In Pursuit of the Gene

176 ¨ SEX CHROMOSOMES
loid set of chromosomes, producing a new parthenogenetic egg, and the
cycle began anew.
After so brilliantly divining the secrets of the phylloxeran chromosomes,
there was something magnificently consistent in Morgan’s refusal to
abandon his cytoplasmic theory of sex determination. Because the transition
from parthenogenetic female to sexual female did not involve a change
in chromosome number, Morgan now argued, the loss of chromosomes
need not necessarily be the cause of the sex of the males. In fact, he suggested,
it could be just the other way around: the loss of the two sex chromosomes
might actually be a consequence of the cells being already male. 41
Though his argument was defensible, his heart no longer seemed to be
quite so much in opposing, and in the end Morgan granted, “The chromosomal
relations, however, remain still a fact and a very extraordinary fact. It
is hard to conceive that these relations have no connection with sex determination.”
42 Contrary to his deepest wishes, Morgan had finally succeeded
in clearing the path for the chromosome interpretation by reconciling a set
of seemingly irreconcilable facts. 43
¨ WHILE PURSUING the Phylloxera study, Morgan had been moving forward
on several other fronts, most importantly with research on De Vries’s
mutation theory, in which he saw the potential for creative evolutionary
change. He had visited De Vries’s experimental garden in 1902 and been
greatly impressed by the mutating Oenothera, but he had missed De Vries
himself. 44 In his 1903 Evolution and Adaptation, Morgan had expressed his excitement
over the mutation theory, arguing that natural selection could not
possibly have played the central role in evolution that Darwin claimed
for it. As Morgan saw it, it was the mutations themselves that dictated the
direction of evolution, not the selection of minute differences. 45 While
De Vries had granted that selection had the ability to pick and chose among
the new mutant forms, Morgan denied it any formative role at all, allowing
only that selection might eliminate mutations that were positively
harmful.
The two men seem to have met for the first time in California during
the summer of 1904 when De Vries was lecturing in the Berkeley summer