About the middle of the last century Louis de Vilmorin showed that it
was possible to subject plants to the methods of amelioration of races
then in use for domestic animals, and since that time atavism has played
a large part in all breeding-processes. It was considered to be the
greatest enemy of the breeder, and was generally spoken of as a definite
force, working against and protracting the endeavors of the
horticulturist.
No clear conception as to its true nature had been formulated, and even
the propriety of designating the observed phenomena by the term atavism
seemed doubtful. Duchesne used this word some decades ago to designate
those cases in which species or varieties revert spontaneously, or from
unknown internal causes, to some long-lost characters of their
ancestors. Duchesne's definition was evidently a sharp and useful one,
since it developed for the first time the idea of latent or dormant
qualities, [186] formerly active, and awaiting probably through
centuries an occasion to awaken, and to display the lost characters.
Cases of apparent reversion were often seen in nurseries, especially in
flower culture, which under ordinary circumstances are rarely wholly
pure, but always sport more or less into the colors and forms of allied
varieties. Such sporting individuals have to be extirpated regularly,
otherwise the whole variety would soon lose its type and its uniformity
and run over to some other form in cultivation in the vicinity. For this
reason atavism in nurseries causes much care and labor, and consequently
is to be dealt with as a very important factor.
From time to time the idea has suggested itself to some of the best
authorities on the amelioration of plants, that this atavism was not due
to an innate tendency, but, in many cases at least, was produced by
crosses between neighboring varieties. It is especially owing to Verlot
that this side of the question was brought forward. But breeders as a
rule have not attached much importance to this supposition, chiefly
because of the great practical difficulties attending any attempt to
guard the species of the larger cultures against intermixture with other
varieties. Bees and humble-bees fly from bud to bud, and carry the
pollen from one [187 ] sort to another, and separation by great
distances would be required to avoid this source of impurity.
Unfortunately the arrangements and necessities of large cultures make it
impossible to isolate the allied varieties from each other.
From a theoretical point of view the origin of these impurities is a
highly important question. If the breeders' atavism is due to crosses,
and only to this cause, it has no bearing at all on the question of the
constancy of varieties. And the general belief, that varieties are
distinguished from true species by their repeated reversion and that
even such reversibility is the real distinction of a variety, would not
hold.
For this reason I have taken much trouble in ascertaining the
circumstances which attend this form of atavism. I have visited a number
of the leading nurseries of Europe, tested their products in various
ways, and made some experiments on the unavoidable conditions of
hybridizing and on their effect on the ensuing generations. These
investigations have led me to the conclusion, that atavism, as it is
generally described, always or nearly always is due to hybridization,
and therefore it is to be considered as untrue or false atavism.
True atavism, or reversion caused by an innate latent tendency, seems
to be very rare, [188] and limited to such cases as we have spoken of
under our last heading. And since the definition, given to this term by
its author, Duchesne, is generally accepted in scientific works, it
seems better not to use it in another sense, but rather to replace it in
such cases by another term. For this purpose I propose the word
vicinism, derived from the Latin vicinus or neighbor, as indicating the
sporting of a variety under the influence of others in its vicinity.
Used in this way, this term has the same bearing as the word atavism of
the breeders, but it has the advantage of indicating the true cause
thereof.
It is well known that the term variability is commonly employed in the
broadest possible sense. No single phenomenon can be designated by this
name, unless some primary restriction be given. Atavism and vicinism are
both cases of variability, but in wholly different sense. For this
reason it may be as well, to insert here a short survey of the general
meanings to be conveyed by the term variation. It implies in the first
place the occurrence of a wide range of forms and types, irrespective of
their origin, and in the second place the process of the change in such
forms. In the first signification it is nearly identical with
polymorphy, or richness of types, especially so when these [189] types
are themselves quite stable, or when it is not at all intended to raise
the question of their stability. In scientific works it is commonly used
to designate the occurrence of subspecies or varieties, and the same is
the case in the ordinary use of the term when dealing with cultivated
plants. A species may consist of larger or smaller groups of such units,
and they may be absolutely constant, never sporting if hybridization is
precluded, and nevertheless it may be called highly variable. The
opium-poppy affords a good instance. It "varies" in height, in color of
foliage and flowers; the last are often double or laciniated; it may
have white or bluish seeds, the capsules may open themselves or remain
closed and so on. But every single variety is absolutely constant, and
never runs into another, when the flowers are artificially pollinated
and the visits of insects excluded. So it is with many other species.
