THE PECTEN AND ITS ANALOGUES 657 



have rather low fold numbers — 7-14. This is hard to explain away, for 

 most parrots are active diurnal birds and fly a good deal. They would 

 seem not to have high visual-acuity requirements, however; for, with the 

 exception of the notorious sheep-killing (?) kea, they restrict themselves 

 more to gross vegetable food than do any other birds. The eyes of parrots 

 bear other surprises, and would be well worth intensive study : they have 

 the narrowest known binocular fields (p. 295), and lack the customary 

 red oil-droplets (pp. 499-500). It can be said, though, that within the 

 parrot group the number of pecten folds varies as one might expect, for 

 the nocturnal owl-parrot (Strigops habroptilus) has only four to six folds. 



Rather low numbers (i.e., less than 12) occur in many sea-birds, shore- 

 birds, herons etc. Some of these are nocturnal, others not. One clear-cut 

 example of correlation — which could be multiplied — is the stone-curlew 

 (Burhinus cedicnemus) , which feeds only at night, has very large eyes, 

 and has only eight pecten folds. 



The general correlation of large, elaborate pectens with diurnality and 

 of reduced folds with nocturnality was noticed by Wagner back in 1837. 

 This was thirty years before the formulation of the Duplicity Theory; 

 and it was only long after 1867 that it was first realized that cones and 

 rods might have very different metabolic rates and requirements. Wag- 

 ner studied 108 species of birds, and though all the additional species 

 examined since have only borne him out, his idea has been quite ignored 

 or forgotten since the turn of the century when Virchow last accorded 

 it a few words in print. Jokl did not know of it when, in 1923, he per- 

 ceived the physiological interchangeability of the various S N d's — but 

 thought that the metabolic level of a retina, determining the need or 

 dispensability of an s N D, was governed by the activeness of the animal. 

 Thus, he explained the absence of a conus in both turtles and croco- 

 dilians on the basis of sluggishness (forgetting Sphenodon, which also 

 lacks a conus and is sluggish as well as nocturnal) , and he accounted for 

 the reduced pectens of Apteryx, Casuarius, and Struthio {sic) on anal- 

 ogous grounds — i.e. flightlessness. The ostrich has plenty of 'folds'; and, 

 though flightless, it is very far from sluggish. 



Wagner and Jokl were each on one rail of the right track. From all 

 present indications it does not appear that we need ascribe to the pecten 

 any 'intentional' activity other than the giving off of nutrients for the 

 retina to absorb from the vitreous. That it gives off heat (which however 

 is not needed) goes without saying. That considerable water escapes 

 from it also is clear from certain of Abelsdorff and Wessely's experi- 



