In the scientific study of the result of crosses, the most essential
point is the distinction of the several characters of the parents in
their combination in the hybrids and their offspring. From a theoretical
point of view it would be best to choose parents which would differ only
in a single point. The behavior of the differentiating character might
then easily be seen.
Unfortunately, such simple cases do not readily occur. Most species, and
even many elementary species are distinguished by more than one quality.
Varieties deviating only in one unit-character from the species, are
more common. But a closer inspection often reveals some secondary
characters which may be overlooked in comparative or descriptive
studies, but which reassume their importance in experimental crossings.
In a former lecture we have dealt with the qualities which must be
considered as being due to the acquisition of new characters. If we
[277] compare the new form in this case with the type from which it has
originated, it may be seen that the new character does not find its
mate, or its opposite, and it will be unpaired in the hybrid.
In the case of retrogressive changes the visible modification is due, at
least in the best known instances, to the reduction of an active quality
to a state of inactivity or latency. Now if we make a cross between a
species and its variety, the differentiating character will be due to
the same internal unit, with no other difference than that it is active
in the species and latent in the variety. In the hybrid these two
corresponding units will make a pair. But while all other pairs in the
same hybrid individuals consist of like antagonists, only this pair
consists of slightly unlike opponents.
This conception of varietal crosses leads to three assertions, which
seem justifiable by actual experience.
First, there is no reason for a diminution of the fertility, as all
characters are paired in the hybrid, and no disturbance whatever ensues
in its internal structure. Secondly, it is quite indifferent, how the
two types are combined, or which of them is chosen as pistillate and
which as staminate parent. The deviating pair will have the same
constitution in both cases, being [278] built up of one active and one
dormant unit. Thirdly this deviating pair will exhibit the active unit
which it contains, and the hybrid will show the aspect of the parent in
which the character was active and not that of the parent in which it
was dormant. Now the active quality was that of the species, and its
latent state was found in the variety. Hence the inference that hybrids
between a species and its retrograde variety will bear the aspect of the
species. This attribute may be fully developed, and then the hybrid will
not be distinguishable from the pure species in its outer appearance. Or
the character may be incompletely evolved, owing to the failure of
cooperation of the dormant unit. In this case the hybrid will be in some
sense intermediate between its parents, but these instances are more
rare than the alternate ones, though presumably they may play an
important part in the variability of many hybrid garden-flowers.
All of these three rules are supported by a large amount of evidence.
The complete fertility of varietal hybrids is so universally
acknowledged that it is not worth while to give special instances. With
many prominent systematists it has become a test between species and
varieties, and from our present point of view this assumption is
correct. Only the test is of little use in practice, as fertility may be
diminished [279] in unbalanced unions in all possible degrees, according
to the amount of difference between the parents. If this amount is
slight, if for instance, only one unit-character causes the difference,
the injury to fertility may, be so small as to be practically nothing.
Hence we see that this test would not enable us to judge of the doubtful
cases, although it is quite sufficient as a proof in cases of wider
differences.
Our second assertion related to the reciprocal crosses. This is the name
given to two sexual combinations between the same parents, but with
interchanged places as to which furnishes the pollen. In unbalanced
crosses of the genus _Oenothera_ the hybrids of such reciprocal unions
are often different, as we have previously shown. Sometimes both
resemble the pollen parent more, in other instances the pistil-parent.
In varietal crosses no such divergence is as yet known. It would be
quite superfluous to adduce single cases as proofs for this rule, which
was formerly conceived to hold good for hybrids in general. The work of
the older hybridists, such as Koelreuter and Gaertner affords numerous
instances.
Our third rule is of a wholly different nature. Formerly the distinction
between elementary species and varieties was not insisted upon, and the
principle which stamps retrograde changes [280] as the true character of
varieties is a new one. Therefore it is necessary to cite a considerable
amount of evidence in order to prove the assertion that a hybrid bears
the active character of its parent-species and not the inactive
character of the variety chosen for the cross.
We may put this assertion in a briefer form, stating that the active
character prevails in the hybrid over its dormant antagonist. Or as it
is equally often put, the one dominates and the other is recessive. In
this terminology the character of the species is dominant in the hybrid
while that of the variety is recessive. Hence it follows that in the
hybrid the latent or dormant unit is recessive, but it does not follow
that these three terms have the same meaning, as we shall see presently.
