( xlvi ) 



of the definitions of jiencra and liiglier nnits. For tlie degree of exactness 

 depends here (1) on tiie more or less intrinsic, study of a larger number of 

 forms than in tlie case of species and varieties, and (2) on the correctness of 

 tlie author's inductive reasoning. 



The classificatory category coming in our system of classification next to 

 the species is the genus. Some authors iuterjiohxte between genus and species 

 a category they call subgenus. From a comparison of a number of subgenera 

 which have been defined, we are able to state that they had either no standing, 

 being groups of species quite arbitrarily put together, or they represented well- 

 defined homogeneous groups — i.e. were equal to a genus. We do not see any 

 possibility of distinguishing between a defined genus and a defined subgenus. 

 To call one defined group a genus and another defined group a subgenus is quite 

 arbitrary. As there is a gap between every two species, and mostly between 

 the varieties as well, and, further, as the species fall into groups different in 

 extent or dififerent in composition, according to this or that organ being taken 

 as the basis of the groujjing, the limitation of the genera would be entirely left 

 to the personal opinion and ability of each individual classifier, if there was no 

 general definition of what kind of classificatory unit a genus is meant to be. 

 Systematists differ, indeed, very much in the extent given to genera, some 

 authors adhering to large unwieldy groups of the Linnean type, and others 

 erecting a genus for almost every species, and sometimes even for subspecies. 

 In order to check arbitrariness, to escape uncertainty as far as possible, and 

 to 'make generic classification more stable, we think it advisable to define a 

 genus as a classijieatonj unit one category higher than species comprising one 

 definable group of' species. 



In many cases the group contains only one species by the otiier members 

 having become extinct, or by there being as yet only one sjiecies known, the 

 other species being still undiscovered. 



In the present Revision of the Sphingidae it has been our special endeavour 

 to give a solid foundation to the genera, supplementing and rectifying the vague 

 or faulty definitions with which the workers in this gronj) of insects have 

 contented themselves. Though many genera hitherto considered to be valid 

 have been shown by us to have no standing, the number of genera of 

 Sphingidae has been much enlarged, owing to the closer examination of the 

 insects proving many groups of apjiarently similar species to be heterogeneous. 

 We have laid special stress upon the genera as conceived by us representing 

 stages in the evolution of the Sphingidae. 



There is nowadays a tendency among British Lejiidopterists to imitate 

 some American leading spirits in Lepidopterology in shifting the term " family " 

 (J'amilia ; designated by the ending -viae according to common consent) to 

 a lower category than that to wliich it was originally applied. We do not 

 see what good it serves to call, for instance, all the Hawk Moths together 

 a superfamily, and the next divisions of it families. It is an entirely 

 superfluous innovation, and only leads to confusion, like all shifting of 



