1913] New Application of Taxonomic Principles, 229 



genus. It has gradually become evident that this category is 

 a natural prime division of the subtribe, demanded in young 

 stocks where transitional forms are numerously present but 

 not as a rule called for in older stocks where such transitionals 

 are infrequent. 



It may be pointed out by way of illustration that we know 

 many insect stocks whose component forms are well differ- 

 entiated from each other; we know other insect stocks whose 

 forms are less markedly differentiated among themselves, and 

 we know still further stocks which comprise masses of closely 

 similar forms. The first are old stocks, the second are middle- 

 aged stocks practically in their prime; the last are young stocks, 

 still undergoing evolution and characterized by the presence of 

 many transitional forms. The same system of taxonomy is 

 not applicable to all these classes of stocks. The three classes 

 mentioned are of course not clearly delimited, for certain 

 stocks are bound to be intermediate between them. But each 

 stock can always be treated on its own merits. For the first 

 class in general, the current system of taxonomy answers fairly 

 well — that is to say, the tribes are usually quite easily divided 

 directly into genera. In the second class, comprising in 'general 

 the stocks of middle age, we need the subtribal category between 

 the tribe and the genus. In the youngest stocks we need to 

 employ still another category, as an elementary grouping of 

 genera, between the subtribe and the genus. This is what has 

 been termed the group-unit, for it is both theoretically and 

 practically the unit of taxonomic groups. 



So far as it has been possible to work out the status of the 

 group-unit to date, its value appears to correspond to a fractional 

 part of the contracted subtribal value and the whole or a part 

 of the transitional subtribal value, as these values are exhibited 

 in young superfamilies and stocks undergoing evolution. The 

 group-unit therefore corresponds to the well marked genus in 

 the old stocks, plus its intergeneric space which is conceived to 

 be a fixed quantity covering certain transitionals that have 

 dropped out. The well marked genus itself corresponds to the 

 typic genus of the group-unit, while the latter has associated 

 with it various transitional or atypic genera which are not 

 represented in the old stocks but must here be fitted into the 

 taxonomic system. These transitionals or atypic genera are 

 not subgenera of the typic genus. They are subordinated to 



