At the time when the North European Sea flooded a portion of England, Scandinavia was still connected with Scotland, and the latter with Ireland (Fig. 6, p. 126). There is no doubt that the food-supply in the Arctic Regions was decreasing with an increase of snowfall and with the gradual lowering of the land, which reduced also the habitable area.

Arctic species therefore were driven south in search of fresh pastures.

But it need not be supposed that anything like a vast destruction of the fauna of the Arctic Regions took place. Only fewer mammals were able to find food in a given s.p.a.ce than heretofore. This southward migration may have commenced, in the case of plants and the invertebrates, at a much earlier time,--during the Miocene or Pliocene Epochs,--but it is doubtful whether the mammals and birds which we find in our pleistocene and recent deposits began to travel south much before the commencement of the Glacial period. The beginning of the Glacial period in England, I think, is indicated by the deposition of the Red Crag, though the latter is generally regarded as belonging to the pliocene series. Much of the northward migration from the British Islands of Lusitanian and other forms had then ceased, but we have in Scandinavia, just as in these islands, a southern relict fauna and flora, plants and animals which had wandered across what is now the German Ocean from Scotland to Scandinavia, and have never become extinct in that country to the present day. I need only mention the Red Deer, the Badger, and Slugs of the genus _Arion_.

Professor Blytt directs attention to some such southern relict species of plants now only found in the extreme south-west of Scandinavia, such as _Asplenium marinum_, _Hymenophyllum Wilsoni_, _Carex binervis_, _Scilla verna_, _Erica cinerea_, _Conopodium denudatum_, _Meum athamantic.u.m_, and _Rosa involuta_ (p. 28).

The Arctic fauna and flora in Scandinavia--that is to say, the descendants of those species which migrated direct from Greenland and Spitsbergen, as we have seen, are numerous. They of course persisted throughout the Glacial period in the country, and are now in many localities being exterminated partly by change of climate, partly by a keen compet.i.tion with more vigorous rivals which have come to Scandinavia from the east. It is a curious circ.u.mstance, as pointed out by Professor Blytt, that the Arctic plants in the Botanic Gardens at Christiania are able to stand almost any amount of sunshine, but are very liable to be injured by the frost, and have to be covered in the winter. A similar observation has been made in the case of the Alpine plants at Kew Gardens, which have to be wintered in frames, though their homes are either in the high Alps--among the everlasting snows--or in the intensely cold climate of Greenland. Many of the Scandinavian plants exhibit instances of discontinuous distribution, thus showing their ancient origin; and there is altogether nothing in the fauna and flora of that country which might lead us to believe that these were exterminated during the Glacial period and reintroduced subsequently.

The climate during that period in Scandinavia was probably more equable and moister,--with a greater snowfall in winter and with less sun to melt the snow during summer,--so that the development of glaciers took more formidable dimensions, chiefly on the east side. The lowlands of Sweden were covered by the sea, whilst many of the valleys were choked with great glaciers, which cast off portions of ice as they reached the sea, just as the Greenland and other northern glaciers do (_vide_ p.

237). A country which at the present day probably somewhat resembles the former Scandinavia climatically is Tierra del Fuego, in the extreme south of South America. Though there is an abundant snowfall, so that glaciers reach the sea in many parts of the country, the flora has been described by travellers as luxuriant; and it appears that the fauna also is richer than might be expected from the cheerless climate.

Towards the latter part of the Glacial period the land-connection between Scandinavia, Spitsbergen, and Greenland broke down, and the waters of the Arctic and Atlantic Oceans joined. Whether it was at this time or later that the other land-connection between Scandinavia and Scotland collapsed is difficult to determine; but it is certain, I think, that Scotland was still united with Ireland even after these two great land-bridges ceased to exist.

SUMMARY OF CHAPTER IV.

