SEXT^AL DIMORPHISM IN lU'TTEEFLlES. 101 



presence o£ reoular alternations of season, and not always 

 then. I still venture to think that a probable view concerning 

 many of them is that briefly expressed by me some years ago 

 in ' Nature ' (Vol. Ix., 1899, p. 98), viz., that polymorphism, 

 however it may have arisen, is capable of being brought more 

 or less into relation with locality and season under the 

 influence of natural selection. On the other hand, it is 

 conceivable that in some cases at all events the forms in 

 question may have first arisen as adaptations to the seasonal 

 changes, and afterwards, in consequence of extending their 

 range, or some other alteration of conditions, may have 

 partly or entirely lost their correspondence therewith. These 

 are questions that must, I think, for the present remain 

 unanswered ; though whatever the solution may be, there 

 seems no need to anticipate that it will w-eaken the case for 

 selective adaptation.'^ 



To my mind "Migration'^ is the simple and sufficient 

 explanation of these irregularities in Seasonal Uiinorphism, 

 in certain regions, of species which are seasonally dimorphic 

 in the greater piirt of their range. It will be noticed that 

 with the exception of Melanitis leda, L., all the species 

 referred to by Dr. Dixey belong to the subfamily Fieri nae. 



Mr. Distant in " Rhopalocera Malayana/' discussing the 

 habits of the Fierin?e (page 285), recounts the following 

 instances to illustrate their proclivities as migrants, quoting- 

 Mr. Jones' account of swarms of Terias lisa which visited 

 Bermuda in October 1874 : " Several persons living on the 

 North side of the main island perceived, as they thought, a 

 cloud coming over from the N.W. which drew nearer and 

 nearer to the shore, on reaching which it divided info two 

 parts, one of which went eastward and the other westward, 

 gradually falling upon the land. They were not long in 

 ascertaining that what they had taken for a cloud was an 

 immense concourse of T. lisa, BoiscL, which flitted about 

 all the grassy open patches and cultivated grounds in a lazy 

 manner as if fatigued after their long voyage over the deep. 

 They did not stay long on the island.-" A similar phenomenon 

 on the same island occurred on 10th October 1847. During 



