Biological Molecules 



is not by any means the only cause that deter- 

 mines the colouration of the organism. If it 

 were, young creatures in their first plumage 

 would invariably resemble the parents, the two 

 sexes would always be alike, and there would be 

 no such phenomenon as seasonal dimorphism. 



As a matter of fact, the portions of the egg (we 

 call them, for the sake of clearness, colour-produc- 

 ing biological molecules) which give rise to the 

 pcecilomeres exhibit themselves merely in the 

 shape of tendencies ; the ultimate form the 

 colouring will take depends to a large extent 

 upon other and extraneous circumstances, such 

 as the secretion of hormones. 



Thus it is that organisms seem to display an 

 almost endless diversity of colouration. But 

 beneath all this diversity we see something like 

 order. It occasionally happens (why, we do not 

 know) that one, or more, of the biological mole- 

 cules which make up the nucleus of the fertilised 

 ovum becomes altered in the sexual act, with 

 the result that a discontinuous variation or muta- 

 tion appears in the resulting organism. The 

 mutation may be a favourable one, or one which 

 does not affect in any way the chances of an 

 organism in the struggle for existence, or an 

 unfavourable one. In the last of the three cases 

 the organism will perish early and not leave 

 behind any offspring exhibiting its peculiarity. 



It is thus that natural selection acts. Natural 

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