The fauna of the Arctic Regions is much poorer than that of the other regions which are dealt with in this work. In some groups, such as Reptiles and Amphibia, there are no representatives at all, but no doubt a larger number of species existed there in earlier Tertiary times. At least we have fossil evidence that during the Miocene Epoch plants of many families flourished in Greenland of which no vestige is now left in the Polar area. Climatic conditions must therefore have changed, as in Europe. A gradual refrigeration took place, owing probably to the slow withdrawal of the current which supplied the Arctic Sea with warmth.

Greenland and Europe were then connected, and the Arctic Ocean was separated from the Atlantic. This land-connection is supposed to have lain far north between Scandinavia, Spitsbergen, and Greenland, and must have persisted until towards the end of the Glacial period.

As the temperature decreased and the land-area available in the north diminished, the surplus population, consisting of animals and plants, and possibly also of human beings, moved southward. We have traces in Europe, and especially in the British Islands, of a very early migration from the north in the so-called American plants and in the freshwater sponges. The geographical distribution of some of the Arctic species of mammals is referred to in greater detail, to show how the relative age of their entry into Europe can be determined. Two forms of Reindeer, resembling the Barren-ground and Woodland varieties, have been met with in European deposits, but only the former occurs in Ireland and the south of France, whilst eastward the other becomes more common, and finally is the only one found. It is believed that the Barren-ground is the older form as far as Europe is concerned, and that it came to us with the Arctic migration, and that the other Reindeer reached Europe much later from Siberia, when Ireland had already become detached from England. The range of the Arctic Hare is equally instructive. It must have been a native of Europe since early glacial or pre-glacial times--before the common English Hare had made its appearance in Central Europe. Along with other Arctic forms, it entered Northern Europe directly from the Arctic Regions, by means of the former land-connection which joined, as I remarked, Lapland with Spitsbergen, Greenland, and North America. There need not have been a post-glacial connection between Europe and Greenland; the present flora of that country may have survived the Glacial period in the Arctic Regions, as has been maintained by some botanists and other authorities. Professor Forbes argued from the occurrence of the same species of sh.o.r.e mollusca on the coast of Finmark and Greenland that these two countries were not long ago joined, so that a slow migration from west to east along an ancient coast-line could have taken place. That such a migration actually occurred is further made probable, judging from the presence of American mollusca in the Crag deposits on the east coast of England. These came into the North Sea in the first place direct from the Arctic Ocean at a time when the two oceans freely communicated with one another across the lowlands of Northern Russia, Northern Germany, and Holland. Arctic sh.e.l.ls are also found below the boulder-clay on the Baltic coast, and a free communication such as indicated is generally held to have taken place at no very distant date. The so-called "relict species"--marine animals left in freshwater lakes in districts formerly covered by this sea--lend some support to this view. But the view that the continental boulder-clay is a marine deposit is not now held except by a few, though I here bring it forward again, as it seems to me to fit in so much better with the known facts of distribution. The sea just referred to probably existed throughout the greater part of the Glacial period; and icebergs, which originated from the Scandinavian glaciers, would have brought detritus and boulders to the lowlands. Scandinavia was then connected with Scotland, and England with France.

FOOTNOTES:

[1] A very interesting piece of information has been given us, recently, by Mr. Barrett-Hamilton on the Arctic Fox of Spitsbergen. In comparing the skulls of Spitsbergen Foxes with those of Europe, he found that the former are much smaller, and represent a distinct race or sub-species.

This small race he believes to be confined to Greenland, Iceland, Spitsbergen, and Novaya Zemlya, whilst the larger one occurs in Europe, Asia, and on the Commander Islands. This fact favours the view which I have advocated in Chapter V., that the Arctic Fox in Europe is a Siberian migrant, and did not come from the north with the Reindeer and Arctic Hare.

[2] I have already expressed this view on p. 120.

[3] The occurrence of this species in Lough Neagh in Ireland, pointing to a connection between the Irish Sea and the Baltic, will be referred to later on; as also that of two allied forms in the Caspian Sea.

[4] For additional species with a similar range, _vide_ Nordquist.

