162 MIMICRY IN N. AMERICAN BUTTERFLIES 



steps. The evidence for this conclusion will be 

 clearer when some of these steps have been 

 described in detail (see pp. 164-8). 



THE LINE OF MIGRATION BY WHICH DANAIDA 

 ORIGINALLY ENTERED AMERICA 



There can be little doubt that D. plexippus 

 invaded America by way of the north, probably 

 following the line of the Aleutian Islands. In 

 North America it possesses an astonishing distri- 

 bution for a member of so tropical a group, 

 ranging immensely further north than any other 

 Danaine in the world. Furthermore, D. genutia, 

 the probable representative of its Old World 

 ancestor, extends far beyond the tropics into 

 Western and Central China. A study of the 

 distribution of the Asclepiad food-plants on the 

 eastern coast of Asia might perhaps throw light 

 on the problem. D. plexippus was certainly the 

 earlier of the two invaders of the New World. 

 This is clearly shown by the extent of its own 

 modification no less than by the changes it has 

 itself produced. Its immense size, the shape of 

 the hind-wing cell, and the form of the fore wings 

 indicate that it is far more widely separated 

 than is D. berenice from the nearest Old World 

 species. It has furthermore been resident in North 

 America long enough to effect profound changes 

 in the pattern of an indigenous Nymphaline 

 butterfly, rendering it an admirable mimic ; 

 whereas D. berenice, and probably its form strigosa 



