On the Diptera of the Amber-fauna. 319 
group may be formed of the likewise very numerous species 
which, although so like some European species as to be at first 
glance mistaken for them, show upon nearer examination very 
definite plastic characters. The discovery of these characters 
often requires a great deal of attention; nevertheless they are of 
such a nature that the comparison of even single specimens 
leaves no doubt as to their specific differences.’ 
The large number of species contained in all the four groups 
shows that the Dipterous fauna of North America is not only 
very much like the European fauna, but that there is between 
them a relationship of a more intimate kind, which is to be com- 
pared only with that uninterrupted succession offered by the Dip- 
terous fauna of the whole northern part of the Old World. 
mon, w 
only slightly different in coloring; otherwise their faunas have 
no p 
which they can live and their brood can prosper. As the Le 
lulide show in this respect the nearest approach to them, the laws 
of their geographical distribution may also be the nearest to 
f the Diptera. The latter laws differ from those of the 
other orders of insects, by the wide area of distribution of the 
Single species and by the configuration of these areas. The 
not nearly the same for the species of all families, but vary ac- 
cording to families, so that the climatic character 18 most clearly 
5 pe : " 
AS instances of such species may be named: Chrysopila guadrata Say and 
Chrysopila nubecula Fall, Leptix vertebrata Say, and Leptis annulata Deg.. Leptis 
e Het ella bombylans Linn., Helom é an 
“eOmyza, lateritia Helo Meig., Sepedon pusillus Lew. and 
Paden spinipen Scop Phibporic opporita Laow and Philygria. punctato-nervosa Fall, 
