II THE PIGMENTS OF ORGANISMS 41 



said to be obtained from the diatoms of their food, 

 used to be given as an example ; another is the case 

 worked out recently by Mr. Poulton (1893), showing 

 that the green pigment of some caterpillars is derived 

 from the green leaves upon which they live. Another 

 example, which in some degree resembles Mr. 

 Poulton's case, is one mentioned by Zopf (1892). In 

 studying the colouring-matter of the fungus Pilobolus 

 already mentioned, he found that a parasite growing 

 on the fungus took up not only the drops of oil but 

 also the pigment associated with the oil, the result 

 being that parasite and host were similarly coloured. 

 This is interesting, because it probably has some 

 bearing on certain of the cases of colour-resemblance 

 which are now so numerous in the literature of colour. 

 The above five sets of pigments have been con- 

 sidered in some detail, because it is quite possible 

 that ultimately the greater number of the pigments 

 at present known may be found to fall into one or 

 other of these categories. In the meantime, however, 

 there is no proof of this, and there are very many 

 pigments which it is quite impossible to classify. On 

 this account, we shall, simply consider them system- 

 atically under the organisms in which they occur. 

 Before doing this it may be well, in the absence 

 of a complete classification of pigments, to consider 

 for a little the distribution of pigmental colours in 

 organisms. 



The Distribution of Pigmental Colours 



Blueas a pigmental colourisexceedinglyrare among 

 the more complex animals,but is not uncommon among 



