82 EVOLUTION OF THE HUMAN EYE 



stereoscopic vision. A movement of the eyes may be 

 observed in some of the Ungulata, when their attention is 

 suddenly attracted, which is an approach toward conver- 

 gence, but which is really more a movement from diver- 

 gence toward parallelism. It is exceedingly well depicted 

 in a' plate in Chalmers Mitchell 's book on The Childhood of 

 Animals, which shows a group of spring-bok facing the 

 artist, in the characteristic attitude of attention, at the 

 London Zoological Gardens a few hours after a kid was born. 

 In the Carnivora more obvious attempts at convergence 

 may be noted; when holding food with their front paws 

 they tear it with their teeth. Dogs can be trained to con- 

 verge by having a lump of sugar placed on their nose "on 

 trust." 



The associated evolution of acute central form-sense, 

 accommodation and convergence, is of considerable interest 

 when we consider the intimate dependence of each one upon 

 the others for purposes of stereoscopic vision. If any one 

 of these three faculties becomes seriously interfered with, 

 so that the balance between them is disturbed, then the 

 recently acquired capacity of fusing the images seen with the 

 two eyes is given up, and one eye only is employed for fix- 

 ation, the other being allowed to wander in the direction of 

 least resistance, the condition we speak of as concomitant 

 strabismus being established. Concomitant strabismus may 

 be the outcome of several different causes, but three well- 

 established predisposing factors are: (1) A defect in the 

 development of the central form-sense in one eye, i. e., con- 

 genital amblyopia; (2) the presence of hypermetropia, 

 which leads to an excessive amount of accommodative effort 

 being made in the focussing of the images of near objects; 

 (3) the presence of heterophoria, which leads to undue 

 amount of effort on the part of the recti muscles to maintain 

 the requisite amount of convergence. 