They are at the same time wholly stable and very variable.
The terms variation and variety are used frequently when speaking of
hybrids. By crossing forms, which are already variable in the sense just
mentioned, it is easy to multiply the number of the types, and even in
crossing pure forms the different characters may be combined in
different ways, the resulting combinations [190] yielding new, and very
often, valuable varieties. But it is manifest that this form of
variation is of quite another nature from the variations of pure races.
Many hybrid varieties are quite constant, and remain true to their type
if no further crosses are made; many others are artificially propagated
only in a vegetative way, and for this reason are always found true.
Hybrid varieties as a rule were formerly confused with pure varieties,
and in many instances our knowledge as to their origin is quite
insufficient for sharp distinctions. To every student of nature it is
obvious, that crossing and pure variability are wholly distinct groups
of phenomena, which should never be treated under the same head, or
under the same name.
Leaving aside polymorphy, we may now discuss those cases of variability,
in which the changes themselves, and not only their final results play a
part. Of such changes two types exist. First, the ever-recurring
variability, never absent in any large group of individuals, and
determining the differences which are always to be seen between parents
and their children, or between the children themselves. This type is
commonly called "individual variability" and since this term also has
still other meanings, it has of late become customary to use instead the
term "fluctuating variability." [191] And to avoid the repetition of the
latter word it is called "fluctuation." In contrast to these
fluctuations are the so-called sports or single varieties, not rarely
denominated spontaneous variations, and for which I propose to use the
term "mutations." They are of very rare occurrence and are to be
considered as sudden and definite steps.
Lastly, we have to consider those varieties, which vary in a much wider
range than the ordinary ones, and seem to fluctuate between two opposite
extremes, as for instance variegated leaves, cultivated varieties with
variegated or striped flowers, double flowers and some other anomalies.
They are eversporting and ever-returning from one type to the other. If
however, we take the group of these extremes and their intermediates as
a whole, this group remains constant during the succeeding generations.
Here we find once more an instance of the seemingly contradictory
combination of high variability and absolute constancy. It means that
the range of variability has quite definite limits, which in the common
course of things, are never transgressed.
We may infer therefore that the word variability has such a wide range
of meanings that it ought never be used without explanation. [192]
Nothing indeed, is more variable than the signification of the term
variable itself.
For this reason, we will furthermore designate all variations under the
influence of neighbors with the new and special term "vicinism." It
always indicates the result of crossing.
Leaving this somewhat lengthy terminological discussion, we now come to
the description of the phenomenon itself. In visiting the plantations of
the seedsmen in summer and examining the large fields of garden-flowers
from which seed is to be gathered, it is very rare to find a plot quite
pure. On the contrary, occasional impurities are the rule. Every plot
shows anomalous individuals, red or white flowers among a field of blue,
normal among laciniated, single among double and so on. The most curious
instance is afforded by dwarf varieties, where in the midst of hundreds
and thousands of small individuals of the same height, some specimens
show twice their size. So for instance, among the dwarfs of the
larkspur, _Delphinium Ajacis_.
Everywhere gardeners are occupied in destroying these "atavists," as
they call them. When in full bloom the plants are pulled up and thrown
aside. Sometimes the degree of impurity is so high, that great piles of
discarded plants of the same species lie about the [193] paths, as I
have seen at Erfurt in the ease of numerous varieties of the Indian
cress or _Tropaeolum_.
Each variety is purified at the time when it shows its characters most
clearly. With vegetables, this is done long before flowering, but with
flowers only when in full bloom, and with fruits, usually after
fertilization has been accomplished. It needs no demonstration to show
that this difference in method must result in very diverging degrees of
purity.
We will confine ourselves to a consideration of the flowers, and ask
what degree of purity may be expected as the result of the elimination
of the anomalous plants during the period of blooming.
Now it is evident that the colors and forms of the flowers can only be
clearly distinguished, when they are fully displayed. Furthermore it is
impossible to destroy every single aberrant specimen as soon as it is
seen. On the contrary, the gardener must wait until all or nearly all
the individuals of the same variety have displayed their characters, as
only in this way can all diverging specimens be eliminated by a single
inspection. Unfortunately the insects do not wait for this selection.
They fertilize the flowers from the beginning, and the damage will have
been done [194] long before the day of inspection comes around. Crosses
are unavoidable and hybrid seeds will unavoidably come into the harvest.
