16 



VARIATION AND CORRELATION IN THE CRAYFISH. 



Each joint of leg in is larger than the corresponding joint of leg n, 

 by an amount which is in every case many times its probable error. Not 

 only are legs n and in differentiated from one another in absolute size, 

 but also in their proportions, as the following ratios show. 



100 X Carpopodite 



Meripodite 

 100 X Carpopodite 



Propodite 

 100 X Propodite 

 Meripodite ' 



, leg ii =61.2 per cent.; leg in = 69.6 per cent. 

 , leg ii = 68.0 per cent.; leg in = 67.5 per cent, 

 leg n = 90.0 per cent.; leg in 103.1 per cent. 



How do these differentiated appendages compare with respect to rela- 

 tive variability ? Does the degree of variability run parallel to the degree 

 of morphological specialization, or does the reverse relation hold? To 

 show the bearing of our results on these questions, table 5 has been pre- 

 pared. This gives the coefficient of variation for each joint of each leg. 

 On account of the differentiation of the legs in absolute size it is idle to 

 use the standard deviation as a measure of comparative variability. 



TABLE 5. Comparative variability of different appendages coefficients of 



variation. 



From this table we note the following points : 



(a) Leg i is the most variable of the three; leg n stands next, and 

 leg in is the least variable, but the differences between the last two are 

 practically insignificant. 



(6) The differences between corresponding joints of the different legs 

 in respect to relative variability are, on the whole, small as compared with 

 their probable errors. The only marked exception is the propodite in leg I. 



(c) The considerable excess of the mean variability of the joints of 

 leg i over the means for legs ii and in is largely due to the great 

 variability of the propodite of leg i (i. e., the great chela). This is by 

 far the most variable of any of the characters studied. The high varia- 

 bility of the great chela has been noted by other students of variation in 

 the Crustacea (cf. Yerkes, 1901, and Schuster, 1903). 



