33i; 



Variation and Corrclatiou in Arctila 



as the subject character and thc outer dianieter as the relative character. The 

 nican outer dianieter for cach of the 10 "age" arrays was thcn coniputcd. Tlic 

 rcsults arc shown in Table IX. 



TAKLE IX. 



Mean Unter Diameters uf Arcellae ut Saccessive Stades in the Development 



of the Culture. 



It will bc sccn at once that there is very littlc definite shift of tlie mean 

 towanls a higher valuL' in the higher "age" classes. The straight line best fitting 

 tlicse nieans is almost exactly vcrtical. To get a numerical idoa of thc aniount 

 of the shift, the average of the five means of the first 2.5Ü iudividuals may be 

 conipared with the average of the five niean.s of the 2.50 iudividuals last measured. 

 The mean dianieter for the first 2.50 individnals measured is .5.5-632 mikrons ; 

 for thc last 250 it is .5.5-912 mikrons. Thc difference between these values is 

 -280 mikron, which is evidently a practically insensible difference. 



It may bo coucluded that in a pcriod of time of approximately three weeks 

 the average sizc of Arcella shells in a culture, as estiinatcd by random samplcs 

 of 50, does not sensibly increase. 



IV. ÖUMM.VUV. 



The present paper comprisos the rcsults of a Statistical study of Variation and 

 corrclatiou in the shell of the common rhizopod, Arcella vulgaris. 



The points especially investigated were : (i) the Variation in the total diameter 

 i)f thc shell, (ii) the Variation in thc opening of thc shcll, (iii) thc Variation in thc 

 colour of tlic shell, (iv) thc corrclatiou bctwccn the total diameter of thc .shcll aiid 

 the diameter of thc opening, (v) the corrclatiou between the colour and the total 

 diameter of the shell. The frequency distributions and the eonstants dedueed 

 from them, and the corrclatiou tables and coefficients for thc various characters 

 are prcsented. 



