38 A BOOK OF INSECTS 



of the specimens from these localities exhibit characters 

 which, in the southern counties, are rarely seen. The 

 black spots in the centre of the fore-wings are smaller, 

 and surrounded by white ; the orange marginal spots are 

 indistinct, while the black spots on the underside of the 

 wings are small, or altogether wanting. At this stage 

 the butterfly is known popularly as the Castle Eden or 

 Durham argus, and to men of science as the variety 

 salmacis. If we go still farther northward, to the Clyde 

 in the west and to Aberdeen in the east, we shall find 

 that these characters are intensified, and that the southern 

 English form of the butterfly does not occur at all. In 

 these Scotch forms the central spot in the fore-wing is 

 pure white, the orange marginal spots are absent (in a 

 few specimens they are dimly discernible), while the black 

 spots on the underside of the wings have entirely dis- 

 appeared. We have now the Scotch brown argus, the 

 variety artaxeroces of science. 



" These typical Scotch insects " (writes Professor Car- 

 penter) " differ so markedly from those found in the south 

 of England that they were formerly believed to belong 

 to a distinct kind. This opinion received confirmation in 

 the fact that while the southern form has two life-cycles 

 in the year (the June butterflies laying eggs which develop 

 into a fresh generation of butterflies in August, the off- 

 spring of these surviving the winter as caterpillars), the 

 northern form has but a single brood (the butterflies 

 appearing in June and July, and the caterpillars hatched 

 from their eggs not pupating until the following spring). 

 But the study of the insect in the north of England 

 (especially in Durham) has shown, as we have seen, that 

 both the Scotch and southern forms occur together, and that 

 every intermediate link between them can be found. More- 

 over, all these diverse forms can be reared from the same 



