320 On the Diptera of the Amber-fauna. 
plicity of the conditions required for the existence of a speci 
still some families show in this respect peculiarities which do 
not find a satisfactory explanation in those two causes, 
On account of the very great extent of the area of distribu: 
tion of the Diptera in general, the faunze of distant countries 
have many more species of this order in common, than of any 
other order of insects, The same causes on which this extent of 
distribution depends facilitate even in our days the importation 
of Diptera much more than that of other insects, through the 
intercourse between countries. It is well known that Musca do- 
ions of smoked meat and cheese along with him, Piophila Pea 
sionts and Casei have accompanied him. They occur in Green- 
beantiful Symmictus costatus is found together with them, from 
in to the southern extremity of Africa. The barrier of a 
and the a uction of Medeterus inequalipes, common on th 
shores of 
with them, as for instance several of Oscinis and Chlorops with the 
cereals, also the noxious Cécidomyia destructor. Petalophora cap- 
wala occurs wherever the orange and the lemon are cultiva 
and with the extension of the culture of the olive-tree, Dacus 
Olee has followed it. | 
Those 
dis- 
nently colonize them. It is no wonder therefore that An 
which for a considerable period of time has been in connie bs 
always increasing intercourse with Europe, should have wit}. 
2 species in common. It would be m 
a 
a 
