658 BIRDS 



ments; but this serves only to provide water which, in a pectenless eye, 

 the vitreous would get from the ciliary epithelium anyway. The need for 

 a pecten (or for any other s N d) , and for a large one or a small, seems 

 to depend solely upon the rate-of-living of the sensory retina. Some of 

 the factors, at least, which heighten this rate are diumality, activity, and 

 high retinal temperature (owed chiefly to warm-bloodedness, but assisted 

 by the absorption of photopic images in the contiguous pigments) . Con- 

 versely, it is depressed by the elimination of cones in nocturnality, by 

 sluggishness, and by low retinal temperature. 



The interplay of these factors is various. Probably the turtle, though 

 diurnal, needs no conus because it is sluggish. Probably the large geckoes, 

 though nocturnal, need one because they are extraordinarily active. Prob- 

 ably the chameleon's conus is tiny because the animal, though diurnal, is 

 sloth-like in all its movements except the extension of its fly-catching 

 tongue. Probably the flying-squirrel, though nocturnal, needs retinal 

 capillaries because it is active and warm-blooded. But these are guesses — 

 we have no cold figures on the retinal metabolism of these forms, and of 

 their close relatives which have different habits and different S N D con- 

 ditions. 



Before any final ballot is taken on the prosaic theory offered here in 

 explanation of the S N D, in general, and the many more glamorous and 

 intrinsically more 'attractive' interpretations of the pecten, we need very 

 badly to know more about the true sizes of pectens — the area over which 

 they expose the blood circulating in them, the rate at which the blood is 

 changed for fresh, the permeability of their vessels, and so on. Then, 

 such data must be compounded with the status of the chorioid, with the 

 area, thickness, and histology of the retina, and with the results of in 

 vitro determinations of the metabolism of unit pieces of retinal tissue 

 from various birds and various other vertebrates, wisely selected in the 

 light of the whole S N D situation. 



These are problems for a physiologist to attack : he has the apparatus 

 and the methods*; and he can be assured in advance that his findings 

 will be of great value in themselves even if they do not yield correlations 

 which take the mystery out of the pecten. To date, ornithologists and 

 ophthalmologists have been too content to sit back and speculate about 

 the pecten, though they were told by von Husen, back in 1913, that only 

 physiological experimentation would reveal the whole meaning of the 

 structure. As Mark Twain said of the weather, everyone talks about the 



*See, for example, the paper of Lindeman (1940). 



