IX THE MARKINGS OF ORGANISMS 199 



seems to account in some degree for their usual uni- 

 form distribution in the tissues or structures in which 

 they occur. We may note here that in the only 

 case in which a dark (melanin) pigment has been 

 subjected to a thorough examination, it was found 

 that the insolubility was not intrinsic, but was due to 

 intermixed substances which acted a^ mordants ; if, 

 however, such substances tend always to occur in 

 association with the pigment, the practical result is 

 the same. The question whether the numerous 

 dark pigments found in the animal kingdom are 

 related to one another is an interesting one, but has 

 not yet been properly attacked. 



It should be noticed in leeches that in general 

 terms each segment tends to repeat the colour, as it 

 does the structure, of the others. The segments are 

 here the similar parts which tend to resemble one 

 another ; we have already (p. 108) considered the in- 

 genious theory which seeks to correlate the colour 

 with the detailed structure of the muscles. 



Arthropoda. — In the Arthropoda we have again 

 the same contrast between uniformly, often brightly, 

 coloured organisms on the one hand, and on the 

 other those showing wonderful arrangements of 

 colour-markings, with black or brown as a basis. In 

 a rough way the contrast again lies between uni- 

 formly coloured marine forms — the Crustacea, and 

 patterned terrestrial forms^ — the Insects. The con- 

 trast is only roughly true, however, for we have 

 already seen how many insects there are, especially 

 in the less specialised orders, which are uniformly 

 coloured in greens or browns. 



As to the patterns and markings of insects, if we 



