i866— 1872] SEXUAL SELECTION 89 



special selection. I am very ..lad to hear of case of female Letter 452 

 Birds of Paradise. 



I have never in the least doubted possibility of modifyii 

 female birds alone for protection, and I have long b< 

 it for butterflies. 1 have wanted only evidence for the 

 female alone of birds having had their colour modified 

 for protection. Hut then I believe that the variations by 

 which a female bird or butterfly could get or has 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. 



To B. D. Walsh. 1 Letter 453 



Down, October 31st, 1868. 

 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. 2 Professor Asa Gray and Dr. Hooker 

 were staying here, and I told them of the facts. Th 



1 For a biographical note see Vol. I., p. 248. 



* A short account of the Periodical Cicada (C. septenrfecim) is given 

 by Dr. Sharp in the Cambridge Natural History, Insects II., p. 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 \N.S."\ofth* I. 

 Department of Agriculture, 1898. The Cicada lives for long periods 

 underground as larva and pupa, so that swarms of the adults of one 

 race {septendecini) 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 1808, 

 who published a joint paper in the American Ento/n Vol. I., p. 



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 [p. 14"), he published a letter to Mr. Darwin in which he speaks 

 of the 13-year form as an incipient species ; see 1>: 

 Entomolog. Reports Bull. 6, i'.S.E.C. p. - is given by Marlatl 

 With regard to the cause of the difference in period of the two forms, 

 Marlatt (pp. 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 ersav/ith 13-year eggs. The results 

 were, however, not known at the time of publication of Marlatt's paper. 



