What big eyes you have, Grandma!

When Little Red Riding Hood saw the Big Bad Wolf dressed in her Grandma’s nightgown (Fig. 1), she quickly had to interpret several observations. ‘‘Grandma’’ had very big eyes which, the wolf claimed, were the better to see her with. And big ears, the better to hear her with. And big, big teeth, the better to eat her with! Little Red Riding Hood seemed to accept the wolf’s explanations, perhaps invoking an evolutionary biologist’s use of the principle of parsimony, that the simplest explanation for a highly organized trait is adaptive natural selection. Eyes are for seeing, ears for hearing, and hard, pointy things in the mouth are for crunching food. Her reason for accepting ‘‘Grandma’s’’ explanations were generally right, though she should have realized that slashing teeth are not a primate’s natural apparatus, so that ‘‘Granny’’ wasn’t really Granny. Fortunately (for her, not the wolf), the woodsman arrived in time. The evolutionary idea expressed here is simple. When we see an organ made of light-sensitive cells fronted by a clear disc that focuses light onto those cells, and the cells interconnect and travel to specific areas of the brain in a way that maintains the two-dimensional array structure of those retinal cells, it is reasonable to