Vol,. 6, 1920 
GENETICS: S. WRIGHT 
329 
In the diagram, the pattern of each guinea-pig is represented as de- 
termined by three factors, H (heredity), E (environment common to Utter 
mates before birth) and D (the residue, largely irregularity in develop- 
ment). Our problem is to determine the degree of determination by each 
of these factors. 
In a forthcoming paper, a method of estimating the degree to which a 
given effect is determined by each of a number of causes will be discussed 
at some length. 
Figure 6 is meant to illustrate a system in which the variations of two 
quantities X and Y are determined in part by independent causes, such as 
A and D, respectively, and in part by common causes such as B and C. 
These common causes may be correlated with each other as in the figure. 
It is assumed that all of the relations are approximately linear and that 
the influence of the various causes are combined approximately by addi- 
FIG. 6 
Diagram^ illustrating'twof effects (XY) which are determined in part by the same 
correlated causes (BC). 
tion. The path coefficient, measuring the importance of a given path of 
influence from cause to effect, is defined as the ratio of the variability of 
the effect to be found when all causes are constant except the one in ques- 
tion, the variability of which is kept unchanged, to the total variability. 
Variability is measured by the standard deviation. The path coefficients 
in the figure are represented by small letters. 
It can be shown that the squares of the path coefficients measure the de- 
gree of determination by each cause. If the causes are independent of 
each other, the sum of the squared path coefficients is unity. If the causes 
are correlated, terms representing joint determination must be recognized. 
The complete determination of X in figure 6 by factor A and the corre- 
lated factors B and C, can be expressed by the equation : 
a^-hb' + c' + 2bcrBc = 1 (D 