Their number may be limited by an early eradication of the vicinists, or
by the elimination of the first ripe seeds before the beginning of the
regular harvest, or by other devices. But some degree of impurity will
remain under ordinary circumstances.
It seems quite superfluous to give more details. In any case in which
the selection is not done before the blooming period, some impurities
must result. Even if it is done before that time, errors may occur, and
among hundreds and thousands of individuals a single anomalous one may
escape observation.
The conclusion is, that flower seeds as they are offered in commerce,
are seldom found absolutely pure. Every gardener knows that he will have
to weed out aberrant plants in order to be sure of the purity of his
beds. I tested a large number of samples of seeds for purity, bought
directly from the best seed growers. Most of them were found to contain
admixtures and wholly pure samples were very rare.
I will now give some illustrative examples. From seeds of a yellow
snapdragon, I got one red-flowered specimen among half a hundred [195]
yellow ones, and from the variety "Delila" of the same species two red
ones, a single white and two belonging to another variety called
"Firefly." _Calliopsis tinctoria_ has three varieties, the ordinary
type, a brown-flowered one and one with tubular rays. Seeds of each of
these three sorts ordinarily contain a few belonging to the others.
_Iberis umbellata rosea_ often gives some white and violet examples. The
"Swan" variety of the opium-poppy, a dwarfish double-flowered form of a
pure white, contained some single-flowered and some red-flowered plants,
when sown from commercial seed are said to be pure. But these were only
occasional admixtures, since after artificial fertilization of the
typical specimens the strain at once became absolutely pure, and
remained so for a series of generations, as long as the experiment was
continued. Seeds of trees often contain large quantities of impurities,
and the laciniated varieties of birch, elder and walnut have often been
observed to come true only in a small number of seedlings.
In the case of new or young varieties, seed merchants often warn their
customers as to the probable degree of purity of the seeds offered, in
order to avoid complaints. For example the snow-white variety of the
double daisy, _Bellis perennis plena_, was offered at the start as
containing [196] as much as 20% of red-flowered specimens.
Many fine varieties are recorded to come true from seed, as in the case
of the holly with yellow fruits, tested by Darwin. Others have been
found untrue to a relatively high degree, as is notorious in the case of
the purple beech. Seeds of the laciniated beech gave only 10% of
laciniated plants in experiments made by Strasburger; seeds of the
monophyllous acacia, _Robinia Pseud-Acacia monophylla_, were found to be
true in only 30% of the seedlings. Weeping ashes often revert to the
upright type, red May-thorns (_Crataegus_) sometimes revert nearly
entirely to the white species and the yellow cornel berry is recorded to
have reverted in the same way to the red berries of the _Cornus Mas_.
Varieties have to be freed by selection from all such impurities, since
isolation is a means which is quite impracticable under ordinary
circumstances. Isolation is a scientific requirement that should never
be neglected in experiments, indeed it may be said to be the first and
most important requisite for all exact research in questions of
variability and inheritance. But in cultivating large fields of allied
varieties for commercial purposes, it is impossible to grow them at such
distances from each other [197] as to prevent cross-pollination by the
visits of bees.
This purification must be done in nearly every generation. The oldest
varieties are to be subjected to it as well as the latest. There is no
regular amelioration, no slow progression in the direction of becoming
free from these admixtures. Continuous selection is indispensable to
maintain the races in the degree of purity which is required in
commerce, but it does not lead to any improvement. Nor does it go so far
as to become unnecessary in the future. This shows that there must be a
continuous source of impurities, which in itself is not neutralized by
selection, but of which selection can only eliminate the deteriorating
elements.
The same selection is usually applied to new varieties, when they
occasionally arise. In this case it is called "fixing," as gardeners
generally believe that through selection the varieties are brought to
the required degree of purity. This belief seems to rest mainly on
observations made in practice, where, as we have seen, isolation is of
very rare application. Most varieties would no doubt be absolutely pure
from the first moment of their existence, if it were only possible to
have them purely fertilized. But in practice this is seldom to be
obtained. Ordinarily the breeder is content with such slow [198]
improvement as may be obtained with a minimum of cost, and this mostly
implies a culture in the same part of the nursery with older varieties
of the same species. Three, four or five years are required to purify
the novelty, and as this same length of time is also required to produce
sufficient quantities of seed for commercial purposes, there is no
strong desire to shorten the period of selection and fixation. I had
occasion to see this process going on with sundry novelties at Erfurt in
Germany. Among them a chamois-colored variety of the common stock, a
bluish _Clarkia elegans_ and a curiously colored opium-poppy may be
mentioned. In some cases the crossfertilization is so overwhelming, that
in the next generation the novelty seems entirely to have disappeared.
