28 



VARIATION AND CORRELATION IN THE CRAYFISH. 



Similar combiuations may be made for joint a of leg b and joint a' 

 of leg c. The whole system so formed will afford a comparison between 

 (a) the correlation of a joint in a first leg with its homologue in a second 

 leg, and (/3) the correlation of the first joint with the non-homologous 

 joints of the second leg. It is clear that in such a system of comparison we 

 avoid entirely any possible conflicting influence of the two factors homol- 

 ogy and contiguity of parts within the metamere. In order to detect 

 what the relative influence may be of homology and contiguity in an 

 antero-posterior direction (i. e., between metameres), we have merely to 

 separate the data in the appropriate way in the tables. In all of these 

 tables a plus ( + ) entry in the column headed "excess" means that the 

 constants for a pair of homologous joints is greater than that for the corre- 

 sponding non-homologous pair. The data are given in table 12. 



Table 12. 

 (A) MERIPODITES OF LEGS I AND II, AND II AND III (CONTIGUOUS METAMERES). 



Homologous 

 Joints. 



Meripodite i 

 with meripo- 

 dite II (aa' ).' 



Meripodite ii 

 with meripo- 

 dite III (a' 

 a"). 



Coefficient. 



0.9665 ± 0.0026 



.9729 ± .0021 



Excess. 



+ ( 8.60) 

 + ( 2.07) 

 + ( 3.54) 

 + ( 4.41) 

 + (10.55) 

 + ( 4.25) 

 + ( 9.19) 

 + ( 1.79) 



Coefficient. 



0.8943 ± 0.0080 



.9578 ± .OOaS 



.9502 ± .0039 



.9440 ± .0044 



.8632 ± .0102 



.9559 ± .0035 



.9021 ± .0075 



.9670 ± .0026 



Non-homologous Joints. 



Meripodite i with car- 



popodite II [ah' ). 

 Meripodite i with pro- 



podite II {ao.'). 

 Meripodite ii with car- 



popodite I (a'b). 

 Meripodite ii with pro- 



podite I (a'c). 

 Meripodite ii with car- 



popodite III (a'b"). 

 Meripodite ii with pro- 



podite III (a'c"). 

 Meripodite iii with 



carpopodite ii (a''b'). 

 Meripodite ix with pro- 



podite I (a"c' ). 



(B) MERIPODITES F LEGS I AND III (NON-CONTIGUOUS METAMERES). 



Meripodite i "] 

 with meripo 

 dite III (aa"). 



0.9686 ± 0.0025 



+ (9.94) 



+ (2.61) 



+ (2.18) 



+ (3.42) 



0.8652 ± 0.0101 

 .9579 ± .0033 

 .9601 ± .0031 

 .9539 ± .0036 



Meripodite i with car- 

 popodite II (ab"). 



Meripodite i with pro- 

 podite III (ac"). 



Meripodite iii with 

 carpopodite i (a"b). 



Meripodite iii with 

 propodite i (a"c). 



1 The letters in parenthoses in this and succeeding tables refer to fig. 7. 



