In the foregoing lectures I have tried to show that there is a real
difference between elementary species and varieties. The first are of
equal rank, and together constitute the collective or systematic
species. The latter are usually derived from real and still existing
types. Elementary species are in a sense independent of each other,
while varieties are of a derivative nature.
Furthermore I have tried to show that the ways in which elementary or
minor species must have originated from their common ancestor must be
quite different from the mode of origin of the varieties. We have
assumed that the first come into existence by the production of
something new, by the acquirement of a character hitherto unnoticed in
the line of their ancestors. On the contrary, varieties, in most cases,
evidently owe their origin to the loss of an already existing character,
or in other less frequent cases, to the re-assumption of a quality [248]
formerly lost. Some may originate in a negative, others in a positive
manner, but in both cases nothing really new is acquired.
This distinction holds good for all cases in which the relationship
between the forms in question is well known. It seems entirely
justifiable therefore to apply it also to cases in which the systematic
affinity is doubtful, as well as to instances in which it is impossible
to arrive at any taxonomic conclusions. The extreme application of the
principle would no doubt disturb the limits between many species and
varieties as now recognized. It is not to be forgotten however that all
taxonomic distinctions, which have not been confirmed by physiologic
tests are only provisional, a view acknowledged by the best
systematists. Of course the description of newly discovered forms can
not await the results of physiologic inquiries; but it is absolutely
impossible to reach definite conclusions on purely morphologic evidence.
This is well illustrated by the numerous discords of opinion of
different authors on the systematic worth of many forms.
Assuming the above mentioned principle as established, and disregarding
doubtful cases as indicated, the term progressive evolution is used to
designate the method in which elementary species must have originated.
It is the [249] manner in which all advance in the animal and vegetable
kingdoms must have taken place, continuously adding new characters to
the already existing number. Contrasted with this method of growing
differentiation, are the retrogressive modifications, which simply
retrace a step, and the degressive changes in which a backward step is
retraced and old characters revived. No doubt both of these methods have
been operative on a large scale, but they are evidently not in the line
of general advancement.
In all of these directions we see that the differentiating marks show
more or less clearly that they are built up of units. Allied forms are
separated from each other without intermediates. Transitions are wholly
wanting, although fallaciously apparent in some instances owing to the
wide range of fluctuating variability of the forms concerned, or to the
occurrence of hybrids and subvarieties.
These physiologic units, which in the end must be the basis for the
distinction of the systematic units, may best be designated by the term
of "unit-characters." Their internal nature is as yet unknown to us, and
we will not now look into the theories, which have been propounded as to
the probable material basis underlying them. For our present purpose the
empirical evidence of the general occurrence of [250] sharp limits
between nearly related characters must suffice. As Bateson has put it,
species are discontinuous, and we must assume that their characters are
discontinuous also.
Moreover there is as yet no reason for trying to make a complete
analysis of all the characters of a plant. No doubt, if attained, such
an analysis would give us a deep insight into the real internal
construction of the intricate properties of organisms in general. But
taxonomic studies in this direction are only in their infancy and do not
give us the material required for such an analysis. Quite on the
contrary, they compel us to confine our study to the most recently
acquired, or youngest characters, which constitute the differentiating
marks between nearly allied forms.
Obviously this is especially the case in the realm of the hybrids, since
only nearly related forms are able to give hybrid offspring. In dealing
with this subject we must leave aside all questions concerning more
remote relationships.
It is not my purpose to treat of the doctrine of hybridization at any
length. Experience is so rapidly increasing both in a practical and in a
purely scientific direction that it would take an entire volume to give
only a brief survey of the facts and of all the proposed theories.
[251] For our present purposes we are to deal with hybrids only in so
far as they afford the means of a still better distinction between
elementary species and varieties. I will try to show that these two
contrasting groups behave in quite a different manner, when subjected to
crossing experiments, and that the hope is justified that some day
crosses may become the means of deciding in any given instance, what is
to be called a species, and what a variety, on physiologic grounds. It
is readily granted that the labor required for such experiments, is
perhaps too great for the results to be attained, but then it may be
possible to deduce rules from a small series of experiments, which may
lead us to a decision in wider ranges of cases.
To reach such a point of view it is necessary to compare the evidence
given by hybrids, with the conclusions already attained by the
comparison of the differentiating characteristics of allied forms.
