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gene for white that the two never separated, and suppose 

 the new mutation affected the eye so that its effect could 

 be observed (for if not the change would not concern us). 

 The new mutation would behave towards white in the 

 same way as do all pairs of allelomorphs and yet in a 

 strict sense is not allelomorphic. It is not necessary to 

 elaborate here this idea, because fortunately in the case 

 of Drosophila there is strong evidence to show that multi- 

 ple allelomorphs do not arise in this way. The evidence 

 for this statement will be given in Chapter XVII. 



