The Combinatory of Parental Characters
Koelreuter's conception of hybrid production is inseparable from his account of normal generation. He rejected ovist and spermist theories of preformation; he also rejected Linnæus' theory of cortical and medullar layers—views he considered more clever than correct.
According to Koelreuter, two homogenous fluids of different kinds determined by the Creator for union with one another join to produce organization. The male agent in flowering plants is a product of the pollen grain; the female agent, Koelreuter believed initially, is a sticky secretion of the stigma. These fluids, male and female, differ essentially; that is, "the force of one must be different from the force of the other." From the union of these two fluids results another mean fluid, with a mean force compounded from the two simple forces. This purposive agent which emerges from the organization of the reproductive fluid as a whole is the source of organization for the future plant. For each class of organized beings, a specific compound force produces a determinate structure and specific nature.
Although Koelreuter made many observations and experiments concerning the process of fertilization, the mass of quantitative evidence he accumulated to support theories of epigenesis, equality of parental contributions, and sterility of interspecific offspring constitutes the novelty of his contribution to late 18th-century generational theory.
Koelreuter's work contained an important ambiguity concerning the source of organization in the living body. It is impossible to determine unequivocally whether the source of organization is material or nonmaterial. Koelreuter draws an analogy between the union of two seed materials and the production of salt crystals. When acid and alkaline substances unite, a third, intermediate salt results. In the same way, he argues, the intermediate fluid resulting from the union of male and female seed materials either constitutes the origin or the firm foundation of the vital machine, or produces this vital machine out of itself. Neither the male nor the female seed matter suffices to produce this result by itself, no more than an acid or alkaline substance can in and of itself produce the intermediate salt or form crystals. The formation of the plant requires both the compound of two specific seed materials and the composite active and purposive force resulting from that compound.
Nature works in the same way to produce a cross between two species. Once the male and female seed matters unite, formation proceeds rapidly from the nucleus to the flower, and in the process
the sharpest eye can find no more imperfection than in the natural plant. The resulting bastard, composed from the seed matter of two separate species, reflects twofold nature in both its intermediate form and its absolute or relative infertility.
When Koelreuter first worked to produce a cross between Nicotiana paniculata and N. rustica , he experimented with many flowers. Each time his fertilization succeeded. The result was perfect, somewhat divergent seeds. From 110 seeds he produced 78 plants; of these, he kept 21 over the winter, and in March 1761 they flowered. In the spread of their branches, situation and color of the flower, and individual floral parts, each plant exhibited a mean between the two natural parental species. In repetitions of these experiments, the first-generation bastards consistently exhibited characteristics in "almost geometrical proportion" between differences in the parental species. Koelreuter tabulated his measurements:
Experiments with seven other genera supported the inference of intermediate size in first-generation hybrids. Koelreuter found that even the time of flowering and the odor of hybrid offspring were intermediate between the characteristics of the parents.
The intermediate character of first-generation hybrids, Kolereuter argued, supported the Aristotelian doctrine of reproduction by means of two seed matters. Koelreuter also used it to argue against the contemporary doctrine of generation, that is, "the doctrine of animalcula, or of original embryos and nuclei in the ovaries of animals and plants activated by male seed."
All offspring of the first cross, Nicotiana paniculata × N. rustica , were identical. When Koelreuter reversed the direction of the cross, N. rustica × N. paniculata , the offspring "agreed all together with the plants of the first experiment, and reacted in the same way to the experiments performed upon them." This phenomenon, now called the "identity of reciprocal crosses," also spoke to another contention about the contributions of parents to offspring. The hybrids with which naturalists were best acquainted, plant and animal, show a greater resemblance to the mother than to the father, leading naturalists to assume that this is a universal character of hybrids. Linnæus used the idea in his two-layer theory of generation: "a bastard offspring is with respect to its inner medullar [essential] substance the exact image of the mother, but in leaves and other outer [nonessential] parts [the image of] the father."
Koelreuter's reciprocal crosses and detailed measurements established, however, that parental contributions could not be distinguished so readily. Since Koelreuter did not come across any examples of sex-linked characters in his experimental plants, his inference that each parent contributed equally to an intermediate result was to an extent justifiable.
