1913] New Application of Taxonomic Principles. 231 



with transitionals and hence their component categories are 

 difficult of deHmitation. 



These facts and the consequent necessity for a fractional 

 subtribal category must be apparent to anyone who studies 

 these flies assiduously. In many cases the natural tribes and 

 subtribes can not be defined on the external characters of the 

 adult, nor can they be defined in other than a very complex 

 and thus highly unsatisfactory manner on all characters, due 

 to the presence of the transitionals, and we are thus forced to 

 employ more restricted group categories in order to make a 

 taxonomic system fit them. The conditions which we face 

 here are those that obtain at any given time during the active 

 evolution of new and young stocks. If we had all the indi- 

 viduals that have been produced during the evolution of any 

 subtribe of insects, arranged before us in the order of their 

 descent, we would be totally unable to classify them into 

 either group-units, genera, subgenera or species, simply because 

 no lines of division would be indicated for such separations. 

 They would be found to form a mass of transitionals in a 

 gradual and spreading transition from first to last; through 

 their roots all would be found to connect by gradual transitions 

 with each other. But at any given point in their development 

 by excluding their predecessors, the remnant would be amenable 

 to separation into categories after the group unit plan. These 

 conditions actually obtain in certain young stocks today, and 

 it is only due to the fragmentary nature of the material which 

 we are able to secure out of their totals of countless individuals 

 produced that we are able to attempt a classification of the 

 residue. We do not have to fit their predecessors into the 

 taxonomic system, since they are all lost to us except recent 

 material which agrees with the present. Therefore we are able 

 to draw lines of separation, but the transitionals present demand 

 that the lines be drawn closely. Here lies the necessity for 

 the group-unit category. Its province is to represent the 

 transitional subtribal forms in their true relationships to the 

 typical divisions of the subtribe proper, in young and new 

 stocks now undergoing evolution. 



The term group-unit was chosen because the value of the 

 category to which it is applied is bound to be the unit of group 

 values. Species and genera are both taxonomic units, since 

 both enter into the concept and construction of the binomial. 



