J. Davidson 145 



samples of a group with mean value at 570, the observed standard 

 deviation is 105 while the calculated one is 127. 



The nine varieties (Nos. VIII, IX, X, XI, XII, XIII, XIV, XV, XVI) 

 with infestation numbers 370 to 451 are distributed about a mean of 

 407, the observed standard deviation is 103 as against a calculated value 

 108. The two remaining varieties (Nos. XVII and XVIII) do not seem 

 to be assignable to any other group, and may be regarded at least pro- 

 visionally as each representing a separate grade of immunity; the very 

 low susceptibihty and the high variability of the last variety (No. XVIII) 

 especially makes it worth a more extensive study. 



(Received Oct. 25th, 1921.) 



