opinions will differ. Doubtless other orders have many such 
cases, but my studies enable me to speak of the two-winged 
flies only. In but a very few families of flies, in reality I may 
say in but one or two, do we have even a tolerable knowledge of 
the North American fauna. In quite a number, however, our 
knowledge is sufficient to base fairly good conclusions as regards 
distribution, and these conclusions lead me to the belief that al- 
most invariably species of flies common to the two continents 
have an unusually wide distribution in this country. Ten per 
cent, of our species of Syrphidae, a family of flies that comes 
rarely into direct relation with man's economy, are common to 
the two continents. Of the thirty species thus known very 
nearly all are found from the Atlantic to the Pacific, forming very 
nearly a half of the species that are known to occur across the 
United States. In the family of Tabanidae, or horse-flies, not a 
single one of the hundred and fifty species is known to be com- 
mon to the two continents, and very few species in the United 
States have a wide distribution. Among the AsiiidiE, a large 
family of predaceous flies, one species, and one only, is known to 
extend into the two continents, and this one species is one of the 
four or five that are found on the Pacific and Atlantic coasts. In 
numerous other cases I have observed similar facts, and always 
confidently expect to find such .species reappearing in the West- 
ern fauna. What conclusions may we draw from such facts ? 
That their distribution has been due to commerce ? Or, that they 
are indigenous throughout their extended habitats, persistent 
forms that have survived unchanged from preglacial times? 
Among the desmids, out of about three hundred species ac- 
credited to the United States, only about one-third are said to be 
peculiar to our fauna, the others common to all parts of the 
world, though chiefly European. As among other insects, I 
have found species of flies occurring only in the White Moun- 
tains and the Pacific fauna, which indicates the persistency of 
their types from different geological and climatic conditions. The 
circumpolar habitat of many such species may, as Osten Sacken 
suggests, account for their occurrence on the two sides of the 
v^ell as in Europe, but it is purely grc 
