The Making of Species 
But, it may be objected, if the colour of an 
organism be derived from one of these so-called 
biological molecules, how is it that it affects the 
whole organism, or, at any rate, several of the 
other unit characters? The objection may be 
met in several ways. In the first place, the 
colour-forming molecules may split up into as 
many portions as there are units which it affects, 
and each portion may attach itself to a unit. Or 
the property which we call colouration may not 
be derived from a molecule, it may be an ex- 
pression in the relative positions of the various 
molecules in the fertilised egg. Or the colour- 
determining molecule may secrete a ferment or 
a hormone, and this may be the cause of the 
particular colouring of the resulting organism. 
We do not pretend to say which (if any) 
of these alternative suppositions is the correct 
one. But it seems to us that some such con- 
ception as that which we have set forth is forced 
upon us by observed facts. This conception 
should be regarded not as a theory, but rather as 
an indication of the lines along which we believe 
the study of inheritance could best be made. 
The fertilised ovum has nothing of the shape 
of the creature to which it will give rise. It is 
merely a potential organism, a something which 
under favourable conditions will develop into an 
organism. 
In the higher animals each individual is either 
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