In Pursuit of the Gene

SEX CHROMOSOMES © 171
Not only had Stevens independently discovered a theory that was strikingly
like Wilson’s idiochromosome theory, but she had provided a crucial
piece of confirming evidence that Wilson had lacked, and immediately after
receiving her manuscript he devoted himself to an extended review of the
cytological slides that were the basis of the claim that males in Anasa contained
an even number of chromosomes. 19 Although he did not directly acknowledge
that he had read her results and that they had influenced his
thinking, Wilson did add a second footnote acknowledging Stevens’s independent
discovery of the fact that there were two types of sperm distinguished
by the presence or absence of the small chromosome, as well as
what he referred to as “the significant fact” that the small chromosome was
found in all the somatic cells of the male, while the female somatic cells
contained only matched pairs of larger chromosomes. 20
In October 1905, Morgan wrote Driesch that Wilson’s new findings
“make it look at first sight as though the chromosomes were the thing.”
But upon reflection he had realized that the logic of the paper “lands you
in an absurdity.” 21 Though he did not spell out the details, Morgan had indeed
discovered a flaw in Wilson’s argument that followed from his assumption
that sex chromosomes conferred specific male and female qualities.
22 When he realized the flaw in Wilson’s theory, Morgan continued, he
had felt a great relief. Once again comfortable in his conviction that chromosomes
were not the sine qua non, Morgan argued that the simplest way
to account for the fact that there were two kinds of sperm “was to assume
that there were two kinds of protoplasm.” 23
In the meantime, however, Morgan had forged ahead with his own
study of sex determination on a different track. Already in the spring of
1905, Morgan had begun collecting various species of an aphid-like insect
group known as the Phylloxera. As was the case with aphids, a colony was begun
by a single insect, which hatched from a winter egg. This “stem
mother” then reproduced asexually, giving rise to a generation of winged
migrants, also parthenogenetic, that laid male and female eggs. Finally, the
union of the resulting male and female insects gave rise once again to a
winter egg, out of which would hatch a new parthenogenetic stem mother
the following spring. But what excited Morgan about the phylloxerans,