6 Prof. W. F. R. Weldon. Certain [Mar. 3, 



Table I. — Mean Value of Post-spinous Carapace Length (ps.) for 

 every observed Value of Total Carapace Length (c) in Plymouth. 

 (1000 individuals.) 



M c = 249 -63 ; Q c = 4 '55 M ps = 177 "53 ; = 3 '50 



Length of 

 c. 



1 PTi erf", n 00.9 



c — Mc 



ps — Mp S 



Length of ps 

 when 

 r = 081. 



Difference 

 observed, 



VS. 



uver aov 



188 *41 



+ 2 *95 



+ 3 ^ll 



185 -87 



+ 2-54 





185 "41 



+ 2*28 



+ 2 -29 



184 *00 



+ 1-41 



£iOXJ 



183 "25 



+ 2 -06 



+ 1*63 



183 '38 



-0-13 



258 



182 "25 



+ 1 -84 



+ 1-35 



182 -76 



-0-51 



9^7 



182 *34 



+ 1-62 



+ 1*37 



182*13 



+ 0-21 





182 '22 



+ 1-40 



+ 1-31 



181-51 



+ 0-71 



255 



181 "14 



+ 1-18 



+ 1-03 



180 *88 



+ 0-26 





179 *98 



+ 0'96 



+ '70 



180-26 



-0-28 



9"^ 



179 *50 



+ 0*74 



+ *56 



179 -63 



— 0-13 



252 



179 "17 



+ 0*52 



+ 0-47 



179 *01 



+ 0-16 



251 



178 -68 



+ 0*30 



+ 0*33 



178 -38 



+ 0-30 



250 



177-71 



+ 0-08 



+ 0-05 



177-76 



-0-05 



249 



177 -39 



-0*13 



-0-05 



177 -26 



+ 0-10 



248 



176 -64 



-0-36 



-0-25 



176 -01 



+ 0-53 



247 



175 -36 



-0-58 



-0*62 



175 -88 



-0-52 



246 



175 '20 



-0-80 



-0-67 



175 -26 



-0-06 



245 



173 -56 



-1-01 



-1-13 



174 -66 



-1-10 



244 



173 -31 



-1-24 



-1-21 



174 -01 



-0-70 



243 



173 -33 



-1-46 



-1-20 



173 -38 



-0-05 



242 



172 -81 



-1-68 



-135 



172 -76 



+ 0-05 



241 



171 -30 



-I -90 



-1-78 



172 -13 



+ 0-83 



240 



169 -57 



-2-12 



-2-27 



171 -51 



+ 1-06 



Under 240 



170 '33 



-2-81 



-2-20 



169 -55 



-0-78 



The mean value of r, as determined from these two tables, is 0"81 ; 

 that is to say, if all those individuals be chosen in which the deviation 

 of one of the two organs from its average length is m times the prob- 

 able error of that organ, the mean deviation from the average size of 

 the other organ in the individuals chosen will be f 81m times its 

 probable error. 



If the variations in size occurred in each of these two organs with 

 a frequency equal to that indicated by a probability curve, then every 

 pair of entries in each table should give the same value of r. As the 

 conformity between the observed frequency and that indicated by 

 probability is only approximate, the individual determinations give 

 values which differ slightly from one another ; but these variations 

 are on the whole small, as may be shown in three ways. 



In fig. 4, all the determinations of r are indicated : the deviation of 

 the organ whose value is fixed in each case is measured parallel to the 

 axis of y, the mean deviations of the associated organ parallel to the 

 axis of x : so that each pair of values in Tables I and II determines 



