I received your former
paper on Phytophagic variety (186/1. For "Phytophagic Varieties and
Phytophagic Species" see "Proc. Entomolog. 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 th