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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