ACCOMMODATION IN MAMMALS 287 



change of setting. If an eye is one diopter hypermetropic, it needs only 

 one diopter of accommodation in order to make itself emmetropic, and 

 thus obtain sharp images all the way to the horizon. And with only two 

 diopters of accommodation, it can give itself a near-point at one meter — 

 ordinarily quite close enough, for any animal that cannot read! 



Among zoo animals and domesticated ones, just as with auto-domesti- 

 cated — i.e., 'civilized' — man, anything may happen. In fact, it is wholly 

 unsafe to draw ecological conclusions from any situation in domestic 

 species. Less than fifty per cent of horses are emmetropic; and though 

 myopia is most unusual for a wild mammal, it is extremely common in 

 zoo animals and barnyard varieties. 



Along with their normal slight hypermetropia, ungulates usually show 

 a slight horizontal astigmatism, probably a consequence of their efforts to 

 widen the visual field horizontally by every possible means (see pp. 299- 

 300). The extent of accommodation is very low indeed in mammals — 

 often zero — except in the primates. The cat, which is the nearest com- 

 petitor of the simians in this regard, has but half the accommodation of 

 a thirty-year-old man and loses even this in old age. Human accommo- 

 dation being 'tops' for mammals (Beer found no more than ten diopters 

 in any ape), it is desirable to turn back to the graph (Fig. 15, p. 35) 

 showing its extent at various ages. The senescent diminution of the 

 power of accommodation in mammals is bound up with the accommo- 

 datory method itself. Certainly in the Ichthyopsida no such falling-off 

 is to be expected, for the lens in these animals may become even harder 

 with age than it is in the young, without this affecting the range of 

 accommodation a particle. In the Sauropsida, the direct action of the 

 ciliary muscle probably accomplishes an effective alteration of lens form 

 at ages where, if the animal were a mammal of the same relative age, 

 the lesser force of the elasticity of the lens capsule could no longer make 

 headway against the sclerosis of the lens fibers. 



A special situation arises in small-eyed mammals. The squirrels are 

 exceptional among the rodents, in having some accommodation, which 

 we should expect from their diurnality and high visual acuity. The Eu- 

 ropean squirrel may be emmetropic or as much as one-half diopter hyper- 

 metropic, and can accommodate from one to one and one-half diopters. 

 As the size of the eye diminishes from that of a cat to that of a mouse 

 (Fig. 71, p. 173), the increasing (relative) size and firmness of the lens 

 and its (relative) recession toward the retina results not only in the 

 reduction of accommodation from a couple of diopters to nothing, but 



