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NATURE 



[October 28, 1897 



The fauna of Ireland as well as the flora is found to consist 

 mainly of two elements, one of which came from the north and 

 the other from the south. In the fauna of Great Britain the 

 same two elements occur, but there is, in addition, a third— an 

 eastern one— chiefly confined to the eastern counties. The 

 southern element contains animals which came originally from 

 south-western, and others which migrated to the British Islands 

 from the South and Central Europe. The former are confined 

 to the south-western counties of England and Ireland, whilst the 

 latter are chiefly found in the south-west of England, Wales, 

 Ireland, and the west of Scotland. The northern element chiefly 

 occurs in Scotland, the north of England, and the north and 

 west of Ireland. 



Though it may be admitted that a small percentage of the 

 British fauna reached the British Islands by occasional means of 

 dispersal, the bulk of it migrated on land. A land-connection 

 must therefore have existed formerly between Great Britain and 

 Ireland, and the continent of Europe. 



The late Edward Forbes believed that the Lusitanian or 

 south-western element in the Irish flora (it was not known at the 

 time that there was also a similar fauna) came to Ireland in 

 Miocene times and survived the Glacial Period on a now sunken 

 land which lay to the south west of that island. Almost all 

 other authorities are convinced that both flora and fauna were 

 entirely exterminated in Ireland during the Pleistocene Epoch, 

 and that what exists there now, migrated to it after the Glacial 

 Period. 



A short statement of the general conclusions arrived at with 

 regard to the geographical changes in Europe during later 

 Tertiary times, and the chief migrations of animals now follows, 

 so as to facilitate the comprehension of the principal arguments 

 advanced in favour of the view that there were two distinct 

 invasions of northern species, and that the Irish fauna is alto- 

 gether pre-Glacial. 



To judge from the range of the south-western European 

 plants and animals in Ireland, it is evident, as Forbes suggested, 

 that they came long before the other southern species or the 

 northern ones. Last of all came the eastern or Siberian migrants. 

 These never reached Ireland, but as we have such abundant 

 evidence of the time of their arrival in Europe, the history of 

 their migration is of great importance, since it furnishes us with 

 a clue to the date of earlier migrations. 



We have geological evidence that a vast migration proceeded 

 from Siberia, and entered Europe between the Caspian and the 

 Ural Mountains. A large number of mammals came with this 

 Siberian invasion, and no fewer than twenty-nine species reached 

 England, ten of which still inhabit Great Britain. There is no 

 evidence that any of them ever lived in Ireland. 



I have endeavoured to ascertain the causes of that migration 

 and its geological date. Both Tcherski and Brandt, the two 

 highest authorities, are of opinion that the present Siberian 

 fauna lived in the country already in preGlacial times, and that, 

 with the addition of some now extinct forms, such as the 

 mammoth, it flourished as far north as the New Silserian Islands. 

 Since the advent of the Glacial Period, the fauna is supposed to 

 have very gradually retreated from these high northern latitudes, 

 which are now almost uninhabitable. 



Against these views it has been urged that, to some extent, 

 the mammalian bones and carcases found in the New Siberian 

 Islands, rest on a solid layer of ice, and that as this ice was 

 probably formed during the Glacial Period, the migrations must 

 have taken place in post-Glacial times. This presupposes an 

 extraordinary amelioration of climate in Siberia, the effects of 

 which certainly would have been felt in Europe, but of which 

 we have no evidence. 



There is geological evidence that a marine transgression took 

 place in Northern Russia in early Pleistocene times, and that at 

 the same time the united waters of the Caspian Sea and the Sea 

 of Aral covered a large tract of the central parts of that country. 

 It is supposed by some naturalists (and in favour of this view I 

 have collected some additional facts) that the White Sea and 

 this large inland sea were connected right across Russia, thus 

 forming a barrier by means of which the Siberian fauna was 

 prevented from migrating to Europe. It is also suggested that 

 the whole of the continental boulder clay is a marine deposit, 

 and that its maximum southward extension approximately marks 

 the shores of a north European ocean. 



In support of this view are quoted a number of Caspian 

 species which must have come from the Arctic Ocean, and the 

 fact of the occurrence of Dreyssensia polymorpha, in the lower 



and not in the upper continental boulder-clay, thus proving that 

 a migration took place in both directions. 