A second essential character of bastard offspring, fully as important as their intermediate form, is their absolute or relative infertility. Although the formation of the bastard tobacco parallels that of the natural species and its flowers are brilliant, the bastard plant is deficient in the most important character of all: fertility, the final cause of all formation. Koelreuter found the pollen containers of the Nicotiana bastard to be markedly smaller than in the natural species. What pollen they contained was white and dry; the grains did not cohere with one another as in the natural species. When examined under a microscope, the pollen grains proved to be irregular and shrunken. They contained scarcely any fluid, and most were empty husks. These observations led Koelreuter to doubt the fertility of the bastard plant; his experimental results reinforced this suspicion. Of the many flowers on the bastard plants, not one succeeded in bearing a single seed, even after dusting with a large quantity of the plant's own pollen: "instead of 50,000 [they] contained not a single one, and more than a thousand flowers, one after the other, fell without leaving a single capsule behind." In every sense of the word the tobacco proved to be "a true, and as far as I know, the first botanical mule produced by art."
Koelreuter thought infertility only a relative imperfection, which, from the point of view of ultimate consequences, proved to be a positive good. Nature wields sterility to preserve the order established at the Creation. "What an astonishing confusion would the peculiar and unchanged hybrid characters, and consistent fertility of such plants give rise to in nature? What a monstrous swarm of imperfections would they bear, and what evil and inevitable consequences would ensue?"
Koelreuter's pollination experiments, however, quickly convinced him that the infertility of the bastard was only relative. When pollinated with either of the parent species, the Nicotiana hybrid produced some ripe seed; much more was produced when plants of the
original natural species were self-pollinated. For each cross, Koelreuter raised ten plants from the bastard. The results were no longer intermediate between the two parent species, but measurably resembled the pollen parent. In Koelreuter's terminology, the plants were metamorphosed or transformed (his verb is verwandeln ): they again approached the fundamental nature (Grundwesen ) of which the initial crossing had deprived them. Again his measurements told the tale of transformation—in the size of plants; the spread of branches and flowers; and the form, size, and number of flowers.
Koelreuter's evidence for the relative infertility of hybrids and for the progressive reversion of hybrid offspring to the parental species parallels Georges Louis Leclerc Buffon's conjectures concerning animals published three years later. Under certain circumstances, Buffon maintained, a male mule can engender progeny and a female mule can conceive and give birth. Buffon also found a wide range in the productivity of different species of animals. This showed him that fertility is variable. Just as species vary in productivity, so, too, must hybrids—a conjecture that squared with Buffon's transformist views.
Koelreuter was less comfortable with the transformist position. Thus, when his experiments began to produce hybrids with varying degrees of fertility, he attempted to read the results as so many potential reversions to the parental species. He carefully constructed a table of his experimental results with respect to fertility. This was, he emphasized, not just another "useless, hasty, and absurd list of chimeric bastards," but the first systematic catalogue according to a theory of generation certified by experiment. Koelreuter's class of "perfect" bastards includes offspring of two or three natural species of a single genus; normally they are infertile in the highest degree, although some products of only two species prove fertile on the female side when pollinated by either parent, or were fertile to a
diminished degree on both sides when crossed with parental species. "Imperfect" bastards are offspring of two natural species produced when a tincture of pollen from the female supplemented pollen from the male. These offspring are characterized by a diminished degree of fertility on both sides. Finally, what Koelreuter called "varietal" bastards are completely fertile. Koelreuter denied that the parents were different species. He drew a forceful conclusion:
Such a bastard in the true sense is either wholly infertile, or at most in a very limited and unequally diminished degree, by comparison with the true natural species from which it was produced is fertile. On the other hand, a mere bastard variety retains the degree of fertility of its parents, or at least loses nothing observable of this. Thus, I regard the experiment of crossing [species] in every respect as the only true, certain, and infallible touchstone of all separate species and varieties.
Koelreuter also distinguished between first- and second-generation hybrids. At an early stage in the experimental series, for example, he noticed a marked contrast in uniformity and stability. First-generation bastards for any single cross are all alike—intermediate in form between the parental species and either wholly or relatively sterile. Second-generation hybrids, even when produced from a single ovary, tend to be less like the parental bastard and more like the grandparents. After he fertilized perfect bastards of the first generation with pollen from Nicotiana paniculata or from N. rustica , he classified the results of such back crosses as descending (to the natural maternal species) or ascending (to the natural paternal species). Later he succeeded in producing second-generation hybrids from the self-pollination of the tobacco hybrid. Thus, contrary to expectation, pollen from the hybrid tobacco finally proved to have a slight degree of fertility. In Dianthus and Mirabilis bastards he found a greater degree of fertility on both sides.
