divergence; in the periodical cicada. 229 



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 nar- 

 row belt that there is reason to believe that it occurs wherever the two 

 species are brought into contact. If our exposition of the segregations 

 to which these species have been subjected is correct, they are cumu- 

 lative 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 com- 

 plete 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 selection of different forms 

 producing transformation, which, acting upon sections of the species 

 that are prevented from crossing, result in ever-increasing divergence. 



3. Divergent Evolution in the Periodical Cicada (Cicada septendecim).* 



In Cicada septendecim 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 seventeen years and one or two 

 months. The imago appears late in May, and for a little more than a 

 month the males make the woods ring with their shrill stridulations. 

 The eggs, which are deposited in the green twigs of trees, mature dur- 

 ing the latter part of July, and each newly hatched larva, dropping to 

 the ground, takes up a solitary subterranean life, which it follows till 

 its period of seventeen years is nearly complete. It then appears 

 above the ground, passes into its winged stage, and enters on a few 

 weeks of social life which closes its career. This species is widely dis- 

 tributed in that part of the United States that lies between the Atlan- 

 tic shores and the Rocky Mountains, but does not occur in Minne- 

 sota, Northern Michigan, or Northern New England. It is, however, 

 represented by two races in every respect the same, except that one 

 has a life-history of thirteen and the other of seventeen years. The 

 thirteen-year race prevails in the Gulf States, but in New England and 

 the Middle States the seventeen-year race is alone found. In Illinois, 

 Missouri, Kansas, and in several of the Southern States the two races 

 occur in the same localities ; but it is evident that even in such localities 

 it is only once in 2 2 1 years that there will be any opportunity for cross- 

 ing between them, and we are informed by those who have made 



* My information is chiefly derived from Bulletin No. 8, Division of Entomology, 

 U. S. Department of Agriculture, by Dr. C. V. Riley. 



