226 APPENDIX ir — INTBNSIVe SBGRBGATION. 



The distribution of these three species is of great interest, as it illus- 

 trates divergence both with and without local segregation. Basil- 

 archia archippus ranges over nearly the whole United States and over 

 the southern portion of Canada. B. astyanax occupies the valley of 

 the Mississippi and eastward to the Atlantic from the Gulf of Mexico 

 on the south to the lakes on the north. B. arthemis is distributed from 

 Newfoundland and Nova Scotia on the east, over New England, Can- 

 ada, the region of the lakes, away to the northwest, toward the con- 

 fines of Alaska. It will be observed that the area of distribution of B. 

 archippus includes the whole of that of B. astyanax and a large portion 

 of that of B. arthemis; while the areas of B. astyanax and B. arthemis 

 overlap along the whole northern border of the territory occupied by 

 B. astyanax. This area of overlapping distribution in which the three 

 species are associated is about i,ooo miles in length and from loo to 

 250 miles in width. 



(2) Forms of Isolation that separate Basilarchia archippus from B. 

 astyanax and B. arthemis. — It is evident that, in the present condition 

 of distribution, geographical barriers and territorial separation have 

 nothing to do with the integrity of B. archippus as a separate species. 

 In other words, it is not under the influence of geographical or local 

 isolation. Whatever may have been its past history, these certainly 

 are not the causes that at present prevent it from interfusing with 

 other species of Basilarchia with which it is associated. 



Again, seasonal isolation seems to have but little influence; for 

 though B. archippus seems to appear 1 5 or 20 days earlier than the 

 other species, the remainder of the breeding season, which extends 

 over many weeks, is coincident. 



The habits and feeding instincts of this species must tend to sepa- 

 rate it somewhat from B. arthemis, for this latter species frequents 

 forest regions, especially when elevated and hilly, while B. archippus 

 is found in the open country in fields and meadows, especially in low 

 levels. The eggs of B. arthemis are chiefly deposited on the species of 

 birch and willow that are found on the highlands ; while the eggs of B. 

 archippus are chiefly deposited on the willows and poplars found on 

 the lowlands, though on the White Mountains it occasionally extends 

 its range to as high levels as B. arthemis. There is, therefore, between 

 these species a slight degree of industrial isolation ; but this partial 

 segregation does not prevent their being often found in the same 

 fields, and unless held apart by sexual instincts and by partial infer- 

 tility, hybrids, which are now very rare, would be very common. 



We are, therefore, led to beUeve that diversity of sexual instincts, 

 accompanied by a considerable degree of cross-steriUty, is the chief 



