No. 5.] THE INTRODUCED LITTORINA. 257 



A Fifth Test of Variability. 



It will be noted on Chart XIV * that the three British curves 

 lie directly under each other. The curves are not only alike 

 in their general configuration, but the ideal weights, as indi- 

 cated by the vertical curve, are approximately the same. Among 

 the American curves it will be noted that there is more varia- 

 tion both in the contour of the curves and, what is more impor- 

 tant, in the vertical curve of ideal means. No manipulation of 

 data or combination of three American localities can be made 

 which will not give an amplitude of variation according to 

 weight which is greater than any combination of the three 

 series of British shells. 



A Si.vt/t Test of Variability. 



It is, unfortunately, impracticable to express variations in 

 color either by curves or mathematical formulae. Color aver- 

 ages are uncertain and the estimation of extremes difficult. 



The bands of color of the British shells are generally clearly 

 defined throughout their entire length, and give a distinct cast 

 to the British shells, whatever their age. The limits of the 

 bands and the edges of the lines are much more irregular and 

 indefinite in the American forms, and the amount of pigment 

 and its distribution in the individual shells is more variable. 

 Thus, the American shells have a mottled appearance and a 

 suggestion of indefiniteness of color that the British shells 

 seldom possess. 



1 On this chart the Roman numerals, at the right, indicate the localities from 

 which the shells are collected, and the Arabic numerals, at the left, indicate the 

 number of specimens used in the plotting of each curve. The lower curve is based 

 on 200 specimens from Tenby, Chart I ; the upper curve is based on 183 speci- 

 mens from Beverly, Chart VI. In the middle of each horizontal curve is a num- 

 ber which indicates the mean weight. The shells from Tenby have a mean weight 

 of i. 86 grams ; those from Beverly have a mean weight of 1.43 grams. A vertical 

 curve connects the several mean points, and since it bends tp the left, as it ascends, 

 it indicates that the American shells are lighter than the European. The very 

 irregular curve of relative bulks, or displacements, drawn at the right, is intended 

 to show the lack of dependence of the factor of weight upon bulk. 



