LIGHT-SENSE 



47 



of daylight an animal exposes- itself to and the number of 

 cones met with in its retina correspond fairly accurately. 

 The Simise, who, to obtain their food in their arboreal resorts 

 (consisting, as it often does, of minute objects), require a 

 high degree of illumination, have an area of the retina in 

 which only cones are present, and in which they are closely 

 congregated together. It would seem probable that it is 

 due to this congregation of the cones together at the macula 

 that man and monkeys can stand exposure to brighter day- 

 light than any other mammal. Man 's eyes cannot, however, 

 stand exposure without damage to the same degree of 



FIG. 8. Headlight fish. ^Ethoprora lucida. 



direct sunlight as some birds; eagles, for example, as they 

 rise to fly will gaze directly at the sun without flinching. 

 In the diurnal rapacious birds the retina has been found to be 

 richer in cones than anv other birds, or than anv mammals. 



*/ ^ 



Various means, supplementary to those in the retina itself, 

 have been acquired by mammals to fit them for an environ- 

 ment having low degrees of luminosity. Some deep-sea 

 fishes and cave-animals, who live in absolute darkness, have 

 no visual sense and no visual organs, their survival depend- 

 ing on the acuity of their other senses. Other deep-sea 

 fishes have evolved their own illuminating appliances, and 



