How the eye got its brain

Evolution provides an explanatory framework for biology. Our usual formula is to explain a trait by equating its present function with its adaptive reason for being, invoking natural selection as the causal force. But since there is no trait that we won’t try to explain by such necessarily post hoc scenarios of deterministic molding by selection, evolutionary explanations are often derided by critics as Just So stories. This is irritating, because we think we are going Kipling one better by saying How the Leopard Really Got His Spots (see Murray, 1997). But if we examine our stories more closely, it is difficult to know when we have the true ones right. The evolution of eyes illustrates this in many ways.