222 Variation and Correlation in the Earthworm 
The variation constants for position of clitellum deduced from Table IV. have 
the following values : 
Mean = 30-762 ± -01 3 somites. 
Standard Deviation = "436 + 009 somites. 
Coefficient of Variation = 1-416 + '030 %. 
For the number of somites in the clitellum the values are : 
Mean = 6-238 + -015 somites. 
Standard Deviation = "oOO + Oil somites. 
Coefficient of Variation = 8-019 ± '173 
It is at once evident that the variation in re.spect to number of somites in the 
clitellum is significantly greater, as indicated both by the standard deviation and 
the coefficient of variation, than the variation in the position of the clitellum. 
The difference between the coefficients of variation in the two cases is very large 
(G-603 The difference between the two standard deviations is absolutely 
small (-064), but relatively it is large enough to be significant, being more than 
three times its probable error (which is + -014). The probable reason for the 
difference in variability between these two characters will be discussed later. 
Turning now to the correlation between position and number of somites in the 
clitellum, we have the following table: 
TABLE VI. 
Correlation between Number of Somites in Clitellum and 
Position of that Organ. 
Number of Somites in Clitellum. 
° a 
•j3 cu 
7 
8 
Totals 
29 
1 
1 
30 
39 
62 
16 
117 
31 
354 
21 
1 
376 
32 
1 
1 
Totals 
394 
84 
17 
495 
The value of the coefficient of correlation deduced from this table is 
r = - -629 ± -018. 
This rather high negative value for the coefficient is of considerable interest 
from the morphogenetic standpoint. It shows that as the number of somites 
anterior to the clitellum increases, the number included in the clitellum itself 
tends to decrease, and vice versa. Or in other words, it means that the posterior 
end of the clitellum is a definitely fixed morphological point, subject to very little 
variation. 