On this ground we first have to inquire what may be expected respecting
the internal nature and the outcome of the process of crossing in the
various cases cited in our former discussion.
We must always distinguish the qualities, which are the same in both
parents, from those that constitute the differentiating marks in every
single cross. In respect to the first [252] group the cross is not at
all distinguished from a normal fertilization, and ordinarily these
characters are simply left out of consideration. But it should never be
forgotten that they constitute the enormous majority, amounting to
hundreds and thousands, whereas the differentiating marks in each case
are only one or two or a few at most. The whole discussion is to be
limited to these last-named exceptions. We must consider first what
would be the nature of a cross when species are symmetrically combined,
and what must be the case when varieties are subjected to the same
treatment. In so doing, I intend to limit the discussion to the most
typical cases. We may take the crosses between elementary species of the
same or of very narrowly allied systematic species on the one side, and
on the other, limit treatment to the crossing of varieties with the
species, from which they are supposed to have sprung by a retrograde
modification. Crosses of different varieties of the same species with
one another obviously constitute a derivative case, and should only be
discussed secondarily. And crosses of varieties with positive or
depressive characters have as yet so rarely been made that we may well
disregard them.
Elementary species differ from their nearest allies by progressive
changes, that is by the acquirement [253] of some new character. The
derivative species has one unit more than the parent. All other
qualities are the same as in the parent. Whenever such a derivative is
combined with its parent the result for these qualities will be exactly
as in a normal fertilization. In such ordinary cases it is obvious that
each character of the pollen-parent is combined with the same character
of the pistil-parent. There may be slight individual differences, but
each unit character will become opposed to, and united with, the same
unit-character in the other parent. In the offspring the units will thus
be paired, each pair consisting of two equivalent units. As to their
character the units of each single pair are the same, only they may
exhibit slight differences as to the degree of development of this
character.
Now we may apply this conception to the sexual combination of two
different elementary species, assuming one to be the derivative of the
other. The differentiating mark is only present in one of the parents
and wanting in the other. While all other units are paired in the
hybrid, this one is not. It meets with no mate, and must therefore
remain unpaired. The hybrid of two such elementary species is in some
way incomplete and unnatural. In the ordinary course of things all
individuals derive [254] their qualities from both parents; for each
single mark they possess at least two units. Practically but not
absolutely equal, these two opponents always work together and give to
the offspring a likeness to both parents. No unpaired qualities occur in
normal offspring; these constitute the essential features of the hybrids
of species and are at the same time the cause of their wide deviations
from the ordinary rules.
Turning now to the varieties, we likewise need discuss their
differentiating marks only. In the negative types, these consist of the
apparent loss of some quality which was active in the species. But it
was pointed out in our last lecture that such a change is an apparent
loss. On a closer inquiry we are led to the assumption of a latent or
dormant state. The presumably lost characters have not absolutely, or at
least not permanently disappeared. They show their presence by some
slight indication of the quality they represent, or by occasional
reversions. They are not wanting, but only latent.
Basing our discussion concerning the process of crossing on this
conception, and still limiting the discussion to one differentiating
mark, we come to the inference, that this mark is present and active in
the species, and present but dormant in the variety. Thus it is present
in both, and as all other characters not differentiating [255] find
their mates in the cross, so these two will also meet one another. They
will unite just as well as though they were both active or both dormant.
For essentially they are the same, only differing in their degree of
activity. From this we can infer, that in the crossing of varieties, no
unpaired remainder is left, all units combining in pairs exactly as in
ordinary fertilization.
Setting aside the contrast between activity and latency in this single
pair, the procedure in the inter-crossing of varieties is the same as in
ordinary normal fertilization.
Summarizing this discussion we may conclude that in normal fertilization
and in the inter-crossing of varieties all characters are paired, while
in crosses between elementary species the differentiating marks are not
mated.
In order to distinguish these two great types of fertilization we will
use the term bisexual for the one and unisexual for the other. The term
balanced crosses then conveys the idea of complete bisexuality, all
unit-characters combining in pairs. Unbalanced crosses are those in
which one or more units do not find their mates and therefore remain
unpaired. This distinction was proposed by Macfarlane when studying the
minute structure of plant-hybrids in comparison with that of their
parents (1892).
