VARIATION AND CORKELATION IN ARCELLA. 



203 



Tlie fre(j[uencj polygon for the outer diameter variation is exhibited 



in Fig-. 1, 



EXPLANATION OP FIGURE. 



Fig. 1. Frequency polygon of variation in outer diameter of Arcella vulgaris. Abscissas, dimen- 

 TSions in mikrons; ordinates, number of individuals. S. D. = Standard deviation. 



Arcella was shown to be approximately equal in amount of variation 

 to Paramoecium. The values for Paramoecium were determined by 

 Simpson.* 



The relations between the values of the coefficient of variability in the 

 two cases are shown in the following table : 



COEFFICIENT OP VARIATION. 



The correlation in variation between the outer and inner diameter 

 w^as discussed. The correlation table and a model of the correlation sur- 

 face were exhibited. The value of the coefficient of correlation between 

 these two characters, outer and inner diameter was 



r = .83G 



•showing a comjraratively high degree of correlation in this simple rhizo- 

 pod. The value of the coefficient of the correlation between the length 

 and breadth of Paramoecium (Simpson, loc. cit.) is .421. Correlation 

 is much closer in the firm, definitely formed shell of Arcella than in the 

 body of Paramoecium. 



*SLmpson, J. Y. The relation of Binary Fisson to Variation. Biometrika. Vol. I, pp. •100-464. 1902 



