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 assumed 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 accumulation 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 (Turdus 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 (Turdus 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. europÆa) 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 Titmouse (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 cÆsia), 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 Japan. 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 IguanidÆ, 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 Japan, 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 shores 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 (CicindelidÆ). 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 CicindelidÆ 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 CicindelidÆ 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 shores of the Caspian and then across Russia to the shores 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 (CarabidÆ) 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 CarabidÆ 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 Butterflies 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 Butterflies 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 Butterflies 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 principal 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 Butterflies 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 Butterflies 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 assume, 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):—

Of the arrival of many of these in Europe we have geological proof, as they have left their bones in recent pleistocene deposits, and are unknown from older European strata. The remote ancestors of others, such as Sorex and Lagomys, no doubt lived in Europe, but the recent species probably had their original homes in Asia. It is evident that in recent geological times there existed no active centre of origin for mammals in Europe, and that our continent was largely dependent on the neighbouring one for the supply of its mammalian fauna. A shifting of the centre of development from Europe to Asia appears to have taken place occasionally, as already mentioned (p. 45). Mr. Lydekker has drawn attention to the fact that though the remote ancestors of the ElephantidÆ resided in Europe, neither the latter continent nor North America was the home of the direct ancestor of any of the true Elephants. Similarly, though we have had our Sorex in Europe from the Upper Eocene and Lagomys from the Middle Miocene, the geographical distribution of Sorex vulgaris and Lagomys pusillus does not support the view that they are of European origin and have migrated to Asia. Their absence from most of the European islands indicates either an extremely recent origin or a recent immigration from Asia, and the latter view seems to me much the more probable.

No less than twenty-six species of the Siberian mammals penetrated as far west as the British Islands, and nine of these still inhabit Great Britain. Some of the remaining seventeen species probably lived only for a very short time in England, and the rest gradually became extinct one by one. This process of extinction of the aliens still continues. The Beaver (Castor fiber) has died out within recent historic times. We possess legends and uncertain historic records pointing to the existence of the Reindeer in Scotland as recently as about seven centuries ago. But much the same state of things has happened on the Continent. The Glutton (Gulo luscus), which still lived in Northern Germany last century, has now entirely vanished from that country, as also the Reindeer. The Lemmings have found an asylum in Scandinavia. The Musk-Ox (Ovibos moschatus) has disappeared not only from Europe but also from Asia, and is now confined to Arctic America and Greenland. The Horse no longer occurs in Europe in the wild state, and the Saiga Antelope (Saiga tartarica) has retreated to the Steppes of Eastern Europe and Western Siberia.

As we proceed more and more eastward across Central Europe, we find that a larger and larger percentage of the Siberian migrants have adopted the new country as their permanent home, though in France and Germany, as well as in Austria, we have evidence that a great number of Siberian species, which formerly lived there, have either become entirely extinct, or have retreated towards the land of their origin. There is a prevalent belief that these migrants have taken refuge on the higher European mountain ranges, but this idea is altogether erroneous, as will be shown in the chapter dealing with the origin of the Alpine fauna.

One of the Jerboas (Alactaga jaculus) occurs fossil as far west as Western Germany, but it is now confined to Russia and Western Siberia. The Bobak marmot (Arctomys bobak), which has a similar range now, probably inhabited France in former times. A Siberian species which has retreated but little is the Hamster (Cricetus vulgaris). Its fossil remains have been found in Central France, but it does not now occur west of the Vosges Mountains.

It appears, therefore, as if a wave of migration had swept over Central Europe from east to west, that those species which were able to adapt themselves to the new surroundings had remained, and as if the rest had died out or were gradually retreating to the east.

