XXIV PREFACE. 



principal works on North American Diptera, beginning with the 

 Catalogue etc. of 1858, have been published. There is not the 

 slightest exaggeration in saying that, without the encouragement 

 and the support, received from the Smithsonian Institution for 

 the last 20 years, the study of North American diptera would 

 have remained far behind the stage which it has reached at present. 

 The inherent limitation of a Catalogue like the present con- 

 sists in the fact, that although it is more than a mere compilation, 

 it is less than a monograph. In many respects, the task of the 

 monographer had to be encroached upon : synonymies established, 

 species transferred to the proper genera, European species, occurr- 

 ing in North America, recognised and introduced in the lists etc. 

 The amount of latent labor of this kind, accomplished in this 

 Volume, will reveal itself to those, who will take the trouble 

 to compare it with my earlier Catalogue (for instance in the 

 Asllidae or Syrpliidae). There is some danger in carrying this 

 kind of anticipatory epuration too far, because in performing it, 

 we cannot expect to attain the thoroughness of a monograph. 

 And it is in the belief, that I have reached the point, where it 

 is time for me to stop, that I hand over my work to the public, 

 with a full sense of its imperfections. 



C. R. OSTEN SACKEN. 

 HEIDELBERG, Germany 

 June 1878. 



