is present and visible. In our exposition of the facts we have of course
limited ourselves to the observable features of the phenomena without
searching for a further explanation. For a more competent inquiry
however, and for an understanding of wider ranges of facts, it is
necessary to penetrate deeper into the true nature of the implied
causes.
Therefore we must try to show that elementary species are distinguished
from each other by the acquisition of new qualities, and that varieties
are derived from their species either by the reduction of one or more
characteristics to the latent state, or by the energizing of dormant
characters.
Here we meet with a great difficulty. Hitherto varieties and subspecies
have never been clearly defined, or when they have been, it was [225]
not by physiological, but only by morphological research. And the claims
of these two great lines of inquiry are obviously very diverging.
Morphological or comparative studies need a material standard, by which
it may be readily decided whether certain groups of animals and plants
are to be described or de-nominated as species, as subspecies or as
varieties. To get at the inner nature of the differences is in most
cases impossible, but a decision must be made. The physiological line of
inquiry has more time at its disposal; it calls for no haste. Its
experiments ordinarily cover years, and a conclusion is only to be
reached after long and often weary trials. There is no making a decision
on any matter until all doubtful points have been cleared up. Of course,
large groups of facts remain uncertain, awaiting a closer inquiry, and
the teacher is constrained to rely on the few known instances of
thoroughly investigated cases. These alone are safe guides, and it seems
far better to trust to them and to make use of them for the construction
of sharp conceptions, which may help us to point out the lines of
inquiry which are still open.
Leaving aside all such divisions and definitions, as were stamped with
the name of provisional species and varieties by the great systematist,
[226] Alphonse De Candolle, we may now try to give the proofs of our
assertion, by using only those instances that have been thoroughly
tested in every way.
We may at once proceed to the retrogressive or negative varieties. The
arguments for the assumption that elementary species owe their origin to
the acquisition of new qualities may well be left for later lectures
when we shall deal with the experimental proofs in this matter.
There are three larger groups of facts, on which the assumption of
latent characters in ordinary varieties rests. These are true atavism,
incomplete loss of characters, and systematic affinity. Before dealing
with each of these separately, it may be as well to recall once more
that in former lectures we have treated the apparent losses only as
modifications in a negative way, without contemplating the underlying
causes.
Let us recall the cases of bud-atavism given by the whitish variety of
the scarlet _Ribes_, by peaches and nectarines, and by conifers,
including _Cephalotaxus_ and _Cryptomeria_. These and many other
analogous facts go to prove the relation of the variety to the species.
Two assumptions are allowable. In one the variety differs from the
species by the total loss of the [227] distinctive character. In the
other this character is simply reduced to an inactive or dormant state.
The fact of its recurrence from time to time, accompanied by secondary
characters previously exhibited, is a manifest proof of the existence of
some relation between the lost and the resumed peculiarity. Evidently
this relation cannot be accounted for on the assumption of an absolute
disappearance; something must remain from which the old features may be
restored.
This lengthy discussion may be closed by the citation of the cases, in
which plants not only show developmental features of a former state, but
also reproduce the special features they formerly had, but seemingly
have lost. Two good illustrative examples may be given. One is afforded
by the wheat-ear carnation, the other by the green dahlias, and both
have occurred of late in my own cultures.
A very curious anomaly may from time to time be observed in large beds
of carnations. It bears no flowers, but instead of them small green
ears, which recall the ears of wheat. Thence the name of "Wheat-ear"
carnation. On closer inspection it is easily seen how they originate.
The normal flowers of the carnations are preceded by a small group of
bracts, [228] which are arranged in opposite pairs and therefore
constitute four rows.
In this variety the flower is suppressed and this loss is attended by a
corresponding increase of the number of the pairs of bracts. This
malformation results in square spikes or somewhat elongated heads
consisting only of the greenish bracts. As there are no flowers, the
variety is quite sterile, and as it is not regarded by horticulturists
as an improvement on the ordinary bright carnations, it is seldom
multiplied by layering. Notwithstanding this, it appears from time to
time and has been seen in different countries and at different periods,
and, what is of great importance for us, in different strains of
carnations. Though sterile, and obviously dying out as often as it
springs into existence, it is nearly two centuries old. It was described
in the beginning of the 18th century by Volckamer, and afterwards by
Jaeger, De Candolle, Weber, Masters, Magnus and many other botanists. I
have had it twice, at different times and from different growers.
