Biological Molecules and Colour 
Several fly-catchers of the genus Pezorhynchus 
(males only). 
We have said sufficient to show that certain 
combinations of colours recur in nature in species 
which are neither nearly related to one another 
nor subjected to similar environment. For such 
phenomena it is difficult, if not impossible, to 
account on the theory that natural selection, 
acting on minute variations, is responsible for 
all the varied colouring of the animal kingdom. 
The facts, however, are in accordance with the 
supposition that the organism is the result of the 
growth and development of a number of units or 
biological molecules which exist in the fertilised 
egg. 
If there be any truth in the supposition, 
the colouration of every animal must be due to 
the development of one or more of these mole- 
cules. Colouration may be expression of the 
arrangement of all the molecules in the fertilised 
egg, or it may be due to the development of a 
number of molecules whose function is to deter- 
mine the colouring of an organism, or it may be 
the result of the development of one such mole- 
cule, which perhaps splits up in such a way that 
a portion attaches itself to each of the other 
molecules. 
But it is idle to speculate on this point. As 
we have already insisted, the tendency to build 
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