Ornithologists are well acquainted with the fact that in some years there is an unusually large exodus from Eastern Europe and Siberia of birds; and that species like the Waxwing (Ampelis garrulus) then appear in great numbers. But the appearance of this bird in Western Europe is not looked upon as so remarkable as that of Pallas's Sandgrouse (Syrrhaptes paradoxus, Fig. 3, p. 42), a typical inhabitant and resident of the Arctic Steppes. The last great irruption took place in 1888, and many birds reached even the extreme west of Ireland in May and June of that year. A few weeks before, it had been announced to the German papers that large flocks of this peculiar pigeon-like bird had arrived in the eastern provinces; and though the vast majority vanished as quickly as they had come, a certain number remained for a year or so in the newly visited countries, and some even bred in England.

Twenty-five years before, in 1863, a similar migration had occurred, though not perhaps on quite such a vast scale, and a few small flocks had made their appearance in Western Europe on several occasions between these dates.

It may not be generally known that no other bird has been honoured by our Government in a like manner, for it is the only animal for whose protection a separate Act of Parliament has been passed. In spite of this unusual precaution, the species has not survived to add another member to the resident British fauna. The wave of migration from the east has come and vanished again just like so many others with which history is familiar.

These migrations from the east occurring at the present day give us some idea of those of which we have fossil evidence, and which all had their origin in Central and Northern Asia. Almost all the species of mammals to which I have referred as being of Siberian origin have been found in the fossil state in comparatively recent geological deposits within a certain very limited area. None of the typical species have ever been found in Southern Europe proper, including the Mediterranean islands. It must be remembered that though the Reindeer is a Siberian migrant, the form of the Reindeer which was found in the Pyrenees belonged to a distinct variety—in fact, to a much earlier migration which issued from the Arctic European Regions, and to which I have referred in detail (pp. 150-158). Curiously enough, no deposits of these typical Siberian mammals have ever been obtained in Scandinavia—only in Russia, Austria, Switzerland (the lowlands), Germany, Belgium, France, and England. To facilitate a study of the extent of these migrations, I have constructed a map on which the probable course taken across Central Europe is roughly indicated by dots (Fig. 16).

In the migrations of to-day we perceive the same tendency as in the older ones of which we have fossil evidence, viz., generally a spreading of species on a large scale over new territory, and then a gradual shrinkage towards their original home, with an occasional survival of small colonies in the invaded part. It must not be supposed that this observation applies alone to the Siberian migration. In the case of the Arctic one, precisely the same thing has happened, and we shall see that the Southern (migration from the south) agrees in this respect with the others.

As for the immediate cause of these migrations, it is to be looked for either in the scarcity of food dependent upon a temporary or permanent change of climate, or in an excessive increase in numbers of a particular species. I do not propose to trace back migrations beyond the Pliocene Epoch, or indeed much beyond the beginning of the Glacial period, which is regarded as a phase of the most recent geological epoch, viz., the Pleistocene. During the period in question, we have indirect evidence of one vast migration from Siberia into Europe across the lowlands lying to the north of the Caspian and to the south of the Ural Mountains. There is a general consensus of opinion that this migration took place in Pleistocene times. Professor Nehring thinks that there can be no doubt (p. 222) that the Siberian migrants arrived in Northern Germany after the first stage or division of the Glacial period, and lived there probably during the inter-glacial phase which occurred between the first and second stages—if indeed we look upon this period as being divisible into two distinct stages.

Judging from the evidence of distribution of mammals in pleistocene Europe, Professor Boyd Dawkins came to the conclusion (p. 113) that the climate of our continent "was severe in the north and warm in the south, while in the middle zone, comprising France, Germany, and the greater part of Britain, the winters were cold and the summers warm, as in Middle Asia and North America." "In the summer time the southern species would pass northwards, and in the winter time the northern would swing southwards, and thus occupy at different times of the year the same tract of ground, as is now the case with the Elks and Reindeer." Very different are the views of Professor Nehring on this subject. According to him, the climate in Germany must have been extremely cold and damp, resembling that of Greenland, though perhaps not quite so arctic. Professor Nehring does not at all believe that southern and northern species of mammals could have lived in Central or Northern Europe at the same time; though of this we have undoubted geological evidence (pp. 72-75). He thinks that the supposed commingling of southern and northern types, which has actually been shown by Professor Dawkins to occur, is either due to careless observation or to the fact that some of the species need not necessarily have lived where their bones were found (p. 133).