So far as I have been able to ascertain reversions of this curious
carnation to normal flowers have not yet been recorded. Such a
modification occurred last summer in my garden on a plant which had not
been divided or layered, but on which the slender branches had [229]
been left on the stem. Some of them remained true to the varietal type
and bore only green spikes. Others reverted wholly or partially to the
production of normal flowers. Some branches bore these only, others had
spikes and flowers on neighboring twigs, and in still other instances
little spikes had been modified in such manner that a more or less well
developed flower was preceded by some part of an ear.
The proof that this retrograde modification was due to the existence of
a character in the latent state was given by the color of the flowers.
If the reverted bud had only lost the power of producing spikes, they
would evidently simply have returned to the characteristics of the
ordinary species, and their color would have been a pale pink. Instead
of this, all flowers displayed corollas of a deep brown. They obviously
reverted to their special progenitor, the chance variety from which they
had sprung, and not to the common prototype of the species. Of course it
was not possible to ascertain from which variety the plant had really
originated, but the reproduction of any one clearly defined varietal
mark is in itself proof enough of their origin, and of the latency of
the dark brown flower-color in this special case.
A still better proof is afforded by a new type of green dahlia. The
ordinary green dahlia [230] has large tufts of green bracts instead of
flowering heads, the scales of the receptacle having assumed the texture
and venation of leaves, and being in some measure as fleshy. But the
green heads retain the form of the ordinary flower-heads, and as they
have no real florets that may fade away, they remain unchanged on the
plants, and increase in number through the whole summer. The new types
of green dahlia however, with which I have now to deal, are
distinguished by the elongation of the axis of the head, which is
thereby changed into a long leafy stalk, attaining a length of several
inches. These stalks continue growing for a very long time, and for the
most part die without producing anything else than green fleshy scales.
This long-headed green dahlia originated at Haarlem some years ago, in
the nursery of Messrs. Zocher & Co. It was seen to arise twice, from
different varieties. Both of these were double-flowered, one a deep
carmine with white tips on the rays, the other of a pale orange tint,
known by the name of "Surprise." As they did not bear any florets or
seeds, they were quite sterile. The strain arising from the carmine
variety was kindly given to me by Messrs. Zocher & Co., and was
propagated in my garden, while the other was kept in the nursery. In the
earlier cultures both remained true to [231] their types, never
producing true florets. No mark of the original difference was to be
seen between them. But last summer (1903) both reverted to their
prototypes, bearing relatively large numbers of ordinary double
flowerheads among the great mass of green stalks. Some intermediate
forms also occurred consisting of green-scaled stalks ending in small
heads with colored florets.
Thus far we have an ordinary case of reversion. But the important side
of the phenomenon was, that each plant exactly "recollected" from which
parent it had sprung. All of those in my garden reverted to the carmine
florets with white tips, and all of those in the nursery to the pale
orange color and the other characteristics of the "Surprise" variety.
It seems absolutely evident, that no simple loss can account for this
difference. Something of the character of the parent-varieties must have
remained in the plant. And whatever conception we may formulate of these
vestigial characters it is clear that the simplest and most obvious idea
is their preservation in a dormant or latent state. Assuming that the
distinguishing marks have only become inactive by virescence, it is
manifest that on returning each will show its own peculiarities, as
recorded above. Our second point was the incomplete loss of [232] the
distinguishing quality in some varieties. It is of general occurrence,
though often overlooked. Many white varieties of colored flowers give
striking instances, among them many of the most stable and most prized
garden-flowers. If you look at them separately or in little bouquets
they seem to be of irreproachable purity. But if you examine large beds
a pale hue will become visible. In many cases this tinge is so slight as
to be only noticeable in a certain illumination, or by looking in an
oblique direction across the bed; in others it is at once evident as
soon as it has been pointed out. It always reminds the observer of the
color of the species to which the variety belongs, being bluish in
violets and harebells, reddish in godetias and phloxes, in _Silene
Armeria_ and many others. It proves that the original color quality of
the species has not wholly, but only partly disappeared. It is dormant,
but not entirely obliterated; latent, but not totally concealed;
inactive, but only partially so. Our terminology is an awkward one; it
practically assumes, as it so often does in other cases, a conventional
understanding, not exactly corresponding to the simple meaning of the
words. But it would be cumbrous to speak always of partial inactivity,
incomplete latency or half awakening qualities. Even such words as
sub-latent, [233] which would about express the real state of things,
would have little chance of coming into general use.
Such sub-latent colors are often seen on special parts in white
varieties of flowers. In many cases it is the outer side of the petals
which recalls the specific color, as in some white roses. In violets it
is often on the spur that the remains of the original pigment are to be
seen. In many instances it is on the tips of the petals or of the
segments of the corolla, and a large number of white or yellow flowers
betray their affinity to colored species by becoming red or bluish at
the edges or on the outer side.
