28 



VARIATION AND CORRELATION IN THE CRAYFISH. 



Similar combinations 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). 



'Tho letters in parentheses in this and succeeding tables refer to fig. 7. 



