ACCOMMODATION AND CONVERGENCE 81 



As already pointed out, the development of a highly acute 

 spot of central vision for form-sense is correlated in mammals 

 with a change in feeding habits, viz., the use of the hands 

 for picking up food and conveying it to the mouth, instead 

 of in the first place seizing it with the mouth. When the 

 food consists of small things, such as fruits, grains, nuts 

 and insects, acute form-sense, unless accompanied by active 

 accommodative power, would be of very limited use. Hence, 

 like the development of the fovea centralis, the increased 

 range of accommodation, which has necessitated the exten- 

 sive changes in the shape and structure of the eye just men- 

 tioned, has been the outcome of the adoption of arboreal 

 life. ' 



Just as we find the development of active accommodation 

 linked up with the development of acute central form-sense, 

 so we find the power of convergence linked up in its for- 

 mation with the development of active accommodation. 

 Indeed, these three visual factors appear to have evolved, 

 pari passu, with life in the trees. 



As has already been pointed out, the evolution of con- 

 jugate movements of the eyes is associated with the presence 

 of semi-decussation of the fibers of the optic nerves at the 

 chiasma, the receipt of visual impressions from the two 

 eyes on the same side of the brain, and the development of 

 corresponding points in the retina. These conjugate move- 

 ments reach to a high degree of perfection in the Carnivora, 

 but it is onlv when we come to the Primates that we meet 



b 



with a highly developed power of convergence in association 

 with stereoscopic vision. 



Some reptiles and birds are able to converge the axes of 

 the two eyes on objects when feeding, but they have com- 

 plete decussation of the fibers of the two optic nerves. Visual 

 impressions from the two eyes are not received in them on 

 the same side of the brain, so that they do not have true 

 6 



