COLOR VISION IN MAMMALS 517 



while all catarrhines are diurnal and trichromatic, there remains a pos- 

 sibility that the trichromasy known for some cebids (e.g., the squirrel 

 monkey, Saimiri sciurea, and the spider monkey, Ateleus ater) has 

 evolved through a dichromatic phase in other cebids {i.e., Cebus). 



The marmosets are less distinct from lemuroids than are the lowest 

 catarrhines, and may be ancestral to the Cebidae rather than derivatives 

 thereof — no one can be sure. In any case, nothing is as yet known about 

 their color vision. Among the cebids the nocturnal, assuredly achromatic 

 Actus may be the most primitive,* though this honor is usually accorded 

 to the closely-related diurnal genus Callicebus, whose color-vision status 

 is unknown. A case, of sorts, could thus be made out for considering 

 that trichromasy has evolved independently in the catarrhines and platyr- 

 rhines, and through achromatic (Aotus? marmosets?) and dichromatic 

 (Cebus — and Callicebus?) stages in at least the platyrrhine series, if not 

 through equivalent (but missing) links on the catarrhine side. 



Below the primates there lies but one order of placental mammals, the 

 Insectivora, regarded by taxonomists as ancestral to all other placentalia 

 and as immediately ancestral (even osculant, through such forms as 

 Tarsius) to the primates. Some insectivores (the tree-shrews, Tupaia) 

 are strongly diurnal; but their vision has yet to be investigated. Only the 

 common European hedgehog, Erinaceus europceus, which is nocturnal, 

 has had attention. 



Herter and Sgonina reported on this animal in 1933 and 1934. They 

 could not get their hedgehog to go to a yellow paper and avoid a blue 

 one — it insisted on going to the blue, so the investigators allowed that to 

 be the positive stimulus. Subsequent substitutions of other colored and 

 gray papers for the original stimuli revealed that the animal would 

 usually choose the darker of any two stimuli. The results suggested that 

 the hedgehog could see yellow, but no other color, as a quality distinct 

 from gray; but this conclusion hinged upon the outmoded Hessian 

 assumption that equal brightnesses for man are equal brightnesses for 

 animals. This is extremely unlikely in the case of the hedgehog, a noc- 

 turnal, apparently pure-rod animal. Miss Locher has offered other criti- 

 cisms, which Herter and Sgonina have failed to eliminate in their second 

 contribution. The hedgehog may have a color-life comparable with that 

 of Locher's second squirrel, but it probably has no color vision at all. 



*Though the fact that its tapetum is utterly different from that of the lemuroids (p. 233) 

 suggests rather that the noctumality of Aotus is secondary. 



