INTEKSIVE SEaREaATION. 3G3 



instincts, accompanied by a considerable degree of cross-sterility, 

 is the chief cause preserving the independent character of this 

 species. Excej)t for the sexual Segregation and Segregate 

 Fecundity there is every reason to believe that this species 

 could never have arisen, or, if it had arisen as a variety iu-some 

 isolated locality, would have been submerged in the allied forms 

 when its wider distribution was reached. This conclusion, 

 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 difler but slightly, those of 

 B. ArcJiippus are considerably divergent. This is an index of 

 the psychological and physiological 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 iu the 

 male genitalia, wliich is a form of segregate structure, is an index 

 of Sexual Segregation and Segregate Fecundity. 



The partial Segregation o/B. Astyanax and B. Arthemis. 



In the relations of these two species we find examples of 

 segregative influences difBering somewhat from those that have 

 just been found in the case of B. ArcJiippus. Eegional Segre- 

 gation, with exposure to difterent climates and adaptations to 

 different food-plants, has undoubtedly had an important in- 

 fluence in the formation of these species; but, in the part of the 

 country where they co-exist, 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. Ai'themis, 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,' pp. 159-160, we find mention of 

 two species {Cercyonis Alope and C. Nephele), in which the cross- 

 sterility must be considerably weaker than between the two 

 species we are now considering; for, iu the intermediate region 



