THE VISION OF VERTEBRATES 601 



which vision plays a subsidiary part are on the whole retained. ]\Iarsu- 

 pials and Placentals, however, have evolved into larger and more active 

 types capable of wandering freely over the wide spaces during the day 

 and therefore depending more and more on vision for their expanding 

 activities. Among the Marsupials this evolution reaches its highest 

 point in the kangaroos, and the same sequence is seen in the great 

 placental family. To the Insectivores, the Chiroptera,i and the 

 " Edentates," vision as a general rule is a subsidiary faculty ; ant- 

 eaters and armadillos, for example, gather their prey with their sticky 

 tongues, never seeing the food they eat (Fig. 736). Even to the 

 Rodents (with the exception of the squirrel family), mostly small 

 creatures of nocturnal habits living near to the ground with a limited 

 horizon, the eyes are usually the fourth most important sense-organ in 

 day-to-day activities, coming after the nose, the ears and the tactile 

 vibrissae. It is true that in some, such as the Lagomorpha (rabbits, 

 hares), vision is eminently useful, but the retina is still simple in struc- 

 ture and the eyes are probably used largely for the avoidance of 

 relatively near objects ; deprived of them, however, the animal 

 becomes immobile. In the Sciuridse (squirrels and particularly 

 marmots), however, the eye with its cone-rich or cone-pure retina, 

 becomes for the first time a dominating organ. Among the Ungulates, 

 also, the eye becomes structurally elaborate and vision more important, 

 although the perception of movement would ai)pear to be biologically 

 more useful to them than that of form ; among the Carnivores, it 

 is equally so although much reliance is placed on the other senses. 

 The hearing of the dog is said to be up to 16 times more acute than 

 that of man, his ability to locate sound t^vice as accurate, and his 

 analysis of tone is good ; but he can recognize his master visually 

 only at the relatively short distance of some 500 metres, while a 

 rabbit excites no attention if it does not move ; normal recognition 

 is essentially by smell. The cat has a less acute sense of smell, but 

 it also does not see a stationary man at a distance of 12 metres, while 

 its vision in the dark, although better than that of man, is not all- 

 dominating, for deprived of its tactile \ibriss8e it walks at night with 

 great hesitancy. The Cetaceans are poorly equipped visually and in 

 the analysis of its environment the whale j^robably relies mainly on 

 the excellent development of its tympanic bullae for the detection of 

 vibratory stimuli. Apart from the squirrels it is only when the Primates 

 and particularly man are reached that vision again dominates conduct 



1 The agility displaj-ed by bats in avoiding obstacles at night, such as strings 

 stretched across a dark room, has given the imjDression of an astonishing acuity in night 

 vision. This feat, however, is due to hearing. Bats in flight emit a series of super- 

 sonic squeaks (with vibrations up to 50,000 per sec), inaudible to man, as frequently as 

 100 times a second or more ; the hearing of the echos from obstructing objects probably 

 provides their essential means of guidance. Bats with their muzzles covered or their 

 ears plugged cannot avoid collision. 



