46 
specialised as compared with the primaries. An explanation for 
this I have offered in that the hind-wings bear more of the weight 
of the body (abdomen), and that they also check the downward 
stroke of the primaries. ‘The cause is thus mechanical, and a 
parallel is suggested with the four-footed Vertebrates in which the 
hind limbs are the more specialised. More than this, in the two 
principal directions by which the evolutionary progress is expressed, 
it is in the hind-wings that the direction is usually, if not always, 
accentuated. It is as if, as I have said elsewhere, “‘a wave passed 
over the wings, coming from the hind pair and breaking over the 
primaries, carrying these frail creatures farther along their airy paths 
into their unknown future.” 
Having thus briefly sketched the general progress of the wings, we 
come now to the special directions which this progress takes in 
the neuration, and which revealed themselves through my compara- 
tive studies by means of enlarged photographs. 
The first of these directions is expressed by the changes in the 
media and its branches. A glance at the accompanying figure of the 
hind-wing of /efza/us (so far as the points here to be considered are 
concerned, it matters not whether we take the fore or hind-wing) 
shows us the media as a furcate vein traversing the median cell from 
the base of the wing to the cross-vein. From the cross-vein three 
separate branches reach the exterior margin. According to the 
embryological studies of Spuler the cubitus 1s two-branched, so that 
the present three separate branches belong to the media. Now this 
whole series of the media is undergoing a systematic change in the 
direction of disappearance. 
Let us take the base of the media first. In most of the specialised 
Lepidoptera this base has already ceased to have any function ; it has 
degenerated into a scar, or it has quite vanished. In some forms 
there are little spurs running back from the cross-vein, indicating 
where the basal portion of the nervure used to join on; but there is 
no more any connection with the base of the wing, and the nutriment 
for the branches is supplied through the cross-vein of the cell. The 
base having gone, the cross-vein itself is the next to disappear. The 
same process is repeated, the vein parting between IV 1 and IV 3, 
so that in the more specialised day butterflies and emperor moths 
the cell opens, and finally all trace of the cross-vein is lost. Accord- 
ing to my friend Dr. Seitz, the disappearance of the cross-vein in the 
butterflies is a recently acquired character. Now what becomes 
of the three branches running from the cross-vein to the external 
margin of the wing? We must compare as many wings as possible 
to fully answer the question. It at last appears that the solution les 
with the fate of the middle one of the three, with vein* IV 2. We 
* The use of the word “vein” to designate the tubular rods which support 
the membrane of the wings has been properly objected to, since these struc- 
tures are not homologous with the veins of the higher animals. The word 
‘“nervure” has been proposed, but this word in its origin from ‘“ nerve” would 