The term recessive only applies to the peculiar state into which the
latent character has come in the hybrid by its pairing with the
antagonistic active unit.
In the first place it is of the highest importance to consider crosses
between varieties of recorded origin and the species from which they
have sprung. When dealing with mutations of celandine we shall see that
the laciniated form originated from the common celandine in a garden at
Heidelberg about the year 1590. Among my _Oenotheras_ one of the eldest
of the recent productions is the _O. brevistylis_ or short [281] styled
species which was seen for the first time in the year 1889. The third
example offered is a hairless variety of the evening campion, _Lychnis
vespertina_, found the same year, which hitherto had not been observed.
For these three cases I have made the crosses of the variety with the
parent-species, and in each case the hybrid was like the species, and
not like the variety. Nor was it intermediate. Here it is proved that
the older character dominates the younger one.
In most cases of wild, and of garden-varieties, the relation between
them and the parent-species rests upon comparative evidence. Often the
variety is known to be younger, in other cases it may be only of local
occurrence, but ordinarily the historic facts about its origin have
never been known or have long since been forgotten.
The easiest and most widely known varietal crosses are those between
varieties with white flowers and the red- or blue-flowered species. Here
the color prevails in the hybrid over the lack of pigment, and as a rule
the hybrid is as deeply tinted as the species itself, and cannot be
distinguished from it, without an investigation of its hereditary
qualities. Instances may be cited of the white varieties of the
snapdragon, of the red clover, the long-spurred violet (_Viola_ [282]
_cornuta_) the sea-shore aster (_Aster Tripolium_), corn-rose
(_Agrostemma Githago_), the Sweet William (_Silene Armeria_), and many
garden flowers, as for instance, the _Clarkia pulchella_, the
_Polemonium coeruleum_, the _Veronica longifolia_, the gloxinias and
others. If the red hue is combined with a yellow ground-color in the
species, the variety will be yellow and the hybrid will have the red and
yellow mixture of the species as for instance, in the genus _Geum_. The
toad-flax has an orange-colored palate, and a variety occurs in which
the palate is of the same yellow tinge as the remaining parts of the
corolla. The hybrid between them is in all respects like the
parent-species.
Other instances could be given. In berries the same rule prevails. The
black nightshade has a variety with yellow berries, and the black color
returns in the hybrid. Even the foliage of some garden-plants may afford
instances, as for instance, the purplish amaranth (_Amaranthus
caudatus_). It has a green variety, but the hybrid between the two has
the red foliage of the species.
Special marks in leaves and in flowers follow the same rule. Some
varieties of the opium poppy have large black patches at the basal end
of the petals, while in others this pattern is entirely white. In
crossing two such varieties, [283] for instance, the dark "Mephisto"
with the white-hearted "Danebrog," the hybrid shows the active character
of the dark pattern.
Hairy species crossed with their smooth varieties produce hairy hybrids,
as in some wheats, in the campion (_Lychnis_), in _Biscutella_ and
others. The same holds good for the crosses between spiny species and
their unarmed derivatives, as in the thorn-apple, the corn-crowfoot
(_Ranunculus arvensis_) and others.
Lack of starch in seeds is observed in some varieties of corn and of
peas. When such derivatives are crossed with ordinary starch-producing
types, the starch prevails in the hybrid.
It would take too much time to give further examples. But there is still
one point which should be insisted upon. It is not the systematic
relation of the two parents of a cross, that is decisive, but only the
occurrence of the same quality, in the one in an active, and in the
other in an inactive condition. Hence, whenever this relation occurs
between the parents of a cross, the active quality prevails in the
hybrid, even when the parents differ from each other in other respects
so as to be distinguished as systematic species. The white and red
campions give a red hybrid, the black and pale henbane (_Hyoscyamus
niger_ and _H. pallidus_) give a hybrid [284] with the purple veins and
center in the corolla of the former, the white and blue thornapple
produce a blue hybrid, and so on. Instances of this sort are common in
cultivated plants.
Having given this long list of examples of the rule of the dominancy of
the active character over the opposite dormant unit, the question
naturally arises as to how the antagonistic units are combined in the
hybrid. This question is of paramount importance in the consideration of
the offspring of the hybrids. But before taking it up it is as well to
learn the real signification of recessiveness in the hybrids themselves.
