80 EVOLUTION OF THE HUMAN EYE 



these myopic monkeys had been kept in confinement pre- 

 vious to having their refraction tested. 



The tendency to the development of myopia as an out- 

 come of confinement in restricted spaces seems to be an 

 example of adaptation of structure to function, i. e., an 

 elongation of the posterior part of the eyeball to meet require- 

 ments of vision for near objects. It is similar to the con- 

 dition of things which is met with in human beings. The 

 incompletely developed eye of the newborn is nearly always 

 hypermetropic, the mean amount met with having been 

 estimated by different observers as 2.3 D. and 3 D. Dur- 

 ing the first decade of life the hypermetropia curve rapidly 

 falls, while the percentage of myopes increases during the 

 first two decades. 



Man's eyes at birth are comparable to those of terrestrial 

 mammals in the natural state, who have not adopted arboreal 

 life. His lenses are then nearly spherical and his vitreous 

 chambers of comparatively little depth, so that his eyes 

 are adapted for vision at long distances, with but little 

 capacity for the focussing of near objects. By expansion 

 of the globes, flattening of the lenses antero-posteriorly and 

 increase in the depth of the vitreous chambers, man's eyes 

 gradually reach that condition which is most suitable for 

 his environment under normal conditions, i. e. y where parallel 

 rays are brought accurately to a focus on the macula when 

 the ciliary muscle is at rest, and where there is an ample 

 capacity of accommodation for near objects. If, while this 

 developmental process, which lasts up to the twentieth year, 

 is still proceeding, man's vision becomes unduly restricted 

 to use at short ranges, then, as in the lower mammals w r hich 

 are kept confined in small compartments, adaptation of 

 structure to function tends to take place, the vitreous 

 chambers become deeper than normal and myopia is 

 developed. 



