as got protective
colouring have probably from the first been variations limited in their
transmission to the female sex. And so with the variations of the
male: when the male is more beautiful than the female, I believe the
variations were sexually limited in their transmission to the males.

LETTER 453. TO B.D. WALSH. Down, October 31st, 1868.

(453/1. A short account of the Periodical Cicada (C. septendecim) is
given by Dr. Sharp in the Cambridge Natural History, Insects II., page
570. We are indebted to Dr. Sharp for calling our attention to Mr. C.L.
Marlatt's full account of the insect in "Bulletin No. 14 [NS.] of the
U.S. Department of Agriculture," 1898. The Cicada lives for long periods
underground as larva and pupa, so that swarms of the adults of one
race (septendecim) appear at intervals of 17 years, while those of the
southern form or race (tredecim) appear at intervals of 13 years.
This fact was first made out by Phares in 1845, but was overlooked or
forgotten, and was only re-discovered by Walsh and Riley in 1868, who
published a joint paper in the "American Entomologist," Volume I., page
63. Walsh appears to have adhered to the view that the 13- and 17-year
forms are distinct species, though, as we gather from Marlatt's paper
(page 14), he published a letter to Mr. Darwin in which he speaks of the
13-year form as an incipient species; see "Index to Missouri Entomolog.
Reports Bull. 6," U.S.E.C., page 58 (as given by Marlatt). With regard
to the cause of the difference in period of the two forms, Marlatt
(pages 15, 16) refers doubtfully to difference of temperature as the
determining factor. Experiments have been instituted by moving 17-year
eggs to the south, and vice versa with 13-year eggs. The results were,
however, not known at the time of publication of Marlatt's paper.)

I am very much obliged for the extracts about the "drumming," which will
be of real use to me.

I do not at all know what to think of your extraordinary case of the
Cicadas. Professor Asa Gray and Dr. Hooker were staying here, and I told
them of the facts. They thought that the 13-year and the 17-year forms
ought not to be ranked as distinct species, unless other differences
besides the period of development could be discovered. They thought the
mere rarity of variability in such a point was not sufficient, and I
think I concur with them. The fact of both the forms presenting the same
case of dimorphism is very curious. I have