Evolution of Eyes and Photoreceptor Organelles in the Lower Phyla

How could. such a complex organ as the vertebrate eye have evolved by natural selection of numerous, successive, slight modifications? Charles Darwin posed this question but could not answer it satisfactorily because of the rather limited knowledge of invertebrate eyes in his day. For the vertebrate eye, though genetically inheritable variations are known, the question cannot be answered even today because of the lack of examples to fill the huge gap between the relatively primitive pigment-cup eyes of chordate ancestors and the fully-developed lens eye of the simplest vertebrates. Fortunately, as we now know, the lens eye has evolved independently several other times, and stepwise evolution is suggested by the existence of intermediate grades along those distinct lines