PARTIAL ISOLATION ILLUSTRATED. PLING 
cause preserving the independent character of this species. Except 
for the sexual segregation and segregate fecundity there is every rea- 
son to believe that this species could never have arisen, or, if it had 
arisen asa variety in some isolated locality, would have been submerged 
in the allied forms when its wider distribution was reached. Thiscon- 
clusion, which has been reached by observing the general relations of 
the species, is confirmed by a minute examination of the structure of 
the threespecies. We find that while the male genitalia of B. astyanax 
and B. arthemvs differ but slightly, those of B. archippus are consider- 
ably divergent. This isan index of the psychological and physiolog- 
ical relations of varieties and species of no small importance; for a 
comparison of many species shows that differences of this kind are 
usually accompanied by corresponding degrees of segregation in sexual 
instincts and of cross-sterility. In other words, we find that difference 
in the male genitalia, which is a form of structural segregation, is an 
index of sexual segregation and segregate fecundity. 
(3) The Partial Isolation of B. astyanax and B. arthemis.—In the 
relation of these two species we find examples of segregative influences 
differing somewhat from those that have just been found in the case of 
B.archippus. Regional isolation, with exposure to different climates 
and adaptations to different food-plants, has undoubtedly had an 
important influence in the formation of these species; but, in the part 
of the country where they coexist, their life-histories correspond com- 
pletely, and cross-unions seem to be frequent. The hybrid form has 
been described as a separate species, and some entomologists have 
classed it asa dimorphic form of B. arthemis; but Scudder gives several 
reasons for believing that it is the result of cross-unions between these 
two species. There are, however, several reasons for believing that 
partial segregate fecundity exists between the two species; for, in the 
strip of territory where the two are associated they do not completely 
coalesce, as would be the case if they were completely cross-fertile. 
In Scudder’s Butterflies of New England, pages 159 and 160, we find 
mention of two species (Cercyonis alope and C. nephele), in which the 
cross-sterility must be considerably weaker than between the two spe- 
cies we are now considering; for, in the intermediate region in which 
their areas overlap, the intergrade forms are comparatively abundant. 
Moreover, the difference in the male genitalia of B. astyanax and B. 
arthemis, though much less than that which appears when either of these 
is compared with B. archippus, is such as indicates a considerable 
degree of infertility. 
In these two species we have, then, a good example of partial isola- 
tion through distribution over areas which, though overlapping, are 