As all the deposits in continental Europe, containing remains 

 of Siberian mammals, are of a later age than the lower boulder- 

 clay, it seemed to me that the connection between the Aralo- 

 Caspian and the White Sea must have ceased to exist during 

 and after the Interglacial phase of the Glacial Period, which 

 would also explain the absence of Dreyssensia in the more 

 recent beds. The Siberian fauna probably began to pour into 

 Europe immediately after the deposition of the lower continental 

 boulder-clay. But since the first Siberian mammals made their 

 appearance in England, during the deposition of the Forest-bed, 

 the British newer Pliocene beds must be contemporaneous with 

 this boulder-clay. Further proof of this will be mentioned 

 later on. 



Some further evidence is now given in favour of the marine 

 origin of the boulder-clay, and the causes of the absence of 

 marine shells in the Russian deposits are explained. 



We have geological proofs that the Siberian fauna migrated 

 to Europe on a tract of country known as the " Tchernosjen " 

 or black earth of Russia, and that this originated from the decay 

 of grass which grew there during long ages This fauna then 

 invaded Central Europe and Great Britain. In France its 

 further progress was arrested by the river Garonne. England 

 and France must therefore have been connected ; whilst the 

 absence of deposits containing Siberian mammals from 

 Scandinavia proved that it was separated from the continent. 



The Northern or Arctic element in the Irish fauna must have 

 come directly from the north. It is more or less confined to 

 the northern and western parts of Ireland, and forms a large 

 proportion of the fauna of Scotland and Scandinavia. It 

 suggests that a land-connection between the latter and the 

 British Islands must have existed. The present and past range 

 of the Arctic hare, the reindeer, and the stoat are discussed in 

 detail to show that such a connection actually united the two 

 countries. Reference is also made to the North American 

 species occurring in Ireland which belong to the same migration. 

 The evidences in favour of a former land-connection between 

 Scandinavia and Greenland via Spitsbergen are now reviewed. 

 It is suggested that the American marine mollusca which 

 have been discovered in late Tertiary deposits of the east coast 

 of England reached that coast, not from the Atlantic, but from 

 the Arctic Ocean by means of the sea which extended from the 

 White Sea to the German Ocean. 



The migration of terrestrial animals and plants from this 

 ancient northern land southward took place chiefly during 

 the deposition of the newer English crags, and of the continental 

 lower boulderclay, that is to say, before the Siberian migrants 

 set foot on British soil. 



The southern migration to the British Islands commenced 

 earlier than either the Arctic or the Siberian. Numerous instances 

 are quoted to prove that the southern fauna is composed of 

 species of south-western and of southern and Central European 

 as well as of Asiatic origin. 



In connection with the origin of the Red Deer, the nature of 

 the geographical changes which the Mediterranean basin has 

 undergone during later Tertiary times are now discussed. 



I have endeavoured to show that Ireland was separated from 

 England at the time while the migration from Southern and 

 Central Europe was in progress. The contradictory evidence 

 from fossil sources as to the climate prevailing at that time in the 

 British Islands are there now discussed. 



The orgin and nature of the Glacial Period is so intimately 

 connected with these faunistic problems, that it has been thought 

 advisable to devote a short chapter to this important era in the 

 life of the direct ancestors of our animals. The prevailing 

 opinions as to temperature and general atmospheric conditions 

 during the period are reviewed in connection with the questions 

 as to the possibility of a survival of the terrestrial fauna and 

 flora chiefly in the British Islands. The British Pleistocene 

 fauna does not indicate the prevalence of Atctic conditions — 

 neither does the flora. 



This fact certainly supports the view formerly held by 

 geologists that the phenomena in Northern Europe, now 

 attributed to land-ice, have been produced by sea with floating 

 icebergs, under conditions somewhat comparable to those at 

 present obtaining in Tierra del Fuego. A succinct statement 

 of my views on the Glacial Period and 'the geographical 

 features of Europe at the time — as derived from a study of the 

 European fauna and of its origin — concludes this memoir. 



NO. 1 46 I. VOL. 56] 



