PREFACE. XI 



that I was struck with the relationship of the Western and of the 

 Mexican fauna and have been able to identify several species, 

 published by me as new, in my Western Diptera. California 

 partakes of this relationship , and shows , at the same time, sin- 

 gular and unexpected coincidences with Europe, not shared by 

 the eastern United States. Future describers of western and 

 southern siecies will have to bear these facts in mind. 



Localities. The scope of this work did not allow much 

 detail in the matter of localities. Still , as much as the given 

 space allowed, I have inserted the data which I possessed on 

 the subject. Describers of insects, and especially of exotic forms, 

 are often very careless about statements of that kind. It is very 

 probable, for instance that many species, described by Macquart 

 as coming from Philadelphia or Baltimore, were merely sent 

 from those cities, but collected somewhere else; some of these 

 species have since been received from Texas only. It is to bs 

 hoped that future describers will be more accurate about localities 

 and their altitudes. California and Mexico, in different altitudes, 

 contain several different faunas and the study of the geographical 

 distribution of insects would reach very erroneous conclusions, 

 if it did not discriminate between these faunas. 



Species common to europe and to north America. A very con- 

 siderable number of European species is also found in North 

 America , without belonging in the number of imported insects. 

 Some of the species, common to both continents, do not show 

 any perceptible differences ; in others, a difference exists, but not 

 such as could be considered a specific character. And thus, by 

 gradations, a point is reached, where the specific difference be- 

 comes evident*). A careful study of almost any species, con- 

 sidered as identical, may unexpectedly disclose a minute, but 

 sometimes important distinctive character. Hence all the species 

 of the class in question must be considered as open to challenge. 



*) About the species common to both continents, and the gradations 

 occurring in the specific differences, compare Loew, in Silliman's Joui'n., 

 Vol. XX.XV.I, p. 317. 