Recessive characters are shown by those rare cases, in which hybrids
revert to the varietal parent in the vegetative way. In other words by
bud-variations or sports, analogous to the splitting of Adam's laburnum
into its parents, by means of bud-variation already described. But here
the wide range of differentiating characters of the parents of this most
curious hybrid fail. The illustrative examples are extremely simple, and
are limited to the active and inactive condition of only one quality.
An instance is given by the long-leaved veronica (_Veronica
longifolia_), which has bluish flowers in long spikes. The hybrid
between [285] this species and its white variety has a blue corolla. But
occasionally it produces some purely white flowers, showing its power of
separating the parental heritages, combined in its internal structures.
This reversion is not common, but in thousands of flowering spikes one
may expect to find at least one of them. Sometimes it is a whole stem
springing from the underground system and bearing only white flowers on
all its spikes. In other instances it is only a side branch which
reverts and forms white flowers on a stem, the other spikes of which
remain bluish. Sometimes a spike even differentiates longitudinally,
bearing on one side blue and on the other white corollas, and the white
stripe running over the spike may be seen to be long and large, or
narrow and short in various degrees. In such cases it is evident that
the heritages of the parents remain uninfluenced by each other during
the whole life of the hybrid, working side by side, but the active
element always prevails over its latent opponent which is ready to break
free whenever an opportunity is offered.
It is now generally assumed that this incomplete mixture of the parental
qualities in a hybrid, this uncertain and limited combination is the
true cause of the many deviations, exhibited by varietal hybrids when
compared with their [286] parents. Partial departures are rare in the
hybrids themselves, but in their offspring the divergence becomes the
rule.
Segregation seems to be a very difficult process in the vegetative way,
but it must be very easy in sexual reproduction, indeed so easy as to
show itself in nearly every single instance.
Leaving this first generation, the original hybrids, we now come to a
discussion of their offspring. Hybrids should be fertilized either by
their own pollen, or by that of other individuals born from the same
cross. Only in this case can the offspring be considered as a means of
arriving at a decision as to the internal nature of the hybrids
themselves. Breeders generally prefer to fertilize hybrids with the
pollen of their parents. But this operation is to be considered as a new
cross, and consequently is wholly excluded from our present discussion.
Hence it follows that a clear insight into the heredity of hybrids may
be expected only from scientific experiments. Furthermore some of the
diversity observed as a result of ordinary crosses, may be due to the
instability of the parents themselves or at least of one of them, since
breeders ordinarily choose for their crosses some already very variable
strain. Combining such a strain with the desirable qualities of some
newly imported species, a new strain may [287] result, having the new
attribute in addition to all the variability of the old types. In
scientific experiments made for the purpose of investigating the general
laws of hybridity, such complex cases are therefore to be wholly
excluded. The hereditary purity of the parents must be considered as one
of the first conditions of success.
Moreover the progeny must be numerous, since neither constancy, nor the
exact proportions in the case of instability, can be determined with a
small lot of plants.
Finally, and in order to come to a definite choice of research material,
we should keep in mind that the chief object is to ascertain the
relation of the offspring to their parents. Now in nearly all cases the
seeds are separated from the fruits and from one another, before it
becomes possible to judge of their qualities. One may open a fruit and
count the seeds, but ordinarily nothing is noted as to their characters.
In this respect no other plant equals the corn or maize, as the kernels
remain together on the spike, and as it has more than one variety
characterized by the color, or constitution, or other qualities of the
grains. A corn-grain, however, is not a seed, but a fruit containing a
seed. Hence the outer parts pertain to the parent plant and only the
innermost ones to the [288] seedling and therefore to the following
generation. Fruit-characters thus do not offer the qualities we need,
only the qualities resulting from fertilizations are characteristic of
the new generation. Such attributes are afforded in some cases by the
color, in others by the chemical constitution.
We will choose the latter, and take the sugarcorn in comparison with the
ordinary or starch producing forms for our starting point. Both sugar-
and starch-corns have smooth fruits when ripening. No difference is to
be seen in the young ripe spikes. Only the taste, or a direct chemical
analysis might reveal the dissimilarity. But as soon as the spikes are
dried, a diversity is apparent. The starchy grains remain smooth, but
the sugary kernels lose so much water that they become wrinkled. The
former becomes opaque, the latter more or less transparent. Every single
kernel may instantly be recognized as belonging to either of the types
in question, even if but a single grain of the opposite quality might be
met with on a spike. Kernels can be counted on the spike, and since
ordinary spikes may bear from 300-500 grains and often more, the
numerical relation of the different types may be deduced with great
accuracy.