CHAPTER V.

THE SIBERIAN MIGRATION.

In dealing with the British fauna in particular, I have drawn attention to the fact that it is chiefly in the south of England that we find fossil remains of eastern species of mammals in recent geological deposits. We can actually trace the remains of these species and their course of migration across part of the Continent towards Eastern Europe, and as none of their bones have been discovered in the southern or northern parts of our Continent, it must be a.s.sumed that their home lay in Siberia, where many still exist to the present day, and where closely allied forms also are found. Some of these Siberian migrants have remained in England and on the Continent to the present day. Many have become extinct. But the animals forming this eastern migration did not all originate in Siberia, though I have sometimes spoken of them collectively as Siberian migrants. There must have been other centres of dispersion of species in Europe. We know that a very active centre of development--at any rate for land-mollusca--lay in South-eastern Europe, either in the Caucasus or in the Balkan peninsula, or more probably in both. The Alps no doubt produced a number of species which have spread north and south, and may in their wanderings have joined the Siberian migrants in their western course, and thus have reached the British Islands. Nevertheless, the majority of the mammals belonging to the eastern element of the British fauna (_vide_ p. 95) have undoubtedly originated in Siberia. The Polecat (_Mustela putorius_) and the Harvest Mouse (_Mus minutus_), for instance, are members of that eastern migration. Both occur throughout Central Europe and a large portion of Siberia, but are absent from the extreme north and south of Europe and also from all the Mediterranean Islands. A Siberian species, which has never penetrated so far west as the British Islands, nor even so far north as Scandinavia or south to Italy, is what is known in Germany as the "Hamster" (_Cricetus frumentarius_), a little Rodent which spends the winter asleep in its burrows, and surrounds itself with a great acc.u.mulation of food-material carried there during autumn. The common English Hare, which I formerly regarded as an instance of a Siberian mammal, must now find a place among the Oriental migrants. Its history is very instructive, and I shall have an opportunity later on to refer to it again. Meanwhile, it may be mentioned that though this Hare inhabits Europe in two varieties or races, one of which, _Lepus mediterraneus_, is confined to Southern Europe, the latter owes its origin to an earlier migration from Asia.

When we come to consider the eastern birds, we have to distinguish between resident species and migratory ones. The Black-throated Thrush (_t.u.r.dus atrigularis_), which has been twice obtained in the British Islands, is a mere straggler to Europe, and is not known to breed there at all. Better known birds, perhaps, are the Golden Thrush (_t.u.r.dus varius_), which has even occurred as far west as Ireland, the Rock-Thrush (_Monticola saxatilis_) and the Scarlet Grosbeak (_Carpodacus erythrinus_), which breed in Eastern Europe, but are known only as occasional visitors in the west.

To judge by their distribution, the Bullfinches (_Pyrrhula_) are of Asiatic origin, for seven species out of ten are confined to that continent. Our common Bullfinch (_P. europaea_) probably came with the Oriental migrants, or perhaps its ancestors did. But the larger Northern or Russian Bullfinch (_P. major_) has no doubt entered our Continent directly from the east. We have in many groups similar instances of closely allied species or varieties, one of which, originating at a somewhat later stage than the other, took a different route of migration from that followed by its near relative.

The Pine-Grosbeak (_Pinicola enucleator_) is only known to British ornithologists as an exceedingly rare visitor. Its real home lies in the northern parts of Europe, Asia, and North America, and it is one of the most typical of the Siberian migrants.

But there are a number of other species of birds, which, though probably not of Siberian origin, only migrated westward recently, and have either not yet reached the British Islands, or which lead one to suppose, from their British range, that they are eastern forms.

Such, for instance, is the Nightingale (_Daulias luscinia_), which is probably of Oriental origin, but only visits England regularly in spring. There is no authenticated record of its ever having migrated either to Scotland or Ireland.

