It can hardly fail to be of interest to compare the observations made
regarding _Pisum_ with the results arrived at by the two authorities
in this branch of knowledge, Kölreuter and Gärtner, in their
investigations. According to the opinion of both, the hybrids in outer
appearance present either a form intermediate between the original
species, or they closely resemble either the one or the other type, and
sometimes can hardly be discriminated from it. From their seeds usually
arise, if the fertilisation was effected by their own pollen, various
forms which differ from the normal type. As a rule, the majority of
individuals obtained by one fertilisation maintain the hybrid form,
while some few others come more like the seed parent, and one or other
individual approaches the pollen parent. This, however, is not the case
with all hybrids without exception. With some the offspring have more
nearly approached, some the one and some the other, original stock,
or they all incline more to one or the other side; while with others
_they remain perfectly like the hybrid_ and continue constant in their
offspring. The hybrids of varieties behave like hybrids of species, but
they possess greater variability of form and a more pronounced tendency
to revert to the original type.
With regard to the form of the hybrids and their development, as a rule
an agreement with the observations made in _Pisum_ is unmistakable. It
is otherwise with the exceptional cases cited. Gärtner confesses even
that the exact determination whether a form bears a greater resemblance
to one or to the other of the two original species often involved
great difficulty, so much depending upon the subjective point of view
of the observer. Another circumstance could, however, contribute to
render the results fluctuating and uncertain, despite the most careful
observation and differentiation; for the experiments plants were mostly
used which rank as good species and are differentiated by a large
number of characters. In addition to the sharply defined characters,
where it is a question of greater or less similarity, those characters
must also be taken into account which are often difficult to define
in words, but yet suffice, as every plant specialist knows, to give
the forms a strange appearance. If it be accepted that the development
of hybrids follows the law which is valid for _Pisum_, the series
in each separate experiment must embrace very many forms, since the
number of the components, as is known, increases with the number of
the differentiating characters in _cubic ratio_. With a relatively
small number of experimental-plants the result therefore could only be
approximately right, and in single cases might fluctuate considerably.
If, for instance, the two original stocks differ in seven characters,
and 100 and 200 plants were raised from the seeds of their hybrids to
determine the grade of relationship of the offspring, we can easily see
how uncertain the decision must become, since for seven differentiating
characters the combination series contains 16,384 individuals under
2187 various forms; now one and then another relationship could assert
its predominance, just according as chance presented this or that form
to the observer in a majority of cases.
If, furthermore, there appear among the differentiating characters at
the same time dominant characters, which are transferred entire or
nearly unchanged to the hybrids, then in the terms of the developmental
series that one of the two original stocks which possesses the
majority of dominant characters must always be predominant. In the
experiment described relative to _Pisum_, in which three kinds of
differentiating characters were concerned, all the dominant characters
belonged to the seed parent. Although the terms of the series in their
internal composition approach both original stock plants equally,
in this experiment the type of the seed parent obtained so great
a preponderance that out of each sixty-four plants of the first
generation fifty-four exactly resembled it, or only differed in one
character. It is seen how rash it may be under such circumstances to
draw from the external resemblances of hybrids conclusions as to their
internal nature.
Gärtner mentions that in those cases where the development was regular
among the offspring of the hybrids the two original species were not
reproduced, but only a few closely approximating individuals. With
very extended developmental series it could not in fact be otherwise.
For seven differentiating characters, for instance, among more than
16,000 individuals--offspring of the hybrids--each of the two original
species would occur only once. It is therefore hardly possible that
these should appear at all among a small number of experimental plants;
with some probability, however, we might reckon upon the appearance in
the series of a few forms which approach them.
We meet with an _essential difference_ in those hybrids which remain
constant in their progeny and propagate themselves as truly as the pure
species. According to Gärtner, to this class belong the _remarkably
fertile hybrids_ _Aquilegia atropurpurea canadensis_, _Lavatera
pseudolbia thuringiaca_, _Geum urbano-rivale_, and some _Dianthus_
hybrids; and, according to Wichura, the hybrids of the Willow species.
