Soc. Philadelphia," November 1864, page 403, also December 1865. The part on gradation is summarised at pages 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."), most of which was new to me. I 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." (186/2. "Unity of coloration": this expression does not seem to occur in the paper of November 1864, but is discussed at length in that of December 1865, page 209.) 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 (186/3. Max Wichura's "Die Bastarde befruchtung im Pflanzenreich, etc:" Breslau 1865. A translation appeared in the "Bibliotheque Universelle," xxiii., page 129: Geneva 1865.) 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 aboriginally gall-makers, and bear the same relation to them which Apathus probably does to Bombus? (186/4. Apathus (= Psithyrus) lives in the nests of Bombus. These insects are said to be so like humble bees that "they were not distinguished from them by the early entomologists:" Dr. Sharp in "Cambridge Nat. Hist. (Insects," Part II.), page 59.) With respect to dimorphism, you may like to hear that Dr. Hooker tells me that a dioecious parasitic plant allied to Rafflesia has its two sexes parasitic on two distinct species of the same genus of plants; so look out for some such case in the two forms of Cynips. I have posted to you copies of my papers on dimorphism. Leersia (186/5. Leersia oryzoides was for a long time thought to produce only cleistogamic and therefore autogamous flowers. See "Variation of Animals and Plants," Edition II., Volume II., page 69.) does behave in a state of nature in the provoking manner described by me. With respect to Wagner's curious discovery my opinion is worth nothing; no doubt it is a great anomaly, but it does not appear to me nearly so incredible as to you. Remember how allied forms in the Hydrozoa differ in their so-called alternate generations; I follow those naturalists who look at all such cases as forms of gemmation; and a multitude of organisms have this power or traces of this power at all ages from the germ to maturity. With respect to Agassiz's views, there were many, and there are still not a few, who believe that the same species is created on many spots. I wrote to Bates, and he will send you his mimetic paper; and I dare say others: he is a first-rate man.
Your case of the wingless insects near the Rocky Mountains is extremely curious. I am sure I have heard of some such case in the Old World: I think on the Caucasus. Would not my argument about wingless insular insects perhaps apply to truly Alpine insects? for would it not be destruction to them to be blown from their proper home? I should like to write on many points at greater length to you, but I have no strength to spare.
LETTER 187. TO A.R. WALLACE. Down, September 22nd [1865].