BEITISH APHIDES. 



INTRODUCTION. 



It is a necessary condition of all Monographs that 

 they should be classificatory. This department of 

 biological science is a connecting link between the 

 work of the anatomist and the theorist, for the labours 

 of both would be wholly nugatory if identification of 

 organisms and their relations to each other were 

 ignored. The systematist is therefore an important 

 contributor to the advance of all science. 



The construction of a true natural classification 

 requires mental attainments of a high order, and 

 involves an acquaintance with extensive groups of 

 living forms. Thus, comparatively few have possessed 

 the requisite powers of comparison, aptitude to see 

 correct lines of afiinity, and acuteness to eliminate 

 characters which, perhaps, showy and obvious in them- 

 selves, are nevertheless of minor importance. Thus, 

 the conception of a race, a tribe, or genus, must partake 

 more or less of an abstract thought, and no two minds, 

 perhaps, will think alike in all particulars. 



In more than one instance I have in this Monograph 

 been induced, somewhat against my wish, to propose 

 a new genus upon the evidence of a single species. 

 Mr. Bentham has warned us of the suspicion which 

 naturally attaches to such a procedure. Still, if we 

 concede that nature is not bound by our conceptions of 

 typical forms, they may be permitted for systematic 