The most reliable conclusions as regards former conditions of vegetation and climate can be drawn, according to Professor Nehring, from the smaller burrowing mammals, such as the marmots, sousliks, etc. He is of opinion that a great portion of Northern Europe, where their remains have been discovered, must have possessed tundras and steppes, as we find them nowadays in Siberia, and a climate similar to that of Northern Asia. It is presumed that the climate, after the maximum cold of the first stage of the Ice-Age, ameliorated so far as to permit these mammals to exist in Europe.

The natural question, however, which is forced upon us in reading Professor Nehring's interesting and suggestive work is, where did all these steppe animals live during the earlier part of the Ice-Age? No traces of their remains have been discovered in Southern Europe, and it can therefore certainly be affirmed that they could not have lived there. If Central and Northern Europe were uninhabitable for mammals, Central and Northern Asia must have been even more so, and we have to fall back upon the Oriental Region as a possible home of these species during the assumed maximum cold of the Glacial period. In invading Europe from the Oriental Region these Siberian mammals would have taken the shorter route by Asia Minor and Greece, which was open to them. This they certainly did not do, which proves that they came directly from Siberia to Europe without retreating first to Southern Asia.

But it seems to me that there is no necessity for assuming such drastic changes of climate to have taken place at all (compare pp. 75-80). We really have no idea under what precise climatic conditions the Siberian mammals lived in their original home. The only thing we can be certain of is that the smaller burrowing mammals would not have chosen a wood to live in, if they could possibly help it. Prairies, or sand-dunes with short grass or shrubs, such as abound in Europe near the sea-coast, would suit these species perfectly. If we suppose Northern Germany to have been covered by sea (p. 156) during part of the Pleistocene Epoch, forests would probably not have grown there for a very considerable time afterwards, owing to the excessive salinity of the soil, but a tract of sandy country would have been left on the retreat of the sea. Possibly a slight change of climate in the original home of these steppe-species may have reduced their habitable area, and thus caused their migration into Europe.

But this migration problem cannot be solved without tracing the mammals to their place of origin and investigating their early history. This I shall attempt to do presently; meanwhile, it would be interesting to note whether other groups of animals support Professor Nehring's steppe-theory.

Among groups other than mammals, the most important, for the purpose of drawing conclusions as to former physical conditions and climate, are the mollusca. Their remains have been well preserved, and are easily identified. Though Professor Nehring argues that the molluscs found along with the small mammals harmonise perfectly with the assumption of a steppe-climate (p. 212), I cannot at all agree with him. He enumerates the following sixteen species as having been discovered by him:—

1. Pupa muscorum.
2. Chondrula tridens.
3. Cionella lubrica.
4. Patula ruderata.
5. Do.rotundata.
6. Helix striata.
7. Do.hispidia.
8. Do.tenuilabris.
9. Helix pulchella.
10. Do.hortensis.
11. Do.obvoluta.
12. Hyalinia radiatula.
13. Succinea oblonga.
14. LimnÆa peregra.
15. Clausilia sp.
16. Pisidium pusillum.

Only two of these can be looked upon as typically northern species, viz., Patula ruderata and Helix tenuilabris, though both of them are still found living locally in Germany. Some of the others are decidedly southern species, like Chondrula tridens, Helix obvoluta, H. rotundata, and H. striata. All the rest live and flourish, for example, in Ireland at the present day, where, as we all know, anything but a dry steppe-climate prevails.

