546 THE WONDER OF LIFE 



and if we turn the motley into a dish and give a choice of 

 seaweed, each variety after its kind will select the one 

 with which it agrees in colour, and vanish. At nightfall, 

 Hippolyte, of whatever colour, changes to a transparent 

 azure blue ; its stolidity gives place to a nervous restless- 

 ness ; at the least tremor it leaps violently and often 

 swims actively from one food-plant to another. This blue 

 fit lasts till daybreak, and is then succeeded by the prawn's 

 diurnal tint. Thus the colour of an animal may express 

 a nervous rhythm '. 



In many cases, both among plants and animals, the 

 range of colour exhibited by one and the same organism 

 is very striking, but it has sometimes a very simple ex- 

 planation. There is a colourless ' chromogen ' substance, 

 or ' mother of pigment ', which takes on different colours 

 according to the amount of oxidation to which it is sub- 

 jected imder the action of a ferment. One of the common 

 colour- evoking ferments is called tyrosinase. The different 

 colours in cases of this sort simply correspond to different 

 rates or rhythms ; and it is easy to understand how this 

 or that punctuation might be fixed by Natural Selection. 



The conunon sea-slater, Lygia oceanica, has numerous 

 much-branched black or dark brown chromatophores in 

 its epidermis, which make it inconspicuous against a dark 

 background of rock. Tait has shown (1910) that if the 

 creature is exposed to light in a black-painted dish, it 

 remains dark, but that if it is exposed in a white dish it 

 becomes lighter in colour and more transparent, so that 

 eventually the heart can be seen beating through the skin. 

 This change is due to a retraction of the black chroma- 

 tophores, which also leaves certain white chromatophores 

 more in evidence. When the eyes are painted over with 



