of certain species of Willow. — Part 2nd. 27!' 



could as soon believe, with the schoolboy in the story, that sometimes 

 the Earth went round the Sun and sometimes the Sun went round the 

 Earth. I could as soon believe, contrary to all ornithological authori- 

 ty, that the European Cuckoo or our North American Cowbird some- 

 times builds a nest for itself and sometimes oviposits in the nests of 

 other birds. I could as soon believe, that the bees belonging to the 

 genera Nomada and Ccelioxys sometimes build and provision nests for 

 their own young, and sometimes surreptitiously oviposit in the nests of 

 Halictus and Megachile. But, unless we believe in such anomalies as 

 these, we are bound to believe that perfectly distinct species may in 

 the imago state be apparently identical, and that the galls form the 

 only distinctive character between them. That these inquilinous Saw- 

 flies were priuiordially identical with the gall-making Saw-flies, and 

 that Osten Sacken's two Gall-flies were priuiordially identical, and the 

 undistinguishable Willow Gall-gnats were priuiordially identical, and 

 the undistinguishable Aphidiaus Phylloxera (?) carysecaulis Fitch and 

 Ph. (?) caryae globuli Walsh, were primordially identical — I fully con- 

 cede. On no other hypothesis can I account for the fact of so many 

 pairs of species being exactly alike, so far as the insects themselves are 

 concerned; just as, when I find two copies of the same book exactly 

 alike, I account at once for the fact by assuming that they were stricken 

 off from the same types. But that is quite another affair from all these 

 pairs of species being identical at the present day. 



Negative facts are always more or less unreliable ; but there is one 

 negative fact, or rather bundle of facts, upon which I scarcely think 

 that I can be mistaken throughout, though I may be, and doubtless 

 am, mistaken in some few of the details. Not only is it the case, as I 

 have already partly shown, (Proc. etc III, p. 635,) that, when a given 

 genus of gall-making insects occurs on a given genus of plants, it is 

 very generally represented thereon — if we include exotic as well as do- 

 mestic insects — by several species and often by very numerous species; 

 not only is it the case, as I have already indicated elsewhere, (Proe. 

 etc. I. p. 310, II, p. 401,) that each gall-making genus of true insects, 

 with the single very remarkable exception of Cecidomyia and its sub- 

 genera, is, as a general rule, restricted to one single genus of plants ; * 

 but it is also the case that — putting the gall-making Cecidomyia which 



* It matters little for my argument, whether we assume that these peculiar 

 forma restricted to particular genera of plants are genera or subgenera or mere 

 generic sections. It is sufficient that they arc proved to be structurally distinct 

 from other forms. The rest, after all, is more a question of words than a ques- 

 tion of things. 



