COIX5RATION IN POUSTES. 



6l 



From .these determinations it is apparent that a positive correlation 

 exists between the degree of melanism of the abdomen and that of the 

 thorax and the clypeus. 



The curves shown in figs. 22, 23, and 26 represent these relations 

 between the clypeus and abdomen graphically. 



This same tendency toward melanism in the northern forms is 

 observed in the rubiginosus group. Here the lighter species, such as 

 texanus and bellicosus, predominate in the narrow coastal region, while 

 the group, so far as it is found to the northward, is represented by the 

 dark uniformly colored rubiginosus. 



Just as striking, perhaps, as the relation between melanism and lati- 

 tude, is the prevalence in the arid regions of Arizona and New Mexico 

 of the yellow species, P. flavus and P. aurifer, and even P. navajoe. 



22 



FIG. 22. Relation between melanism of second abdominal segment and clypeus of female. 

 Cold Spring Harbor collection. 



FIG. 23. Relation between melanism of second abdominal segment and clypeus of female. 



Willow Grove collection. 



These three species stand in the relation of albinic extremes to the 

 three groups rubiginosus, aurifer, and canadensis and appear to have 

 been produced by similar conditions in these three different groups of 

 the genus. Finally, I have been impressed with the mixed condition of 

 the species for the upper half of the Mississippi Valley. The Missis- 

 sippi Valley is emphatically a transition zone between the aurifer type 

 of the West and the pallipes type of the East. Specimens gathered in 

 this valley fall into the species aurifer and variatus or pallipes and 

 metrica, according as they incline to the western or the eastern type of 

 coloring; but, as I have shown (pp. 12-33), representatives of both 

 types are repeatedly obtained from the same nest ; also in different 

 parts of this area the species vary in a fashion parallel to their vari- 

 ation in other parts of the country. Thus the divergence in southern 

 Illinois is, on the one hand, toward the aurifer type with a large 



