Retinal ganglion cell axons become reordered as they pass through the chiasmatic region of the optic pathway. Studies in carnivores and rodents show that the fiber order established in the optic tract is a chronological index of their arrival time during development and that the cause of the reordering may relate to the changing glial environment, as well as to the spatial and temporal distribution of proteoglycans within the developing pathway. Primate optic axons become similarly reordered, allowing one to predict a developmental sequence of ganglion cell genesis from fiber position within the mature optic tract. Fiber position within the tract also anticipates the pattern of geniculate innervation, but a prominent exception to this rule is found in the prosimian Galago. The chronotopic reordering is found in every mammalian species that has been examined, including eutherians and metatherians, suggesting that the mechanism producing it is evolutionarily conserved.