ORIGIN OF COLOUR-PATTERNS 175 



what process they are so laid on, toned, and mixed it 

 it difficult to understand, save on the analogy of a 

 natural photographic process of colour. There is 

 known to physicists a mixture of silver-salts so sensi- 

 tive to light that it is decomposed thereby, and acquires 

 the colour of the rays that fall upon it. By its aid, 

 though at an expense prohibitive to commercial 

 success, a geranium can be reproduced in natural 

 colours, and it may be that some similar composition 

 underlies the skin of these prawns, and undergoes an 

 orderly disarrangement when scattered or evenly 

 diffused and coloured light falls upon it. 



But the skin of the prawn is no mere chemical 

 mixture, nor does light act upon it only as on a sensi- 

 tive plate. It is a highly nervous structure, which is 

 affected both by the sight of the eyes and by the rays 

 that impinge directly upon its surface. The processes 

 of pigment formation and pigment distribution are 

 regulated by the nervous system, acting mainly through 

 the eyes. Permission, as it were, is given or withheld 

 for the development and amount of the several pig- 

 ments, their close-set or more open texture, and for 

 the pattern which they must follow. As a skilled 

 painter almost unconsciously lays down the ground- 

 work of his sketch, manipulating the purity and 

 mixture of his colours, controlling them by eye, and 

 painting in the high lights against a body colour, so does 

 the eye of the prawn unconsciously supervise the 

 painting of the surrounding scene that unseen hands 

 are rendering, both in relief and colour, upon its own 