The examples given may suffice to convey a general idea of the
phenomenon, ordinarily called atavism by gardeners, and considered
mostly to be the effect of some innate tendency to revert to the
ancestral form. It is on this conception that the almost universal
belief rests, that varieties are distinguished, as such, from species by
their inconstancy. Now I do not deny the phenomenon itself. The impurity
of seeds and cultures is so general and so manifest, and may so easily
be tested by every one [199] that it cannot reasonably be subjected to
any doubt. It must be conceded to be a fact, that varieties as a rule
revert to their species under the ordinary circumstances of commercial
culture. And I cannot see any reason why this fact should not be
considered as stating a principal difference between varieties and
species, since true species never sport into one another.
My objection only refers to the explanation of the observed facts.
According to my view nearly all these ordinary reversions are due to
crosses, and it is for this reason that I proposed to call them by a
separate name, that of "vicinists." Varieties then, by means of such
spontaneous intercrossing sport into one another, while species either
do not cross, or when crossing produce hybrids that are otherwise
constituted and do not give the impression of atavistic reversion.
I must not be content with proposing this new conception, but must give
the facts on which this assumption rests. These facts are the results of
simple experiments, which nevertheless are by no means easy to carry
out, as they require the utmost care to secure the absolute purity of
the seeds that are employed. This can only be guaranteed by previous
cultures of isolated plants or groups of plants, or by artificial
pollination.
[200] Once sure of this preliminary condition, the experiment simply
consists in growing a variety at a given distance from its species and
allowing the insects to transfer the pollen. After harvesting the seed
thus subjected to the presumed cause of the impurities, it must be sown
in quantities, large enough to bring to light any slight anomaly, and to
be examined during the period of blooming.
The wild seashore aster, _Aster Tripolium_, will serve as an example. It
has pale violet or bluish rays, but has given rise to a white variety,
which on testing, I have found pure from seed. Four specimens of this
white variety were cultivated at a distance of nearly 100 meters from a
large lot of plants of the bluish species. I left fertilization to the
bees, harvested the seeds of the four whites separately and had from
them the following year more than a thousand flowering plants. All of
them were of the purest white, with only one exception, which was a
plant with the bluish rays of the species, wholly reverting to its
general type. As the variety does not give such reversions when
cultivated in isolation, this sport was obviously due to some cross in
the former year. In the same way I tried the white Jacob's ladder,
_Polemonium coeruleum_ album in the neighborhood of the blue-flowered
species, the distance [202] in this case being only 40 meters. Of two
hundred seeds one became a blue atavist, or rather vicinist, while all
others remained true to the white type. The same was observed in the
white creeping thyme, or _Thymus Serpyllum album_, and the white
self-heal, _Brunella vulgaris alba_, gave even so much as 28% seedlings
with purple corollas out of some 400 specimens, after being cultivated
in close proximity to its parent-species. I have tried many other
species, but always with the same result. Such atavists only arise by
cultivation in the proximity of allied varieties, never in isolation.
They are not real atavists, but only vicinists.
In order to show this yet more clearly, I made another experiment with
the white selfheal. I had a lot of the pinnate-leaved variety with
purple flowers and somewhat stouter stems, and cultivated single plants
of the whiteflowering sort at distances that varied from 2-16 meters.
The seeds of each plant were collected and sown separately, those of the
nearest gave up to 5 or 6 hybrids from the seeds of one parent, while
those of the farthest gave only one purple-flowered plant for each
parent. Evidently the chance of the pollen being carried by bees is much
greater on short than on longer distances.
True hybrids between species may arise in quite the same way, and since
it is obviously impossible to attribute them to an innate tendency to
reversion, they afford an absolutely irrefutable proof of the assertion
that pollen is often brought by insects from one lot of plants to
another. In this way I obtained a hybrid between the common Jacob's
ladder and the allied species _Polemonium dissectum_. With a distance of
100 meters between them I had two hybrid seeds among a hundred of pure
ones. At a similar distance pollen was carried over from the wild
radish, _Raphanus Raphanistrum_, to the allied _Raphanus caudatus_, and
I observed the following year some very nice hybrids among my seedlings.
