PARTIAL ISOLATION ILLUSTRATED. 227 



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 as a variety in some isolated locality , would have been submerged 

 in the allied forms when its wider distribution was reached. This con- 

 clusion, which has been reached by observing the general relations of 

 the species, is confirmed by a minute examination of the structure of 

 the three species. We find that while the male genitalia of B. astyanax 

 and B. arthemis differ but slightly, those of B. archippus are consider- 

 ably divergent. This is an 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 as a 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 andC. 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 



