THE RHOPALOCERA IN THE PYRENEES. 323 



appears to approximate very closely to that of the former, in 

 support of which I could furnish abundant proof. It is not, 

 however, my intention in the present paper to enter into detail ; 

 this perhaps I may be persuaded to do in some subsidiary paper 

 on the same subject later on. I propose here to deal with 

 general conclusions only. 



One interesting fact which has struck me, in making a com- 

 parison between the Khopalocera occurring at the higher alti- 

 tudes in the Pyrenees with their relatives in the Alps, is the 

 comparative less number of melanochroic or alpine forms 

 accompanying the type, or taking their place, than in the latter 

 range. Two theories may be put forward to account for this : 

 1, that the lowland or Austral types, which possess representa- 

 tives in the higher zones in the Alps through being recent 

 immigrants to the Pyrenees, have not had sufficient time to 

 develop them in the latter range ; 2, through competition being 

 less severe or conditions less varied than in the Alps. The 

 former seems to me to be the best view to assume, but possibly 

 both may be to a certain extent correct. 



The number of species of Khopalocera occurring in the 

 Pyrenees is considerably less than that existing in the Alps, this 

 difference being specially noticeable in the alpine forms ; thus in 

 the genus Erehia only about half as many species are found 

 in the Pyrenees as there are in the great mountain range in 

 Central Europe. 



This deficiency probably dates back as far as the glacial 

 period, and is to be accounted for by several hypotheses. In 

 large measure it is undoubtedly due to the isolated position of 

 the chain, coupled with the considerably less extent of area 

 which it occupies in comparison with the Alps. To the glacial 

 period, however, must be attributed an overwhelming influence 

 in deciding the present distribution of the existing Khopalocera 

 fauna. The effect it has produced is probably very profound 

 indeed. As the ice-cap slowly advanced over the plains and low- 

 lands of France, the butterflies gradually receded further south, 

 until they ultimately reached the region under consideration. 

 Here their retreat was finally and effectually cut off by the 

 elevated mountain range rising in their rear like a huge wall, 

 and as the ice-sheet approached its base they were iti consequence, 

 like the rest of the pre-glacial fauna, unable to surmount the 

 barrier, and were compelled to succumb to the increasing cold. 

 Most of the alpine forms which exist upon the higher slopes at 

 the present day probably owe their survival during the glacial 

 epoch to the shelter afforded them in Spain at lower levels on 

 the southern side of the range, while nearly all the lowland 

 forms found in the region now are probably post-glacial immi- 

 grants from countries situated to the east. 



In support of this theory may be brought forward the fact 



