NOTE ON THE DIURNALS

Extract

Mancipium brassicœ.—Dr. Chapman writes me that certain specimens of this common species examined by him showed the very short veinlet III. 3+4. This veinlef constantly diminishes in size, progressing towards the tip of the wing to finally vanish, through many forms fo the Pieridœ. I had indeed expected it to be occasionally prersistent in brassicœ, although my preparations did not show it. It has disappeared in Pontia daplidice, in Nathalis iole, and strange to say in that curious and now isolated Pierid, Gonophlebia paradoxa. This variability, in one and the same species, is interesting because it follows the general evolutionary direction of the changes in the venation. Always the radial branches in the Pierids and other groups tend to diminish in number. Always the disintegration of the Media advances, until it finally disappears, as a system, from the surface of the wing (Rothschildia, Samia, Potamis, etc.). A parallel case to that of brassicœ is offered by Copismerinthus ocellata. In some specimens of this Hawk Moth, vein IV, is still thrown off from the cross vein of the hind wings, instead fo the Radius, which it has usually ascended to beyond the cell. We must regard these as instances of generalization in the individual, of a reversion to what was formerly the rule and is now becoming, by slow degrees, the exception.