BIOMETRY 369 
populations. Opinion differs as to the significance of these findings. 
The more optimistic evolutionists look upon such instances as that of 
Bateson’s earwigs as visual demonstrations of a species actually split- 
ting up into two or more species. It seems quite likely that one of 
these types is a successful mutant type that has not fully segregated 
itself as a true species from the parent-type. Another view of the 
significance of bimodal curves is that the condition results from hybridi- 
zation and that the bimodality is the result of the segregation of 
dominant and recessive types. 
34 5 6 7 8 9 
Fic. 63.—Bimodal polygon plotted from data on the earwig. Mean types 
(X#%) indicated above corresponding modes. Numbers below the base line indicate 
length of pincers in millimeters. (From Bateson and Johannsen.) 
THE COEFFICIENT OF CORRELATION 
Only one more biometrical constant need be mentioned here: the 
“coefficient of correlation.” It is often necessary to discover the 
exact relation that exists between two sets of variables in order to 
discover whether they are totally independent or partially correlated 
with each other. For example, we have found that there is a close 
correlation between stature and head length in man, also between 
color of hair and color of eyes; but it is very important to be able to 
reduce the degree of correlation to a simple arithmetical constant. 
This is called the “coefficient of correlation” (commonly expressed 
as rz, Where x is one variable and y the other). The formula for com- 
puting r,, is as follows: 
NCS) 
