1900. | GROTE—THE DESCENT OF THE PIERIDS. 49 
margin, on the interspace just below Cr. Color and pattern are 
not distinctive, but virtually agree in all these forms. 
The preceding types are all, except JVashalis, too closely allied 
to be generic, and they all agree in one important point, the reduc- 
tion of the humeral spur. The marks of a former presence of the spur 
are to be seen on the shoulder of the subcostal vein, slight, jagged 
protuberances. These areso obscure and so similar in all the cases, 
that it is hardly possible to draw any distinction, and my impres- 
sion, that they are less noticeable in (Va¢halis, may be after all in- 
correct. But it is of no obvious importance; the main fact being 
that the spur has vanished in all these types. That Avrema, Terias, 
Xanthidia and Sphenogona should agree in this is not extraordinary, 
since the neuration is otherwise so similar in all these forms, that 
their generic separation seems quite unnecessary. Nor are there 
any other characters, so far as I am able to see, which would 
authorize us to consider them as types of different genera, and only in 
Sphenogona have we the angulation of external margin of sec- 
‘ondaries to sustain a separate title. But that /Va¢ha/s should not 
differ, leads to the inference that the reduction of the radial 
branches may have happened after the loss of the humeral spur. 
The disappearance of the branchlet suddenly, however, is contra- 
dicted by the mass of instances in which the evidence is clear, that 
the absorption takes place slowly. The stages of progression of 
the branches along the radius are constantly marked in a variety of 
forms, in which we find them but slightly differing in relative posi- 
tion. And when £2 finally leaves its original place above the 
cell, where Ar remains, the inference is plainly that it gradually 
moves along toward the tip of the wing. We see it in fact ad- 
vancing by consecutive stages to the cross-vein, there assuming the 
Trifurcula position, and then, traveling still onward, gradually 
approaching the apices of the primaries. All this shifting, which 
we can now follow, by studying the different stages in different 
types, must be assumed to reflect the action which has formerly ex- 
hausted itself on the radius of secondaries, and of which only 
fragmentary direct evidence remains in the five-branched radius of 
fTepialus, the Micropterygides, and a late stage of reduction in 
Crinopteryx famitiella. After all admissions, that in certain cases 
the present reduction on the primaries may be relatively rapid, the 
very multiplicity of the existing stages is evidence that in general 
it is being slowly accomplished. The movement has been going on 
