26 



VARIATION AND CORRELATION IN THE CRAYFISH. 



From this table it is quite clear that neither the plus ( -f ) nor the minus 

 ( ) set of differences can be discarded as probably merely a result of 

 random sampling. Instead it must be concluded that there are real and 

 definite exceptions to the rule of contiguity when the correlations of non- 

 homologous joints are considered. The most striking exceptions to the 

 rule are the correlations which involve as one variable the carpopodite of 

 leg i. Three out of the four of these correlations give minus ( ) excesses 

 and two are very large, the differences ratios being 7.16 and 5.13, the 

 largest in the table. On the whole we may conclude that in the correla- 

 tion of non-homologous segments of legs belonging to different metameres 

 there is no definite evidence in favor of the rule of contiguity. The 

 most striking exceptions to the rule are the correlations involving the 

 carpopodite of the cheliped. 



We may next consider the problem of the influence of position on cor- 

 relation within the metamere. The question here is: Are contiguous 

 segments of the same leg more highly correlated than non-contiguous 

 segments? To answer this table 11 has been prepared on the same 

 general plan as table 9. As before, the excess is designated plus ( -f- ) 

 when the contiguous joint correlations are the larger. 



TABLE 11. Correlation and position within the same metamere. 



It is evident that we have here the exact opposite of the rule of con- 

 tiguity, except in the case of leg I. There the two correlations involving 

 the carpopodite are high, just as they were found to be in the cross- 

 correlations considered above. In legs n and in the non-contiguous joint 

 correlations are greatly in excess of the contiguous. Too much stress 

 can not be laid on this result, however, because of the fact that we are 

 dealing with only three joints. Both of the contiguous joint correlations 

 necessarily involve the carpopodite as one of the variables. Now, a study 

 of table 3 will show that generally the carpopodite correlations of legs 

 n and in are low, irrespective of the relative positions of the joints cor- 

 related. On the other hand the carpopodite correlations of leg I are 

 relatively high. There is apparently some other special factor influencing 



