456 Prof. E. B. Poulton on the Mimetic N. American 



it is easy to suppose that some of the original archippus 

 with more orange than usual may have escaped capture 

 on occasion from this cause. From such a small begin- 

 ning, such as one may now see every year in B. astyanax, 

 sprang doubtless the whole story, and we now find a 

 butterfly which has for a ground color of both surfaces of 

 the wings an orange which is the exact counterpart of that 

 of Anosia plexippus ; by reason of which in all probability 

 it enjoys a freedom from molestation comparable to that 

 attributed to plexippus, so that it ventures more into the 

 open country than its allies, and thus gains a wider 

 pasturage and surer subsistence " (1. c, p. 714). 



The attempt will now be made to give an account of 

 the changes in detail by which the mimetic pattern has 

 been evolved from the entirely different pattern of the 

 non-mimetic ancestor. The changes are somewhat more 

 complex and probably more important upon the under 

 surface which will therefore be considered first. 



a. Hind-wing Under Surface ofLimenitis arc7ii2)pus. — The 

 black outer border to the white discal band of the hind 

 wing of arthemis persists in archippus, but is much dis- 

 placed inwards as compared with its ancestor. The reason 

 of this is evident. The character in arthemis which 

 initiated the mimicry of Anosia is the submarginal row of 

 reddish spots, commonly found in the hind-wing, more 

 rarely in the fore. The enlargement of these, as well as 

 of the black-and-white-marked margin (also in mimicry of 

 Anosia), has involved the shifting inwards of the black 

 outer border of the white band. In many specimens of 

 arcMppms, traces of the white band of arthemis may still 

 be found for a greater or less distance along the inner 

 edge of the black discal line. This vestige is especially 

 often retained along the costal half of the line : it is occa- 

 sionally Avell developed along its whole length. The 

 original submarginal red spots are still recognizable 

 between the discal line and the margin, being easily dis- 

 tinguishable by their deeper richer tint from the pale 

 ground colour of the wing. 



The black margin has become heavier, the double row 

 of blue crescentic spots larger and whiter, but a faded trace 

 of the original blue tint of arthemis can still be made out 

 in the spots of the inner row, and remains distinct in the 

 outer. 



The basal red patches have vanished, but the pale blue 