[256] In the first place it shows that a species hybrid may inherit the
distinguishing marks of both parents. In this way it may become
intermediate between them, having some characters in common with the
pollen-parent and others with the pistil-parent. As far as these
characters do not interfere with each other, they may be fully developed
side by side, and in the main this is the way in which hybrid characters
are evolved. But in most cases our existing knowledge of the units is
far too slender to give a complete analysis, even of these
distinguishing marks alone. We recognize the parental marks more or less
clearly, but are not prepared for exact delimitations. Leaving these
theoretical considerations, we will pass to the description of some
illustrative examples.
In the first place I will describe a hybrid between two species of
_Oenothera_, which I made some years ago. The parents were the common
evening-primrose or _Oenothera biennis_ and of its small-flowered
congener, _Oenothera muricata_. These two forms were distinguished by
Linnaeus as different species, but have been considered by subsequent
writers as elementary species or so-called systematic varieties of one
species designated with the name of the presumably older type, the _O.
biennis_. Varietal differences in a physiologic sense they [257] do not
possess, and for this reason afford a pure instance of unbalanced union,
though differing in more than one point.
I have made reciprocal crosses, taking at one time the small-flowered
and at the other the common species as pistillate parent. These crosses
do not lead to the same hybrid as is ordinarily observed in analogous
cases; quite on the contrary, the two types are different in most
features, both resembling the pollen-parent far more than the
pistil-parent. The same curious result was reached in sundry other
reciprocal crosses between species of this genus. But I will limit
myself here to one of the two hybrids.
In the summer of 1895 I castrated some flowers of _O. muricata_, and
pollinated them with _O. biennis_, surrounding the flowers with paper
bags so as to exclude the visits of insects. I sowed the seeds in 1896
and the hybrids were biennial and flowered abundantly the next year and
were artificially fertilized with their own pollen, but gave only a very
small harvest. Many capsules failed, and the remaining contained only
some few ripe seeds.
From these I had in the following year the second hybrid generation, and
in the same way I cultivated also the third and fourth. These were as
imperfectly fertile as the first, and in [258] some years did not give
any seed at all, so that the operation had to be repeated in order to
continue the experiment. Last summer (1903) I had a nice lot of some 25
biennial specimens blooming abundantly. All in all I have grown some 500
hybrids, and of these about 150 specimens flowered.
These plants were all of the same type, resembling in most points the
pollen-parent, and in some others the pistil-parent of the original
cross. The most obvious characteristic marks are afforded by the
flowers, which in _O. muricata_ are not half so large as in _biennis_,
though borne by a calyx-tube of the same length. In this respect the
hybrid is like the _biennis_ bearing the larger flowers. These may at
times seem to deviate a little in the direction of the other parent,
being somewhat smaller and of a slightly paler color. But it is very
difficult to distinguish between them, and if _biennis_ and hybrid
flowers were separated from the plants and thrown together, it is very
doubtful whether one would succeed in separating them.
The next point is offered by the foliage. The leaves of _O. biennis_ are
broad, those of _O. muricata_ narrow. The hybrid has the broad leaves of
_O. biennis_ during most of its life and at the time of flowering. Yet
small deviations in the [259] direction of the other parent are not
wanting, and in winter the leaves of the hybrid rosettes are often much
narrower than those of _O. biennis_, and easily distinguishable from
both parents. A third distinction consists in the density of the spike.
The distance between the insertion of the flowers of _O. biennis_ is
great when compared with that of _O. muricata_. Hence the flowers of the
latter species are more crowded and those of _O. biennis_ more
dispersed, the spikes of the first being densely crowned with flowers
and flower-buds while those of _O. biennis_ are more elongated and
slender. As a further consequence the _O. biennis_ opens on the same
evening only one, two or three flowers on the same spike, whereas _O.
muricata_ bears often eight or ten or more flowers at a time. In this
respect the hybrid is similar to the pistil-parent, and the crowding of
the broad flowers at the top of the spikes causes the hybrids to be much
more showy than either of the parent types.
Other distinguishing marks are not recorded by the systematists, or are
not so sharply separated as to allow of the corresponding qualities of
the hybrids being compared with them.