A hybrid-bean between _Phaseolus nanus_ and _P. multiflorus_, and some
hybrids between the yellow daisy, _Chrysanthemum segetum_ and the allied
_Chrysanthemum coronarium_ or ox-eye daisy which also arose
spontaneously in my garden between parents cultivated at recorded
distances, might further be noted. Further details of these experiments
need not be given. Suffice to say, that occasional crosses between
species do occur, and not even rarely, that they are easily recognized
as such and cannot be confused with cases of atavism, and that therefore
they give proof to the assumption that in the same way crosses
ordinarily occur also between varieties [203] of the same species, if
cultivated at small distances apart, say 40-50 meters or even more.
Vicinism therefore, may play a part in all such cultures, enough to
account for all the impurities observed in the nurseries or in
commercial seed-samples.
Of course this whole discussion is limited to such species as are not
only as a rule visited by insects, but are dependent on these visits for
their fertilization. Most of our garden-flowers are included in this
category. If not then we may expect to find the cultures and seeds pure,
irrespective of the distances between allied varieties, as for instance
with peas, which are known to be self-fertilizing. Another instance is
given by the barley. One of the most curious anomalous varieties of this
cereal, is the "Nepaul-barley," with its small adventitious flowers on
the palets or inner scales. It is a very old, widely cultivated sort,
which always comes true from seed, and which has been tested in repeated
experiments in my garden. The spikelets of this curious plant are
oneflowered and provided with two linear glumes or outer scales. Of the
inner scales or palets, the outer one is three-lobed at the summit,
hence the varietal name of _Hordeum vulgare trifurcatum_. The central
lobe is oblong and hollow, covering a small supernumerary floret
inserted [204] at its base. The two lateral lobes are narrower,
sometimes linear, and are often prolonged into an awn, which is
generally turned away from the center of the spike. The central lobe
sometimes bears two florets at its base, although but one is usually
present and it may be incomplete.
I might give one more instance from my own experience. A variety of the
evening-primrose with small linear petals was once found by one of my
sons growing wild near Amsterdam. It was represented by only one
individual, flowering among a great many of the ordinary type with broad
petals. But the evening-primroses open their anthers in the morning,
fertilize themselves during the day, and only display their beautiful
flowers in the evening, after the pollination has been accomplished.
They then allure evening moths, such as _Agrotis_ and _Plusia_, by their
bright color, their sweet honeysmell and their nectar. Since the
fertilization is accomplished many hours before opening, crosses are
effected only in rare instances, and the seeds commonly remain true to
the parent type. The seeds of this one plant, when sown separately in my
garden, produced exclusively flowers with the small linear petals of
their parent. Although I had a hundred individuals bearing many
thousands of flowers, there was not an instance of reversion. And such
would [205] immediately have been observed, had it occurred, because the
hybrids between the cruciate and the normal flowers are not
intermediate, but bear the broad petals of the _O. biennis_.
We may now take up another phase of the question, that of the running
out of new varieties, shortly after their introduction into a new
country, or later.
The most widely known instance of this is that of the American corn in
Baden, recorded by Metzger and quoted by Darwin as a remarkable instance
of the direct and prompt action of climate on a plant. It has since been
considered as a reversion to the old type. Such reversions invariably
occur, according to Wallace, in cases of new varieties, which have been
produced quickly. But as we now know, such reversions are due to
spontaneous crosses with the old form, and to the rule, that the hybrids
of such origin are not intermediate, but assume the features of the
older of the two parents. In the light of this experience, Metzger's
observation becomes a typical instance of vicinism. It relates to the
"Tuscarora" corn of St. Louis, a variety with broad and flat white
seeds.
About the year 1840, this corn was introduced into Baden in Germany, and
cultivated by Metzger. In the first year it came true to type, and [206]
attained a height of 12 feet, but the season did not allow its seeds to
ripen normally. Only a few kernels were developed before the winter.
