156 Dr. T. A. Chapman on 



(not quite invariable) rule in the Sicilian examples and the 

 pale ones from Hyeres. 



I suppose I ought to make some effort to explain why 

 H. hycrana should in fifty years have acquired and developed 

 a melanic tendency. The probability seems to be, in 

 reality, that the change has occurred in a considerably 

 shorter time, but this is mere surmise, founded on the 

 belief that H. hyerana has during that interval been col- 

 lected at Hyeres, and that no record, so far as I can find, 

 exists of the melanic form having been observed. As a 

 matter of fact, however, I am aware of no records of such 

 captures at Hyeres, but this is not perhaps surprising, as 

 no melanic form occurring, the collector had little to add 

 to Milliere's account and so published nothing. My friend 

 M. I. Bourgeois on one occasion bred one or two pale ones, 

 without making any record of the fact anywhere. We 

 must, nevertheless, stick to the fifty years as the period 

 during which the change has taken place. It is, no doubt, 

 highly probable that a long period might elapse before the 

 effective cause, whatever it was, accumulated sufficient 

 effect to produce one or a few dark specimens; but thereafter 

 the change by which about two-fifths of the race became dark 

 was probably fairly rapid. It will be interesting to know 

 whether a few more years produce any further effect, or 

 merely confirm the present position. 



What has produced the melanism ? Hyeres is doubtless 

 a larger place than it was fifty years ago, and therefore 

 more urban; but that that change has been accompanied by 

 the very slightest appreciable change towards making it a 

 smoky district with natural objects blackened, has only 

 to be mentioned to raise a smile at the absurdity of the 

 idea. I think we may therefore reject any trace of identity 

 in causation, with those cases where, in England, urban 

 increase and manufacturing activity are the basal causes of 

 melanic change. I am not aware that there is the slightest 

 evidence that Hyeres is a wetter locality of late years than 

 it used to be. Nevertheless there may be something in the 

 circumstance that the rainy season at Hyeres is in the 

 autumn when the moth is on the wing, although this has 

 always been the case no doubt, and is also a climatic 

 feature, more or less, of the whole area of distribution 

 of the moth. 



The special fact that seems to me to throw most light 

 on the matter is that H. hyerana has its head-quarters in 



