( Ixxxiii ) 



of the whole organic world, as the registers of subtle and 

 elusive change — ever going on, yet never seen, — by means of 

 which forms are slowly becoming different from what they 

 have been in the past. It is the existence of a complex 

 pattern composed of several colours, which renders butterflies 

 and to a less extent moths such a remarkably delicate record 

 of change. As we trace the representative individuals of a 

 community of butterflies over any wide range, the trained eye, 

 and often the inexperienced eye, can detect differences which 

 are not seen to anything like the same extent in the individuals 

 of other Orders with corresponding ranges. If the wings of 

 Hymenoptera, Diptera, or Orthoptera possessed the same 

 elaborate patterns as the Lepidoptera, we cannot doubt that 

 they too would exhibit the same differences in various parts 

 of fheir areas. These continual changes which we find as we 

 study the distribution of Lepidopterous forms in space, is 

 vindoubtedly a measure of the speed with which they have 

 occurred in time. Rapidity of change is essential if it is 

 to keep its adjustment with nicety to the fleeting details of 

 disti'ibution.* Hence we may confidently believe, that if we 



* It is to be observed that I speak of the details as fleeting. The 

 general area of distribution is doubtless extremely ancient in most cases. 

 Thus, although a species of Ueliconius, etc., may have originated within 

 the South American tropics, and never have wandered bej'oid them, the 

 complex shape of its actual area of distribution at any one time cannot be 

 regarded as fixed or ancient. Yet in many a species the variation of the 

 constituent individuals is adjusted with precision to the geograpliical details 

 of the existing range. 



Mr. Roland Trimen, ou reading the above footnote, writes to me 

 January 24, 1904: — "Your note reminds me of the recent appearance 

 on the Natal coast of several conspicuous East-African butterflies, vid. : 

 Picris S2nlleri, Ci-enis rosa, and Godartia xvakcfieldii, all of which are 

 shown to have not only extended their range to a point wliere they were 

 ])reviously quite unknown, but to have also established themselves in the 

 fresh area. This is a good case, as Durban has had, for the last twenty- 

 five years at least, a number of keen collectors of Lepidoptera, v/hom such 

 conspicuous forms could not possibly have escaped had they inhabited the 

 neighbourhood. Besides these species, the last butterfly that my friend 

 and collaborator, the late Colonel Bowker, sent to me (1898) was tlie large 

 and extremely consj)icuous black-and-white Acraea satis, which he took 

 at Malvern, near Durban. This is the only example known to me to have 

 occurred in jSTatal ; but Bowker, who noted the resemblance on tlie win^ 

 to Papilio iiiorcniia, wrote that he had seen one other for certain, and 

 thought that he might very possibly have passed over more examjiles for 

 the common Papilio named. Tliis last case is of special interest (shoirhl 

 it prove one of extended range like the three mentioned), because the 

 Acra^m are so exceptionally slow-flying and gregarious, that they must 

 spread very slowly indeed into fresh areas." 