The Bearded t.i.tmouse (_Panurus biarmicus_) is one of the eastern birds still resident in England, though unfortunately it seems to be on the verge of extinction. It is unknown in Scotland and Ireland. Another resident eastern species is the Nuthatch (_Sitta caesia_), but neither of these is probably of Siberian origin.

The majority of the European Reptiles are probably of eastern origin.

Among our British species, the Common Viper (_Pelias berus_), for example, is a typically eastern form. It is almost unknown in Southern Europe proper--that is to say, in Italy, the Balkan peninsula, and the Mediterranean Islands, but its range extends in the west as far as Spain, and in the east right across the Asiatic continent to j.a.pan. It is well known that the Viper occurs in Scotland, and that neither it nor any other snake is found in Ireland. There is a legend, indeed, that snakes did once exist in Ireland and were banished from the island by St. Patrick, but unfortunately we have no historical evidence that such an interesting event actually took place. The Sand-Lizard (_Lacerta agilis_), another British species, may be looked upon as an eastern form. It is quite absent from Italy, the Balkan peninsula, and the Mediterranean Islands, but extends throughout Central Europe to the east.

Among the species of eastern Reptiles which have a mere local range in Europe might be mentioned the two Lizards, _Phrynocephalus auritus_ and _Agama sanguinolenta_. They belong to the family _Iguanidae_, which includes some very large species. Both of them are Asiatic forms, which have only just penetrated across the eastern steppes into Europe, where they inhabit the arid regions between the Caspian and the River Don in Southern Russia.

The species of Mammals living in Europe at the present day have, with few exceptions, migrated to our continent from other parts of the world.

With regard to the Birds, it is possible that a somewhat larger number proportionally may be of European origin. Still, the great majority are, I think, to be regarded as immigrants. The autochthones are about equal to the immigrant reptiles, but many of the European Amphibians and the majority of the Fishes have probably originated on our continent. Some of the European Amphibia--especially among the tailless forms--appear to be immigrants from Asia. Thus the distribution of _Rana arvalis_ in Europe is remarkably like that of a Siberian migrant. This frog occurs in Siberia, ranging southward as far as Persia and parts of Asia Minor.

Crossing the European border, we find it in Russia, Upper Hungary, North and Central Germany,--being rarer in the south,--Denmark, and Scandinavia. According to Bedriaga, it crosses the Rhine only in Alsace, but occurs no farther west. It only just enters Holland. If we suppose the species to have originated in Central Europe, we should expect to find it in Switzerland, France, and perhaps England. If it had its ancestral home in Eastern Europe, we might expect it to occur on the Balkan peninsula. It seems to me more probable, therefore, that _Rana arvalis_ came with the Siberian migration. This need not cause surprise, as the genus _Rana_ is certainly not European. Out of about 110 species, only four are peculiar to Europe, the rest are scattered over all parts of the globe. Moreover, the fact that these four species are confined to Southern Europe would seem to indicate that the first species entered from the south, and there either became modified or spread over nearly the whole continent, as did, for instance, _Rana esculenta_ and _R.

temporaria_. Neither of these is by any means confined to Europe. _R.

esculenta_ ranges right across the Asiatic continent to j.a.pan, and also enters North Africa, while the other has a wide distribution in northern and temperate Asia.

The various groups of Vertebrates are not dependent on each other in their migrations. Mammals and Birds extend their range with so much greater facility than Reptiles and Amphibians, that the surplus population of our neighbouring continents readily poured into Europe when--owing to changes of climate perhaps--they forsook their original homes.