Dr. Kobelt quite agrees with me in thinking that the remains of the mollusca found along with the so-called "steppe-mammals" afford no proof of a steppe-character of the country at the time when they were alive (p. 166). Nor do the mollusca which have been found in England in the Forest-Bed and the succeeding pleistocene strata support such a view. The Forest-Bed, generally regarded as belonging to the Upper Pliocene, I believe to be an inter-glacial pleistocene deposit—contemporaneous with the loess formation in Germany. Of fifty-nine species of land and freshwater mollusca which have been discovered in this bed, forty-eight species, according to Mr. Clement Reid (p. 186), are at present living in Norfolk, six are extinct, two are continental forms living in the same latitudes as Norfolk, and the other three are all southern forms. Not a single species has a particularly northern range. Of the land and freshwater mollusca of the South of England in the succeeding pleistocene deposits, six species are now no longer living in the British Islands, but only one (Helix ruderata) can be looked upon as an Arctic or Alpine form. After this short digression on the mollusca, I will briefly recapitulate what is known about the early history of the Siberian mammals, which will assist us in tracing the cause of their migration to Europe.

We have in Siberia problems quite as difficult of solution as the European ones. Volumes have been written to explain the former presence of Arctic mammals like the Reindeer in Southern Europe, and the most extraordinary demands on the credulity of the public have been made by some geologists in their attempts to account for this comparatively simple problem. In Northern Asia a somewhat similar phenomenon, but much more difficult of explanation, has taken place. Mammals have been found fossil in recent geological deposits in localities where they do not now occur, and apparently the Siberian and the European deposits are of about the same age. Now, however, comes the extraordinary difference. In Europe the Arctic mammals went southward, but in Siberia the Southern ones went northward. Not only do we find the Saiga-Antelope, Tiger, Wild Horse, European Bison, Mammoth, and Rhinoceros in the extreme north of the mainland of Siberia; their remains have even been obtained in the New Siberian Islands. As these islands are situated in the same latitude as the northern part of Novaya Zemlya,—indeed, not far south of the latitude of Spitsbergen,—the fact of such huge mammals having been able to find subsistence there at apparently quite a recent geological period seems an astounding fact. It may be urged that their bones might have been carried so far north by ice, or by some other equally powerful agency. But Tcherski and all other palÆontologists who have examined these northern deposits are unanimous in the belief that these herbivores and carnivores lived and died where their remains are now found. "It is evident," says Tcherski (p. 451), "that these large animals could only have lived in those extremely northern latitudes under correspondingly favourable conditions of the vegetation, viz., during the existence of forests, meadows, and steppes." He also is of opinion that the moist climate which evidently prevailed in Europe during Post-tertiary (Pleistocene) times must have modified the Siberian climate in so far as to render it milder. The existence of the Aralo-Caspian basin (Fig. 12, p. 156) must also have tended in the same direction. It appears then that, at the time when plants and animals are believed to have retired southward in Europe before the supposed advancing Scandinavian ice-sheet, no agency existed in North Siberia which was able to suppress and to annihilate the forest and meadow vegetation, and drive away the fauna connected with it. We know, continues Tcherski, that such genera as Bison, Colus (Saiga), Rhinoceros, Elephas, and Equus are met with in all horizons of the diluvium of West Siberia. He therefore comes to the conclusion (p. 474), that these and other facts imply that the retreat of the North Asiatic fauna commenced about the end of the Tertiary Era (Pliocene), and that it was continued very slowly throughout the Post-tertiary (Pleistocene) Epoch, without any visible changes in its southward direction, even during the time of the most important glacial developments in Northern Europe. Only after the conditions disappeared which had produced the augmentation of an atmospheric moisture, did the climate of North Siberia become deadly to a temperate fauna and flora. Tundras then spread over the meadow-lands and remnants of forests, whilst arctic animals replaced the large ungulates and carnivores which had wandered far away from their native southern home.

