154 ADAPTATIONS TO ARHYTHMIC ACTIVITY 



serves both of the aperture's functions equally well; but as the animal 

 attempts to make use of the evening and night hours, or to set back the 

 alarm-clock in the morning, the pupil must walk an increasingly precar- 

 ious tight-rope. It must open far enough to permit of stimulation, but 

 not so far that the blurring of the image takes more away from vision 

 than the increased light confers. Fortunately, as the intensity falls very 

 low, the widening pupil finally begins to add more to the quality of the 

 image, in brightness, than it takes away, by aberration. As day passes 

 into night, the now familiar acuity-sensitivity seesaw (p. 69) begins 

 inexorably to move. In any but a very strictly diurnal animal, sensitivity 

 goes up, perhaps very high; and in any and all animals, acuity inevitably 

 comes down. 



A highly mobile, precisely controlled pupil prepares its owner for both 

 night and day. We have seen that the same can be said for a well-devel- 

 oped system of photomechanical changes. An active iris in a given animal 

 may be free to give its whole allegiance to illumination, or its movements 

 may be tied reflexly more closely to accommodation, convergence of the 

 two eyes, or emotional changes, or may even be under the control of 

 the will. The pupil may be so blocked by the lens that its closure is 

 impossible unless the iridic sphincter is able actually to compress the lens 

 (a possibility which, as an adaptation to amphibious habits, is realized 

 in some vertebrates) . In the majority of fishes, the blockage is so com- 

 plete and the lens so firm or so large that any attempt at pupil movement 

 is hopeless and the iris is actually devoid of muscle. 

 Pupillary versus Retinal Adaptation — Being more familiar with 

 movable pupils, we are likely to think of the photomechanical changes 

 in the retina as being able to save such situations, and take the place of 

 extensive pupil mobility. Historically, it has really been the other way 

 'round; for as we saw in the preceding section, the phylogenetic history 

 of the photomechanical changes has been one of fairly steady reduction 

 from a peak in the teleosts to complete absence in the mammals, the 

 birds being exceptional, since in them, despite their high position and 

 their mobile pupils, the photomechanical changes have been resuscitated 

 for special reasons which will later be set forth. 



Concomitantly, there has been a steady development of pupil mobil- 

 ity — among twenty-four-hour and nocturnal animals — from nothing in 

 most fishes to a maximum in the mammals. We may reasonably expect 

 to find then, at any evolutionary level, one or the other of these regula- 

 tory mechanisms in good condition (consult Table II, p. 150). 