We observe much the same differences of origin in the various groups of European Invertebrates. The Central European Molluscan fauna, remarks Dr. Kobelt, had already developed from the pliocene--in almost all its details, as regards formation of species and distribution--when the Ice-Age commenced (_b_, i. p. 162). Certain very interesting dislocations, however, in the range of land mollusca can be proved to have taken place about that time. Thus, as Dr. Kobelt has pointed out, the genus _Zonites_, which is now almost confined to the south-east of Europe, occurs in inter-glacial deposits in the valley of the Neckar, and even as far west as the Seine. If we might judge from this single instance, a molluscan migration from the east to the west seems to have occurred either in early or pre-glacial times. That _Helix pomatia_ has migrated only comparatively recently from the East to Western Europe is rendered probable by its general range in northern and western Europe, but I cannot agree with Dr. Kobelt in the belief that _Helix aspersa_ is of an equally recent origin in the North. No matter whether it has been found fossil or no, its range in the British Islands points to its having penetrated to Ireland when the latter was still connected with the Continent by way of England. Its migration from the Mediterranean dates therefore from early pleistocene or late pliocene times.

In referring to the sixty-five species of Land and Freshwater Mollusca which have been described from the continental "Loess," Dr. Kobelt states (p. 166) that this fauna has certainly not a steppe-character. It does not therefore strengthen Professor Nehring's view that Europe during the deposition of the loess had a climate comparable to that of the Siberian steppes. The Glacial period had hardly any effect on the molluscan fauna of Europe. Dr. Kobelt believes in a certain movement of that fauna from the least favourable areas, with a subsequent re-immigration; but even that could not have taken place on a large scale. Nothing like a destruction of the fauna occurred, as far as we know from fossil evidence.

Not a single species of land or freshwater mollusc can be quoted as having migrated to Europe from Siberia in recent geological times. The molluscan fauna of the latter country is so closely connected with that of Europe, that it is quite impossible to elevate it to the rank of a sub-region of the Holarctic Region. Dr. Kobelt insists that Siberia cannot even claim to be placed into a distinct province. According to the same authority, we find no species in the whole Siberian molluscan fauna which we might regard as having immigrated since the close of the Glacial period. Even to attempt the location of the original homes of many of the species which Siberia has in common with Europe, seems hopeless. Such forms as _Arion hortensis_, which has been obtained in Siberia, and which, as we have seen, must have originated in Western Europe, migrated in pliocene or miocene times, possibly along the sh.o.r.es of the Mediterranean and across Asia Minor. We have evidence, therefore, of an eastward migration among the land and freshwater mollusca in later Tertiary times, but not of a westward one from Siberia.

A very different view is presented to us by the coleopterous fauna of Europe. Many of our European Beetles are Siberian migrants. Let us take, for instance, the Tiger Beetles (_Cicindelidae_). There are over forty species of the genus _Cicindela_ in Europe, five of which reach the British Islands. This seems a large number; but there are altogether no less than 600 species of the genus scattered over the greater part of the world, many of them being Asiatic. The genus is certainly not of European origin, for not only are most of the European species confined to the Caucasus and the south-east generally, but no _Cicindelidae_ whatsoever occur, for example, in Madeira or the Canaries, where we should expect some to have persisted if the genus had originated on our continent. Moreover, of the five tribes into which the large family of _Cicindelidae_ can be sub-divided, only two range to Europe, and one of them is represented by only a single species on our continent.

Some of the _Cicindelas_ may have come with the Oriental migration. I think this was the case with the only Irish species of the genus, _C.

campestris_. It occurs all over continental Europe and Northern Asia, and varieties of the species are known from Corsica, Sicily, Crete, the Cyclades, Sardinia, Asia Minor, Greece, and Spain. Five species of _Cicindela_, as I said, are known from England, of which _C. silvatica_ and _C. maritima_ are certainly Siberian migrants, and perhaps _C.

hybrida_ too. Neither of the two first species is found in Southern Europe or in Spain, where we should expect them to occur had they originated on our continent. _C. silvatica_ and _maritima_ have no doubt entered Europe from Siberia in recent geological times, probably soon after a way was opened up across the Tchornosjem district of Southern Russia--that is to say, in inter-glacial times. The former spread along the Central European plain as far west as the south-east of England when Great Britain still formed part of France. _C. maritima_, which preferred the proximity of the sea, migrated along the sh.o.r.es of the Caspian and then across Russia to the sh.o.r.es of the Baltic and North Sea, and has penetrated a little farther north and west in England than its near relative. _C. litterata_ has a very similar distribution and origin, but instead of wandering so far west as the British Islands, it seems to have preferred extending its range southward, and has just reached Northern Italy.