This is Tcherski's explanation of the extraordinary events which he has chronicled, after years of the most arduous labour and under conditions of peculiar hardship. And though his work cannot be over-estimated, and his opinions should receive the most careful consideration, yet I fear the explanation will not be looked upon as entirely satisfactory. Every one will agree with him that the climate of Siberia must have been greatly moister in pliocene and pleistocene times than it is now. The Aralo-Caspian covered a vast area of South-western Siberia. Freshwater basins existed along the east of the Ural Mountains, while Central Asia was studded over with a number of large lakes, which have now almost entirely vanished. But that the generally assumed refrigeration of Europe must have had a chilling effect on the Siberian atmosphere seems to me evident. That the whole of Northern Europe should have been made uninhabitable owing to the advance of the Scandinavian ice-sheet, while North Siberia at the same time supported forests, meadows, and a temperate fauna, is incredible. At the approach of winter, at any rate, the animals would have been driven southward for thousands of miles to seek shelter from the snows and cold and to obtain nourishment, and it would scarcely have been possible for them to undertake such vast migrations at every season. Professor James Geikie's suggestion (p. 706), that the Mammoth and Woolly Rhinoceros could have survived the Pleistocene Epoch in Southern Siberia, does not appear to solve the problem, as that part of Asia must have participated in the great cold which is said to have prevailed all over Europe.

Let us now concede, for the sake of argument, that the current views regarding the pleistocene climate of Europe are correct. We are told by Professor Geikie that the climate of Scotland during part of the Pleistocene Epoch was so cold, that the whole country was buried underneath one immense mer de glace, through which peered only the higher mountain-tops (p. 67). If this was the state of climate in close proximity to the Atlantic, it must probably have been still more severe on the European continent. Now at the present time Siberia has the reputation of being the coldest country in the world, and the mercury of the thermometer is said to remain frozen for weeks during winter, even in the south.

With the prevailing dampness in pleistocene times the snowfall throughout Siberia would have been much heavier than at present, though it would have modified the temperature to some extent. Under such circumstances Southern Siberia could not have been a desirable place of residence for large mammals. It would have been necessary for the Mammoth and the other species referred to, to wander farther into the extreme south of Asia or Europe to find a suitable refuge during the arctic conditions which are supposed to have prevailed in Northern Europe. To quote Professor J. Geikie's own words (p. 706): "They (Mammoth, etc.) would seem to have lived in Southern Siberia throughout the whole Pleistocene period, from which region doubtless they originally invaded our Continent. But with the approach of our genial forest-epoch (penultimate inter-glacial stage) they gradually vanished from Europe, to linger for a long time in Siberia before they finally died out." It is suggested, therefore, by the author that the Mammoth and the other mammals whose remains have been discovered on the New Siberian Islands found their way there during one of the late inter-glacial stages of the Ice-Age. But there is no astonishment expressed by Professor Geikie at the extraordinary change of climate which must have occurred in Siberia to allow of such migrations. I can find no very definite statement in this author's work as to the nature of the climate in Europe during those inter-glacial phases, but he remarks (p. 129) "that the evidence of the Scottish inter-glacial beds, so far as it went, did not entitle us to infer that during their accumulation local glaciers may not have existed in the Highland valleys." There is no evidence, in other words, of the existence in Europe of a milder climate than that prevailing at present. Still less can there be any ground for the supposition that the climate of the whole of Siberia ameliorated to such an extent that forests and meadows could develop as far north as the New Siberian Islands; for if the temperature in Europe was then about the same as now, that of Siberia could not have been vastly higher than it is at present.

It is highly improbable, therefore, that a sufficiently mild climate prevailed in the extreme north of Siberia during the so-called later inter-glacial periods to induce the mammals to which I have referred to seek fresh pastures there.

