The Making of Species 
To sum up, the observed facts of animal 
colouration seem to indicate that there are in 
each organism some twelve or thirteen centres of 
colouring, which we suggest may correspond with 
portions of the fertilised egg. From each of 
these centres the colour develops and spreads, 
so that every part of the organism is eventually 
coloured. These centres of colouring are not 
altogether independent of one another. Some- 
times they all give rise to the same hue, in which 
case we have a uniformly-coloured organism, such 
as the raven. More often from some one colour 
develops, and from others another colour; if 
these two colours happen to be black and white, 
the result is a pied organism, which displays a 
definite pattern due to the correlation of the 
various colour-producing biological molecules. 
Thus it occasionally happens that two widely 
different organisms exhibit very similar mark- 
ings, and therefore resemble one another. When 
this resemblance is believed to be of advan- 
tage to one or other of the similarly-coloured 
species, naturalists call it mimicry, and assert that 
the likeness is due to the action of natural selec- 
tion; but where neither organism can profit by 
the resemblance, zoologists make no attempt to 
explain it. What we suggest is that the coloura- 
tion of an animal depends upon the structure, or, 
at any rate, the nature, of the parts of the egg 
which produce these centres of colour. But this 
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