62 



COLORATION IN POLISTES. 



amount of yellow, and, on the other, toward the rufous representatives 

 of ihepallipes type characteristic of the Middle Atlantic States, while 

 in southern Wisconsin the divergence is toward the more melanic forms 

 of both the aiirifer z.\\^ pallipes types, characteristic of Oregon and the 

 New England States. At Rockford (north central Illinois) the con- 

 ditions are intermediate between the two just described. 



I suggest that the name P. variatus be retained for all these forms, 

 and regard the species as having been derived by the hybridization of 

 two originally distinct lines of development of southern origin, which 

 in the course of their migrations along the eastern and western sides 



Fig. 24.— Relation between melanism of second abdominal segment and remaining segments 



similarly expressed for 100 specimens from Gotha, Fla. 

 Fig. 25.— Similar relation for material from Willow Grove, Pa. Classes same as in figs. 22 and 23. 



of the continent have come together in the Mississippi Valley. This 

 view is in accordance with the opinion of T. D, A. Cockerell : 



The conclusion is justified that the central region fauna was practically stamped 

 out during the glacial epoch, and that the present fauna is derived from the boreal 

 fauns which survived to the east and to the west and the southern fauna which 

 survived in INIexico. This view seems to be supported by a consideration of the 

 present distribution of species as well as by geological evidence. 



This, in the main, accords with the lines of diffusion as far as made 

 out for other orders of insects. Webster (28), in treating of the dis- 

 tribution of the chinch bug, makes out two principal lines of diffusion, 

 one along the Atlantic seaboard and thence inland, the other up the 



