RETINAL STRUCTURE AND ANIMAL HABITS 65 



The bat, a strictly nocturnal form which begins its activi- 

 ties with approaching dusk, possesses, according to my 

 own findings (Detwiler, 1924), a pure rod retina. There are 

 many other forms with which we are acquainted that are 

 predominately nocturnal, but which can see reasonably 

 well in daytime. The cat, for example prowls and hunts at 

 night but not exclusively so, for we have all observed his 

 success at hunting in daytime. The cat, however, with a 

 vertical slit pupil, a predominance of rods, and a choroidal 

 tapetum, is particularly well adapted for nocturnal vision. 

 According to DeVoss and Ganson (1915), the cat is said to 

 have very defective daylight vision and to be colorblind. 

 Dogs appear to have a much more efficient apparatus for 

 night than for day vision. 



The retina of the horse is known to have both elements 

 present. Horses are essentially diurnal animals, but I have 

 found from personal experience that their capacity for noc- 

 turnal vision is probably greatly superior to that of man. 

 The presence of a tapetum lucidum in many mammals 

 possessing cones as well as rods shows that they are par- 

 ticularly well adapted for nocturnal vision. 



Perhaps one of the most interesting groups of animals 

 demonstrating the correlation between retinal structure and 

 a nocturnal mode of life consists of the nocturnal lemurs. 

 These include the so-called Pottos, Lorises, and Galagos. 

 Galago mala and Nycticebus tardigradus (slow-moving 

 loris), both of which have been under my own observation, 

 were found to be most inactive during the daytime, and did 

 all their feeding at night. Apparently in their natural 

 habitat they leap from bough to bough at night and exhibit 

 marked activity. Nycticebus tardigradus is shown in the 

 frontispiece (Figure 1). This animal was so light sensitive 

 that it was difficult to obtain front-view photographs. The 

 moment one turned on a 60-watt lamp, the creature either 

 turned its head away or buried it between its legs. 



An histological examination of the retina of these forms 

 showed the presence of narrow attenuated rods and the 



