Wars of the Roses j 419419 419
of pure male or female semen that produced traits closely resembling those of the parent.
However erroneous, the “incomplete blending” theory was an ingenious solution to the
problem, consistent with the facts as they were then understood. In a way, it anticipated
Gregor Mendel’s concept of the “particulate” nature of inheritance in peas, the difference
being that Mendel’s interpretation allowed predictions to be made about the progeny of a
cross, whereas Koelreuter’s “accidental” model did not.
A detailed discussion of Koelreuter’s numerous contributions to the sexual theory is
beyond the scope of this discussion, but it is worth noting a few other crossing experiments
because of their importance to the history of genetics.^29 For example, Koelreuter carried out
what we now refer to as a reciprocal cross to demonstrate that the hybrids between Nicotiana
paniculata and Nicotiana rustica were identical regardless of which parents served as the
male or female. This experiment effectively demolished the doctrine of preformationism—
both its ovist and spermist versions. It also disproved Linnaeus’s idea that the female con-
tributed the medulla (pith) of the plant while the male contributed the cortex.
Although most of Koelreuter’s hybrids were sterile, to his surprise a few were self- fertile.
However, even the hybrids with sterile pollen, such as N. paniculata × N. rustica, were fer-
tile if crossed to one of its parents, provided the parent served as the pollen donor. This
phenomenon allowed him to perform what is now referred to as a “backcross experiment”
between, for example, the N. paniculata × N. rustica hybrid and either N. paniculata or
N. rustica as the pollen donor species. Using the language of alchemy applied to metals,
Koelreuter’s claimed that repeated backcrosses caused the “transmutation” of the hybrid
back to the parental type.
Koelreuter made good use of those few hybrids he produced that were self- fertile by cross-
ing them with each other. According to his theory of inheritance based on the blending
of uniform liquids, the initial hybrid (referred to in modern parlance as the “first filial”
or “F 1 ” generation) should contain a uniform mixture of the two essences of the parental
species. If an F 1 plant is crossed with another F 1 plant (i.e., selfed), the next generation is
called the “second filial” or “F 2 generation.” Based on Koelreuter’s uniform liquid theory of
inheritance, mixing two identical homogenous mixtures should produce the same homog-
enous mixture. But this is not what Koelreuter observed when he selfed two hybrids. The
F 2 generation was much more variable than the F 1 generation, and, remarkably, some of the
progeny even reverted to their parental types. This was equivalent to mixing two cans of
identical green paint and having the mixture spontaneously separate into layers of yellow
and blue! Koelreuter was completely baffled:This much is ... quite clear, that matters must be rather uneven or disorderly in the
selfing of hybrids; indeed, it seems as if it would occasionally lead to the production
of monstrosities.Koelreuter went on to note the tendency of the F 2 generation to restore “the original
natural [parental] form and fertility.” Koelreuter had unwittingly recreated the phenom-
enon of the “degeneration” of olive and other cultivated fruit trees into wild forms when
propagated by seed, which had been recorded since ancient times. One hundred years later,
Mendel, who adopted a particulate rather than a fluid model for the hereditary material,
based his first two laws of inheritance, the Law of Segregation and the Law of Independent
Assortment, on similar experimental results in peas.