The closely allied Ground-beetles (_Carabidae_) furnish us with equally interesting and instructive proofs of a migration from Asia. Over 300 species of _Carabus_ are known to science. The number of species inhabiting Asia and Europe are about equal. But the genus does not extend its range to Southern Asia or to South America or Australia. Very few species enter Africa, and only nine North America, of which three also occur in Siberia. The genus is unknown in Madeira, and only represented by three species in the Canary Islands. To judge from its distribution, it has probably originated in Western Asia. Probably some _Carabi_ of European origin have spread into Asia, but the Asiatic--or we might say the Siberian--origin and subsequent migration westward of a number of well-known forms appears to me evident. Such forms as _C.

clathratus_, _C. granulatus_, and _C. cancellatus_ are no doubt of European origin, and have only in recent geological times extended their range across Northern Asia, whilst _C. marginalis_, coming from Siberia, can hardly be said to have invaded Europe, since it has never been met with farther west than the eastern provinces of Prussia.

Among the _Carabidae_ there are altogether very many examples pointing to a migration from Asia to Europe, but I do not wish here to give a list of all such cases, and only refer to a few of the more remarkable ones.

One of the European species of Demetrias (_D. unipunctatus_), known to English entomologists as a south-eastern form, seems to have arrived with the Siberian migration, whilst the closely allied _D.

atricapillus_, which has been able to reach Ireland, has a wider range and came earlier with the Orientals.

Messrs. Speyer state (p. 68) that almost all those species of Central European b.u.t.terflies whose northern limit is deflected southward as we approach the west coast of Europe, inhabit also the Volga country and the adjoining parts of Asia. Many of them are much commoner there than in Central Europe, and it appears probable to the authors of the _Geographical Distribution of b.u.t.terflies_ that these species came from the east. Asia and Central Europe have, according to Messrs. Speyer, no fewer than 156 species in common. Mr. Petersen estimates that no less than 91 per cent. of the Arctic-European b.u.t.terflies also occur in Siberia. He made a special study of the Arctic _Macro-lepidoptera_, and came to the conclusion that Central Asia, not having been glaciated in the Ice-Age, offered a possibility of existence to both animals and plants. Here, he thinks, was the princ.i.p.al centre to which Europe owed its re-population in post-glacial times. Mr. Petersen is of opinion (p.

40) that the Arctic-European _Lepidoptera_ are composed of two elements--the pliocene relics which persisted in Europe during the Glacial period, and the new immigrants from Siberia.

No doubt Siberia supplied Europe with a number of species of b.u.t.terflies and Moths in recent geological times, but we need not necessarily suppose that these arrived only after the Glacial period.

Even the most extreme glacialists admit that large areas on our continent were free from ice at the height of the Ice-Age, Siberia had therefore no particular advantage over Europe in giving an asylum to b.u.t.terflies and Moths which were escaping from the rigours of a supposed arctic climate. But we have already learned (p. 80) that the climate during the Glacial period probably differed but little from that which we enjoy at the present day, and we may a.s.sume, therefore, that the _Lepidoptera_ of Siberia migrated during that time or even earlier to Europe.

Let us for a moment reconsider some instances of mammalian migration from Siberia, with a view to studying more closely the nature of these great events. I mentioned the fact that some of the Siberian migrants have remained in England, that more have settled down permanently on our continent, but that many others have either become entirely extinct or do not live any longer in Europe.

Of the mammals which made their appearance in Great Britain in recent geological times, _i.e._, during and since the deposition of the Forest-Bed for example, the following species probably came direct from Siberia across the plains of Europe, as already mentioned (p. 95):--

Canis lagopus.

Gulo luscus.

* Mustela erminea.

* " putorius.

* " vulgaris.