570 J-. w. (iA.MDLi;. 



concentrated light, both red and green rays act like orange- 

 yellow ones in suppressing pigment. Wh<in diluted, however, 

 with white light, red rays produce pigment and pure green rays 

 do likewise. As a background in daylight, theiefore, the 

 monochromatic rays act in one way ; as a pure incident light 

 they act in an'opposite fashion. This apparently contradictory 

 result is therefore supported by the evidence from experiments 

 on two widely different groups of animals, Crustacea and 

 Insecta. 



What exactly, then, are the factors that determine the 

 extraordiuai-y close sympathetic coloui'-rendering of the 

 environment in the pigmentation of these animals ? First of 

 all in both groups, light is not essential to the production of 

 pigment. Poulton's results, as well as my own, show that 

 dark-kept animals become dark coloured, though somewhat 

 irregularly. lu the case of Hippolyte darkness does not 

 induce the formation of all the pigments. Red (vermilion), 

 the dominant one, and yellow to a less extent (giving a brown 

 coloration), are the only colours formed in the absence of 

 light. In the insect larvge, brown pigment is likewise formed 

 in darkness, and develops as a sheath upon the green sub- 

 epidermal layer. The action of light, then, in both groups is 

 rather directive or inhibitoiy than effective. In the case of 

 insects, the orange-yellow rays are apparently those which, 

 when reflected from backgrounds, inhibit this brown pigment 

 and allow the subjacent green pigment to confer its full 

 value on the colour of the larva or pupa. In Crustacea the 

 case is different ; the action of these rays upon them is at 

 present quite unknown. The colours are pigmentary, con- 

 tained in chromatophores and not "hypodermal" as in 

 insects, but the production of the well-known green, brown, 

 and reddish varieties of Hippolyte is due mainly to manipu- 

 lations of a reddish-yellow coloration Avhich is formed in the 

 absence of definite stimulation. 



The liyht reflected from natural alg-al backgrounds is of a 



O CD <~> 



mixed character, but with some yellow, some green, and 

 varying amounts of red in it. All we have to imagine is 