From this seed plants of a wholly different type came the next year, of
smaller stature, and with more brownish and rounded kernels. They also
flowered earlier and ripened a large number of seeds. The depression on
the outer side of the seed had almost disappeared, and the original
white had become darker. Some of the seeds had even become yellow and in
their rounded form they approached the common European maize. Obviously
they were hybrids, assuming the character of their pollen-parent, which
evidently was the ordinary corn, cultivated all around. The observation
of the next year showed this clearly, for in the third generation nearly
all resemblance to the original and very distinct American species was
lost. If we assume that only those seeds ripened which reverted to the
early-ripening European type, and that those that remained true to the
very late American variety could not reach maturity, the case seems to
be wholly comprehensible, without supposing any other factors to have
been at work than those of vicinism, which though unknown at the period
of Metzger's and Darwin's writings, seems now to be fully understood. No
innate tendency to run out and no changing influence of the climate are
required for an adequate explanation of the facts.
In the observation quoted, what astonishes us most, is the great
rapidity of the change, and the short time necessary for the offspring
of the accidental crosses to completely supplant the introduced type. In
the lecture on the selection of elementary species, closely analogous
cases were described. One of them was the wild oat or _Avena fatua_
which rapidly supplants the cultivated oats in bad years in parts of the
fields. Other instances were the experiments of Risler with the
"Galland" wheat and the observation of Rimpau on "Rivett's bearded"
wheat.
Before leaving the question of vicinism and its bearing on the general
belief of the instability of varieties, which when tested with due care,
prove to be quite stable, it may be as well to consider the phenomena
from another point of view. Our present knowledge of the effects of
crosses between varieties enables us to formulate some general rules,
which may be used to calculate, and in some way to predict, the nature
of the impurities which necessarily attend the cultivation of allied
species in close vicinity. And this mode of cultivation being in almost
universal use in the larger nurseries, [208] we may, by this discussion,
arrive at a more scientific estimation of the phenomena of vicinism,
hitherto described.
The simplest case that may be given, is when an ordinary retrograde
variety is cultivated with the species to which it belongs. For
instance, if dwarfs are cultivated next to the taller type, or a white
variety next to the red or blue-flowering species, or thornless forms in
neighboring beds with the armed species. Bees and Bumble-bees,
butterflies and moths are seen flying from flower to flower, collecting
the honey and carrying pollen. I frequently saw them cross the limits of
the neighboring beds. Loaded with the pollen of the variety they visit
the flowers of the different species and impregnate the stigma with it.
And returning to the variety they bring about similar crosses in the
flowers of the latter. Hybrid seeds will develop in both cases and
become mixed with the crop. We now have to ask the question, what sort
of plants will arise from these hybrid seeds. As a general rule we may
state, first, that the hybrids of either form of cross are practically
the same, secondly that they are not intermediate, but that the
character of one parent prevails to the almost absolute exclusion of the
other and in the third place that the older character dominates the
younger.
[209] The hybrid offspring will therefore, in the main, have the
character of the species and be indistinguishable from it, or show only
such differences as escape ordinary observation. When occurring in the
seeds of the variety they betray themselves as soon as the differential
characters are displayed. Between the thousands of flowering plants of a
white variety the hybrids will instantly catch the eye by their red or
blue corollas. Quite the contrary effect results from the admixture of
hybrids with the seeds of the species itself. Here no difference will
show itself, even in the fullest bloom. The effect of the spontaneous
crosses will pass unobserved. The strain, if pure in the first year,
will seem to be still in the same condition. Or in other terms, the
unavoidable spontaneous crosses will disturb the purity of the variety
in the second year, while they do not seem to interfere at all with the
uniformity of the species. The direct effect of the visits of the
insects is evident in the first case, but passes unobserved in the
latter.
From this it would seem, that spontaneous crosses are hurtful to
varieties, but are innocuous to true species. Certainly this would be
so, were there no selection. But it is easily seen, that through this
operation the effect becomes quite the opposite. For when the fields
[210] are inspected at the time of the fullest display of the varietal
characters, the obvious hybrids will be eliminated, but the hidden ones
will of necessity be spared, as they are concealed among the species by
the similarity of their type. Hence, the harvest of the variety may be
rendered pure or nearly so, while the harvest of the species will retain
the seeds of the hybrids. Moreover it will contain seeds originated by
the spontaneous but numerous crosses of the true plants with the
sparsely intermingled hybrids.
This brings us to the question, as to what will be the visible
consequences of the occurrence of such invisible hybrids in the
following generation. In opposition to the direct effects just
described, we may call them indirect. To judge of their influence, we
must know how hybrid seeds of the first generation behave.
In one of our lectures we will deal with the laws that show the
numerical relations known as the laws of Mendel. But for our present
purpose, these numerical relations are only of subordinate importance.