The late Professor Brandt, one of the highest zoological authorities in Russia, was of opinion that at the commencement of the Glacial period the great mammals of Northern Siberia either perished or migrated southward. From there they gradually penetrated into European Russia. He believed that before the Glacial period a connection existed between the Aralo-Caspian Sea and the Arctic Ocean, carrying warm water northward. The gradual disappearance of this marine channel caused a decrease of warmth in Northern Asia, so that large accumulations of frozen soil and ice were formed, which still more depressed the temperature. This, he suggested, probably took place at the time when the Glacial period commenced in North-western Europe.

It has been urged against these views of Tcherski and Brandt, that the bone beds in the Liakov Islands (New Siberian Islands) rest partly upon a solid layer of ice of nearly seventy feet thick. This mass of ice, it was thought, must have accumulated during the Glacial period. As the bones rest upon it, the mammals could only have lived in those islands in more recent times, after the Ice-Age had passed away. Nothing, apparently, can be clearer, and yet in the face of this seeming proof one feels, as I have mentioned before, that if such an extraordinary revolution of climate as is implied by this admission had taken place, we should be able to perceive the traces throughout the northern hemisphere. In this dilemma, a suggestion made by Dr. Bunge, who visited the New Siberian Islands recently at the instance of the Imperial Academy of St. Petersburg, helps us out of the difficulty. He found that, as a rule, these so-called fossil glaciers contain seams of mud and sand, and he argued that the ice had formed, and is still forming at the present day, in fissures of the earth. I entirely concur with this view. Neither palÆontology nor the geographical distribution of animals lend any support to the other theory, and I think we may conclude that Brandt's view in the main is probably the correct explanation of the phenomena which we have discussed. Some important facts of distribution are more easily explicable on this assumption. Why, for instance, should the Siberian fauna of pliocene times have remained in Siberia and not have migrated to Europe at that time? The pliocene mammals of Siberia are mostly of southern origin. Their range increased enormously during the epoch throughout Northern Asia. We should expect them, therefore, to have crossed the Caspian plains, or even the low-lying Ural Mountains, to pour into the neighbouring continent. But Professor Brandt explained how they were prevented from spreading west. An arm of the sea stretched from the Aralo-Caspian to the Arctic Ocean, thus raising an effectual barrier between the two continents. There is some evidence for the belief, as we shall learn presently, that this marine barrier existed also during the early part of the pleistocene epoch. After having greatly expanded during pliocene times, the fauna of Siberia gradually withdrew from the northern regions during the earlier portion of the succeeding epoch. It was only after the marine connection above referred to ceased to exist, or became disconnected, that an entry into Europe was possible.

A fauna, to some extent composed of species now inhabiting the steppes of Eastern Europe and Siberia, poured into the neighbouring continent. On p. 95 I have given a list of those which reached as far west as the British Islands, but, as I mentioned, many other species came from the east about this time. With regard to the early history of the Siberian mammals, I favour a view somewhat between that of Tcherski and that of Brandt. The outpouring of the fauna into Europe seems to me to indicate that there was a sudden change of climate in Siberia. This was produced, perhaps, by the rupture of the marine connection between the Arctic Ocean and the Aralo-Caspian. Such an event would not only have caused the sudden shrinkage of the area available for food-supply by lowering the temperature in Siberia, it would have acted also as a means in assisting the fauna to enter a new continent where an inconsiderable number of mammals, already established, were mostly dispossessed of their homes by the advancing eastern host.

Brandt's theory, however, of a marine connection between the Arctic Ocean and the Aralo-Caspian is by no means generally accepted. That the Caspian Sea was at that time greatly larger than it is at present, and joined to the Sea of Aral and the Black Sea, is acknowledged by everybody. That the deposits laid down by this huge inland sea reach as far north as the shores of the river Kama, in Central Russia, is also well known to geologists. But what comes rather as a surprise, is that Professor Karpinski, whom we must take as one of the highest authorities on the geology of Russia, asserts that this Aralo-Caspian Sea was probably joined by a system of lakes or channels to the Arctic Ocean (p. 183). He was by no means the first, though, to put forward such a theory. We have already learned that Professor Brandt held a somewhat similar view, though he believed in something more than a connection by mere channels, and Mr. KÖppen, and also the Russian traveller Mr. Kessler, agreed with him. So much was Professor Boyd Dawkins impressed with their arguments at the time, that he wrote (c, p. 148): "Before the lowering of the temperature in Central Europe, the sea had already rolled through the low country of Russia, from the Caspian to the White Sea and the Baltic, and formed a barrier to western migration to the Arctic mammals of Asia."