For the history of the evolution of plants this circumstance is of
special importance, since constant hybrids acquire the status of
new species. The correctness of this is evidenced by most excellent
observers, and cannot be doubted. Gärtner had opportunity to follow
up _Dianthus Armeria deltoides_ to the tenth generation, since it
regularly propagated itself in the garden.
With _Pisum_ it was shown by experiment that the hybrids form egg and
pollen cells of _different_ kinds, and that herein lies the reason
of the variability of their offspring. In other hybrids, likewise,
whose offspring behave similarly we may assume a like cause; for
those, on the other hand, which remain constant the assumption appears
justifiable that their fertilising cells are all alike and agree with
the foundation-cell [fertilised ovum] of the hybrid. In the opinion of
renowned physiologists, for the purpose of propagation one pollen cell
and one egg cell unite in Phanerogams[46] into a single cell, which
is capable by assimilation and formation of new cells to become an
independent organism. This development follows a constant law, which
is founded on the material composition and arrangement of the elements
which meet in the cell in a vivifying union. If the reproductive cells
be of the same kind and agree with the foundation cell [fertilised
ovum] of the mother plant, then the development of the new individual
will follow the same law which rules the mother plant. If it chance
that an egg cell unites with a _dissimilar_ pollen cell, we must then
assume that between those elements of both cells, which determine
the mutual differences, some sort of compromise is effected. The
resulting compound cell becomes the foundation of the hybrid organism,
the development of which necessarily follows a different scheme from
that obtaining in each of the two original species. If the compromise
be taken to be a complete one, in the sense, namely, that the hybrid
embryo is formed from cells of like kind, in which the differences are
_entirely and permanently accommodated_ together, the further result
follows that the hybrids, like any other stable plant species, remain
true to themselves in their offspring. The reproductive cells which are
formed in their seed vessels and anthers are of one kind, and agree
with the fundamental compound cell [fertilised ovum].
[46] In _Pisum_ it is placed beyond doubt that for the formation of
the new embryo a perfect union of the elements of both fertilising
cells must take place. How could we otherwise explain that among
the offspring of the hybrids both original types reappear in equal
numbers and with all their peculiarities? If the influence of the
egg cell upon the pollen cell were only external, if it fulfilled
the _rôle_ of a nurse only, then the result of each artificial
fertilisation could be no other than that the developed hybrid
should exactly resemble the pollen parent, or at any rate do so very
closely. This the experiments so far have in no wise confirmed. An
evident proof of the complete union of the contents of both cells is
afforded by the experience gained on all sides that it is immaterial,
as regards the form of the hybrid, which of the original species is
the seed parent or which the pollen parent.
With regard to those hybrids whose progeny is _variable_ we may perhaps
assume that between the differentiating elements of the egg and pollen
cells there also occurs a compromise, in so far that the formation of a
cell as foundation of the hybrid becomes possible; but, nevertheless,
the arrangement between the conflicting elements is only temporary and
does not endure throughout the life of the hybrid plant. Since in the
habit of the plant no changes are perceptible during the whole period
of vegetation, we must further assume that it is only possible for
the differentiating elements to liberate themselves from the enforced
union when the fertilising cells are developed. In the formation of
these cells all existing elements participate in an entirely free and
equal arrangement, in which it is only the differentiating ones which
mutually separate themselves. In this way the production would be
rendered possible of as many sorts of egg and pollen cells as there are
combinations possible of the formative elements.
The attribution attempted here of the essential difference in the
development of hybrids to _a permanent or temporary union_ of the
differing cell elements can, of course, only claim the value of an
hypothesis for which the lack of definite data offers a wide field.
Some justification of the opinion expressed lies in the evidence
afforded by _Pisum_ that the behaviour of each pair of differentiating
characters in hybrid union is independent of the other differences
between the two original plants, and, further, that the hybrid
produces just so many kinds of egg and pollen cells as there are
possible constant combination forms. The differentiating characters
of two plants can finally, however, only depend upon differences in
the composition and grouping of the elements which exist in the
foundation-cells [fertilised ova] of the same in vital interaction[47].
