In Pursuit of the Gene

178 ¨ SEX CHROMOSOMES
next summer he found that centrifuging insect eggs seemed to have produced
a fly with abnormal wings, and, moreover, the deformed wings were
passed to the next generation. In a letter to Driesch, he expressed a near
certainty that it was the centrifuging that had resulted in the altered fly.
While he could not rule out other causes, the significance of the result had
not escaped him: one way or another he had managed to isolate a mutation
in a fruit fly. 51
A year after the isolation of the mutant with the deformed wings, Morgan
started to work more with the fruit fly Drosophila, which was in many
ways the ideal animal for mutation work. It was easy to grow and easy to
test, the two conditions that De Vries, who took it as an article of faith that
all organisms were subject to brief but intense periods of mutation, had
considered essential for mutation studies. 52 It could be bred all year round
in small milk jars, lived on mushed up bananas, and gave rise to a new generation
every twelve days. Beginning around the summer of 1909, Morgan,
again paying strict attention to De Vries’s emphasis on external conditions,
subjected the flies to wide temperature ranges and various chemicals, as
well as to radium and X-rays. 53
Following De Vries’s injunction that “no differentiating marks, however
slight, should be considered as insignificant,” 54 Morgan painstakingly
examined each fly with a jeweler’s loupe. In the first months, Morgan discovered
that he could select for flies with a dark marking that looked like a
trident across the normally amber-colored thorax, but it was only in this
single character that the flies differed, and the color markings were sometimes
subtle and hard to read. In early 1910 Ross Harrison, who had been a
fellow graduate student with Morgan, recalled visiting Morgan at Columbia
and his waving his hand at a wall full of vials filled with Drosophila and exclaiming:
“There’s two years work wasted. I’ve been breeding flies all that
time and have got nothing out of it.” 55
After Harrison’s visit, Morgan’s prospects suddenly improved when out
of the blue his flies seemed to start sporting. In a stock that had been breeding
true for the absence of the dark trident marking for four generations,
Morgan suddenly discovered that 32 of 199 members of the fifth generation
showed some dark specks just under the wings. Granted it wasn’t the cre-