134 The Classification of Insects 



the knowledge and opinion of the naturalist. And 

 that "variety" and "species" express ideas which 

 grade into each other is shown by the introduction 

 of other terms for less or greater degrees of differ- 

 ence. Irregular variation in individuals, such as the 

 appearance of a white border to the central black 

 spot of the forewings in southern P. astrarche, is styled 

 " aberration." The more definite characters shown 

 by the northern intermediate form are considered 

 worth ranking as a variety and distinguishing by a 

 name, P. astrarche var. sahnacis. And artnxerxes with 

 its comparative constancy over a wide and well- 

 marked area, would be called by some naturalists 

 a "geographical race," by others a "sub-species." 

 Even if the isolation and the extinction of connecting 

 links, which we have imagined, were to happen, some 

 naturalists, finding artaxerxes, though distinct in 

 coloration, inseparable by structure from astrarche, 

 would prefer to consider it a " sub-species " rather 

 than a " distinct species." 



In the majority of cases, however, there will be 

 found general agreement among naturalists in the 

 practical use of the term " species." No one, for 

 instance, is likely to deny that P. icarus is " specific- 

 ally " distinct from P. astrarche. But it may be taken 

 for granted that with increased knowledge comes 

 increased difficulty in distinguishing species from 

 each other. The naturalist who knows only British 

 insects easily discriminates between astrarche and 

 icarus, though the former insect is nearer to the latter 

 than to any other British butterfly. But a study 

 of foreign insects shows us that there are species 

 which help to bridge the gap between them. In 

 order to recall the main differences, we may state 

 them in tabular form. 



