378 THE BIOLOGY OF INSECTS 



a result of the study of regional variation among insects, as 

 among other groups of animals, increased attention is being 

 given to this factor. The famous " Large Copper " Butter- 

 fly (Chrysophanus dispar) of the East Anglian fens, extinct 

 for the last sixty years, is apparently unknown on the 

 Continent, and opinion is divided as to w^hether it should 

 be regarded as an '' insular sub-species " of the central 

 European C. rutilus or a distinct but closely allied species. 

 The large Chinese Saturniid silk-moth Philosamia cynthia 

 is represented in Japan and Java by readily distinguishable 

 forms (P. pryeri and P. insularis respectively), which are 

 also regarded as " good " species by some students and as 

 sub-species by others. Actios selene, an Indian moth of the 

 same family, with wings of a predominantly pale greenish- 

 cream colour, has a sub-species A. calandra on the Andaman 

 Islands with bright yellow wings in the male. Such changes 

 in isolated *' colonies " of certain insects may be explained 

 as due either to strongly impressed environmental influence 

 or to the probability that the germinal constitution of the 

 small isolated group differs from that of the main conti- 

 nental stock. Besides geographical isolation, seasonal 

 change in the breeding season serves to segregate incipient 

 species, and so do modifications in the pairing organs of 

 a variety developing from a parent race ; this last-named 

 feature is well illustrated by the two forms of British wasps, 

 Vespa tufa and V, atistriaca^ described in a previous chapter 

 (pp. 234-6). Such modifications tend to result ultimately 

 in mutual infertility and complete prevention of pairing, 

 as K. Jordan (1896) pointed out in his exposition of 

 " mechanical selection." 



Insects far outnumber both in kinds and individuals all 

 other known animals now living on the earth and in the 

 waters taken together. Their individual lives are relatively 

 short and the numbers of their successive generations in the 

 course of the world's history must therefore have been 

 enormous. Since the far-oflF Devonian Period at least, the 

 evolution of the class has been going on, and long before 

 then the most primitive insects must have diverged from the 