Koelreuter used a quantitative argument to account for these results. Crosses in which the two seed matters are in equal proportion produce second-generation bastards resembling the first-generation parent; crosses of seed matters in unequal proportions produce second-generation bastards resembling more or less closely one or the other original natural species, depending on which seed matter is dominant and to what degree. These three main groups correspond to the three segregating classes of second-generation hybrids in Mendel's experiments with plants differing in one essential character. In other words, Koelreuter had found roughly the three segregating classes for second-generation hybrids. Koelreuter found puzzling, however, the exception to this neat scheme. Among the characters of second-generation Mirabilis bastards, for instance, he saw reversions, completely fresh characters, and colors too varied to be classified.
The contrasting results between first- and second-generation bastards required only a simple adaptation of the theory of normal reproduction. In any given plant the process of formation liberates the compound matter that gave rise to the form of the plant, and divides this compound once again into the two original matters, concentrated in the ovules and the pollen grains. In any natural fertilization the two seed matters and the simple forces inherent in them unite in equal proportions to form the intermediate product with its corresponding compound force, hypostatized during the process of generation in the specific characters of external form. In some original plant species the outer characters are very different, in others very similar; likewise, the seed matters and simple forces of natural species display differing degrees of affinity. Between species with little affinity—where characters and simple forces differ significantly—no crosses can take place. In cases of close affinity, where there is "no slight resemblance between its parents and a suitable agreement of their natures," crosses can occur, although the Creator had not intended the two ground matters to unite.
Both vital functions in first-generation plant bastards differ from the norm: enhanced vegetative function and completely or partially curtailed reproductive function. Where infertility is not absolute, the next generation of offspring shows an unnaturally wide range of variability. The measurable intermediacy that characterizes all perfect bastards of the first generation characterizes only a small number of individuals in the next generation, the product of seed matters united in equal proportions. More often, however, mixture and union of seed matters do not proceed with the regularity typical of natural products and simple bastards. The principle of equality is broken; the seed matters combine in different proportions, and "All kinds of wrong paths result."
Two routes are open to nature according to Koelreuter. On one, "where she has the laws of close affinity as a guide, she again approaches the high road with something like a straight line; on the other path, where she lacks this guide, she strays . . . ever more from the high road." Here again Koelreuter's speculation parallels the best-informed contemporary opinion in zoology. In a discussion of sheep, goats, dogs, and domestic fowl the St. Petersburg academician Peter Simon Pallas noted that inconsistency of form, once introduced, increased from one generation to the next. Pallas supposed it to result from a vice introduced into the generative faculties of the original species by way of crosses. This vice, he said, parallels in effect the alteration of fluids and solids in a living body under the influence of a miasma.
The presence of essential characters resembling the original natural species was Koelreuter's key to the contrast between first- and second-generation hybrids. He concluded that although new varieties might arise through selfing, plant bastards do not establish the new and constant species Linnæus had predicted. Eventually hybrid races will revert to one or the other of the original natural species.
Offspring of Nicotiana rustica × N. paniculata are intermediate in form, produce sterile pollen, and are only somewhat fertile on
the female side. After pollination with N. paniculata , the plants grown from the seed resemble the pollen parent far more than the first generation. Koelreuter measured the essential likenesses with his usual care: situation, form, and substance of the leaves; number of leafless slender branches; shape and size of the corolla; form, color, and breadth of the flower; and form, size, and external perfection of the capsules.
The next summer (1762) Koelreuter pollinated this second generation with N. paniculata , and in 1763 he sowed 128 apparently fertile seeds, most of which germinated. Of this crop he retained 12 plants; when they bloomed they so resembled paniculata "that one could only have differentiated them with difficulty had they not been labeled with separate numbers." Moreover, the fertility of both pollen and seed had increased noticeably, although this was subject to fluctuations. Nicotiana paniculata was well on the way toward dominating rustica , and Koelreuter foresaw that this series of experiments would end by producing true N. paniculata . "In a word," he wrote, "I no longer have the least doubt of the possibility of transforming one natural species into another."
In the summer of 1765, four years after he had initiated this experimental series, Koelreuter achieved the first complete transformation of N. rustica into Nicotiana paniculata in the fifth generation, or, as he put it, in "the fourth ascending degree." It was as if "he had seen a cat emerge in the shape of a lion." A comparable reversion in the direction of the original mother plant, which he had followed in a parallel series of experiments, would, he predicted, require a different although proportional number of generations. He was also able to announce the successful transformation of Dianthus and Mirabilis . From a cross AB indistinguishable from the natural species B,
Koelreuter had thus produced offspring B. Since the maternal plant of AB was A, he reasoned that he had thereby transformed species A into species B. Moreover, he showed that the number of generations required for a successful transformation varied from species to species. He concluded that the transformation of "one plant into another [occurs according] to the greater or lesser degree of fertility [of] the bastards produced from their equality."
