DISTRIBUTION OF THE BRITISH BUTTERFLIES. 273 



linger here and there on some of the African mountains. It is 

 not demanding too much to suppose then that Hyale is at least 

 as old as the Pre -Glacial Period, and probably much older. A 

 similar age must be assigned to Edusa, which, though not 

 present in South Africa, is there represented by a very close ally, 

 as it also is by another in North America, the latter being indeed 

 scarcely distinguishable. 



To the species common to Europe and America a considerable 

 antiquity may be allowed. In connection with them, the in- 

 teresting question arises, " In which hemisphere did they 

 originate?" or how did they pass from Europe to America, or 

 vice versa. Their origin may have been just as well in one as 

 in the other, but I think there is a slight probability in favour of 

 an American origin for at least some of them. Perhaps, however, 

 their birthplace may have been in the circumpolar lands, which 

 seem at one time to have had a rich fauna and flora, and where 

 it is by no means impossible that many palaearctic species were 

 developed. But be this as it may, most of the evidence for the 

 line of passage from America to Europe, or Europe to America, 

 is in favour of Behring's Straits, or rather of the land connection 

 that formerly existed there. Not much can be said in favour of 

 a passage by Iceland and Greenland, for between Greenland and 

 America there seems to have been no direct land connection. 



Let us turn now to the question of the antiquity of the species 

 in Britain. 



In the first place, it may be said that, with perhaps one or 

 two exceptions, there is really no reason why any of the species may 

 have been inhabitants of Britain for more than a few thousand 

 years. Continental Europe is so close, that it is quite within the 

 bounds of possibility that many of the species might have flown 

 over, or been blown over, any time within (say) a thousand years. 

 Still the probabilities are against this, and in favour of their 

 arrival with the rest of our indigenous fauna and flora, which 

 came to us across the dry bed of the German Ocean. 



As regards plants and wingless animals, a certain amount of 

 evidence as to the order of their arrival may be gathered from 

 their existence or non-existence in various of the outlying groups 

 of islands in proximity to Britain. If we find certain species in 

 these islands, the probability is that they arrived there before the 

 land connection was cut off, and that consequently they arrived 



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