16 



VARIATION AND CORRELATION IN THE CRAYFISH. 



Each joint of leg iii is larger than the corresponding joint of leg ii, 

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

 only are legs ii 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 



leg II = 61.2 per cent.; leg iii = 69.6 per cent, 

 leg II = 68.0 per cent.; leg iii = 67.5 per cent, 

 leg II = 90.0 per cent.; leg iii = 103.1 per cent. 



Meripodite 



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 coeflScient 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. 



Joint. 



Leg I. 



Leg II. 



Leg III. 



Meripodite.. 

 Carpopodite 

 Propodite . . . 



Mean. . 



1.3. 151 ±0.379 

 14. 267 ± .413 

 18.047 ± .528 



12.821 ±0.369 

 13. 705 ± ..396 

 12.082± .347 



11.972±0.344 

 13.184± .380 

 11.789± .339 



15.155 



12.869 



12.315 



From this table we note the following points : 



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

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

 practically insignificant. 



(h) 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). 



