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 as 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. 10B) 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 
