Cp?) 
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 their areas. These continual changes which we find as we 
study the distribution of Lepidopterous forms in space, is 
undoubtedly 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 
distribution.* 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 Heliconius, etc., may have originated within 
the South American tropics, and never have wandered beyond 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 geographical details 
of the existing range. 
Mr. Roland Trimen, on 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. : 
Pieris spilleri, Crenis rosa, and Godartia wakefieldii, all of which are 
shown to have not only extended their range to a point where they were 
previously 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, whom 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 the large 
and extremely conspicuous black-and-white Acrwa satis, which he took 
at Malvern, near Durban. This is the only example known to me to have 
occurred in Natal ; but Bowker, who noted the resemblance on the wing 
to Papilio morania, wrote that he had seen one other for certain, and 
thought that he might very possibly have passed over more examples for 
the common Papilio named. This last case is of special interest (should 
it prove one of extended range like the three mentioned), because the 
Acree are so exceptionally slow-flying and gregarious, that they must 
spread very slowly indeed into fresh areas.” 
