18 



VARIATION AND CORRELATION IN THE CRAYFISH. 



Working in this way from formula (i) above, we have for the mean 

 indices, or the mean proportions of the joints of the legs in comparison 

 with cephalothorax length, the results set forth in table 6. 



Table 6. — Mean proportions of the different joints of the legs. 



[Unit = 1 per cent, of cephalothorax length.] 



Joint. 



Meripodite . 

 Carpopodite 

 Propodite . . 



Leg I. 



38.63 

 28.22 

 79.15 



Leg II. 



30.11 

 18.44 

 27.11 



Leg III. 



33.86 

 23.57 

 34.91 



By this table of mean proportions the differentiation between the legs 

 is again clearly shown. In passing it may be remarked that the values 

 for the proportions of the difiPerent joints of the legs as given in this table 

 will perhaps be useful for diagnostic purposes in defining the species, 

 since they represent the mean of a considerable number of specimens. 

 In this connection, however, the point brought out in a later section of 

 the paper, dealing with "index correlations" (pp. 40-45), must be kept 

 in mind. 



Turning to the question of the relative variability in the proportions 

 of the different legs, we have the results given in table 7. The values 

 in this table are the standard deviations of the indices whose means are 

 given in table 6. 



Table 7. — Variability of the proportions of the different joints of the legs. 



[Unit = 1 per cent, of cephalothorax length.] 



From this table the following points are to be noted: 



(rt) Leg I, the most highly developed and differentiated of the three 



under consideration, is the most variable in its proportionate as well as 



in its absolute size. 



