PHYLOGENY, EFFICIENCY; TAPETA AND ACUITY 245 



variations we can see an obvious effect upon tapetal efficacy. This is the 

 distance of the reflective material from the tips of the visual cells. Where 

 this distance is greater, as in chorioidal tapeta (separated from the tips 

 of the rods by the thickness of the choriocapillaris and the pigment epi- 

 thelium), the spreading of the scattered reflected light results in its 

 striking many rods in addition to those which it had originally traversed 

 before reflection. Where the tapetum smoothly and directly contacts the 

 visual-cell palisade, as in the opossum, there is less opportunity for scat- 

 tering to blur the image and detract from the acuity of scotopic vision — 

 low, at best, as it is bound to be. Yet the opossum gives every evidence 

 of having extremely low visual acuity, while the cat is far from being 

 badly off in this respect (see Table V, p. 207). If there were anything 

 logical about the distribution of tapetal types, the cat would have the 

 opossum's, and the opossum, the cat's. 



The Tapetum and Visual Acuity — The tapetum is not always re- 

 stricted in usefulness to the dimmest of illuminations. As was pointed 

 out in Chapter 7, the all-round capacity of twenty-four-hour eyes is not 

 due to a fence-straddling avoidance of specialization, but to a mosaic 

 of compatible specializations for both scotopic and photopic vision. 

 A tapetum is perfectly compatible with an area centralis (though it is 

 never associated with a fovea). Mustelus is matched, among the prim- 

 ates, by Lemur catta and Aotus. One of these is diurnal, the other noc- 

 turnal; but each has both an afoveate area centraUs and a tapetum, while 

 the close relatives of both (other Lemur spp., other Simiae) have neither 

 of these features. 



Notably, the ungulates and some carnivores (lion, polar bear) have 

 large eyes but not particularly small images. They can compensate for 

 the dimness of the large image by means of the tapetum, the size of the 

 image enabling them to attain keen vision despite the low ratio of cones 

 to rods. In dim light the tapetum gives the animal enough sensitivity, 

 and in average light it is still usable because of the size of the image. It 

 is certainly not ordinarily a source of dazzlement as is evidenced by the 

 fact that few ungulates and no large carnivores have any approach to a 

 slit pupil. It is probably no accident that in these animals the area cen- 

 tralis falls within the confines of the tapetum. If cones are indeed concen- 

 trated within these imperfectly known areae centrales, the lowered sensi- 

 tivity of those regions is nicely compensated by the tapeta behind them. 



