1909] Mimicry in tJic Butterflies of North America 21 



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mimicked by male Cethosias and extensively by the females of 

 species of Elymniinae, while incipient Mimicry is seen in the 

 males of some of them. With the exception of Hypolimnas misip- 

 pus, common to both Regions, the Oriental mimics of Danaida do 

 not approach the degree of resemblance attained by the best 

 African mimics of D. chrysippus. It has already been pointed 

 out that the Oriental mimics of this genus are far less numerous 

 than the African. On the other hand, it is a curious fact that the 

 only North American mimic of D. plexippiis, — Limenitis {Basi- 

 larchia) archippiis — reaches a far higher degree of resemblance 

 than that attained by any of the characteristically Oriental 

 mimics of Danaida. 



The evidence as a whole enables us to decide that Danaida is 

 an Old World genus and a compraatively recent intruder into 

 America, while the perfection of the likeness attained by an 

 indigenous American mimic proves that, under favourable cir- 

 cumstances, such resemblances may be rapidly produced. I do 

 not, of course, mean to imply that the transformation was in 

 any way sudden, or by other than minute transitional steps. The 

 evidence for this conclusion will be clearer when some of these 

 steps have been described in detail. (See pp. 214-17.). 



The Line of Migration by Which Danaida Originally 



Entered America. 



There can be little doubt that D. plexippiis invaded America 

 hx w'ay of the north, probably following the line of the Aleutian 

 Islands. In North America it possesses an astonishing distribu- 

 tion for a member of so tropical a group ranging immensely 

 further north than any other Danaine in the world. Further- 

 more, 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. plexippiis 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 /). 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 Nym- 



