A geographical review of the published literature confirmed that red-green color-vision deficiency (Daltonism) is rare near the equator and more prevalent at higher latitudes, as others have reported. A survey of dictionaries of the languages spoken by the populations for which Daltonism data are available confirmed that distinct words for "blue" are also rare near the equator, and more common at higher latitudes. These results were compared to show an even stronger relation between the proportion of languages with a word for "blue" and the prevalence of Daltonism. We believe that the strong correlation between "blue" and Daltonism suggests that an evolutionary, physiological cause for both phenomena.