Law of Regression 
mean whether that departure is due to a mutation 
or a fluctuating variation. 
It is only by watching the effect of the 
peculiarity on the offspring of its possessor that 
we are able to determine the nature of the varia- 
tion. Where the peculiarity is due to a fluctuating 
variation the offspring will display the peculiarity 
in a diminished degree ; but if the peculiarity be 
due to a mutation, the offspring are likely to 
display it in as marked a degree as the parent. 
Fritz Miller and Galton conducted inde- 
pendently enquiries into the amount of the 
regression shown by the progeny of parents 
which have deviated from the average by 
fluctuating variation. 
Miller experimented with Indian corn; Galton 
with the sweet pea. 
Each found that where the deviation of the 
parents is represented by the figure 5, that of 
their offspring is usually 2, that is to say, the 
deviation they display is, on the average, less 
than half that of their parents. 
Applying this rule to the hypothetical case 
given above, if two individuals of species A 
having a length of wing of 20 inches be bred 
together, their offspring will, on an average, have 
a length of wing of 20 inches, since neither 
parents showed any deviation from the mean. 
On the other hand, the offspring of 20-inch-wing 
individuals of species B would show, on an 
F 81 