What interests us here is the fact that hybrids of varieties do not
remain constant in the second generation but usually split as it is
said, remaining hybrid only in part of their offspring, the other
portion returning to the parental types. This however, will show itself
only in those individuals [211] which reassume the character of the
varietal parent, all the others apparently remaining true to the type of
the species. Now it is easy to foresee what must happen in the second
generation if the first generation after the cross is supposed to be
kept free from new vicinistic influences, or from crosses with
neighboring varieties.
We may limit ourselves in the first place to the seeds of the unobserved
hybrids. For the greater part they will repeat the character of their
parents and still remain concealed. But a small number will display the
varietal marks, as for example showing white flowers in a field of blue
ones. Hence, the indirect consequence of the spontaneous crosses will be
the same in the species, as was the direct effect in the variety, only
that it appears a year later. It will then be eliminated in the process
of selection.
Obviously, this elimination conduces only to a partial purification. The
conspicuous plants will be destroyed, but a greater number of hybrids
will remain, still concealed by their resemblance to the general type
and will be spared to repeat the same process next year. So while the
variety may be freed every year from the impurities brought into it in
the preceeding summer, the admixtures of the species [212] will continue
during a number of years, and it may not be possible to get rid of them
at all.
It is an often recurring assertion that white varieties of colored
species are the most stable of all horticultural races. They are often
said to be at least as constant as the species itself, and even to
surpass it in this quality. With our present state of knowledge, the
explanation of this general experience is easily given. For selection
removes the effect of spontaneous crosses from the variety in each year,
and renders it practically pure, while it is wholly inadequate to
produce the same effects on the species, because of the concealed
hybrids.
The explanation given in this simple instance may be applied to the case
of different varieties of the same species, when growing together and
crossed naturally by insects.
It would take too long to go into all the details that present
themselves here to the student of nature and of gardens. I will only
state, that since varieties differ principally from their species by the
lack of some sharp character, one variety may be characterized by the
lack of color of the flowers, another by the lack of pubescence, a third
by being dwarfed, and so on. Every character must be studied separately
in its effects on the offspring [213] of the crosses. And it is
therefore easily seen, that the hybrids of two varieties may resemble
neither of them, but revert to the species itself. This is necessarily
and commonly the case, since it is always the older or positive
characters that prevail in the hybrids and the younger or negative that
lie hidden. So for instance, a blue dwarf larkspur, crossed with a tall
white variety, must give a tall blue hybrid, reassuming in both
characters the essentials of the species.
Keeping this rule in view, it will be easy to calculate what may be
expected from spontaneous crosses for a wide range of occurrences, and
thus to find an explanation of innumerable cases of apparent variability
and reversion in the principle of vicinism. Students have only to
recollect that specific characters prevail over varietal ones, and that
every character competes only with its own antagonist. Or to give a
sharper distinction: whiteness of flowers cannot be expected to be
interchanged with pubescence of leaves.
In concluding I will point out another danger which in the principle of
vicinism may be avoided. If you see a plant in a garden with all the
characteristics of its species, how can you be sure that it is truly a
representative of the species, and not a hybrid? The prevailing [214]
characters are in either case the same. Perhaps on close inspection you
may find in some cases a slight difference, some character being not as
fully developed in the hybrid as in the species. But when such is not
the case, or where the opportunity for such a closer examination is
wanting, a hybrid may easily be taken for a specimen of the pure race.
Now take the seeds of your plant and sow them. If you had not supposed
it to be hybrid you will be astonished at finding among its progeny some
of a wholly different type. You will be led to conclude that you are
observing a sudden change in structure such as is usually called a
sport.
Or in other words you may think that you are assisting at the
origination of a new variety. If you are familiar with the principle of
vicinism, you will refrain from such an inference and consider the
supposition of a hybrid origin. But in former times, when this principle
was still unknown and not even guessed at, it is evident that many
mistakes must have been made, and that many an instance, which until now
has been considered reliable proof of a so-called single variation, is
in fact only a case of vicinism. In reading the sparse literature on
sports, numerous cases will be found, which cannot stand this test. In
many instances crossing must be looked to as an explanation, [215] and
in other cases the evidence relied upon does not suffice to exclude this
assumption. Many an old argument has of late lost its force by this
test.
Returning to our starting point we may now state that regular reversions
to a specific type characterize a form as a variety of that species.
These reversions, however, are not due to an innate tendency, but to
unobserved spontaneous crosses.
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