Glare Island Survey — Aquatic Co [copter a. 29 17 



the centre of production of such a species would have moved southward, and 

 with the retreat of the ice would again have returned northward and the 

 palaeontological record is clearly explicable on this assumption. It is obvious 

 therefore, that the present-day centre of production of any species only 

 indicates the climatic type to which that species belongs and cannot in any 

 way be regarded as the permanent home of the species. It must however be 

 remembered that altitudinal movement is equivalent to latitudinal so that a 

 species domiciled in the mountains of southern Europe during the tropical 

 and warm temperate periods would occupy a temperate or even arctic 

 " island " and could, by descending to the plains as the climate grew colder, 

 maintain its centre of production in the same geographical area. 



With regard to Forbes' 1 Lusitanian species it is not contended that they 

 were " tropical " in the Miocene period. They are not so now and from the 

 fact that at least many of them occur in the mountains in the Spanish 

 peninsula, they are not even warm temperate species. At the time when the 

 British islands enjoyed a tropical climate these " Lusitanians " — if they were 

 then in Spain and Portugal — must have been high up in the mountains and 

 just as effectively isolated from Ireland as if an ocean rolled between these 

 two areas as it does at the present day. Forbes' hypothesis of a land-bridge, 

 to account for these species reaching Britain from the Spanish peninsula 

 during Miocene times is therefore of no avail, unless we assume also the 

 existence of a mountain-range and in western Europe mountain-chains tend 

 to run east and west, not north and south. 



When we remember also that from Miocene to Pleistocene the general 

 movement of the fauna and flora was southward, it seems even less probable 

 that our south-western Irish group came to us from south-western Europe — 

 unless they have reached us since the climax of the Glacial period. At some 

 time previous to the Glacial period our country enjoyed a temperate climate 

 and, assuming that these species survived the cold period here, it seems more 

 reasonable to believe that they came to us from the north and that they 

 reached the Spanish peninsula from the same direction. 



With regard to the western group being a relict fauna and flora, it is to be 

 noted that the species are not in the west confined to the mountains. Many 

 of them, both northern and southern, descend almost to sea-level and are 

 found living side by side. It has been suggested that the damp atmosphere 

 of the west is the dominating clirnatic factor, but this can scarcely affect the 

 water-beetle fauna. It seems more likelv that the small annual range of 



1 I draw a. distinction between his group and that of later writers who do not distinguish between 

 his "Lusitanian" and " Galliean" types. 



B.I. A. PROC, VOL. XXXI. C 29 



