XII PKEFACE. 



Authors differ in their mode of treatment of species, the identity 

 of which is doubtful; some prefer at once to describe them as 

 new, others assume the identity, until the difference is proved. 

 For several reasons of a purely practical kind , I prefer the 

 latter method, thus following the principle, laid down by Fabricius 

 (Philos. entomologica) : Locus natalis speciem nunqiiam distinguit. 

 Once described as a new species, without indication of its 

 distinctive characters, the species escapes attention; on the con- 

 trary, it invites one's notice and challenges criticism, as long as 

 it is quoted as common to both continents. A time will come 

 when it will be possible to subject that wiiole class of species 

 to a thorough comparative study. 



Stnontmt. It has been my effort throughout to make sure, as 

 much as possible, that every name, which figures in the list, 

 should actually represent a different species. This is reached, in a 

 certain measure, for the fauna north of Mexico (with the exception, 

 of course, of those families, which have not been worked at all: 

 the Muscidae calypterae etc.). To attain this result, I have first, 

 made out a number of synonymies by means of an attentive 

 reading of the descriptions; and, secondly, I have visited the 

 Museums in London, Paris, Lille, Berlin, Frankfort, Darmstadt, 

 Turin and Vienna^ and have seen the types of descriptions, which 

 they contain. Any one, who has visited public Museums for the 

 purpose of examining types of descriptions , knows , that even 

 under the most favorable circumstances, that kind of work is 

 not like work done at home (especially in the difficult families). 

 Moreover, the study of types of descriptions must be based upon 

 a previous knowledge, and a thorough one, of the corresponding 

 species. As I had no collection with me for comparison, and 

 had to rely on my memory, and as my knowledge in the different 

 families of diptera is very unequal , and , in some of them very 

 small, I am far from having exhausted the study of the North 

 American types, contained in those Museums. I am also far from 

 believing, that wiiat I made out is always free from error. Those 

 who in future will take up single families for monographic work, 

 are therefore strongly recommended not to take for granted the 



