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 
peecilomeres 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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