18641869] GALLS 26l 



of the progress of millions of years, with every continent Letter 185 

 swarming with good and enlightened men, all ending in this, 

 and with probably no fresh start until this our planetary 

 system has been again converted into red-hot gas. Sic 

 transit gloria mundi> with a vengeance. . . . 



To B. D. Walsh. Letter 1 86 



Down, March 27th [1865]. 



I have been much interested by your letter. I received 

 your former paper on Phytophagic variety, 1 most of which 

 was new to me. 1 have since received your paper on willow- 

 galls ; this has been very opportune, as I wanted to learn 

 a little about galls. There was much in this paper which has 

 interested me extremely, on gradations, etc., and on your 

 " unity of coloration." 2 This latter subject is nearly new to 

 me, though I collected many years ago some such cases with 

 birds ; but what struck me most was when a bird genus 

 inhabits two continents, the two sections sometimes display 

 a somewhat different type of colouring. I should like to hear 

 whether this does not occur with widely ranging insect- 

 genera? You may like to hear that Wichura 3 has lately 

 published a book which has quite convinced me that in 

 Europe there is a multitude of spontaneous hybrid willows. 

 Would it not be very interesting to know how the gall- 

 makers behaved with respect to these hybrids ? Do you 

 think it likely that the ancestor of Cecidomyia acquired its 

 poison like gnats (which suck men) for no especial purpose 

 (at least not for gall-making) ? Such notions make me wish 

 that some one would try the experiments suggested in my 

 former letter. Is it not probable that guest-flies were 



1 For " Phytophagic Varieties and Phytophagic Species " see Proc. 

 Entomolog. Soc. Philadelphia, Nov. 1864, p. 403, also Dec. 1865. The 

 part on gradation is summarised at pp. 427, 428. Walsh shows that a 

 complete gradation exists between species which are absolutely unaffected 

 by change of food and cases where " difference of food is accompanied 

 by marked and constant differences, either colorational, or structural, 

 or both, in the larva, pupa and imago states." 



2 "Unity of coloration": this expression does not seem to occur in 

 the paper of Nov. 1864, but is discussed at length in that of Dec. 

 1865, p. 209. 



3 Max Wichura's Die Bastarde befruchtung ini Pflanzenreich, etc. : 

 Breslau 1865. A translation appeared in the Bibliotheque Universclle, 

 xxiii., p. 129: Geneva 1865. 