This hybrid remains true to the description given. In some years I
cultivated two generations [260] so as to be able to compare them with
one another, but did not find any difference. The most interesting point
however, is the likeness between the first generation, which obviously
must combine in its internal structure the units of both parents, and
the second and later generations which are only of a derivative nature.
Next to this stands the fact that in each generation all individuals are
alike. No reversion to the parental forms either in the whole type or in
the single characteristics has ever been observed, though the leaves of
some hundreds, and the spikes and flowers of some 150 individual plants
have been carefully examined. No segregation or splitting up takes
place.
Here we have a clear, undoubted and relatively simple, case of a true
and pure species hybrid. No occurrence of possible varietal
characteristics obscures the result, and in this respect this hybrid
stands out much more clearly than all those between garden-plants, where
varietal marks nearly always play a most important part.
From the breeder's point of view our hybrid _Oenothera_ would be a
distinct gain, were it not for the difficulty of its propagation. But to
enlarge the range of the varieties this simple and stable form would
need to be treated anew, by [261] crossing it with the parent-types.
Such experiments however, have miscarried owing to the too stable nature
of the unit-characters.
This stability and this absence of the splitting shown by varietal marks
in the offspring of hybrids is one of the best proofs of unisexual
unions. It is often obscured by the accompanying varietal marks, or
overlooked for this reason. Only in rare cases it is to be met with in a
pure state and some examples are given of this below.
Before doing so, I must call your attention to another feature of the
unbalanced unions. This is the diminution of the fertility, a phenomenon
universally known as occurring in hybridizations. It has two phases.
First, the diminished chance of the crosses themselves of giving full
crops of seed, as compared with the pure fertilization of either parent.
And, secondly, the fertility of the hybrids themselves. Seemingly, all
grades of diminished fertility occur and the oldest authors on hybrids
have pointed out that a very definite relation exists between the
differences of the parents and the degree of sterility, both of the
cross and of the hybrid offspring. In a broad sense these two factors
are proportionate to each other, the sterility being the greater, the
lesser the affinity between the parents. Many writers have [262] tried
to trace this rule in the single cases, but have met with nearly
unsurmountable difficulties, owing chiefly to our ignorance of the units
which form the differences between the parents in the observed cases.
In the case of _Oenothera muricata x biennis_ the differentiating units
reduce the fertility to a low degree, threatening the offspring with
almost complete infertility and extinction. But then we do not know
whether these characters are really units, or perhaps only seemingly so
and are in reality composed of smaller entities which as yet we are not
able to segregate. And as long as we are devoid of empirical means of
deciding such questions, it seems useless to go farther into the details
of the question of the sterility. It should be stated here however, that
pure varietal crosses, when not accompanied by unbalanced characters,
have never showed any tendency to diminished fertility. Hence there can
be little doubt that the unpaired units are the cause of this decrease
in reproductive power.
The genus _Oenothera_ is to a large degree devoid of varietal
characteristics, especially in the subgenus _Onagra_, to which
_biennis_, _muricata_, _lamarckiana_ and some others belong. On the
other hand it seems to be rich in elementary species, but an adequate
study of [263] them has as yet not been made. Unfortunately many of the
better systematists are in the habit of throwing all these interesting
forms together, and of omitting their descriptive study. I have made a
large number of crosses between such undescribed types and as a rule got
constant hybrid races. Only one or two exceptions could be quoted, as
for instance the _Oenothera brevistylis_, which in its crosses always
behaves as a pure retrogressive variety. Instead of giving an exhaustive
survey of hybrids, I simply cite my crosses between _lamarckiana_ and
_biennis_, as having nearly the aspect of the last named species, and
remaining true to this in the second generation without any sign of
reversion or of splitting. I have crossed another elementary species,
the _Oenothera hirtella_ with some of my new and with some older Linnean
species, and got several constant hybrid races. Among these the
offspring of a cross between _muricata_ and _hirtella_ is still in
cultivation. The cross was made in the summer of 1897 and last year
(1903) I grew the fourth generation of the hybrids. These had the
characters of the _muricata_ in their narrow leaves, but the elongated
spikes and relatively large flowers of the _hirtella_ parent, and
remained true to this type, showing only slight fluctuations and never
reverting or segregating [264] the mixed characters. Both parents bear
large capsules with an abundance of seed, but in the hybrids the
capsules remain narrow and weak, ripening not more than one-tenth the
usual quantity of seed. Both parents are easily cultivated in annual
generations and the same holds good for the hybrid. But whereas the
hybrid of muricata and biennis is a stout plant, this type is weak with
badly developed foliage, and very long strict spikes. Perhaps it was not
able to withstand the bad weather of the last few years.