The reality of such very slight hues, and their relation to the original
pigment of the species may in some cases be proved by direct experiment.
If it is granted that latency is not an absolute quality, then it will
be readily accepted, that even latency must be subjected to the laws of
gradual variation or fluctuating variability. We will deal with these
laws in a later lecture but every one knows that greater deviations than
the ordinary may be attained by sowing very large numbers and by
selecting from among them the extreme individuals and sowing anew from
their seed. In this way the slightest tinge of any latent color may be
[234] strengthened, not indeed to the restoration of the tinge of the
species, but at least so far as to leave no doubt as to the identity of
the visible color of the species and the latent or sublatent one of the
variety.
I made such an experiment with the peach leaved harebell or _Campanula
persicifolia_. The white variety of this species, which is often met
with in our gardens, shows a very pale bluish hue when cultivated in
large quantities, which however is subject to individual variations. I
selected some plants with a decided tinge, flowered them separately,
sowed their seeds, and repeated this during two generations. The result
was an increase of the color on the tips of the segments of the corolla
in a few individuals, most of them remaining as purely white as the
original strain. But in those few plants the color was very manifest,
individually variable in degree, but always of the same blue as in the
species itself.
Many other instances could be given. Smooth varieties are seldom
absolutely so, and if scattering hairs are found on the leaves or only
on some more or less concealed parts, they correspond in their character
to those of the species. So it is with prickles, and even the thornless
thorn-apple has fruits with surfaces far from smooth. The thornless
horse-chestnut [235] has in some instances such evident protuberances on
the valves of its fruits, that it may seem doubtful whether it is a pure
and stable variety.
Systematic latency may betray itself in different ways, either by normal
systematic marks, or by atavism. With the latter I shall deal at length
on another occasion, and therefore I will give here only one very clear
and beautiful example. It is afforded by the common red clover.
Obviously the clovers, with their three leaflets in each leaf, stand in
the midst of the great family of papilionaceous plants, the leaves of
which are generally pinnate. Systematic affinity suggests that the
"three leaved" forms must have been derived from pinnate ancestors,
evidently by the reduction of the number of the leaflets. In some
species of clover the middle of the three is more or less stalked, as is
ordinarily the case in pinnate leaves; in others it is as sessile as are
its neighbors. In a subsequent chapter I will describe a very fine
variety, which sometimes occurs in the wild state and may easily be
isolated and cultivated. It is an ordinary red clover with five leaflets
instead of three, and with this number varying between three and seven,
instead of being nearly wholly stable as in the common form. It produces
from time to time pinnate leaves, [236] very few indeed, and only
rarely, but then often two or three or even more on the same individual.
Intermediate stages are not wanting, but are of no consequence here. The
pinnate leaves obviously constitute a reversion to some prototype, to
some ancestor with ordinary papilionaceous leaves. They give proof of
the presence of the common character of the family, concealed here in a
latent state. Any other explanation of this curious anomaly would
evidently be artificial. On the other hand nothing is really known about
the ancestors of clover, and the whole conception rests only on the
prevailing views of the systematic relationships in this family. But, as
I have already said, further proof must be left for a subsequent
occasion.
Many instances, noted in our former lectures, could be quoted here. The
systematic distribution of rayed and rayless species and varieties among
the daisy-group of the composites affords a long series of examples.
Accidental variations in both directions occur. The Canada fleabane or
_Erigeron canadensis_, the tansy or _Tanacetum vulgare_ and some others
may at times be seen with ray-florets, and according to Murr, they may
sometimes be wanting in _Aster Tripolium_, _Bellis perennis_, some
species of _Anthemis_, _Arnica montana_ and in a number [237] of other
well-known rayed species. Another instance may be quoted; it has been
pointed out by Grant Allen, and refers to the dead-nettle or Lamium
album. Systematically placed in a genus with red-flowering species, we
may regard its white color as due to the latency of the general red
pigment.
But if the flower of this plant is carefully examined, it will be found
in most cases not to be purely white, but to have some dusky lines and
markings on its lower lip. Similar devices are observed on the lip of
the allied _Lamium maculatum_, and in a less degree on the somewhat
distant _Lamium purpureum_. With _Lamium maculatum_ or spotted
dead-nettle, the affinity is so close that even Bentham united the two
in a single species, considering the ordinary dead-nettle only as a
variety of the dappled purple type. For the support of this conception
of a specific or varietal retrograde change many other facts are
afforded by the distribution of the characteristic color and of the
several patterns of the lips of other labiates, and our general
understanding of the relationships of the species and genera in this
family may in a broad sense be based on the comparison of these
seemingly subordinate characteristics.
