XV 



where this occurs, as In the American Dismorphias and the Malayo-Australian 

 EuPLOEA MiDAMUS L. it is clearly a manifestation of transition from an older 

 condition in this new direction. 



These are the local influences, whose action manifests itself in accordance 

 with the individual susceptibility, which must be kept in mind when there is 

 question of the above mentioned accelerated development which produces the 

 alleged seasonal varieties. 



It is worthy ot being noted here how little variation occurs in migratory 

 butterflies distributed over a great part of the world, such as Anosia (Danais) 

 Plexippus L., Pyrameis Cahdui L., or Lycaena Boeticus L. ; would not this 

 point to a very slight susceptibility to foreign influences in these species, thereby 

 explaining how they can adapt themselves in this manner to different climates ? 



With regard to the peculiar elongated form of wing of the South American 

 Rhopalocera, so characteristic of the family Heuconidae, which is confined to 

 that part of the world, I have already alluded to the fact that this form of 

 wing has been acquired not only by the Neotropidae, a form of Danaid 

 established there for a very long period, but likewise by the progenitors, which 

 must have arrived from Africa much later, of the present genera Lycorea and 

 Ituna, and by several of the later arrivals amongst the Acraeidae, Pieridae, 

 and Nymphaltdae. By these last, however, by no means so generally, sothat 

 some species of the Pierid genus Dismorphia are evidently still in a period 

 of transition which is shown by the fact that in only one of the sexes this 

 change of wing form has commenced. While the Danaidae which must certainly 

 have reached America last, to which Danais (Anosia) Plexippus L. belongs, 

 have retained their original form of wing entirely unchanged. W^here this 

 process of change of form occurs in this manner it can hardly be explained 

 otherwise than that it is caused by the difference in individual susceptibility; 

 that only during a long continued existence under specific uniform conditions 

 this increases to such an extent that the form of wing in all the species becomes 

 thus modified but with an existence of shorter duration does not thus increase 

 in all the species and this modification is, consequently, produced only in 

 certain species and sometimes even only in one of the sexes of such species, 

 while with a still shorter existence it may not occur at all. 



The peculiar modification in the form of wing alluded to must, therefore, be 

 considered as the response to a stimulus, directing the evolution of the form 

 of wing in a specific course. A stimulus whose character we may be unable 

 to define as yet but which is evidently restricted to South America, although 

 the region where its influence is felt is of very great extent. 



With regard to the evolution of pigmental colours the same phenomenon 