In one particular Professor Dawkins's views differ from those of almost all the previous writers. His connection between the Caspian and the Arctic Ocean is placed to the west of the Ural Mountains, while it had always been assumed by the Russian writers to have lain on the eastern or Asiatic side of that mountain range. Thus, when Tcherski in recent years announced that the tract on this eastern side of the mountains was covered by freshwater deposits, his discovery seemed once for all to settle the problem of the arctic marine connection in the negative. As Professor Dawkins's theory has, however, received much additional affirmative evidence by current faunal researches, a connection between the Caspian (or Aralo-Caspian) and the Arctic Ocean (White Sea) may have actually existed within recent geological times.

What relict lakes are, has already been explained (p. 176), and their fauna will again be referred to in a subsequent chapter. I might perhaps be allowed to repeat that such lakes are supposed to have been flooded by, or to have been in close connection with, the sea at some former period. Many of the Swedish lakes are spoken of as relict lakes (Reliktenseen), because they contain a number of marine species of animals which have now become adapted to live in fresh water, but all of whose nearest relatives inhabit the sea. One of these, the schizopod crustacean Mysis relicta,—a shrimp-like creature,—which was formerly believed to inhabit also the Caspian, is of particular interest. More recently, the occurrence of this Mysis in the Caspian was denied, but though this denial has been confirmed by Professor Sars in his memoir on the crustaceans of the great Russian inland sea, he has been enabled to add two new species of Mysis to the list of those already known to science. These are M. caspia and M. micropthalma, and both are closely related to the arctic marine Mysis oculata. According to Professor Sars, the genus Mysis as a whole may be regarded as arctic in character. The occurrence of these two species, therefore, in his opinion, points to a recent connection of the Caspian with the Glacial Sea.

A large number of other crustaceans have been described by the same author from the Caspian. Of the order Cumacea, which is exclusively marine, ten species are mentioned, but none of these seems to range beyond the Caspian. Among the smaller species of crustaceans, a minute pelagic copepod (Limnocalanus grimaldii) also inhabits the Baltic and the Arctic Ocean. The marine isopod Idotea entomon, related to the common wood-louse, has a similar distribution.

Genuine Arctic species of Fishes do not seem to occur in the Caspian, though some, viz., Clupea caspia, Atherina pontica, Clupionella Grimmi, and Syngnathus bucculentus, are almost certainly the descendants of marine forms.

The Seal of the Caspian (Phoca caspica) is closely allied to the Arctic Seal, and its presence alone in that sea indicates that at no very distant date—at any rate since pliocene times—a closer connection with the Arctic Ocean existed than at present.

I am sure it will be readily granted that there is zoological evidence for the belief of such a connection or union between the two great seas. However, it may be urged that owing to the presence of an ice-sheet in Northern Europe during the Glacial period, such a connection must either have been pre-glacial or have existed after that period. But the connection must have occurred at a time when the Caspian extended far to the north—when indeed the so-called post-tertiary Caspian deposits were laid down (Fig. 17). Since the boulder-clay which covers the plain of Northern Russia is assumed to be the ground-moraine of the great northern ice-sheet, we might expect to find that the Caspian deposits were not contemporaneous with it. Curiously enough, it has been shown by Mr. SjÖgren that all observations have pointed to the fact that these two deposits do not overlie one another, but occur side by side, and are therefore contemporaneous. This seems to warrant our belief, that while the boulder-clay was being laid down in Northern Europe, the Aralo-Caspian Sea had some communication with the White Sea.

