INTEISTSTVE SEGREGATTOIS'. 365 



entirely preclude liybrids between B. ArcJiippus and each of the 

 other species. Examples of both tliese hybrids are described by 

 Scudder. 



While this Segregation was being completed, one of the two 

 forms thus created must have become subject to a new set of 

 segregative influences, arising from wider distribution with di- 

 versity of climate and of habits of feeding, reinforced by a slight 

 degree of Segregate Tecundity. £. Astyanax and B. Arthemis 

 are the two species resulting from this last Segregation, and the 

 process is so far from being complete, that wherever the areas 

 of these two species overlap a hybrid form, which has been 

 known as B. Proserpina, appears. That it is a hybrid is proved 

 by the fact that it " varies most toward Astyanax where tliis 

 prevails, and most towards ^?■^7ieM^^5 where that prevails," that it 

 is found only in the narrow belt where the two species are brought 

 into contact, and that it has been reported from so many points 

 in this narrow belt that there is reason to believe that it occurs 

 wherever the two species are brought into contact. If our ex- 

 position of the Segregations to which these species have been 

 subjected is correct, they are cumulative in two respects — first 

 because after one Segregation has been established another is 

 superimposed, and second because a partial segregation established 

 in one generation tends to become more complete in subsequent 

 generations. 



The primary causes in the whole process are the activities of 

 the organisms acting upon each other and upon the environments 

 in such a way as to produce, in the first place. Independent 

 Generation with some degree of divergence, and then Unbalanced 

 Natural Selection and other forms and transformation, which, 

 acting upon selections of the species that are prevented from 

 crossing, result in ever increasing divergence. 



DiVEEGENT Evolution in the Periodical Cicada 

 {Cicada septemdecini) *. 



In this species we have examples of two quite distinct diver- 

 gences, each depending on its own forms of Segregation, which 

 are easily recognized. 



The life-history of this insect covers 17 years and one or two 



* My information is chiefly derived from the U.S. Dei^artment of Agriculture 

 (Division of Entomology), Bulletin No. 8, by Dr. 0. V. Eiley. 