Transmutation of metals served as an analogy for this experimental series. In theory one metal could be transformed into another by taking from it particular properties, conceived as so many independent substances, and substituting other properties for them. Mercury, for example, could be ennobled by removing the two characters upon which its fluidity and volatility rested and substituting other characters for them. In the process two seed matters were needed: the male agent, of a sulfuric nature, possessed the force to make the fluid mercurial female seed matter capable of resisting fire and forming a stable body. Sulfur, in other words, transformed the nature of the mercurial body. The result was a true metamorphosis in which the male, sulfuric agent asserted its superiority (Uebergewicht ) over the mercurial agent.
Koelreuter believed he had achieved the alchemist's dream in botany in the course of a few years. In plants the male seed matter is oily and sulfuric; union with the female seed matter produces a stable organic body, "the initial basis of the future plant." In succeeding generations the male agent gradually takes the upper hand, and in the end the nature of the female has been wholly transformed. The transformation depends on "close affinity, fertility not wholly suppressed in the production of bastards, and dominance in a certain degree."
Koelreuter used his success to argue for the possibility of the transformation of metals. Likewise, one species of animal can probably be transformed into another. From a canary might come a linnet. Experiment had already established that the female retains
fertility in "the second descending degree," and it was therefore probable that she would do so in the "second ascending degree."
Central in Koelreuter's reasoning was the contention that a measurable change of characters in the product of any unnatural union reflects the proportions of the two seed matters. Variability increases in the offspring of bastards since the number of possible combinations of the two seed matters is infinite: that thread of Ariadne, natural affinity, is lacking. Koelreuter did see limits to variation, however. By means of crosses a naturalist can transform one affine species into another, or produce one of the infinite number of possible variations between the forms of the two natural species, but cannot produce wholly new species with entirely new characters. The naturalist thus transforms natural bodies by altering or removing particular characters and substituting others.
Transformations of this kind refuted monoparental heredity, and they showed that crossing was a powerful instrument for change in the world of living forms. But how was change itself interpreted? Koelreuter read his experiments against a conceptual tradition that did not permit development away from the original natural forms. He considered bastard plants artificial products, able to survive only in artificial conditions. "In the orderly arrangement and ordinary situation," he wrote, "established by nature in the plant kingdom, bastard plants would be difficult to produce or even to initiate." Even if we suppose the possibility of a true bastard plant in an open field, "the question would remain whether this chance had not taken place in a region where the natural situation as a whole, either mediately or immediately, had been destroyed or changed: true wilderness as it comes from the hand of nature is one thing; a field, free but in respect to a hundred things often very much altered by the hand of man, is another."
Consider Verbascum , for which the probability of natural crosses is great. Koelreuter did not believe that unnatural products could transform the natural species, even when, as in Verbascum , the
natural species cross regularly. In an open field, where its own and alien pollen reach the stigmata at approximately the same time, a plant will accept the male agent intended for it by nature, excluding the alien matter from fertilization. Neither ancients nor moderns spoke of bastard Verbascum in the field. Linnæus, it is true, mentioned a Verbascum bastard, but this was likely a product of the unnatural conditions in the Uppsala botanical garden. Koelreuter continued, "It is to be wished, nonetheless, that Herr von Linné had given us a more careful description, and more according to nature than according to his fantastic theory of generation, which contradicts nature."
As a naturalist, Koelreuter may have sensed the limits and assumptions of his conceptual tradition, but his was a stubborn defense of the traditional essentialist concept of a species. Koelreuter used his experiments to throw light on the nature of the species as a whole, not the inheritance of individual characters. He set out to prove that a cross between two species does not produce new and constant species. We may imagine that his findings were in this respect a relief. He regarded the essence of species as monolithic; the intermediacy that characterized all first generation bastards confirmed this interpretation. However, second-generation and back-crossed hybrids did not lend themselves to the same neat explanations. Here Koelreuter took comfort from the predominance of forms resembling the original natural species; from this he concluded that hybrid offspring will revert sooner or later to one or the other of the original species. They cannot, he thought, from new and constant species.