312 ADAPTATIONS TO SPACE AND MOTION 



on monkeys has shown that all four recti can be shifted about, the eyes 

 becoming re-conjugated in a few days.* The recovery takes place, though 

 more slowly, even when the animal is kept in darkness. 



It is the predator which visually pursues its prey, and the inquisitive 

 primate picking up this object and that for manipulation at close range, 

 which have the greatest need for the accurate estimation of distance 

 which sharp binocular vision alone confers. Such vision is obviously aided 

 as much by frontality as by the improvement of the area and the final 

 creation of a fovea. According to Lindsay Johnson and Elliott Smith, no 

 non-simian mammal can converge its eyes, though it is perhaps significant 

 that cats and dogs can be taught to do so — the cat being perhaps closer to 

 the verge of producing a fovea than other arhythmic mammals. Nicolas, 

 however, states that the dog converges naturally. Other authors have 

 claimed that many mammals do converge when excited in the pursuit of 

 prey or in fleeing from an enemy, thus widening the binocular field when 

 it will do the most good. Such convergence is not necessarily voluntary, 

 however. Even in the rabbit, which has no voluntary eye movements, the 

 angle between the optic axes is less when the animal is excited than when 

 it is undisturbed (see Table X, p. 298) . 



The squirrels, and especially the marmots with their 'universal macu- 

 larity', constitute a rather special and interesting case. The marmot or 

 prairie-dog's eyes are strongly lateral and are but slightly movable. But 

 the retina has everywhere as high a resolving power as many another 

 animal's fovea, so there is no need of fixative, aiming movements of the 

 eyes. As Rochon-Duvigneaud has pointed out, the marmot can explore 

 space without betraying itself by the slightest movement, even of its eyes. 

 It is thus far from being in the same class with such forms as the rat or 

 the frog. The latter keep their eyes still not because their retinal reso- 

 lution is everywhere so excellent, but because it is everywhere so poor. 



The case of the marmot is the only one which prevents us from gener- 

 alizing that the spontaneous eye motility of vertebrates is correlated with 

 high visual acuity as such. We still must say that such movements occur 

 only where there is high acuity of vision within a restricted area of the 

 retina (p. 305), which must be directed toward an object if the latter is 

 to be seen at all well. 



*The investigators (Leinfelder and Black) found however that if the superior oblique was 

 disturbed there was no re-coordination even after months. The meaning of this is not yet clear. 



