(19) 



There remain only spine number and the two size characters, 

 which must be considered together, since they are closely 

 correlated , 



Shell size, as has been shown, gives the highest paren- 

 tal coefficient of correlation, both in the population and 

 in line 30. In th;e other lines t?ie coefficient was often 

 negative. Spine number shows a decided correlation in the 

 population and a significant one in line 30, and in all the 

 other lines the coefficieiit was always positive. It is easy 

 to imagine a way in whicli shell size could be "inherited" 

 in a purely mechanical sense, since the old shell acts as 

 a limit to the possible growth of the organism within it, 

 and this organism then serves as a mold over the surface of 

 which the new shell is secreted. Finally, spine number is 

 the character which appears to have the least relation to 

 the physiological activity of the organism, and is the one 

 whichx can be most accurately and easily determined. For 

 these reasons, spine number was chosen as the subject for 

 selection experimerits . 



After the objections which have just been raised to the 

 use of shell size as a subject for selection experiments, it 

 is advisable to determine whether variations in shell size 

 are correlated with variations in spine number. This is 

 especially necessary, since in our population we find that 

 the large lines have few spines and the small lines many 

 spines. Within the clone, however, this does not hold. 

 The coefficient of correlation between shell size and spine 

 number for the 745 individuals of line 30 is .019 ± .025, 

 that is, for all practical purposes, zero. 



