The Making of Species 
average, a length of wing of only about 138} 
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 18} 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 20-inch wing, their offspring 
will nevertheless vary zuter se, some of them 
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