In Pursuit of the Gene

SEX CHROMOSOMES © 173
theory of sex determination had also led Morgan to begin to question the
basic assumption of Mendelism. In particular, the recent finding in mice
that it was impossible to isolate a pure-breeding yellow-coated mouse led
him to question the entire notion of the purity of the germ cells. 30 He began
to doubt that it was possible to cleanly extract the dominant factor
from a recessive, and that the idea that factors combined to give a 3:1 ratio
was only a convenient fiction. His dictum, as he explained it to his friend
Davenport, was “once crossed always mixed,” and he insisted that there was
“no evidence to show that pure extracted races do not carry latent characters,
and a good deal of evidence to show that they do.” 31 As in the case of
the two sexes, he believed that the contrasting characters coexisted, and local
conditions determined which was expressed.
Swimming directly against the ever-strengthening tide of Mendelism,
he also attacked the stability of the Mendelian factors in his address to
a conference on sex determination in December 1906. 32 As he had suggested
in regard to sex determination, the view that characters resided fully
formed in the embryo was an anachronism. It did not make sense to treat
them as “entities that could be shuffled, but seldom get mixed.” Continuing
with the metaphor, he rejected the idea that with “each new deal the characters
are separated, one germ cell getting one character and another the
contrasted characters.” 33 With an uncanny precision, he put his finger on
the formal properties of Mendel’s factors that made possible an exact theory
of heredity—that they could live together for countless generations
without producing any influence on each other—and then proceeded to
reject them without a pang of doubt. In fact, it hardly seemed to faze him
that he now stood opposed to the three major developments in contemporary
biology—Darwinism, the chromosome theory of sex determination,
and Mendelism. It was this immunity to the prevailing opinions and, in certain
cases, to the power of logic to persuade, that would lead some of his
colleagues and students to despair.
The following year Driesch was slated to be the lead speaker in the
session on experimental zoology at the Seventh International Zoological
Conference, which was held in Boston, but he had withdrawn at the last
minute because his wife had fallen ill. “Embryology and Experimental Em-