1889.] A Nczv Cattlc-Pest. 589 
that it will account for all, and the notable case brought forward 
by the same author of Catabomba pyrastri is a pertinent one. 
This European species occurs in abundance in the western 
United States and in Chili, but has never been found east of the 
Missouri river. So, too, I doubt not that the European Eristalis 
tenax was at home, at least for awhile, on the Pacific coast be- 
fore it suddenly spread eastward about 1870. 
On the other hand many species that we should naturally 
expect to find on the two continents are yet confined to the one. 
Some, if not many, of these have failed to migrate simply because 
a good opportunity has never occurred, and our Hmnatobia is 
evidently of this class. But for others other explanations must 
be sought for. As the black rat and the Norway will not abound 
in the same region, so it is not unreasonable to suppose that the 
incompatibility, if one may so put it, of many species will pre- 
vent their living in common. Again, too, possibly the numerous 
parasites of insects may find an adaptabilty to newly introduced 
forms that may not only keep them in check, but actually keep 
them from obtaining a foothold. More potent causes undoubtedly 
are the climatic conditions and food supplies. As before inti- 
mated, those families of flies having the widest range of distribu- 
tion for their species have generally the largest number of " for- 
eign " species, while those in which the habitats are restricted 
have but few such species. A possible explanation for the latter 
is that a greater struggle for existence has weeded out the poorly 
favored ones and adapted the remainder more closely to the im- 
mediate environments. Certain it is that many of those fami- 
lies that are confessedly difficult to the systematist are the ones 
having fewer " foreign " species. 
However, the very extensive family of parasitic Tachinidae 
have remarkably extended habitats for their species, while I do 
not recall a single species common to the two continents, though 
a number reach through the two Americas. This non-identity 
of forms may be more apparent than real, yet it is very singular 
that none have been recognized, while in the related family of 
Anthomyidc-E nearly a third of the recognizable hundred or so 
species are " European," and the family has, if anything, been 
