HEADLEE | A STUDY IN BUTTERFLY WING-VENATION 289 
The variation in the position of this spur nearer the outer end of 
the discal cell is due, when it lies on radius, to the distance to which 
the coalescence between F,,, and R has proceeded toward the base 
of the wing. It may have continued to a point just proximad of the 
discal cell apex, as in Semmnopsyche diana, or it may have continued 
until, as shown in Diethria clymena, R,,, both arises from and again 
fuses with radius before R, arises (pl. Lx1, figs. 13 to 15). 
In all these cases, however, that part of R,,, within the limits of 
the discal cell and not coalesced is always more or less completely 
atrophied. In order to understand how this has come about, it is 
necessary to sketch some of what I believe to be the phylogenetic 
history of this vein. 
Primitively R,,; was unconnected with radius except at point of 
origin. Later a connection between it and R was established at 
or near the apex of the present discal cell. This connection, which 
took in some cases the form of a short cross-vein and in others the 
form of a coalescence, must have preceded any weakening of the 
former vein because, so long as R,,, was the sole air passage to and 
brace-rod of the veins Rk, and R,, natural selection would tend to 
preserve it, but when this connection became established that part 
of R,,, between the origin and the connection became not only use- 
less but, through its weight and the energy consumed in forming it, 
a positive handicap to the species possessing it. Tendencies toward 
atrophy of this part were siezed upon by natural selection and in 
the forms that we know, it has everywhere disappeared as a distinct 
vein, leaving only more or less well-preserved vestiges. 
From the fact that this modification of radius, the variation and 
loss of which we have just seen, seems to have come about through 
a splitting back of the vein in question, I shall refer to it as the 
split-back condition of R,,,.. It is also to be noted that the places at 
which the other branches of radius originate have moved, if at all, 
toward the outer edge of the wing. 
Spuler found this condition of radius in the pupal wings of Vanessa 
io and Papilio machaon and gave good figures of pupal and adult 
wings showing it. He, however, considered the state of radius in 
Dasychira pudibunda, in which trachea R,,, arises from trachea R 
proximad of the origin of R,, as comparable to although less de- 
veloped than, that in Vanessa and Papilio (pl. Lx1, fig. 16). This is 
not the case, for the actual point of origin of R,,, is not greatly 
changed, as may be seen by a comparison of it with the forking of 
media. This deceptive appearance is due to the coalescence of R, 
