THE GENUS DENDROCTONUS. 25- 



connected with the anatomical problem, it was this state of confusion 

 which led the writer to make a study of the subject in order to deter- 

 mine the facts and principles involved and to establish a basis for 

 his future systematic and economic work on the scolytid and other 

 beetles. 



There appear to be two opposing ideas regarding the origin and 

 evolution of the primary and secondary elements of the insect seg- 

 ment. One involves the principle of reduction of several primitive 

 segments into one, on the theory that the transverse divisions rep- 

 resent modifications of several primitive segments. The other involves 

 the principle of complex modification from a simple undivided primi- 

 tive segment into many primary and secondary divisions, on the 

 theory that this has been brought about more or less independently 

 through the influence of the requirements of function to meet the 

 demands of peculiar life activity in different forms or species, and 

 that this plan of modification has been controlled and limited by 

 the fundamental plan of structure in the hexapodal type of organism, 

 and by the principle of relative proportions and correlation of parts 

 so as to conform to the general modification of the entire body in 

 the evolution of the species. 



The writer does not deem it advisable, in this connection, to dis- 

 cuss the relative merits of these theories or any of the other theories 

 advanced by different authors. He is inclined to believe that while 

 it is important to utilize any good evidence relating to the probable 

 origin and homology of parts, it is more important for present needs 

 to deal with the facts as they are found in existing forms and stages 

 and to so name and define the major and minor divisions or elements 

 of the segment that they may be readily recognized and utilized in 

 any comparative study of their modification and in the description 

 and identification of species, genera, and larger groups. Therefore 

 the writer's interpretation of the recognizable elements in the thoracic 

 segments of Dendroctonus does not involve any theory of origin or 

 evolution, but is based on the recognition of a dominant tendency 

 in the insect segment to represent a system of four longitudinal and 

 four transverse divisions, any one or all of which may or may not be 

 clearly represented in one or more segments of the same insect. 



With this conception of a prevailing principle as a guide to the 

 location of the possible primary and secondary divisions of the ana- 

 tomical elements as they are indicated in any given segment, and to 

 the recognition of the possible range of modification and distinction 

 as manifested in the different segments of the same insect or in the 

 corresponding segment of different insects, many of the difficulties 

 and confusing factors relating to the proper definition of parts and 

 application of names are eliminated. 



