COLOR VISION IN MAMMALS 515 



given, the souslik preferred the blue strongly, and white and black 

 equally. Given red, white, and blue pieces, it took blue slightly oftener 

 than white, and either about twice as often as red. This order of prefer- 

 ence — blue first, then white and black equally, then red — does not quite 

 check with the tree-squirrel's preference always for the darker of two 

 stimuli. The lens of Citellus is so strongly yellow that blue paper should 

 appear darkened, as red does to Sciurus rulgaris or to nocturnal rodents. 

 Even the vulgaris lens, which is probably pallid compared with those of 

 other squirrels, absorbs some light from A,436m|l on — all light from 

 A,400m|i, onward, according to Merker. But the equal value of white and 

 black for the souslik seems a paradox. 



Leaving the rodents, we come at last to the primate order. Here, as 

 with the birds, there has never been any doubt of the occurrence of color 

 vision in all its glory. Among the species in the higher (Anthropoidea) 

 sub-division of the order, the chimpanzee, the Guinea baboon iPapio 

 papio), the pig-tailed macaque (Nemestrinus nemestrinus), Pithecus 

 jascicularis, the rhesus monkey, the sooty mangabey {Cercocebus tor- 

 quatus) , squirrel and spider monkeys have all been studied. The work of 

 Kinnaman (1902), K6hler (1918), Bierens de Haan (1925), Kohts 

 (1928), Trendelenburg and Schmidt (1930), KlUver (1933), Brecher 

 (1936), and Grether (1939, 1940, 1941) on these forms has shown that 

 their hue systems are identical with the human one to all intents and 

 purposes. None of this work whatever is negative in implication. 



The few investigations to date upon the lower primates, the Lem- 

 uroidea or Prosimiae as opposed to the Anthropoidea or Simiae, have 

 yielded only negative results. In general, this is to be expected, for most 

 of these lower forms are strongly nocturnal whereas all of the higher 

 primates except the douroucoulis or night monkeys (genus Aotus =Nycti- 

 pithecus) are diurnal. Some, at least, of the prosimians are pure-rod. 



But among these lower primates there are two groups of genera whose 

 habits are opposite to those of all other lemuroids — just as Aotus stands 

 out as a rebel among the anthropoids. These are in the sub-families 

 Indrisins and Lemurinse. In the former, the avahis (genus Lichanotus) 

 are strictly nocturnal, but Propithecus is diurnal and crepuscular and 

 the black indris ilndri indri) is diurnal. Among the lemurines there are 

 also several nocturnal genera; but Hapalemur is diurnal and so are all 

 of the many species of Lemur itself. 



One of the true lemurs, Lemur mongoz, was investigated in 1930 by 

 Bierens de Haan and Prima, who fully expected the species to exhibit a 



