48 
times pulled this way and sometimes that way. Therefore the 
evolution of the branches of the media may be described as running 
up into a contest between radius and cubitus for the possession of 
these, the residue of the medial system, after the disappearance of 
the base of the nervure traversing the cell, and of the cross-vein 
connecting the main systems above and below the media and 
supporting its three branches. 
But this struggle is not in every case decided as between radius 
and cubitus. Sometimes, as in the Lyceenidze and Hesperiadee, 
the cross-vein retires on either side of the middle branch, leaving 
it stranded. It thus maintains its generalised and central position, 
resisting the attractions of both of the opposing systems. It is 
left to its certain fate. Deprived of nutriment by the degeneration 
of the cross-vein, it dwindles to a fold or scar, and finally disappears 
altogether. The breaking up of the medial system is not stayed. 
The upper and lower branches become part and parcel of the 
radial or cubital systems, and if the middle branch yields and goes 
over finally to one or the other, its existence as a vein is pro- 
longed, otherwise not. Evidently, therefore, we have in the 
condition and relative position of this middle branch of the media, 
vein IV 2,a gauge for the amount of specialisation. The more it 
becomes united with either the radial or cubital system, the greater 
the extent of specialisation. In a general view this process of the 
evolution of the medial system may be regarded as one of ab- 
sorption, either through degeneration or a fusion with other veins 
or systems of veins. So that here, and before dismissing our con- 
sideration of this frst direction in which we find a movement among 
the veins of the wing we can set up our thesis—~he extent of the 
absorption is everywhere the measure of the specialisation.* 
We now come to the second direction. It affects the radius of the 
fore-wings especially, which we have seen to be three- to five-branched, 
and it relates to the disappearance of these branches. The more 
generalised condition of the radius is clearly that in which it has 
five branches reaching the margins of the wing. But in the more 
specialised of the forms one or two of the branches have disappeared. 
These branches are those which arise from the upper side of the 
radius, and which reach to the costa and apex. We find this 
disappearance in Parnassius, Pierts,t Thecla, and the emperor moths. 
The radius remains five-branched in Papilio, Nymphalis, Hesperia, 
Sphinx. JT am merely citing prominent examples. We have a 
condition in which all five branches are distinct and_ separate 
(Hesperia), then one in which III 4 and III 5 are furcate, etc. ; then 
various combinations, principally affecting veins III 3 to III 5, the 
=< Canw Ent.) 29) 17/5. 
y It is perhaps on account of this suppression of the radial veins that 
Mr. Meyrick brings Lycena and Pieris together, in any case overlooking the 
fact that the plan of the Pierid wing is Nymphalid; the plan of the Lyczenid 
wing is Hesperid. 
