Historical Constraints and the Evolution of Development 1125
lens, cone and pigment cells and an electroretinogram as it is typical for
photoreceptor cells can be recorded, when the ectopic eyes are exposed to light."
Nonetheless, as these ectopic eyes are not neurologically "wired up," the fly
presumably cannot use them for vision. (To give some sense of the excitement and
weirdness of these results upon their initial discovery—for we have, a mere five years
later, already become accustomed to such findings—I include as Fig. 10-22 the "Post-
it" note that Gehring penciled when he sent me his first reprint announcing this
achievement.)
But this conserved developmental pathway for insect and vertebrate eyes,
however surprising in the light of previous assumptions about the impossibility of
such genetic homology between phyla, did not yet directly address the theoretical
issue of convergence in evolution. After all, the single-lens eye of vertebrates bears
little anatomical similarity to the multifaceted fly eye, and no claims for convergence
had been staked upon this case. But the discovery
10 - 22. A personal touch expressing the excitement of this discovery: when Gehring sent me his
1996 reprint on inducing ectopic fly eyes with mouse genes, he inserted this Post-it just above
his finding that the ectopic eyes are morphologically normal and react to light in the normal
way. I added the marginal notation—not to pour water on a great discovery—that these eyes
are not wired to the brain and therefore will not function.