106 



THE CHROMOSOMES 



is to give rise to an ' incipient species ' it must not be 

 lethal or upset the genie balance of the organism too 

 profoundly. If, however, the original balance is only 

 slightly upset subsequent mutation and/or recom- 

 bination of existing genes may re-establish a new 

 ' secondary balance '. While the origin of * incipient 

 species ' is thus independent of natural selection, the 

 latter is an important factor in the subsequent 

 evolution of taxonomic differences between the new 

 form and the original one (' species-differentiation '). 

 Gene -mutation remains the only fundamental mech- 

 anism of morphological change in organisms, but it 



