SENSES, INSTINCTS, AND INTELLIGENCE 327 



dwindling away, but it is larger in some races than in others, 

 and it occasionally shows a minute internal cartilage, a tell- 

 tale evidence of man's filiation to a mammalian stock in 

 which the third eyelid was large and useful. 



Birds have, like crocodiles, well-developed lachrymal 

 glands, but it is rare for them to shed tears. Mr. Beebe 

 describes a flamingo " weeping copiously " in fear of a 

 condor (" The Bird," p. 211). The secretion of the lachry- 

 mal gland exudes below the upper eyelid, is spread over the 

 cornea, and drained away into the nasal chamber. There 

 is also a pair of Harderian glands, whose secretion escapes 

 below the third eyehd. 



In most birds the eyes are placed very markedly to each 

 side of the head, and the right eye looks to the right, the 

 left to the left. This gives rise to monocular vision, and 

 it is plainly very different from the state of affairs in man 

 and monkeys where the axes of the two eyes are both 

 directed forwards. The advantage of monocular vision is 

 that it allows those birds that have always to be on the 

 look-out two extensive visual fields. 



Dr. Lucas writes : " The moment an object of interest 

 is detected the bird does not direct both eyes toward it, 

 but there is a concentration of one eye, the vision of the 

 other being suppressed at will. In some diseases of man, 

 where the axis of one eye has departed from the parallel 

 of the other, each eye sees a field which does not correspond 

 with the other, yet diplopia, or double vision, is not present, 

 as the one or the other field of vision is suppressed according 

 to the automatic concentration in one or the other eye. 

 Note a group of pheasants or pigeons watching the same 

 object ; one eye only will be directed toward the position. 

 Watch a fowl or a pigeon gazing upward at a hawk ; one 

 eye will be skyward, the other toward the ground. In such 

 cases the vision of the downward eye is being suppressed. 

 If suppression were not possible in birds a condition similar 

 to diplopia would be present. An idea of this condition 

 can be gained by pressing one's eye, thus shifting the visual 

 axis of one eye, when a double image is obtained." Some 



