228 Annals Entomological Society of America [Vol. VI, 



dimly comprehended before. The one who uses this key con- 

 scientiously and with fair judgment must get this insight. It 

 only remains to bring the taxonomy into accord with the 

 conditions. This is no simple matter, but it is capable of 

 adjustment. 



A careful comparative study of muscoid conditions by the 

 writer, extending over the past five or six years and beginning 

 before the reproductive and early-stage criteria became available 

 has resulted in what may be called the typic-atypic application 

 of taxonomic principles. The idea was dimly comprehended 

 in 1907 from a study of the external adult anatomy alone and 

 published in May, 1908 (Tax. Musd. FHes), while a clearer 

 perception of it was gained and the foundation for its practical 

 application laid during the next few months and the results 

 published in September, 1908 (Rec. Res. from Rear, and Dis. 

 Tach.). From that time to the present the typic-atypic idea 

 in taxonomy has kept pace with the progress of the investiga- 

 tions into the reproductive and early-stage characters of the 

 muscoid flies as compared with their external adult morphology. 

 The working out of the scheme of application with /the view of 

 ultimately bringing it to a point of completeness has been 

 laborious in the extreme, and many mistakes and new starts 

 have been made. Theoretical phylogeny and a taxonomic 

 application to match have been constantly checked up by 

 practical and actual phylogeny, thus showing errors that have 

 had to be corrected. 



The writer has been still further fortunate in being able to 

 spend some time during the past three years, 1910 to 1912, in 

 several districts of the Andean montanya in Peru and Ecuador, 

 perhaps the most favored biotic region on earth and thus the 

 best adapted to illustrate the working out of phylogenetic 

 principles in nature. Here he has been tremendously impressed 

 with the extreme richness in transitional forms displayed by 

 certain of the youngest muscoid stocks, which have furnished 

 additional proofs of the soundness of the typic-atypic system of 

 treatment. A paper on these forms is forthcoming (New Gen. 

 and Spp. Muse. Flies, chiefly Hystriciidae from the And. Mon- 

 tanya) . 



The typic-atypic system calls into use the new group-unit 

 category, which includes the typic genus and such atypic 

 genera as approach more closely to it than to any other typic 



