A behavioral tracking technique was used to investigate dark adaptation in two American red squirrels (Tamiosciurus hudsonicus). Increase of sensitivity was followed over a range of about 4 log units. A break typical of duplex retinas appeared in the dark-adaptation curve about 18 min after the beginning of adaptation. More than 30 min was required for complete adaptation, with the final threshold of the squirrel about 1.5 log units above that of the human measured on the same apparatus.According to duplicity theory, the visual systems of some species are made up of two such systems, which take their origin with two types oe receptors, the rods and the cones. The rod and cone systems in duplex retinas typically mediate vision at different levels of illumination. Both systems increase their sensitivity in darkness, but not to the same degree. The cone system adapts more rapidly and over a smaller range than does the rod system. As a result, one obtains discontinuous dark-adaptation curves with two sectors, each reflecting one o f t he receptor activities, from organisms whose retinas possess both systems.

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