[47] “_Welche in den Grundzellen derselben in lebendiger
Wechselwirkung stehen._”
Even the validity of the law formulated for _Pisum_ requires still to
be confirmed, and a repetition of the more important experiments is
consequently much to be desired, that, for instance, relating to the
composition of the hybrid fertilising cells. A differential [element]
may easily escape the single observer[48], which although at the outset
may appear to be unimportant, may yet accumulate to such an extent
that it must not be ignored in the total result. Whether the variable
hybrids of other plant species observe an entire agreement must also
be first decided experimentally. In the meantime we may assume that in
material points a difference in principle can scarcely occur, since the
unity in the developmental plan of organic life is beyond question.
[48] “_Dem einzelnen Beobachter kann leicht ein Differenziale
entgehen._”
In conclusion, the experiments carried out by Kölreuter, Gärtner,
and others with respect to _the transformation of one species into
another by artificial fertilisation_ merit special mention. A special
importance has been attached to these experiments, and Gärtner reckons
them among “the most difficult of all in hybridisation.”
If a species _A_ is to be transformed into a species _B_, both must be
united by fertilisation and the resulting hybrids then be fertilised
with the pollen of _B_; then, out of the various offspring resulting,
that form would be selected which stood in nearest relation to _B_ and
once more be fertilised with _B_ pollen, and so continuously until
finally a form is arrived at which is like _B_ and constant in its
progeny. By this process the species _A_ would change into the species
_B_. Gärtner alone has effected thirty such experiments with plants of
genera _Aquilegia_, _Dianthus_, _Geum_, _Lavatera_, _Lychnis_, _Malva_,
_Nicotiana_, and _Œnothera_. The period of transformation was not alike
for all species. While with some a triple fertilisation sufficed,
with others this had to be repeated five or six times, and even in
the same species fluctuations were observed in various experiments.
Gärtner ascribes this difference to the circumstance that “the specific
[_typische_] force by which a species, during reproduction, effects
the change and transformation of the maternal type varies considerably
in different plants, and that, consequently, the periods within which
the one species is changed into the other must also vary, as also the
number of generations, so that the transformation in some species is
perfected in more, and in others in fewer generations.” Further, the
same observer remarks “that in these transformation experiments a good
deal depends upon which type and which individual be chosen for further
transformation.”
If it may be assumed that in these experiments the constitution of
the forms resulted in a similar way to that of _Pisum_, the entire
process of transformation would find a fairly simple explanation.
The hybrid forms as many kinds of egg cells as there are constant
combinations possible of the characters conjoined therein, and one
of these is always of the same kind as the fertilising pollen cells.
Consequently there always exists the possibility with all such
experiments that even from the second fertilisation there may result a
constant form identical with that of the pollen parent. Whether this
really be obtained depends in each separate case upon the number of
the experimental plants, as well as upon the number of differentiating
characters which are united by the fertilisation. Let us, for
instance, assume that the plants selected for experiment differed in
three characters, and the species _ABC_ is to be transformed into the
other species _abc_ by repeated fertilisation with the pollen of the
latter; the hybrids resulting from the first cross form eight different
kinds of egg cells, viz.:
_ABC_, _ABc_, _AbC_, _aBC_, _Abc_, _aBc_, _abC_, _abc_.
These in the second year of experiment are united again with the pollen
cells _abc_, and we obtain the series
_AaBbCc_ + _AaBbc_ + _AabCc_ + _aBbCc_ + _Aabc_ + _aBbc_ + _abCc_ +
_abc_.
