OVERLAPPING FORMS 161 



possible to suggest what the actual factors producing these ap- 

 pearances may be. 



There are, naturally, many other cases among birds which 

 are suspected of being in reality comparable, but in most of them 

 the evidence is still inadequate. Among Lepidoptera also there 

 are a few of these ; perhaps the most striking is that of Basilarchia 

 "proserpina." 1 * The genus is well known to European col- 

 lectors under the name LimenitiSj of which we in England have 

 one species, L. sibylla, the "White Admiral." A species very 

 like sibylla in general appearance is common in the northern 

 parts of the United States, ranging through Canada and Northern 

 New England, but rarely south of Boston. This species has 

 the conspicuous white bands across both wings like our sibylla. 



There is also a more Southern type known as astyanax, 

 which is very different in its appearance, being without the white 

 bands and having a broad irroration of blue scales on the posterior 

 border of the hind wings. The two are so distinct that one would 

 not be tempted to suspect any very close relation between them. 

 In its distribution astyanax is described by Field as replacing 

 arthemis south of latitude 42. About Boston it is much more 

 common than arthemis. 



The two forms encroach but little on each other's territory, 

 but where they do coexist, a third form, known as proserpina, is 

 found which is almost intermediate, with the white bands much re- 

 duced. There is now no doubt that this proserpina is a hetero- 

 zygous form, resulting from a combination of the characters of 

 arthemis and astyanax. Field succeeded in rearing a brood of 16 

 from a proserpina mother caught wild which laid 31 eggs, and of 

 these, nine (five males, four females) resembled the mother, 

 being proserpina, and seven (four males, three females) were 

 arthemis. There can be no question therefore that the mother 

 had been fertilised by a male arthemis and that no-white-band 

 is a factor partially dominant over the white band. Another 

 point of interest which Field observed was that the proserpina 

 female refused to lay on birch, poplar or willow, but accepted 



13 For these facts I am indebted to Mr. W. L. W. Field, who has lately published 

 an account of his observations and experiments. See especially, Psyche, 

 XVII, No. 3, where full references to previous publications are given. 



12 



