The Making of Species 
up elaborate theories on very slender foundations 
is a too frequent failing of zoologists. We desire 
merely to emphasise the fact that the phenomena 
of animal colouration almost force us to the con- 
clusion that the colouring of each organism is the 
result of the development of a number of units. 
It may be objected that, if this be the case, the 
number of the units which contribute to the colour 
of any organism must be exceedingly large, since 
we see in nature an almost limitless number of 
different schemes of colouring. If the colour of 
each animal be the result of the development of 
a few units, it might be thought, firstly, that the 
diversity of schemes of colouration which we 
observe in nature could not possibly occur ; 
and secondly, that, under such circumstances, 
the colour pattern of a bird or beast should 
be of the nature of a mosaic, each colour being 
sharply defined and separated from every other 
colour, instead of the colours shading one into 
the other, as is so frequently the case. 
Such objections would be based on a miscon- 
ception as to the nature of the units which com- 
bine to produce the colouration of an organism. 
These units show themselves as centres of develop- 
ment of colour, as points from which the colour 
or colouring they represent spreads, until it 
meets and mingles with other patches of colour 
which are being developed from other centres. 
The colour produced at one centre may spread 
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