94 Mr. W. L. Distant's Geographical Distribution 



it is more tlian probable that many otber specimens of 

 D. Archippus have reached our shores^ though possibly 

 not till and about this time. Whether it becomes a 

 thorough colonist in this country or not, if we examine 

 some of the factors which must necessarily be of the greatest , 

 advantage in aiding the wide distribution of Lepidoptera, 

 it will be clearly seen that D. Archippus is peculiarly 

 favoured and enjoys a preponderance of those conditions 

 which tend to its dispersal and favour its survival. 



The causes and conditions which effect and qualify the 

 distribution of Lepidoptera have, as regards insects, 

 generally been fully treated by Mr. Wallace, and with his 

 usual appreciation of efficient causes;* and may, perhaps, 

 be summarized as means of dispersal and conditions which 

 are favourable to their survival in a new habitat. 



As regards the "means of dispersal" of this butterfly 

 we need have no recourse to past geological changes, how- 

 ever great a part those depressions and u|)heavals may have 

 performed in the range of other insects, and we are justified 

 in considering the principal and only factors, as winds, cur- 

 rents, and the agency of man. With the first two means 

 of dispersal is involved a phase of the question which is 

 still of the greatest obscurity, i. e., as to voluntary or in- 

 voluntary migration. By the term voluntary migration is 

 understood those (to us) apparently purposeless flights of 

 butterflies, often in immense swarms, of which we have 

 had graphic accounts from such travellers as Darwin when 

 as far south on the east coast of S. America as the Bay of 

 San Bias, by Bates and Spruce on the Amazon, by Belt 

 at Nicaragua, by Jones at Bermuda, and numerous other 

 excellent and qualified observers. It is in our present 

 ignorance of the impelling cause of these movements that 

 Ave give them the name of voluntary in contradistinction 

 to those occasional migrations through the force of gales 

 of wind and like causes, or by the eggs or puj)£e being 

 carried on the branches or in the roots of trees, down the 

 length of great rivers or across the sea. 



The original habitat of this species is the New World, 

 though whether it first appeared in the Nearctic (as is 

 probable) or Neotropical region, it is not to our pre- 

 sent purpose to inquire, and it will be convenient to trace 

 its migration thence in two lines: firstly, eastwards to 



* Geo. Dist. of Animals, vol. i. pp. 33, 34. 



