EXCRETORY PIGMENTS 167 



power, that arsenic, sulphur, and gouty phosphates 

 are transferred from the deeper to the more super- 

 ficial parts of the body ; and the more insight we gain 

 into the colouring of the skin, the more clearly do we 

 see that it is the seat of purification, in consequence 

 of the complexity and bulk of the animal body. 



White butterflies owe their white and yellow colours 

 to one of these ' excretory ' poisonous substances, 

 and the silvery whiteness of fish is due to a similar 

 though harmless substance. Thus it comes about that 

 similarly coloured pigments are not necessarily of 

 similar nature. The yellow butterfly, the tiger, and 

 the frog contain three different substances, and we 

 have to apply tests before we can separate the old 

 historical yellow of the frog from the later and 

 derived pigment of the tiger and fly. 



It is to these three functions — nutrition, respira- 

 tion, and excretion — and to nutrition first and fore- 

 most, that the pigments of animals owe their origin. 



In each case the original significance of the pig- 

 ment may be lost, and a later and apparently merely 

 decorative one be attained. To the study of skin- 

 painting and surface-colouring much has yet to be 

 contributed before we can, as it were, restore the 

 colouring and realise the design. But in such a quest 

 nothing is insignificant, and he who can explain freckles 

 will solve one of the most baffling problems of human 

 or bestial colouration. 



The Secondary Meanings of Animal Colouration. — 

 Having thus obtained a grasp upon the meanings 



