Biological Molecules and Colour 



Several fly-catchers of the genus Piezorhynchus 

 (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 



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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