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.