Three hypotheses have been elaborated based upon the examination of afferent projections to the diencephalon of six vertebrate classes: the first, that the target diencephalic areas should be considered homologous in all the classes; the second, that olfaction does not dominate the sensory input to the telencephaIon or diencephalon in nonmammalian forms as previously thought, but that a strong case can be made that vision does; and the third, that overlap can be inversely correlated with specialization more readily than with a hypothetical evolutionary position.