THE FIELD OF VISION 39 



whole macular region on one side included in the blind 

 area. 



Conjugate movements of the eyes in the vertical plane 

 are innate in the human infant, but in the horizontal plane 

 are only gradually acquired during the first six months of 

 life. 



From the early development of conjugate movements in 

 the vertical plane we may infer that man's mammalian 

 forbears were exposed to dangers which attacked them from 

 overhead, in the way in which small rodents are now attacked 

 by carnivorous birds. The development of conjugate 

 movements in the lateral plane followed later, with the 

 shrinkage of the snout and the translation of the eyes 

 forward in the head. 



The development of acute, central, stereoscopic vision 

 for form, together with corresponding points in the two 

 retinae, introduces a fresh factor in connection with the 

 field of vision— that is, the field of fixation. Though no 

 mammals below monkeys have a capacity for distinguishing 

 detail, such as is associated with the presence of a fovea 

 centralis, there is evidence to show that in some, especially 

 the Carnivora, there is an area centralis in the retina more 

 acutely sensitive to form than the periphery. Such animals 

 can fix an object and make movements of the eyes to main- 

 tain fixation, e. g., when a cat follows the movements of a 

 mouse. The field of fixation becomes, however, of greater 

 importance with the increased acuity of central stereoscopic 

 vision, such as has been developed in association with arboreal 

 life. Still more is it important to man, who has assumed a 

 fully erect posture and become exposed to the dangers of 

 terrestrial life by his descent from the trees. The rapid 

 movement of the highly-developed spots with acute form- 

 sense, from one object to another in his circumference, 



