A SEVENTEEN-YEAR RACE AND A THIRTEEN-YEAR RACE. ii 
acteristics and habits, but also on the ground of external structure, 
no material difference in this respect having been noted between the 
two races, although it was known that the individuals did not cross 
when they appeared together. Doctor Walsh was very firmly of the 
opinion, on the other hand, that they represent two distinct species, 
yet in a letter to Mr. Darwin he described the 13-year race as an 
incipient species, to which, for convenience, it is desirable to give a 
distinctive name.? His published views on the subject, given in a 
posthumous paper, are quoted below.? Reterring to the impossi- 
bility of distinguishing species in certain genera by a mere compari- 
son of the periect specimens, he says: . 
Upon the same principle I strongly incline to believe that the 17-year form of the 
periodical Cicada (C. septendecim Linn.) is a distinct species from the 13-year form 
(C2 tredectn (Walsh and Riley¢) Riley), although it has been impossible for me, on 
the closest examination of very numerous specimens, to detect any specific differ- 
ence between these two forms. It is very true that the 13-year form is confined to the 
more southerly regions of the United States, while the 17-year form is generally, but 
not universally, peculiar to the Northern States; whence it has been, with some show 
of plausibility, inferred that the 13-year form is nothing but the 17-year form accel- 
erated in its metamorphosis by the influence of a hot southern climate. But, 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 out in the same year, we should 
certainly expect that if the forms belonged to the same species they would occasionally 
intercross, whence would arise an intermediate variety having a periodic time of 
14, 15, or 16 years. As this does not appear to have taken place, but, on the contrary, 
there is a pretty sharp dividing line between the habits of the two forms, without 
any intermediate grades of any consequence, I infer that the internal organization of 
the two forms must be distinct, although externally, when placed side by side, they 
are exactly alike. Otherwise, what possible reason could there be for one and the 
same species to lie under ground in the larva state for nearly 17 years in one county 
and in the next adjoining county to lie under ground in the larva state for scarcely 13 
years? I 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 Doctor Walsh give a strong basis of proba- 
bility to the theory of the specific distinctness of the two races, and 
particularly the fact that where the broods overlap there seems to be 
no interbreeding. Doctor Walsh’s position has been upheld by 
Dr. Wm. H. Ashmead, who states that in a very careful examination 
24See Index to Missouri Entomological Reports, Bul. 6, U. 8S. Ent. Comm., p. 58. 
6 American Entomologist, Vol. II, p. 335. 
¢Taking the ground that Doctor Phares can not be credited with the race name 
‘‘tredecim”’ on account of the ephemeral character of the journal in which he employed 
it, the credit should go to Walsh-Riley, since the article in the American Entomolo- 
gist of December, 1868, where it was next suggested, was a joint or editorial one. 
Professor Riley himself sanctions this course in the Bibliography of Economic 
Entomology, Part II, p. 61, No. 474. 
31117—No. 71—07 
D) 
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