A goodly number of constant hybrids are described in literature, or
cultivated in fields and gardens. In such cases the essential question
is not whether they are now constant, but whether they have been so from
the beginning, or whether they prove to be constant whenever the
original cross is repeated. For constant hybrids may also be the issue
of incipient splittings, as we shall soon see.
Among other examples we may begin with the hybrid alfalfa or hybrid
lucerne (_Medicago media_). It often originates spontaneously between
the common purple lucerne or alfalfa and its wild ally with yellow
flowers and procumbent stems, the _Medicago falcata_. This hybrid is
cultivated in some parts of Germany on a large scale, as it is more
productive than [265] the ordinary lucerne. It always comes true from
seed and may be seen in a wild state in parks and on lawns. It is one of
the oldest hybrids with a pure and known lineage. The original cross has
been repeated by Urban, who found the hybrid race to be constant from
the beginning.
Another very notorious constant hybrid race is the _Aegilops
speltaeformis_. It has been cultivated in botanic gardens for more than
half a century, mostly in annual or biennial generations. It is
sufficiently fertile and always comes true. Numerous records have been
made of it, since formerly it was believed by Fabre and others to be a
spontaneous transition from some wild species of grass to the ordinary
wheat, not a cross. Godron, however, showed that it can be produced
artificially, and how it has probably sprung into existence wherever it
is found wild. The hybrid between _Aegilops ovata_, a small weed, and
the common wheat is of itself sterile, producing no good pollen. But it
may be fertilized by the pollen of wheat and then gives rise to a
secondary hybrid, which is no other than the _Aegilops speltaeformis_.
This remained constant in Godron's experiments during a number of
generations, and has been constant up to the present time.
[266] Constant hybrids have been raised by Millardet between several
species of strawberries. He combined the old cultivated forms with newly
discovered types from American localities. They ordinarily showed only
the characteristics of one of their parents and did not exhibit any new
combination of qualities, but they came true to this type in the second
and later generations.
In the genus _Anemone_, Janczewski obtained the same results. Some
characters of course may split, but others remain constant, and when
only such are present, hybrid races result with new combinations of
characters, which are as constant as the best species of the same genus.
The hybrids of Janczewski were quite fertile, and he points out that
there is no good reason why they should not be considered as good new
species. If they had not been produced artificially, but found in the
wild state, their origin would have been unknown, and there can be no
doubt that they would have been described by the best systematists as
species of the same value as their parents. Such is especially the case
with a hybrid between _Anemone magellanica_ and the common _Anemone
sylvestris_.
Starting from similar considerations Kerner von Marilaun pointed out the
fact long ago that many so-called species, of rare occurrence, [267]
standing between two allied types, may be considered to have originated
by a cross. Surely a wide field for abuse is opened by such an
assertion, and it is quite a common habit to consider intermediate forms
as hybrids, on the grounds afforded by their external characters alone,
and without any exact knowledge of their real origin and often without
knowing anything as to their constancy from seed. All such apparent
explanations are now slowly becoming antiquated and obsolete, but the
cases adduced by Kerner seem to stand this test.
Kerner designates a willow, _Salix ehrhartiana_ as a constant hybrid
between _Salix alba_ and _S. pentandra_. _Rhododendron intermedium_ is
an intermediate form between the hairy and the rusty species from the
Swiss Alps, _R. hirsutum_ and _R. ferrugineum_, the former growing on
chalky, and the other on silicious soils. Wherever both these types of
soil occur in the same valley and these two species approach one
another, the hybrid _R. intermedium_ is produced, and is often seen to
be propagating itself abundantly. As is indicated by the name, it
combines the essential characters of both parents.
