The Making of Species 



average, a length of wing of only about i8J 

 inches. They tend to return to that mode from 

 which their parents had departed. 



But suppose that the deviation of the parents 

 in this case had been due, not to fluctuating 

 variation, but to a mutation ; this would mean 

 that, owing to some internal change in the egg 

 that produced each parent, 20 inches became the 

 normal length of wing ; that the normal length of 

 wing had suddenly shifted from 17 inches to 

 20 inches. 



The result of this would be that their offspring 

 would have on an average a wing-length of 

 20 inches instead of i8J inches, that the centre 

 of variation as regards length of wing had 

 suddenly shifted from 17 to 20, that, in future, 

 all fluctuating variations would occur on either 

 side of 20 inches, instead of on either side of 

 17 inches as heretofore. 



Thus a variation is a fluctuating one or a 

 mutation according as it does or does not obey 

 Galton's Law of Regression. 



De Vries says that it is of the essence of 

 mutations that they are completely inherited. 

 This statement, although substantially true, fails 

 to take into consideration the factor of fluctuating 

 variation. For example, in the above instance 

 if the two individuals of species B had mutated 

 into forms with a 2O-inch wing, their offspring 

 will nevertheless vary inter se, some of them 



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