202 ADAPTATIONS TO DIURNAL ACTIVITY 



an essentially senescent change, and its optical usefulness is the sheerest 

 of accidents. 



Oil-droplets and yellow corneae both appear first in the chondrostean 

 and holostean fishes respectively. The oil-droplets of the sturgeon are 

 colorless, though they were not necessarily always so. They have been 

 used as an argument that the oil-droplet was first evolved as a colorless 

 focusing device; but the sturgeon has a nocturnally-adapted eye, and one 

 would expect its oil-droplets to be colorless even if they had been pig- 

 mented in some diurnal ancestor. 



In the diurnal Amia, the whole cornea is yellow, with the color intensi- 

 fied dorsally. The pigment itself has not been studied, but it is probably 

 the same ichthyocarotin which occurs in many of the dermal chromato- 

 phores of fishes generally. Some teleosts, notably the markedly diurnal 

 pikes (Esox spp.) have as strongly yellowed corneae as Amia. In a spe- 

 cies of darter from Georgia, so new to science that as yet it has no name, 

 Hubbs reports a central, homogeneous, deep yellow coloration in the 

 cornea, opposite to and co-extensive with the pupil. Other teleosts have 

 various, usually diffuse and pallid yellow colorations; but in most species 

 the cornea is quite colorless. However much a fish may prefer bright sun- 

 light, that light is dimmer through water than it would be on land. No 

 fish can see more than a few rods at best owing to the extinction of 

 light in water, hence few can afford the luxury of a yellow filter unless 

 they are content to use their eyes mostly near the surface. Most fishes 

 enter deep, dim water at some time of the year. They must also do with- 

 out vision when beneath a covering of ice and snow. The eels are 

 exceptional, below the mammals, in having retinal capillaries; but these 

 are not intended as a filter — their significance is a very special one (see 

 pp. 405-6). 



Of the amphibians, only the frogs approach diurnality, and these have 

 oil-droplets which may be colorless, or yellowed by the same carotenoid 

 pigment which colors the animal's fat. Other amphibians lack even color- 

 less droplets. 



Among the reptiles Sphenodon is at once conspicuous since, though 

 nocturnal, and with its visual cells almost all converted from cones into 

 massive rods (see Chapter 16, section C), the oil-droplets and some yel- 

 low pigmentation thereof have been retained. The reader will remember 

 that Sphenodon has also kept the fovea of its diurnal forebears. (Fig. 

 82, p. 189). The turtles have ruby-red, orange, and lemon-yellow oil- 

 droplets. The crocodilians, like the similarly nocturnal toads, have got- 



