28 GENETICS 



frequencies divided by n, according to the formula 



.. * 7 s (**?) 

 -\ -T-> 



in which x represents the deviation of each class from 

 the arithmetical mean; f, the number of individuals in 

 each separate class ; 2, the sum of the classes ; and n, 

 the total number of individuals. 1 



In the present instance the standard deviation is 

 .846, a number that has valuable significance only 

 when brought into comparison with standard deviations 

 similarly derived from other groups of starfishes. 



Such a variation polygon as the above expresses the 

 law that the farther any single group is from the 

 mean of all the groups making up the polygon, the 

 fewer will be the individuals representing it. 



7. THE INTERPRETATION OF VARIATION CURVES 

 a. Relative Variability 



The statistical determination of the relative vari- 

 ability of two lots of organisms with respect to a cer- 

 tain character may be illustrated by the case of the 

 oyster-borer snail, Urosalpinx cinereus, as seen in the 

 accompanying table on page 29. 



The obvious conclusion to be drawn from this table 

 is that the snails which were unintentionally carried 

 from the Atlantic coast to California in the transplan- 

 tation of oysters show more variation in their new 

 habitat than they did in the old one with respect to the 



1 For directions explaining the use of such formulae see Daven- 

 port's "Statistical Methods." 



