One approach to studying the evolution of behavior is to compare the functions of homologous structures in living species which approximate different evolutionary stages. Identifying homologous structures depends in turn on revealing similarities which are so extensive that they are indicative of descent from a common ancestry. In the present paper the visual pathways to the telencephalon are compared in birds, reptiles and mammals. All three vertebrate classes share two pathways, one which projects from the retina to the dorsal thalamus and then to the telencephalon and one which travels first to the optic tectum and from the tectum to the dorsal thalamus before projecting to the telencephalon. The similarities among these pathways in each class seem sufficient to support the proposition that pathways were already established in the common ancestors of these groups. Comparative studies of the structure and function of these pathways may provide a basis for tracing the evolution of behavior.