Causes of Variation 1 39 



species from the European continent ; and within the 

 British Isles, this tendency to dark variation is most 

 marked in moths from Ireland and the hilly districts 

 of northern England and Scotland. In correspond- 

 ence with these facts we know that the British Isles 

 have a moister climate and a lower summer tempera- 

 ture than Continental Europe, while these conditions 

 are most marked in those highland and western 

 regions where the darkest moths are found (I43)- 



Many other direct effects of surroundings on the 

 variation of insects doubtless await discovery, but the 

 greatest care has to be exercised in making generalisa- 

 tions on the subject. We have a common butterfly 

 in these countries, Chrysophatius phloeas, whose wings 

 are of a metallic copper-colour with black spots. In 

 southern Europe a summer-brood of this insect occurs 

 in which the black has largely overspread the copper, 

 and this dark form has been artificially produced by 

 subjecting British pups to a high temperature. We 

 must admit, therefore, that the darkening of wing- 

 patterns cannot always be due to the action of cold. 



Another exception is furnished by the insect used 

 to introduce the subject of varieties and species; it 

 will be remembered that the black centres of the 

 wing eye-spots of Polyommatus astrarche disappear in 

 the Scottish race artaxerxes. In this case however it 

 is possible to suggest an explanation. It is commonly 

 taken for granted, since the type-form rtj-/rart-/6^ occurs 

 throughout Europe, extending into northern England, 

 while artaxerxes is confined to Scotland and western 

 Ireland, that the latter is an offshoot of the former. 

 But it is far more likely that an animal confined to 

 the western and north-western outposts of Europe is 

 not a new offshoot but a very old type which has 

 become extinct elsewhere. And this view of the 

 case is confirmed when we consider wherein artaxerxes 



