The structural patterns of vertebrate eyes have been undergoing intelligent scrutiny for about a century and a half. In that time, and more and more rapidly toward the present, men have been learning much about the functional meanings of those patterns, and their roles in the lives of the animals which have produced them. It has seemed to me that it is time an attempt was made to interpret comparative ocular biology as a whole to those who want to know what the eye is all about, but are repelled by the pedantic terminology of anatomy texts, the mathematics of physiological optics, the scatteredness of the ecological literature, and the German language. In this book, I have made such an attempt.