8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



marked generic types, such as CBstrus, Cuterebva, Dcxia, Macrony- 

 chia, P/iasia, Trichopoda, Meigenia, Masicera, PJwroccra, Tachina, 

 Gonia, Belvosia, Plagia, Thryptocera, Phania, CEstrophasia, Milto- 

 gramma, Pyrrhosia, Ocyptera, Gymnosoma, Bchinomyia, Hystricia, 

 Dejeania, Sarcophaga, Calliphora, Musca, Stomoxys, Glossina, and 

 at least a hundred others. These types correspond in value to the 

 more settled genera of the older superfamilies, where intermediate 

 form- are largely lacking. In the present superfamily, however, it 

 is quickly seen that massed in between these many typical forms 

 are numerous intermediate ones, which collectively vary in all direc- 

 tions and combine certain of the characters of the various types. 

 These intermediates are the bridges for the passage of genera, so to 

 speak — the inevitable precursors and resultants in the process of the 

 evolution of genera. The same holds good of species. Numerous 

 intergrades are found to group naturally around and between the 

 various species. That these intermediates and intergrades are pres- 

 ent is due to the fact that the Muscoidea are now — at the present 

 day, geologically speaking — in their period of greatest prolificacy, a 

 period characterized by a condition of multiform development. 

 After the lapse of a great space of time, many of these intermediate 

 forms will have dropped out of the struggle, leaving a residue more 

 or less well defined from each other and thus much more amenable 

 to taxonomic treatment. This is now the case with the older dip- 

 terous superfamilies. which have long since passed their period of 

 greatest prolificacy. 



It should be explained that the term "intermediates" is used to 

 designate forms of generic rank or higher, and "intergrades" to 

 designate those which are only of specific rank. The further term 

 "intergradants" may be employed to designate individuals which 

 connect species, but upon which it is not practicable to bestow names. 



The Muscoidea are of very recent evolution — in fact, their evolu- 

 tion is still going on. Here are species, genera, and families in the 

 making. The whole superfamily is one enormous assemblage of 

 thousands upon thousands of forms distinguishable from each other 

 by only slight differences and exhibiting characters which intergrade 

 in all directions. That such a multitude of closely similar forms is 

 exceedingly difficult to classify goes without saying. These forms 

 can not be classified in the ordinary way. but demand special treat- 

 ment adapted to the conditions. 



The key to the whole situation, when it comes to methods of tax- 

 onomic treatment in this superfamily, is that we have here the task 

 of defining not only the numerous well-marked types corresponding 

 to the existing forms in the older and less specialized dipterous 



