492 i Flora Unveiled
Darwin’s belief that the variability in the F 2 generation is caused by the trauma of hybrid-
ization is reminiscent of Henschell’s discredited view that Koelreuter’s hybrids were actu-
ally monsters caused by emasculation of the flowers and root stress induced by growing the
plants in pots. Even genius has its limits, and Darwin was no closer to an explanation of
“degeneration” than any of his predecessors.
Meanwhile, the obscure monk Gregor Mendel was laboring away in his garden in the
Augustinian monastery in Brünn, the provincial capital of what was then Moravia, Austria.
Mendel had been carrying out crossing experiments with peas to try to answer the same
questions about the causes of variability that Darwin was struggling with in his Origin of
Species. But whereas Darwin thought of variability in physiological terms, Mendel, who had
studied combination theory in mathematics— a method for determining all the possible
arrangements of any group of objects— took a statistical approach to the problem. He had
begun by studying albinism in mice, crossing wild- type mice with albinos and observing
the colors of the progeny. However, after being reprimanded by the local bishop, Anton
Ernst Schaffgotsch, he switched to plants. Bishop Schaffgotsch thought it unseemly and
“unnecessarily titillating for a priest who had taken vows of chastity and celibacy” to be
spending his time watching mice copulate.^21 “I turned from animal breeding to plant breed-
ing,” Mendel later explained with a sly smile, because “the bishop did not understand that
plants also have sex.”^22
To apply his statistical methods to the study of heredity, Mendel divided heredity into
discrete, physical units that could form different combinations with each other. Such a
“particulate” theory of heredity represented a marked departure from the essentialist doc-
trine that envisioned crossing as the mixing of miscible fluids. For this approach to work,
Mendel’s hereditary units had to be distinct from each other and stable in their character-
istics. After poring over the studies of Gaertner and other plant hybridizers, he chose peas
as his experimental material because he was able to identify several phenotypic traits that
behaved like his ideal stable hereditary units.
In a nutshell, Mendel’s crossing experiments led him to discover the rules that govern the
appearances of both the F 1 and F 2 generations. The hereditary material was not a uniform
essence, but a collection of individual hereditary units. These units were later called genes.
The pea genes Mendel studied existed in different forms, called alleles, which determined
particular traits, such as smooth versus wrinkled peas. Crossing peas with different traits
led to a combination of the hereditary units of the male and female parents, and the progeny
(F 1 generation) all contained the same combination. However, when the F 1 generation was
selfed, the hereditary units all came apart and formed new combinations. Because some
of the alleles were dominant and some recessive,^23 the new combinations led to phenotypic
variability. In modern terms, we say that Mendel discovered the two basic principles of the
segregation and independent assortment of alleles during sexual reproduction.^24
It is tantalizing to imagine what direction Darwin’s work would have taken had he been
aware of Mendel’s results, which were originally published in 1866, only seven years after the
Origin of Species. It is likely that he would have immediately realized that Mendel’s new laws
governing inheritance could explain the injurious effects of selfing as the accumulation and
subsequent expression of deleterious recessive alleles that are not expressed in the progeny of
outcrossed plants. This is thought to be one of the reasons why outcrossing was selected for