ON THE SHELL OF LITTORINA LITOREA AS 

 MATERIAL FOR THE STUDY OF VARIATION. 



R. P. BIGELOW AND ELEANOR P. RATHBUN. 



The common periwinkle, Lit tori na litorca, seems at first 

 especially suitable for a statistical study of the effect of a new 

 environment upon type and variability. For the best evidence, 

 according to Morse ('Hot and Ganong ('86), goes to show that 

 this species has been introduced from Europe into Nova Scotia 

 and New England within the last fifty years. It was first 

 reported from Halifax by John Willis in 1857. Previous to this 

 date no mention is found of the species in any list of the shells 

 of Nova Scotia, New England, or the Gulf of St. Lawrence. 

 Furthermore the shell is not found fossil in these regions, and is 

 not found in the prehistoric shell heaps. The species seems 

 therefore to have been introduced into America somewhere about 

 the middle of the nineteenth century. 



After its introduction it spread gradually southward along the 

 coast of New England as far as Long Island Sound. In every 

 place where it has appeared north of Gape God it has increased 

 enormously and in many cases nearly driven out the indigenous 

 species of Littorina, until it is now probably the most abundant 

 gasteropod on the coast. But south of the Cape it is only fairly 



Thus we have in I., litorca a species which has been intro- 



One might expect a priori that this species on migrating into a 

 new area would find new conditions and be subjected to new 

 factors of natural selection, which would tend to establish a new 

 type, or else that the species, having escaped, perhaps, from 

 some of the checks that limited its variability in its former 

 environment, might show a greater range of variation under 

 the new conditions. By a statistical study of L. litorea one 

 might hope to determine whether one or both of these supposi- 

 171 