Since the form _abc_ occurs once in the series of eight components,
it is consequently little likely that it would be missing among the
experimental plants, even were these raised in a smaller number,
and the transformation would be perfected already by a second
fertilisation. If by chance it did not appear, then the fertilisation
must be repeated with one of those forms nearest akin, _Aabc_, _aBbc_,
_abCc_. It is perceived that such an experiment must extend the farther
_the smaller the number of experimental plants and the larger the
number of differentiating characters_ in the two original species;
and that, furthermore, in the same species there can easily occur a
delay of one or even of two generations such as Gärtner observed.
The transformation of widely divergent species could generally only
be completed in five or six years of experiment, since the number of
different egg cells which are formed in the hybrid increases in square
ratio with the number of differentiating characters.
Gärtner found by repeated experiments that the respective period of
transformation varies in many species, so that frequently a species
_A_ can be transformed into a species _B_ a generation sooner
than can species _B_ into species _A_. He deduces therefrom that
Kölreuter’s opinion can hardly be maintained that “the two natures
in hybrids are perfectly in equilibrium.” It appears, however, that
Kölreuter does not merit this criticism, but that Gärtner rather has
overlooked a material point, to which he himself elsewhere draws
attention, viz. that “it depends which individual is chosen for further
transformation.” Experiments which in this connection were carried out
with two species of _Pisum_ demonstrated that as regards the choice of
the fittest individuals for the purpose of further fertilisation it
may make a great difference which of two species is transformed into
the other. The two experimental plants differed in five characters,
while at the same time those of species _A_ were all dominant and
those of species _B_ all recessive. For mutual transformation _A_
was fertilised with pollen of _B_, and _B_ with pollen of _A_, and
this was repeated with both hybrids the following year. With the
first experiment _B_/_A_ there were eighty-seven plants available in
the third year of experiment for the selections of individuals for
further crossing, and these were of the possible thirty-two forms;
with the second experiment _A_/_B_ seventy-three plants resulted,
which _agreed throughout perfectly in habit with the pollen parent_;
in their internal composition, however, they must have been just as
varied as the forms of the other experiment. A definite selection was
consequently only possible with the first experiment; with the second
some plants selected at random had to be excluded. Of the latter only
a portion of the flowers were crossed with the _A_ pollen, the others
were left to fertilise themselves. Among each five plants which were
selected in both experiments for fertilisation there agreed, as the
following year’s culture showed, with the pollen parent:--
1st Experiment. 2nd Experiment.
2 plants -- in all characters
3 " -- " 4 "
-- 2 plants " 3 "
-- 2 " " 2 "
-- 1 plant " 1 character
In the first experiment, therefore, the transformation was completed;
in the second, which was not continued further, two more fertilisations
would probably have been required.
Although the case may not frequently occur that the dominant characters
belong exclusively to one or the other of the original parent plants,
it will always make a difference which of the two possesses the
majority. If the pollen parent shows the majority, then the selection
of forms for further crossing will afford a less degree of security
than in the reverse case, which must imply a delay in the period of
transformation, provided that the experiment is only considered as
completed when a form is arrived at which not only exactly resembles
the pollen plant in form, but also remains as constant in its progeny.
Gärtner, by the results of these transformation experiments, was led to
oppose the opinion of those naturalists who dispute the stability of
plant species and believe in a continuous evolution of vegetation. He
perceives in the complete transformation of one species into another
an indubitable proof that species are fixed within limits beyond which
they cannot change. Although this opinion cannot be unconditionally
accepted we find on the other hand in Gärtner’s experiments a
noteworthy confirmation of that supposition regarding variability of
cultivated plants which has already been expressed.
Among the experimental species there were cultivated plants, such as
_Aquilegia atropurpurea_ and _canadensis_, _Dianthus caryophyllus_,
_chinensis_, and _japonicus_, _Nicotiana rustica_ and _paniculata_, and
hybrids between these species lost none of their stability after four
or five generations[49].
[49] [The argument of these two last paragraphs appears to be that
though the general mutability of natural species might be doubtful,
yet among cultivated plants the transference of characters may be
accomplished, and may occur by integral steps until one species is
definitely “transformed” into the other.]