Phylogeny of the Pierinee. 271 
or by its freedom from admixture with the darker scales 
that often give the hindwing in this genus a compara- 
tively dusky appearance (as in some specimens of C. 
edusa). In many species a small companion is visible 
in the interspace between the second subcostal and dis- 
coidal nervure. On the under surface the discoidal spot 
is much more distinct. Its centre is silvery white, and 
it is usually encircled with a brown or orange ring, 
which in some species is sharply marked off both 
internally and externally by thin marginal lines of a 
darker colour. The companion reproduces the discoidal 
spot on a smaller scale. It always occupies an inter- 
space, whereas the chief discoidal spot is situated upon 
the second disco-cellular nervule. (Fig. 20, D 4.) 
No doubt is likely to arise as to the identity of the 
discoidal spot in Synchloe with that in Colias so far as 
the forewing is concerned. The chief difference between 
them is that in the former genus the spot is less sharply 
defined and less regular in outline ; moreover in Synchloe 
it frequently reaches forward to the costa, and it may 
extend backward to the space between the first and 
second median nervule; whereas in Colias, as we have 
seen, it does not pass the limits set by the subcostal 
nervure and the third median nervule. But there can be 
no reasonable question that the spots are homologous in 
the two cases; and a comparison of the two genera 
further suggests the probability that in Synchloe we 
have an older, in Colias a newer and more specialised 
form of the marking. ‘This conclusion accords with 
what has been already said about the marginal and sub- 
marginal series of markings, which in Colvas, especially 
in the males, have departed somewhat further from the 
original type than in Synchloe; and it will be seen to 
receive further confirmation from what is to follow. 
With regard to the discoidal spot on the hindwing, 
although it is perhaps not at first sight clear that the 
markings in the two genera are really homologous, it is 
nevertheless quite possible, on an attentive examination 
of allied forms, to trace the steps by which the passage 
from one to the other is effected. In the hindwing of 
many species of Synchloe there is no indication of a 
discoidal spot at all; in others, however, a definite 
aggregation of black scales is visible about the disco- 
cellular nervules. In some species, such as S. protodice 9, 
