A SKVKNTKKN-YKAK KACK ANM) A ll 1 1 K TKKN-^ K A K HACK. 17 
actcrist ics and liahils. l)iii also on ilic ^i-oiiiid of cxlci-iial >l riicl iirc. 
no material dillVrcMicc in I his rc^piMi liaNin^- been noted hclwccn the 
two rac(vs. althoni:-li it was known that the indixidiiaK did not ci-oss 
\vh(Mi they a|)|)(>ai'ed toi:(Mh(M-. Doetoi" Walsh was \cv\ lirnd\ of the 
o|)inion, on th(> olhei- hand, that they re|)res(Mit two distinct spccio. 
y(>t in a letter to Mr. Dai'win he d(>^^eI•il)ed the l."!-year I'aee a> an 
inei|)ient spi'cies. to which, I'oi- eon\-enienc(\ it is desirable to '/we a 
(list incli\(> name." ills ])nl)lishe(l \iews on t li(> sid)ject, u;i\-en in a 
I)o>tlHimons paper, are (pioled below.'' KelcM-rin^^ to the impossi- 
bility ()( (list inii:iiishiiit^ species in certain ^-enera by a mere compai-i- 
son o^ the pcM'icM't spociiiicns, lie says: 
r])Mii ilir ^aiiic priii(i])le I strongly incline to believe that the l7-ye:ii- lonii of ilic 
j)erio(.lical Ciciula {C. scjftcmkcim Linn.) i.s a di.slinct species from lh<- IM-ycar inim 
1 r. (rcdccihi (Walsh and Riley'" ~i Riley), although it has been iinpossiblc lor nie, on 
ilie closest examination ol very numerous specimens, to detect any specific differ- 
riice between these two forms. It is very true that the 13-year form is confined lo ilic 
more southerly regions of the United States, while the 17-year form is geneially. bin 
not universally, peculiar to the Northern States; whence it has been, with some show 
of i)lausibility, inf(.'rred that the 13-year form is nothing but the 17-year form accel- 
erated in iis metamorphosis by the influence of a hot southern climate. But, as these 
I wo forms interlock and overlap each other in various localities, and as it fre(iuently 
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 
I he two forms must be distinct, although externally, when placed side by side, they 
are exactly alike. Otherwise, what jjossil^le 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 })roods overlap there seems to be 
no interbreeding. Doctor Walsh's position has been upheld l)y 
Dr. Win. 11. Aslunead, who states that in a very careful examination 
«See Index to Missouri Entomological Reports, Bui. 6, U. S. Ent. Comm., p. 58. 
& American Entomologist, Vol. II, p. 335. 
c Taking the ground that Doctor Phares can not l)e crt dilecl willi thf> race name 
"tredecim" on account of the ephemeral cliaracicroi" i he journal in w liidi he ciniiloyed 
it, the credit should go to Walsh-Riley, since the article in the American 1-niomolo- 
gist of December, 18(58, where it was next suggested, was a joini or cdiiorial one. 
Professor Riley himself sanctions this course in the r)iblio'_Maj)liy of Economic 
Entomology, Part II, p. 01, No. 471. 
31117— No. 71—07 2 
