DISTRIBUTION 379 
Silpha lapponica is strictly arctic in Europe, but is distributed 
over most of North America; Silpha opaca, on the contrary, 
is common all over Europe, but is strictly arctic in North 
America. Silpha atrata, common throughout Europe and 
western Siberia, was introduced into North America, but failed 
to establish itself. 
Southwest.—Very many species have come to us from Cen- 
tral America and even from South America. South America 
appears to be the home of the genus Halisidota, according to 
Webster, who has traced several of our North American spe- 
cies as offshoots of South American forms. Many of our 
species may be traced back to Yucatan. AH. cinctipes ranges 
from South America to Texas and Florida; H. tessellaris has 
spread northward from Central America and now occurs over 
the middle and eastern United States, while a form closely like 
tessellaris ranges from Argentina to Costa Rica; H. carye 
follows tessellaris, and appears to have branched in Central 
America, giving off Hl. agassizu, which extends northward 
into California. Similarly in the case of the Colorado potato 
beetle (Leptinotarsa decemlineata) and its relatives. Accord- 
ing to Tower, the parent form, L. uwndecemlineata, seems to 
have arisen in the northern part of South America, to have 
migrated northward and, in the diversified Mexican region, to 
have split into several racial varieties. The parent form 
grades into L. multilineata of the Mexican table lands, which 
in turn, in the northern part of the Mexican plateau, passes 
imperceptibly into L. decemlineata, which last species has 
spread northward along the eastern slope of the western high- 
lands, west of the arid region. In the lower part of the Mex- 
ican region the parent form may be traced into L. juncta, 
which has spread along the low humid Gulf Coast, up the Miss- 
issippi valley to southern Illinois, and along the Gulf Coast 
and up the Atlantic coast to Maryland, Delaware and New 
Jersey. In general, the mountains of Central America and 
Mexico and the plateau of Mexico have been barriers to the 
northward spread of many species, which have reached the 
