1914.] NATURAL SCIENCES OF PHILADELPHIA. 187 



paler shade of the ground-color outside the discal black stripe on 

 both surfaces of both wings. The change of shade follows the 

 suggested direction of the black line to the inner margin of the fore 

 wing, although near this border it is not sharply demarcated as on 

 the rest of the wing. Such an abrupt change in the depth of the 

 color is very rarely to be seen on the upper surface of archippus. 

 Scudder has looked on the reddish spots of arthemis, occupying the 

 ■very position of this paler shade in obsoleta, as the foundation from 

 which the mimetic form arose (6, p. 714), and I have followed him 

 (21, 23). If we are right, and the transformation occurred first in 

 this area and only later in the area inside the white discal stripe, it is 

 easy to understand why there should be a difference in the shade of 

 the ground-color for natural selection to seize upon. The Arizona 

 Danaida strigosa is also paler on the outer than it is on the inner part 

 of the wings, although the transition is gradual and not sharp as in 

 ■obsoleta. On the under surface of the fore wing archippus is, in this* 

 very respect, more strikingly ancestral than obsoleta, the pattern of 

 the model having been such as to emphasize the feature. Archippus 

 is also commonly ancestral as compared with obsoleta in the distinct 

 indication by a reddish-brown tint of the red submarginal spots on 

 the under surface of both wings (31; p. 456). 



The white mark in area la of the fore wing has this further interest, 

 that it indicates the point at which the outer edge of the discal 

 band of the hind wing met that of the fore, reconstructing for us a 

 pattern like that of weidemeyeri and arthemis in which the band of 

 the hind wing is placed much further from the outer margin than it is 

 in the other wing. The evolution of the, marginal pattern of both 

 surfaces of both wings of obsoleta from a condition like that of arthemis 

 appears to have been the same as in archippus (31, pp. 456-459) and 

 to have reached nearly the same result. The slight differences cor- 

 respond with those between the respective models and are doubtless 

 due to mimicry. 



The two white spots in the fore wing cell on the under side were 

 present in all the males of obsoleta. The females showed greater 

 variability, the basal spot being sometimes absent, but generally 

 much larger than in the males. On the upper surface of the same 

 wing the distal spot was large, for this feature, in 6 females, small in 3, 

 minute in 1. In 14 males it was sharp and distinct, though small, 

 and it could be detected in 8 of the others. In the remainder the 

 triangular black mark in which the white spot lies could be made 

 out by looking carefully for it. White scales were probably origin- 



