18 THE PERIODICAL CICADA. 
of the material in the National Museum he has observed small but 
constant differences between the two races in the shape of the last 
ventral segment of both the male and the female. 
For the present purpose, however, it seems wiser to consider the 
13-year broods as representing a race merely, or an incipient species, 
as suggested by Walsh, because of the absolute resemblance in prac- 
tically every feature of structure, coloration, and habit, in the two 
forms, which exhibit the single important point of difference repre- 
sented by the four years’ variation in the length of their subterranean 
lives. 
While in the matter of interbreeding they may be distinct, as the 
records seem to prove conclusively, the two races represent one 
species for all practical purposes and differ in a very striking manner 
from all other species of the family Cicadidz. One race is unques- 
tionably the offshoot of the other, the original differentiation being 
probably caused by some variation in climatic conditions. 
It is, perhaps, a hopeless task, and at best only a matter of conjec- 
ture, to attempt to explain the phenomenon of what is practically the 
same insect requiring in one part of the country seventeen years for 
its underground development through its preliminary stages and in 
another section thirteen years, in the face of the fact that while, in the 
main, the two sections are, ee northern and southern, yet at 
the point of juncture the broads of the two races overlap. That the 
17-year period does not depend so much on the greater severity of the 
northern winters is evident, protected as the insect is by the depth of 
its burrows, and the natural explanation is that the longer period of 
warmth in the South hastens the development of the insect, or, in other 
words, that the difference in the length of the warm growing period 
during which the insect can thrive and increase in size in the southern 
half of its range enables it to go through its development in four years 
less time than in the North, where shorter summers and consequently 
shorter periods of growth occur. The chief objections to this theory, 
but not necessarily controverting it, are those made by Doctors 
Smith and Walsh in the quotations given. The problem is, however, 
a very interesting one, and some light may be thrown upon it by 
further experiments similar to those described under the head fol- 
lowing. 
RELATION OF CLIMATE TO THE RACES. 
The anomaly presented of two distinct periods for the completion of 
the adolescent stages of the periodical Cicada, exhibited by the 13-year 
and 17-year races, and its apparent basis in climate led Professor Riley 
to institute some careful experiments in transferring the eggs of the 
13-year race, collected in various Southern States, to different locali- 
ties in the North, and conversely, eggs of the 17-year race collected in 
the North to localities in the South, to determine the actual influence 
