^38 [December 



on the Walnut (Juglans), the Hickory (Carya) and the Persimmon 

 (Diospyros), and the latter, so far as hitherto known, feeding exclu- 

 sively on the Honey-locust (Grleditschia). Now, from the fact that 

 there are two of these Dri/ocampa which do not inhabit the Oak, it is 

 manifest that there can be no generic peculiarity of structure which 

 compels the entire genus to confine themselves to that tree. Why then, 

 out of five or six Dryocampa, do as many as three or four inhabit the 

 Oak ? Why are they not scattered round amongst our Elms and Ashes 

 and Cherries and Plums and Thorns and Crabs and Willows and Pop- 

 lars and Beeches 't The Theory of Chances demonstrates that this can- 

 not be a merely fortuitous event. There must be some cause for it. 

 What is that cause? The Creative Theory is dumb, or tells us that it 

 is so, because it is so ; the Derivative Theory answers promptly, clearly 

 and loudly, that it is because all Dryocampadse sprang ages ago from 

 some one pre-existing species, which inhabited the Oak or some pre- 

 existing form closely allied to the Oak ; and that certain nascent types, 

 in the course of ages, ceased more or less, and at a more or less early 

 period, to feed on the Oak. so as to become isolated from their breth- 

 ren at a comparatively early date, and have consequently deviated more 

 or le.ss, but always in a far greater degree than the others, from the 

 primordial type, and run into what I have called Phytophagic Species. 

 Look, again, at the cases of the N. A. Grall-gnats {Cecidomyla) which 

 form galls on the Willow, and of the N.*A. Grall-flies {Gi/inps) which 

 form galls on the Oak. I know from my own observation of both these 

 two groups that, as a general though not as a universal rule, each spe- 

 cies is limited to a particular species of the genus of Plants which it 

 inhabits. In the case of the latter, Osten Sacken has shown the same 

 thing, (Fror. Ent. Soc. Fhll. I. p. 50.) and as to the former, both 

 Loew and Osten Sacken assert it of the whole family of Cecidomi/idse. 

 (Amber-Dipt. Sill. Journ. xxxvil. p. 309. Dipt. N. A., p. 179.) 

 It cannot be said that there is some peculiarity in their generic organi- 

 zation, which limits them thus to one or other particular species either 

 of Oak or Willow; for there are probably certain species of (rail-gnats 

 which inhabit several species of Willow, and there are most indubitably 

 certain species of Grall-flies which inhabit several species of Oak.* Con- 



*The N. A. Oaks (quercus), are divided by Gray into two sections which almost 

 attain a subgeneric value, from the circumstance of the acorns either ripening the 