The same holds good in many other cases, and systematists have often
become uncertain [238] as to the true value of some form, by its
relationship to the allied types in the way of retrogressive
modification. Color-differences are so showy, that they easily
overshadow other characters. The white and the blue thorn-apple, the
white and the red campion (_Lychnis vespertina_ and _diurna_) and many
other illustrative cases could be given, in which two forms are
specifically separated by some authors, but combined by others on the
ground of the retrograde nature of some differentiating mark.
Hitherto we have dealt with negative characters and tried to prove that
the conception of latency of the opposite positive characteristics is a
more natural explanation of the phenomenon than the idea of a complete
loss. We have now to consider the positive varieties, and to show that
it is quite improbable that here the species have struck out for
themselves a wholly new character. In some instances such may have been
the case, but then I should prefer to treat these rather as elementary
species. But in the main we will have to assume the latency of the
character in the species and its reassumption by the variety when
originating, as the most probable explanation.
Great stress is laid upon this conception by the fact, that positive
varieties are so excessively rare when compared with the common
occurrence [239] of negative ones. Indeed, if we put aside the radiate
and the color-varieties of flowers and foliage, hardly any cases can be
cited. We have dealt with this question in a former lecture, and may now
limit ourselves to the positive color-varieties.
The latency of the faculty of producing the red pigment in leaves must
obviously be accepted for nearly the whole vegetable kingdom. Oaks and
elms, the beautiful climbing species of Ampelopsis, many conifers, as
for instance _Cryptomeria japonica_, some brambles, the Guelder-rose
(_Viburnum Opulus_) and many other trees and shrubs assume a more or
less bright red color in the fall. During summer this tendency must have
been dormant, and that this is so, is shown by the young leaves of oaks
and others, which, when unfolding in the spring show a similar but paler
hue. Moreover, there is a way of awakening the concealed powers at any
time. We have only to inflict small wounds on the leaves, or to cut
through the nerves or to injure them by a slight bruising, and the
leaves frequently respond with an intense reddening of the living
tissues around and especially above the wounds. _Azolla caroliniana_, a
minute mosslike floating plant allied to the ferns, responds to light
and cold with a reddish tinge, and to shade or warmth with a pure green.
The foliage [240] of many other plants behaves likewise, as also do
apples and peaches on the insolated sides of the fruits. It is quite
impossible to state these groups of facts in a more simple way than by
the statement that the tendency to become red is almost generally
present, though latent in leaves and stems, and that it comes into
activity whenever a stimulus provokes it.
Now it must be granted that the energizing of such a propensity under
ordinary circumstances is quite another thing from the origination of a
positive variety by the evolution of the same character. In the variety
the activity has become independent of outer influences or dependent
upon them in a far lesser degree. The power of producing the red
pigments is shown to be latent by the facts given above, and we see that
in the variety it is no longer latent but is in perfect and lasting
activity throughout the whole life of the plant.
Red varieties of white flowers are much more rare. Here the latency of
the red pigment may be deduced partly from general arguments like those
just given, partly from the special systematic relations in the given
cases. Hildebrand has clearly worked out this mode of proof. He showed
by the critical examination of a large number of instances that the
occurrence of the red-flowered varieties is contingent upon the [241]
existence of red species in the same genus, or in some rare cases, in
nearly allied genera. Colors that are not systematically present in the
group to which a white species belongs are only produced in its
varieties in extremely rare cases.
We may quote some special rules, indicated by Hildebrand. Blue species
are n the main very rare, and so are blue varieties of white species
also. Carnations, Asiatic or cultivated buttercups (_Ranunculus
asiaticus_), _Mirabilis_, poppies, _Gladiolus_, _Dahlia_, and some other
highly cultivated or very old garden-plants have not been able to
produce true blue flowers. But the garden-anemone (_Anemone coronaria_)
has allies with very fine blue flowers. The common stock has bluish
varieties and is allied to _Aubretia_ and _Hesperis_, and gooseberries
have a red form, recalling the ordinary currant. In nearly all other
instances of blue or red varieties every botanist will be able to point
out some allied red or blue species, as an indication of the probable
source of the varietal character.
Dark spots on the lower parts of the petals of some plants afford
another instance, as in poppies and in the allied _Glaucium_, where they
sometimes occur as varietal and in other cases as specific marks.
