A SEVENTEEN-YEAR AND A THIRTEEN-YEAB BA4 15 
as these two forms interlock and overlap each other in various localities, and as it 
frequently happens that particular broods of the two forms come ont in the - 
year, we Bhonld certainly expect that, if the forms belonged to the same species, 
they would occasionally intercross, whence would arise an intermediate variety hav- 
ing a periodic time of fourteen, fifteen, <>r sixteen year-. As this does :."t appear 
to have taken place, but, «>n the contrary, there is a pretty sharp dividing line 
between the habits of the two forms, without any intermediate grades of any con- 
sequence, I inter that the internal organization of the two form-, must he distinct, 
although externally, when placed side by side, they are exactly alike. Othei 
what possible reason could there be for one and the same species to lie under ground 
in the larva stare tor nearly seventeen years in one county and in the next adjoining 
county to lie under ground in the larva state for scarcely thirteen \ presume 
that even the most bigoted believer in the old theory of species would allow that, 
if it can once be proved to his satisfaction that two apparently identical forms are 
always structurally distinct, whether in their external or their internal organization. 
they must necessarily be distinct species. 
The reasons urged by Dr. Walsh give a strong basis of probability 
to the theory of the specific distinctness of the two races, and particu- 
larly the fact tbat where the broods overlap there seems to be no inter- 
breeding. Dr. Walsh's position has been recently upheld by Mr.W. II. 
Ashmead. who states that in a very careful examination of the material 
in the National Museum he has observed small but constant dift'ere: 
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, for the reason of the absolute resemblance in 
practically 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 conclusively prove, the two races represent one species 
for all practical purposes and differ in a very striking manner from all 
other species of Cicada. One race is unquestionably 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 
mam. the two sections are. respectively, northern and southern, yet at 
the point of juncture the broods of the two races overlap. That the 
17-year period does not depend so much on the greater severity ot' the 
northern winters is evident, protected as the insect is by the depth o[' 
its burrows, and the natural explanation is that the longer period (^' 
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, 
20110— No. U li 
