The single-locus X-chromosome theory was proposed to explain the unique intra- and inter-species variation in the colour vision found in New World monkeys. The colour vision of only five species of these monkeys has been studied in any detail, and although this data does show some minor deviations from the theory, it is generally consistent with the theory. It has been suggested that the colour vision of New World monkeys may represent an intermediate stage in between the dichromatic colour vision of non-primate mammals and the uniform trichromacy of Old World primates. Alternatively, the colour vision system of New World monkeys may represent a method of obtaining the maximum variation in the colour vision of closely related family groups.