The yellow fails in many highly developed [242] flowers, which are not
liable to produce yellow variations, as in _Salvia_, _Aster_,
_Centaurea_, _Vinca_, _Polygala_ and many others. Even the rare pale
yellowish species of some of these genera have no tendency in this
direction. The hyacinths are the most remarkable, if not the sole known
instance of a species having red and blue and white and yellow
varieties, but here the yellow is not the bright golden color of the
buttercups.
The existence of varietal colors in allied species obviously points to a
common cause, and this cause can be no other than the latency of the
pigment in the species that do not show it.
The conception of latency of characters as the common source of the
origination of varieties, either in the positive or in the negative way,
leads to some rules on variability, which are known under the names
given to them by Darwin. They are the rules of repeated, homologous,
parallel and analogous variability. Each of them is quite general, and
may be recognized in instances from the most widely distant families.
Each of them is quite evident and easily understood on the principle of
latency.
By the term of repeated variability is meant the well-known phenomenon,
that the same variety has sprung at different times and in different
[243] countries from the same species. The repetition obviously
indicates a common internal cause. The white varieties of blue- and
red-flowered plants occur in the wild state so often, and in most of the
instances in so few individuals that a common pedigree is absolutely
improbable. In horticulture this tendency is widely and vexatiously
known, since the repetition of an old variety does not bring any
advantage to the breeder. The old name of "conquests," given by the
breeders of hyacinths, tulips and other flower-bulbs to any novelty, in
disregard of the common occurrence of repetitions, is an indication of
the same experience in the repeated appearance of certain varieties.
The rule of parallel variations demands that the same character
occasionally makes its appearance in the several varieties or races,
descended from the same species, and even in widely distinct species.
This is a rule, which is very important for the general conception of
the meaning of the term variety as contrasted with elementary species.
For the recurrence of the same deviation always impresses us as a
varietal mark. Laciniated leaves are perhaps the most beautiful
instance, since they occur in so many trees and shrubs, as the walnut
tree, the beech, the birch, the hazelnut, and even in [244] brambles and
some garden-varieties of the turnip (_Brassica_).
In such cases of parallel variations the single instances obviously
follow the same rules and are therefore to be designated as analogous.
Pitchers or ascidia, formed by the union of the margins of a leaf, are
perhaps the best proof. They were classified by Morren under two heads,
according to their formation from one or more leaves. Monophyllous
pitchers obey the same law, viz.: that the upper side of the leaf has
become the inner side of the pitcher. Only one exception to this rule is
known to me. It is afforded by the pitchers of the banyan or holy
fig-tree, _Ficus religiosus_, but it does not seem to belong to the same
class as other pitchers, since as far as it has been possible to
ascertain the facts, these pitchers are not formed by a few leaves as in
all other cases, but by all the leaves of the tree.
In some cases pitchers are only built up of part of the leaf-blade. Such
partial malformations obey a rule, that is common to them and to other
foliar enations, viz.: that the side of the leaf from which they emerge,
is always their outer side. The inner surface of these enations
corresponds to the opposite side of the leaf, both in color and in
anatomical structure. The last of the four rules above mentioned is
[245] that of the homologous variability. It asserts that the same
deviation may occur in different, but homologous parts of the same
plant. We have already dealt with some instances, as the occurrence of
the same pigment in the flowers and foliage, in the fruits and seeds of
the same plant, as also illustrated by the loss of the red or blue tinge
by flowers and berries. Other instances are afforded by the curious fact
that the division of the leaves into numerous and small segments is
repeated by the petals, as in the common celandine and some sorts of
brambles.
It would take too long to make a closer examination of the numerous
cases which afford proof of these statements. Suffice it to say that
everywhere the results of close inspection point to the general rule,
that the failure of definite qualities both in species and in varieties
must, in a great number of cases, be considered as only apparent. Hidden
from view, occasionally reappearing, or only imperfectly concealed, the
same character must be assumed to be present though latent.
In the case of negative or retrogressive varieties it is the transition
from the active into a dormant state to which is due the origin of the
variety. Positive varieties on the contrary owe their origin to the
presence of some character [246] in the species in the latent state, and
to the occasional re-energizing thereof.
Specific or varietal latency is not the same thing as the ordinary
latency of characters that only await their period of activity, or the
external influence which will awake them. They are permanently latent,
and could well be designated by the word perlatent. They spring into
activity only by some sudden leap, and then at once become independent
of ordinary external stimulation.
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