_Linaria italica_ is a hybrid toad-flax between _L. genistifolia_ and
_L. vulgaris_, a cross which I have repeated in my garden. _Drosera
obovata_ [268] is a hybrid sundew between _D. anglica_ and _D.
rotundifolia_. _Primula variabilis_ is a hybrid between the two common
primroses, _P. officinalis_ and _P. grandiflora_. The willow-herb
(_Epilobium_), the self-heal (_Brunella_) and the yellow pond-lilies
(Nuphar) afford other instances of constant wild hybrids.
Macfarlane has discovered a natural hybrid between two species of sundew
in the swamps near Atco, N.J. The parents, _D. intermedia_ and _D.
filiformis_, were growing abundantly all around, but of the hybrid only
a group of eleven plants was found. A detailed comparison of the hybrid
with its parents demonstrated a minute blending of the anatomical
peculiarities of the parental species.
Luther Burbank of Santa Rosa, California, has produced a great many
hybrid brambles, the qualities of which in many respects surpass those
of the wild species. Most of them are only propagated by cuttings and
layers, not being stable from seed. But some crosses between the
blackberry and the raspberry (_R. fruticosus_ and _R. idaeus_) which
bear good fruit and have become quite popular, are so fixed in their
type as to reproduce their composite characters from seed with as much
regularity as the species of _Rubus_ found in nature. Among them are the
"Phenomenal" and the [269] "Primus." The latter is a cross between the
Californian dewberry and the Siberian raspberry and is certainly to be
regarded as a good stable species, artificially produced. Bell Salter
crossed the willow-herbs _Epilobium tetragonum_ and _E. montanum_, and
secured intermediate hybrids which remained true to their type during
four successive generations.
Other instances might be given. Many of them are to be found in
horticultural and botanical journals which describe their systematic and
anatomical details. The question of stability is generally dealt with in
an incidental manner, and in many cases it is difficult to reach
conclusions from the facts given. Especially disturbing is the
circumstance that from a horticultural point of view it is quite
sufficient that a new type should repeat itself in some of its offspring
to be called stable, and that for this reason absolute constancy is
rarely proved.
The range of constant hybrids would be larger by far were it not for two
facts. The first is the absolute sterility of so many beautiful hybrids,
and the second is the common occurrence of retrogressive characters
among cultivated plants. To describe the importance of both these groups
of facts would take too much [270] time, and therefore it seems best to
give some illustrative examples instead.
Among the species of _Ribes_ or currant, which are cultivated in our
gardens, the most beautiful are without doubt the Californian and the
Missouri currant, or _Ribes sanguineum_ and _R. aureum_. A third form,
often met with, is "Gordon's currant," which is considered to be a
hybrid between the two. It has some peculiarities of both parents. The
leaves have the general form of the Californian parent, but are as
smooth as the Missouri species. The racemes or flower-spikes are densely
flowered as in the red species, but the flowers themselves are of a
yellow tinge, with only a flesh-red hue on the outer side of the calyx.
It grows vigorously and is easily multiplied by cuttings, but it never
bears any fruit. Whether it would be constant, if fertile, is therefore
impossible to decide. _Berberis ilicifolia_ is considered as a hybrid
between the European barberry (_B. vulgaris_) and the cultivated shrub
_Mahonia aquifolia_. The latter has pinnate leaves, the former undivided
ones. The hybrid has undivided leaves which are more spiny than those of
the European parent, and which are not deciduous like them, but persist
during the winter, a peculiarity inherited from the _Mahonia_. As far as
I [271] have been able to ascertain, this hybrid never produces seed.
Another instance of an absolutely sterile hybrid is the often quoted
_Cytisus adami_. It is a cross between the common laburnum (_Cytisus
Laburnum_) and another species of the same genus, _C. purpureus_, and
has some traits of both. But since the number of differentiating marks
is very great in this case, most of the organs have become intermediate.
It is absolutely sterile. But it has the curious peculiarity of
splitting in a vegetative way. It has been multiplied on a large scale
by grafting and was widely found in the parks and gardens of Europe
during the last century. Nearly all these specimens reverted from time
to time to the presumable parents. Not rarely a bud of Adam's laburnum
assumed all the qualities of the common laburnum, its larger leaves,
richer flowered racemes, large and brightly yellow flowers and its
complete fertility. Other buds on the same tree reverted to the purple
parent, with its solitary small flowers, its dense shrublike branches
and very small leaves. These too are fertile, though not producing their
seeds as abundantly as the _C. Laburnum_ reversions. Many a botanist has
sown the seeds of the latter and obtained only pure common _C. Laburnum_
plants. I had a lot of nearly a hundred seedlings [272] myself, many of
which have already flowered, bearing the leaves and flowers of the
common species. Seeds of the purple reversions have also been sown, and
also yielded the parental type only.
