20 



THE EYE IN EVOLUTION 



illumination or darkness.^ Gamble and Keeble (1900) first reported 

 such a cyclic diurnal colour change which persisted under constant 

 illumination in the prawn, Hippolyte varians, but although subsequent 

 work has not confirmed this particular observation (Kleinholz and 

 Welsh, 1937), the phenomenon has been demonstrated in a number of 

 species of both Invertebrates and the lower Vertebrates.- In some 

 cases these diurnal changes are largely masked by other factors such 

 as pigmentary changes adopted to mimic the background,^ but the 



Figs. 9 and 10. — Diurnal Rhythms in the Pigment of the Crab, Uca. 



The black and white segments at the top of the graphs and in the corre- 

 sponding position immediately below the graphs represent the normal rhythm 

 of daylight and darkness. The second tier of markings below indicate the 

 experimental variations introduced. Ordinates : the degree of pigmentation 

 expressed in Hogben and Slome's scale, 1 representing complete concentration 

 of pigment, i.e., the light phase, and 5 its complete dispersal, i.e., the dark 

 phase (Brown and Webb, 1949). 



Fig. 9. — The normal diurnal rhythm of 

 pigmentation (dark through the day 

 and light at night) is seen to continue 

 uninterruptedly after the animal has 

 been 9 days in darkness. 



Fig. 10. — At the beginning of the experiment Uca 

 was exposed to continuous illumination (80 foot 

 candles) from A to B. There is a decrease in 

 amplitude and then a gradual inhibition of the 

 rhythm until eventually the chromatophores 

 change irregularly. At B the animals were trans- 

 ferred to continuous darkness, whereupon the 

 chromatophores becaine almost completely con- 

 centrated and thereafter a normal 24-hour rhythm 

 in phase with solar day-night was observed. 



effect of the underlying rhythm is seen in the increased rapidity of 

 these secondary responses when they are in phase with the primary 

 diurnal cycle and their sluggishness when they antagonize it. In other 

 animals the fundamental rhythm is preponderant so that secondary 



1 8 to 9 weeks in the beach-louse, Idotea — Menke, (1911) ; 18 days in the lizard, 

 Anolis — Rahn and Rosendale (1941) ; and so on. 



^ Several Invertebrates such as the black sea-urchin, Diadema antillarum — Millott 

 (1950) ; many Crustaceans in addition to Idotea : the prawn, Paloemon — Keeble 

 and Gamble (1904), the fiddler crab, Uca — Abramowitz (1937), the Isopod, Ligia — 

 H. Smith (1938) ; a few Insects such as the stick-insect, Dixippus — Schleip (1910). 

 Compare, for example, Figs. 64-68. 



A number of Vertebrates, particularly in their youth (Cf. Figs. 70 and 73) ; Cyclo- 

 stomea such as the lampern, Lampetra — Young (1935), Jones (1955) ; Amphibians 

 such as salamander larva* and frogs — Hooker (1914), Welsh (1938) ; and Reptiles such 

 as the American horned "toad", Phrynosortui — Redfield (1918), the lizard, Anolis — 

 Rahn and Ro i»ndale (1941), and the chameleon — Zoond and Eyre (1934). 



3 p. 82. 



