72 CELL-DIVISION AND GROWTH II. 2 



given for each age, as determined by length of shell, for the 

 English and American periwinkles separately and also for the 

 complete series. It will be seen that the variability increases 

 slightly and then diminishes again. This is the case also in 

 the American examples, where the fall at the end of growth is 

 greater still, but in the British specimens there is only a slight 

 fall, at 20-21 mm., followed by a considerable rise. The 

 possible significance of this difference in the behaviour of the 

 same species on the two sides of the Atlantic we shall discuss 

 in a moment. 



TABLE V 



Showing the alteration in the variability of the ventricosity of the shell 

 of the Periwinkle (Littorina littorea) during growth. 



Coefficient of variability (- x 100 V 



In the meantime let us consider another case, the Crab, 

 Carciniis moenas, the variability of the frontal breadth of 

 which was examined by the late Professor Weldon. Weldon 

 found that the variability, as measured by the quartile error (Q), 

 first increased and then suddenly diminished with age (as deter- 

 mined by carapace length). If the variability is measured by 

 the coefficient of variability (easily calculated from Weldon's 

 data) the result is the same. This will appear from the table 

 (Table VI). 



TABLE VI 

 Showing the change in the variability of the frontal breadth with age in 



Carcinus nwenas. (After Weldon.) 

 Carapace Length Q. "'inn 



in mm. j^ x *00. 



7-5 942 1-64 



8-5 9-83 1-76 



9-5 9-51 L73 



10-5 9-58 1-78 



11-5 10-25 1-93 



12-5 10-79 2-06 



13-5 10-09 1-95 



For the calculation of the variability at different ages in man 

 data have been provided by Roberts, Bowditch, and Boas. Some 



