Charles Darwin once wrote to Asa Gray, "The eye to this day gives me a cold shudder, but when I think of the fine known gradation my reason tells me I ought to conquer the odd shudder." This quotation is well known. But Matt Ridley, a columnist for the weekend edition of the Wall Street Journal, devoted last week's column to Darwin and the eye, and he has me thinking abou the matter anew, too. The eye was presumably giving Darwin shudders because it is a very complex organ. One has a tough time imagining any single "random variation" that would give a newborn of some mammalian breed a pair of eyes that its parents had not had. THAT would be an implausible jump. Thus, the eye must have come about through increments. Yet it seems difficult to construct a chain of slow increments from no sight at all to the full carbon-based cameras that mammals suggest. As Ridley asks rhetorically, "What use is half an eye?" I was going to write something more he...