INTENSIVE SEGHEaATIOK. 361 



problems of divergent evolutiou ; for they fiirnisli strong indica- 

 tions tliat organisms wliicli are witli dilficulty distinguished from 

 each other by external form or colour, may, nevertheless, be -Aell 

 established species — segregated presumably by sexual instincts 

 corresponding to sexual characters by which those of opposite 

 sexes of the same species readily recognize each other, and 

 |)robably cut off from the possibility of producing hybrids through 

 incompatibility of physiological endowments. In the origin of 

 some of these species Geographical Segregation may have had an 

 important influence ; but concerning others there can hardly be 

 a doubt that the segregative influences^ holding apart species 

 that occupy the same districts, were, from the first, peculiarities 

 of their sexual instincts and constitution. The reason for ac- 

 cepting this view of their origin is found in the fact that, though 

 slightly divergent in other points, the characters by which they 

 are clearly distinguished are found in the forms of the male 

 genitalia ; and in the characters of these organs we find clearly 

 marked species, for the most part free from the intergrading 

 forms which would certainly be presented if the difierent species 

 were not prevented from crossing by sexual instincts or consti- 

 tution. 



A full description of these genera, with observations on the 

 asymmetrical development of the right and left sides of tl.e 

 genital armature in Thanaos, will be found in Scudder's 'Butter- 

 flies of New England; ' see also Mem. of the Boston Sue. Nat. 

 Hist. ii. (1874), and Proceedings of the same Society for April 27, 

 1870, vol. xiii. p. 282 (1871). 



DivEEGENT Species of BasilarcMa. 



Basilar cilia (Scudder) is an attractive genus of butterflies 

 peculiar to North America, where it is represented by four or 

 five species. Three of these are found in New England, and are 

 minutely described in Scudder's ' Butterflies of New England,' 

 from which I draw my information (pp. 250-305). 



The distribution of these three species is of great interest, 

 as it illustrates divergence both with and without Local Segrega- 

 tion. B. ArcMppus ranges over nearly the whole of the United 

 States and over the southern portion of Canada. B. Astyanax 

 occupies the valley of the Mississippi and eat^tward to the 

 Atlantic from the Gulf of Mexico on the south to the lakes 

 on the north. B. Artlicmis is distributed from Newfoundland 



24* 



