19H,] 235 



Magazine (I.e., p. 222) as to its ability to traverse the spaces of ocean 

 that divide its American home from the nearest lands east and west, by 

 means of its own good wings, with no other assistance than the favour 

 of the elements. To begin with, its flight, if not as rapid as that of 

 many other butterflies, is powerful, sustained, and untiring ; its habit 

 of assembling in great swarms, and of mounting high in air, have been 

 alluded to by every writer on its life-history ; and finally, the toughness 

 of its integuments, as well as its hardiness, vitality, and longevity, are 

 exceptional even for the highly protected family to which it belongs. 

 Thus everything is in favour of some individuals at least of a swarm of 

 these butterflies, blown by a heavy gale out to sea and probably carried 

 up to a great height by ascending air-currents, surviving for days, if not 

 for weeks, and eventually " making the land," as indeed was so nearly 

 the case with those recorded from the Atlantic by Mr. Barrett (ante, 

 p. 231). Borne on the wings of the wind, with no more exertion than 

 would be necessary to enable them to keep their balance, they would 

 incur very little if any damage or " waste of tissue," and on their 

 arrival at a foreign shore, even a " female heavy with eggs " would no 

 doubt be in sufficiently good case to found a colony, provided always 

 that the necessary food-plant were at hand. 



As it is, to say the least, very unlikely that the insect under these 

 circumstances would be accompanied by any one of its very few 

 parasites or other enemies, its rapid increase in many of its new homes 

 is readily accounted for. This increase would not be checked, except 

 as in New Caledonia by the wholesale destruction of the food-plant 

 by the larva (ante, p. 190), or as at Sydney (Ent. Mo. Mag., vol. XLI, 

 p. 220) by one or other of the local parasitic insects finding out and 

 adapting itself to the new-comer. 



It is the arrival of the food-plant before that of the butterfly that, 

 in my opinion, is the determining factor of the ability of the latter to 

 establish itself in any new region. In every locality where it has been 

 noted as more than a casual visitor, it will, I think, be found that 

 either Asclepias or Gomphocarpus has arrived there before it. Now 

 both these plants, although troublesome weeds to tropical cultivation 

 and possessed of decidedly poisonous properties, are sufficiently orna- 

 mental to cause them to be sometimes grown as garden flowers or 

 curiosities, and thus they may in some cases be deliberately introduced 

 into new localities, as Gomphocarpus has been into New Zealand and 

 the Azores. As regards Asclepias curassavica, it is not easy to fix even 

 an approximate date for its introduction into the Pacific Islands ; but 



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