356 THE BIOLOGY OF INSECTS 



Polyommatus astrarche (now sometimes known as Aricia 

 medon)j which is a common insect in most parts of England. 

 Though a member of the Lycaenidae, or family of the 

 ** Blues," it has no obvious blue scaling on the wings, which 

 are blackish-brown above with a terminal row of orange-red 

 spots, and greyish below with a set of the black white-ringed 

 eye-spots characteristic of the " blue " group. In Scotland 

 this typical form of astrarche is replaced by the variety or 

 sub-species artaxerxes in which the terminal row of orange 

 spots are faint in the female and wanting in the male, 

 while the forewing shows a central white spot above, and 

 all the spots beneath the wings are entirely white showing 

 no black centres (Plate XIII). Along the coastal region of 

 Northumberland and Durham the ranges of the two forms 

 overlap, and here may be found intermediate phases, several 

 of which have received distinct varietal names. The true 

 connection between these different forms has long been a 

 subject of discussion among students of insects. A study 

 of the *' mixed population " of these butterflies in the 

 north-east of England might suggest a case of continuous 

 variation. After careful study of the subject J. W. H.Harrison 

 and W. Carter (1924) have given good reason for concluding 

 that artaxerxes arose as a mutation from astrarche; the two 

 forms breed true and artaxerxes is found nowhere but in 

 northern Britain and western Ireland, while astrarche 

 ranges right across the Euro- Asiatic continent. The mixed 

 assemblage of intermediate phases of the insect found in 

 Northumberland and Durham is due to mixed breeding, 

 which indicates Mendelian inheritance because segregation 

 of the parent races occurs, since " pure medon and artaxerxes 

 are to be taken constantly and in goodly numbers." Com- 

 plete geographical isolation of the artaxerxes race or its 

 infertility with astrarche would result in definite specific 

 distinction. 



The appearance of a stable variety or mutant which may 

 form the material for a new species is due, as we have seen, 

 to a change within the germ-cells whence the creature 

 develops, but as to the nature and origin of such germinal 



