VARIATION 



47 



mean of all the groups making up the polygon, the 

 fewer will be the individuals that represent it. 



7. THE INTERPRETATION OF VARIATION POLYGONS 

 a. Relative Variability 



The statistical determination of the relative vari- 

 ability of two lots of organisms with respect to a 

 certain character may be illustrated by the case of 

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

 the accompanying table. 



ATLANTIC AND PACIFIC SHELLS COMPARED 



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 



