Progressive melanism on the Riviera. 157 



South Spain, North Africa, Southern Italy, Sicily, etc., 

 and that Hyeres is the extreme northern limit of its range. 

 I pointed out in my previous notes how restricted the 

 habitat of H. hyerana is on the Riviera, and that it is quite 

 absent from many places, apparently suitable to it, as 

 regards food-plant, etc. These places, however, are all of 

 fractionally more northern climate than Hyeres, which has 

 the mildest climate of any French station west of Marseilles, 

 This slight difference in climate between Hyeres and the 

 Riviera further east is probably the efficient cause of its 

 localization to that one spot. How does this affect the 

 matter ? I do not desire to suggest that the northern 

 position is the cause of the melanism, I do not think 

 I should agree with the arguments likely to be adduced in 

 support of such a contention. My suggestion amounts to 

 this, that fifty years ago H. hyerana was a recent immi- 

 grant at Hyeres, possibly for the first time, more likely it 

 had previously established itself there on several occasions, 

 but the locality being at the northern limit of its distribu- 

 tion, a few adverse, or even one very bad season might suffice 

 to destroy it, and its place would be vacant until the arrival 

 of individuals from some other locality. The new arrival 

 would multiply in the form it had maintained in the locality 

 it came from, until, after a sufficient lapse of time, it was 

 modified to suit Hyeres conditions. I suppose then that 

 Milliere found it unmodified, but that now it has a form 

 adapted to Hyeres conditions. If the hypothesis of 

 occasional extinction at Hyeres be accepted, it might no 

 doubt equally be suggested, that the species has disap- 

 peared during the last fifty years and the present form is 

 the result of new arrivals (^marginata. occurs at Gibraltar), 

 or such arrivals may have produced the change without the 

 original race having disappeared. The objection to this is 

 that it seems to postulate immigration, say, probably about 

 every twenty-five years, whereas, change during the past fifty 

 years postulates it, say, only once in about a hundred years, 

 or more. The mixed melanic race would thus be the one 

 suitable to Hyeres, probably owing to the nature of the 

 objects on wiiich the moth would rest. The Capri form 

 being so much paler than the Sicilian, would appear to prove 

 that the dominating influence was not climate but locality. 

 I advance this hypothesis simply because it seems incum- 

 bent on me to produce some sort of a rushlight to illuminate 

 the way of further inquiry. The real interest of the case 



