PERFLATION OF OUE LEAFHOPPER FAUNA. 17 
that we are far from having sufficient data to warrant very positive 
conclusions. Nevertheless, the known facts concerning a number of 
the genera or subfamilies seem to point in certain directions and a 
cursory review of these would seem in place. If we compare, in a 
very general survey, the American with European or Asiatic leaf- 
hopper groups, we are perhaps first struck with numerous funda- 
mental similarities, and, second, with the comparatively few cases 
in which there seems to be specific identity; a condition which would 
indicate common origin for the groups in general, and, further, a 
common development through a long period with a separation only 
long enough to result in the minor separations of species. While 
migrations may account for some of the agreements, there are many 
in which such explanation seems unwarranted — and we have a few 
cases in which a comparatively recent introduction seems quite 
certain. 
In the genus Deltocephalus, which is practically of world-wide dis- 
tribution, a comparison two decades ago might have led one to believe 
the genus essentially European, as more than a half hundred species 
are listed there. But within the last 20 years species for North 
America have been discovered and described in large numbers until 
now there are nearly a hundred known from the United States alone, 
a number which far outweighs the European showing and, if Judged 
by number of species, we will be obliged to consider America as the 
home of the group and postulate a distribution from here to other 
geographic regions. Certainly the immense variety of forms with 
their wide range in latitude and altitude must be accepted as evi- 
dence of great antiquity. A great many of the species are boreal or 
alpine in distribution and while perhaps some allowance should be 
made for more extended collection and study in the North, it appears 
evident that the center of abundance and of variety of adaptation 
is to be found in the plain and plateau region of the Mississippi 
Valley and among the Rocky Mountains. From such a center the 
species diminish in number to the southward, few being known 
from Central America and South America. Dispersal then may have 
proceeded by northward routes to Europe and Asia, and southward 
through Central America and into South America. As for the spe- 
cies which have a common distribution in America and Europe it 
is as easy to assume migration from America to Europe as the reverse. 
DeltocepJialus abdominalis Fab. and D. minki Fieb., which occur in 
northern Europe and America, may thus have migrated in one 
direction or the other, but our D. debilis Uhl., which may be a deriva- 
tive from abdominalis, has apparently had its origin in this country. 
In rather striking contrast to the Deltocephalus group we may 
take the genus Agallia, which, with some 25 or 30 species for America, 
shows a strong preponderance both in number of species and adapta- 
tion for environment in the southern United States and especially 
