226 APPENDIX II—-INTENSIVE SEGREGATION. 
The distribution of these three species is of great interest, as it illus- 
trates divergence both with and without local segregation. Baszl- 
archia archip pus 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 lakesonthenorth. B.arthemzs 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. 
archip pus includes the whole of that of B. astyanax anda 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. ‘Thisarea of overlapping distributionin which the three 
species are associated is about 1,000 miles in length and from roo to 
250 miles in width. 
(2) Forms of Isolation that separate Basilarchia archippus from B. 
astyanax and B. arthemis.—lIt 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 15 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. arthemzs, 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 believe that diversity of sexual instincts, 
accompanied by a considerable degree of cross-sterility, is the chief 
