PRELIMINARY CLASSIFICATION OF SCOLYTOIDEA. 217 



found intermingled with the broods of the typical forms and the 

 characters are not safhciently constant to distmgiiish them as separate 

 species they should simply be considered as coming witliin the range 

 of specific variation, but in the writer's opinion these varieties should 

 not be distmguished by a trmomial. 



In some genera and species the sexes may so differ in whole groups 

 of characters as to indicate, in some cases, different genera. But 

 there is always some character or set of characters common to both 

 sexes which would point to the same species even if they were not 

 found associated in the same brood. 



There is e"\ddence that in some of the species of Xylehorus and 

 allied genera in the Cryphalinse there may be occasional individuals 

 which represent a degenerate form or a caste m the social relations of 

 a brood or colony, with uniform but radically different characters 

 from those of either sex, and that on account of the radical differences 

 Avhich separate them from the species with which they have been 

 found they have heretofore been recognized as good species. Xyle- 

 horus planicollis Zimm. may be such an odd member of the Xylehorus 

 inermis colony and Xylehorus viduus Eichh. an odd type of the 

 Xylehorus fuscatus colony, but further observations will be needed 

 to settle the question. If tliis should be true in these cases it may 

 hold with isolated cases in other social species, like those of Hyjjoilie- 

 nemus, Stephanoderes, Dryocxies, etc. 



THE GENUS. 



There is more latitude, perhaps, for the selection of distinguishing 

 characters of the genera than there is for the species, but the same 

 principle applies. There mast be some single character or group of 

 characters common to a group of allied species which will serve to 

 distinguish the group readily from all other allied groups. The 

 range of departure or variation from the type of the genus is restricted 

 in some genera, but in other genera with many species there may be 

 a very wide range, so that the species will fall in distmct divisions 

 and subdivisions, which are designated by some authors as subgenera. 



Unfortunately there is a wade range in the opinions of different 

 authors as to the limits of a genus. Some go to the extreme in 

 restrictmg it to closely allied species wliile others go to the opposite 

 extreme (Hagedorn, 1910) and include a large number of genera or 

 so-called subgenera. It is plain to the writer that of the two extremes 

 the latter is the more objectionable because it will certainly contrib- 

 ute more than the former tow^ard retarding than advancmg knowl- 

 edge. The writer believes that there is a middle ground on w^hich 

 systematists should endeavor to get together in order that there may 

 be more uniformity in the conception and definition of the genus. 



