CHAPTER IV 

 THE EFFECT OF LIGHT ON PIGMENTATION 



The dramatic effects of light on the pigments of plants and 

 animals have long been recognized. The yellowish-white pallor 

 assumed by plants containing chlorophyll confined in darkness is well 

 known, while the diatoms of the Lakes of Anvergne, equipped with 

 green chlorophyll and brown diatomin, change colour according to the 

 depth of the water in which they find themselves (Heribaud, 1894) ; 

 but the most dramatic effects are evident in the integumentary 

 pigments of 2^oikilochromic ^ animals. The spectacular and rapid 

 changes in colour between black and green seen in the chameleon were 

 noted in the fourth century B.C. by Aristotle, while Pliny described 

 somewhat similar changes in the dying mullet. Since classical times a 

 considerable amount of observation and research has been devoted to 

 the subject and a surprisingly wide range of colour changes has been 

 recorded in a large variety of animals — a euglenoid Protozoon, 

 polycheete worms, leeches, Echinoderms, Cephalopods, Crustaceans, 

 Insects, and among Vertebrates, numerous Fishes, Amphibians and 

 Reptiles. 2 



Biologically these changes may be assumed to serve two purposes, 

 one the antithesis of the other — cryptic or protective and 'phayieric or 

 demonstrative. The protective function is the more fundamental and 

 the more common, the demonstrative is a later and more rare 

 acquisition.^ 



The PROTECTIVE FUNCTION is designed in general to allow the 

 animal to adapt itself to its environment and shows three main 

 modifications. In its most primitive form such a variation in jsigment 

 probably developed as a light -absorptive function to provide protection 

 against deleterious light and heat ; occasionally pigmentary variations 

 are apparently thermo-regulatory — an early attempt at thermostasis — 

 as is seen strikingly in some desert lizards in which colour changes 

 may be induced experimentally by changes in temperature alone 

 (Parker, 1906-38 ; Bauer, 1914 ; Kriiger and Kern, 1924 ; -and others). 

 The most common and dramatic colour variations, however, have 

 evolved as an adaptive phenome7ion allowing the animal to become as 



1 TToiKi'Aof, varied ; xpuiyi'^, colour. 



2 For extensive reviews see van Rynberk (1906), Fuehs (1914), Hogben (1924), 

 Parker (1930-5r.) and Brown (1950). 



^ These are r xamples of a large group of phenomena termed allcesihetic by Huxley 

 (1938) which exer! their biological effect through the agency of the distance receptors 

 of another individual — sight, hearing or smell. 



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