ScienceDaily (May 24, 2009) — In the developing fetus, cell growth follows a very specific schedule. In the eye's retina, for example, cones -- which help distinguish color during the day -- develop before the more light-sensitive rods -- which are needed for night vision.
But minor differences in the timing of cell proliferation can explain the large differences found in the eyes of two species -- owl monkeys and capuchin monkeys -- that evolved from a common ancestor.
http://www.sciencedaily.com/releases/20 ... 213952.htm