47 
must first arrive at a conception of the most generalised position for 
this middle branch. Again appealing to Hepzalus, we find this 
position to be a central one. But in many forms the branch is 
drawn towards the radius ; in others towards the cubitus. Whereas, in 
Ty+9 
Fig. 2—A SpECIALISED WING (typical). 
Fore-wing of Attacus atlas, natural size, from a photographic impression (to 
illustrate the first and second directions of evolutionary change). The 
middle branch of the media, vein IV 2, has yielded to the attraction of the 
radius. The cross-vein has disappeared. All traces of the media as a 
distinct system have vanished. The two upper branches have become 
completely fused with the radial system; the lower branch, IV 3, with the 
cubital. Absorption of the upper radial veins has also commenced. The 
branches III 1 and III 2 are represented by a single vein, II] 1 + 2. The 
vein III 3 is greatly reduced, and indicates an absorption by III 4. 
the breaking up of the cross-vein, the upper branch a/ways follows 
the radius except in /Yesferia, where it shares the isolated fate of 
IV 2, and the lower follows the cubitus, the middle branch is some- 
also not be perhaps strictly applicable. The word “vein” is used in a popular 
sense for these structures, as also for the ribs of the leaf. In German they are 
called “ Rippen,” z.e. “ribs,” but of course they are equally not homologous 
with bony structures. The word “ vein” is not only shorter, but it seems to be 
familiar, and universally understood when used in this connection. It is here 
hardly a matter of using a word securing a better definition of the object as 
between ‘‘vein”’ and “nervure.’’ My aim here, as always, is to be understood 
by using plain terms. 
