330 



Variation and Correlation in Aredia 



B. Variation in Colour. 



The colour detenninations wcre niado in the way already described (p. 325) for 

 371 individuals. The frequency distribution for colour is givcn iu Table V. 



TABLE V. 

 Frequency Distribution of Colour. 



Class A includcs individuals of the lightest colour; class D the darkest indi- 

 viduals, and classes B and C intormcdiato gradations. As was expectcd the largest 

 nuiaher of individuals feil in class D. A niajority of all the indivi(hials fall 

 within this darkest colour class. The nuiuber of lighter coloured individuals is, 

 howcver, larger than probably would havc bcen predicted from general Observation. 



The correlation bctween colour and sizc will be discussed in a latcr section. 



C. Correlation. 



The correlations principally studicd werc those e.xisting between the outer and 

 inner diameters, and between the outer diameter aud colour. 



1. Correlation between outer and inner diameters. 



The correlation table showing the relation between the total diameter of the 

 sheil and the diameter of the opening is e.xhibited in Table VI. 



The value of the cocfHcient of correlation deduced from this table by the usual 

 formula 



S{xy) 



IS 



r = 



n<Ti o-j 



,•=•836 +-007. 



A correlation cocfficient of this magnitude of course iiidicates a high degree 

 of correlation between the outer and inner diameters in Arcella. The value is 

 considerably greater than that obtained by Simpson* for the correlation between 

 the length and bresidth of iudividual l'uramoecia. His value for r iu this case is 



* Loc. cit. p. 403. 