Why this most curious hybrid sports so regularly and why others always
remain true to their type is as yet an open question.
But recalling our former consideration of this subject the supposition
seems allowable that the tendency to revert is not connected with the
type of the hybrid, but is apt to occur in some rare individuals of
every type. But since most of the sterile hybrids are only known to us
in a single individual and its vegetative offspring, this surmise offers
an explanation of the rare occurrence of sports.
Finally, we must consider some of the so called hybrid races or strains
of garden-plants. _Dahlia_, _Gladiolus_, _Amaryllis_, _Fuchsia_,
_Pelargonium_ and many other common flowers afford the best known
instances. Immeasurable variability seems here to be the result of
crossing. But on a closer inspection the range of characters is not so
very much wider in these hybrid races than in the groups of parent
species which have contributed to the origin of the hybrids. Our
tuberous begonias owe their variability to at least seven original
parent species, [273] and to the almost incredible number of
combinations which are possible between their characters. The first of
these crosses was made in the nursery of Veitch and Sons near London by
Seden, and the first hybrid is accordingly known as _Begonia sedeni_ and
is still to be met with. It has been superseded by subsequent crosses
between the _sedeni_ itself and the _Veitchi_ and _rosiflora_, the
_davisii_, the _clarkii_ and others. Each of them contributed its
advantageous qualities, such as round flowers, rosy color, erect flower
stalks, elevation of the flowers above the foliage and others. New
crosses are being made continuously, partly between the already existing
hybrids and partly with newly introduced wild species. Only rarely is it
possible to get pure seeds, and I have not yet been able to ascertain
whether the hybrids would come true from seed. Specific and varietal
characters may occur together in many of the several forms, but nothing
is as yet accurately known as to their behavior in pure fertilizations.
Constancy and segregation are thrown together in such a manner that
extreme variability results, and numerous beautiful types may be had,
and others may be expected from further crosses. For a scientific
analysis, however, the large range of recorded facts and the written
history, which at first sight [274] seems to have no lacunae, are not
sufficient. Most of the questions remain open and need investigation. It
would be a capital idea to try to repeat the history of the begonias or
any other hybrid race, making all the described crosses and then
recording the results in a manner requisite for complete and careful
scientific investigations.
Many large genera of hybrid garden-flowers owe their origin to species
rich in varieties or in elementary subspecies. Such is the case with the
gladiolus and the tulips. In other cases the original types have not
been obtained from the wild state but from the cultures of other
countries.
The dahlias were cultivated in Mexico when first discovered by
Europeans, and the chrysanthemums have been introduced from the old
gardens of Japan. Both of them consisted of various types, which
afterwards have been increased chiefly by repeated intercrossing.
The history of many hybrid races is obscure, or recorded by different
authorities in a different way. Some have derived their evidence from
one nursery, some from another, and the crosses evidently may have been
different in different places. The early history of the gladiolus is an
instance. The first crosses are recorded to have been made between
_Gladiolus_ [275] _psittacinus_ and _G. cardinalis_, and between their
hybrid, which is still known under the name of gandavensis_ and the
_purpureo-auratus_. But other authors give other lines of descent. So it
is with _Amaryllis_, which is said by De Graaff to owe its stripes to
_A. vittata_, its fine form to _A. brasiliensis_, the large petals to
_A. psittacina_, the giant flowers to _A. leopoldi_, and the piebald
patterns to _A. pardina_. But here, too, other authors give other
derivations.
Summarizing the results of our inquiry we see in the first place how
very much remains to be done. Many old crosses must be repeated and
studied anew, taking care of the purity of the cross as well as of the
harvesting of the seeds. Many supposed facts will be shown to be of
doubtful validity. New facts have to be gathered, and in doing so the
distinction between specific and varietal marks must be taken strictly
into account. The first have originated as progressive mutations; they
give unbalanced crosses with a constant offspring, as far as experience
now goes. The second are chiefly due to retrograde modifications, and
will be the subject of the next lecture.
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