Coming now to our experiment, both starchy [289] and sugary varieties
are in this respect wholly constant, when cultivated separately. No
change is to be seen in the spikes. Furthermore it is very easy to make
the crosses. The best way is to cultivate both types in alternate rows
and to cut off the staminate panicles a few days before they open their
first flowers. If this operation is done on all the individuals of one
variety, sparing all the panicles of the other, it is manifest that all
the plants will become fertilized by the latter, and hence that the
castrated plants will only bear hybrid seeds.
The experiment may be made in two ways; by castrating the sugary or the
starchy variety. In both cases the hybrid kernels are the same. As to
their composition they repeat the active character of the starchy
variety. The sugar is only accumulated as a result of an incapacity of
changing it into starch, and the lack of this capacity is to be
considered as a retrogressive varietal mark. The starch-producing unit
character, which is active in the ordinary sorts of corns, is therefore
latent in sugar-corn.
In order to obtain the second generation, the hybrid grains are sown
under ordinary conditions, but sufficiently distant from any other
variety of corn to insure pure fertilization. The several individuals
may be left to pollinate [290] each other, or they may be artificially
pollinated with their own pollen.
The outcome of the experiments is shown by the spikes, as soon as they
dry. Each spike bears two sorts of kernels irregularly dispersed over
its surface. In this point all the spikes are alike. On each of them one
may see on the first inspection that the majority of the kernels are
starch-containing seeds, while a minor part becomes wrinkled and
transparent according to the rule for sugary seeds. This fact shows at
once that the hybrid race is not stable, but has differentiated the
parental characters, bringing those of the varietal parent to perfect
purity and isolation. Whether the same holds good for the starchy
parent, it is impossible to judge from the inspection of the spikes,
since it has been seen in the first generation that the hybrid kernels
are not visibly distinguished from those of the pure starch-producing
grains.
It is very easy to count the number of both sorts of grains in the spike
of such a hybrid. In doing so we find, that the proportion is nearly the
same on all the spikes, and only slight variations would be found in
hundreds of them. One-fourth of the seeds are wrinkled and three-fourths
are always smooth. The number may vary in single instances and be a
little more or a little less than 25%, ranging, for [291] instance, from
20 to 27%, but as a rule, the average is found nearly equal to 25%.
The sugary kernels, when separated from the hybrid spikes and sown
separately, give rise to pure sugary race, in no degree inferior in
purity to the original variety. But the starchy kernels are of different
types, some of them being internally like the hybrids of the first
generation and others like the original parent. To decide between these
two possibilities, it is necessary to examine their progeny.
For the study of this third hybrid generation we will now take another
example, the opium poppies. They usually have a dark center in the
flowers, the inferior parts of the four petals being stained a deep
purple, or often nearly black. Many varieties exhibit this mark as a
large black cross in the center of the flower. In other varieties the
pigment is wanting, the cross being of a pure white. Obviously it is
only reduced to a latent condition, as in so many other cases of loss of
color, since it reappears in a hybrid with the parent-species.
For my crosses I have taken the dark-centered "Mephisto" and the
"Danebrog," or Danish flag, with a white cross on a red field. The
second year the hybrids were all true to the type of "Mephisto." From
the seeds of each artificially self-fertilized capsule, one-fourth
(22.5%) [292] in each instance reverted to the varietal mark of the
white cross, and three-fourths (77.5%) retained the dark heart. Once
more the flowers were self-pollinated and the visits of insects
excluded. The recessives now gave only recessives, and hence we may
conclude that the varietal marks had returned to stability. The dark
hearted or dominants behaved in two different ways. Some of them
remained true to their type, all their offspring being dark-hearted.
Evidently they had returned to the parent with the active mark, and had
reassumed this type as purely as the recessives had reached theirs. But
others kept true to the hybrid character of the former generation,
repeating in their progeny exactly the same mixture as their parents,
the hybrids of the first generation, had given.
This third generation therefore gives evidence, that the second though
apparently showing only two types, really consists of three different
groups. Two of them have reassumed the stability of their original
grandparents, and the third has retained the instability of the hybrid
parents.
The question now arises as to the numerical relation of these groups.
Our experiments gave the following results: [293]