VARIATION 



47 



mean of all the groups making up the pol^^gon, 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 



Locality 



Woods 

 Hole 



West Shore 

 Penzance Point 

 Nobska Point 

 Nobska Point 

 Nobska Point 

 Barnacle Beach 

 Big Wepecket 

 Mid-Wepecket 



Average for Mass 



Cali- 



f Belmont Beds 



fornia '[ San Francisco Bay 



Average for Cal. 



Difference 



Number 

 OF Shells 



1,001 



1,002 



1,002 



1,001 



496 



998 



1,006 



500 



1,008 

 520 



A.M. 



58.928 

 61.718 

 61.737 

 61.944 

 66.944 

 63.932 

 57.426 

 57.606 



61.066 



59.051 



60.892 



59.664 



2.339 

 2.737 

 2.152 

 2.234 

 2.366 

 2.604 

 2.052 

 2.098 



2.335 



3.023 

 3.361 



3.138 



.803 



Prob- 

 able 

 Error 



±.0352 

 ±.0412 

 ±.0324 

 ±.0337 

 ±.0507 

 ±.0393 

 ±.0308 

 ±.0447 



±.0386 



±.0454 

 ±.0703 



±.0538 



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 



