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 broughtinto 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 environmentsin sucha 
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 yearsand one or two 
months. The imago appears late in May, and fora 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. Itis, 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 prevailsin the Gulf States, butin New England and 
the Middle States the seventeen-year race isalone 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 
itis only once in 221 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. 