Fig. 17.—Map of European Russia (after Karpinski). The faintly dotted parts indicate the areas covered by boulder-clay, the strongly dotted ones those exhibiting Aralo-Caspian and other post-pliocene deposits.

The boulder-clay of Northern Continental Europe, as already stated, is now generally recognised to be the product of a huge ice-sheet which invaded the lowlands of Continental Europe from the Scandinavian mountains. Though Alpine glaciers at the present day produce little or no ground moraine, these ancient larger ice-sheets, or "mers de glace," are believed to have deposited immense layers of mud containing scratched and polished stones. Many of the latter have been carried great distances from their source of origin. The Scandinavian ice-sheet is supposed to have advanced as far south as the line indicated on the map, after which it gradually retreated. On this point, however, as in almost every detail connected with the Glacial period, geologists are at variance. Professor James Geikie maintains, that there were no less than four Glacial periods, separated from one another by milder inter-glacial phases. On the Continent the view of two Glacial and one inter-glacial period is, I think, more generally adopted. Professor Geikie's four periods seem to me to have originated in a desire to correlate the British pleistocene deposits with the continental ones, and at the same time to retain the old view of the inter-glacial position of the Forest-Bed. The two theories agree in so far as that in both the glacial conditions culminate in a maximum glaciation, followed by a more temperate phase of climate, with consequent retreat of the ice-sheets, and finally by a renewed advance of the glaciers.

We are told that there is not the slightest doubt about it that a marked but gradual decrease of temperature took place all over Europe either during the beginning of the Pleistocene or towards the end of the Pliocene Epoch.

We might reasonably suppose, then, that a similar climatic effect was produced in Siberia, in consequence of which the fauna would have been obliged to retreat from the extreme northern latitudes southward. No doubt great efforts would have been made by the members of the Siberian fauna—at any rate by those possessing strong power of locomotion—to extend their range in other directions. But we have no evidence that a migration from Siberia came to Eastern Europe at that time. It seems, therefore, as if the barrier referred to by Brandt, KÖppen, Boyd Dawkins, and others (p. 222), had existed at this time. This would have effectually prevented an overflow of the fauna from Siberia. Only in deposits later than the lower continental boulder-clay do we find traces of a Siberian migration. The time of maximum glaciation had then passed away; the great glacier which was believed to have invaded the lowlands of Northern Europe had again retreated, before the Siberian mammals made their appearance in Germany.

It has been stated above (p. 226) that while the Russian boulder-clay was being laid down, the Aralo-Caspian probably had some communication with the White Sea.

But how can this view be reconciled with the existence of a huge mer de glace in the northern plains of Russia? The existence of the ice-sheet has been conjured up in order to explain the presence of the boulder-clay. But not long ago a very different interpretation of the origin of this clay was given; and one, I may say, which explains the history of the Siberian and the European fauna in a more satisfactory manner than is done by the ice-sheet hypothesis. It is that the boulder-clay is not the product of land-ice, but has been deposited by a sea with floating icebergs. Thus the latter hypothesis does not deny the existence of glaciers, but allows the mud to be deposited on the floor of a turbid sea, instead of beneath an immense mer de glace. I need hardly mention that this view, which was formerly universally accepted by geologists, is now scouted by almost every authority, both British and Continental. I should scarcely venture the attempt to revive old memories and stir up again long forgotten controversies, were it not for the fact that many new points have arisen in the course of the above inquiries, which appear to me so very difficult to explain by the land-ice hypothesis, while they are comparatively easy to understand when we adopt the old theory of the marine origin of the boulder-clay. But a few geologists even at the present day, while believing in the land-ice theory, recognise that the marine hypothesis should have some consideration shown to it. I need only remind glacialists of the work recently published by Professor Bonney. "The singular mixture," he remarks (p. 280), "and apparent crossing of the paths of boulders, as already stated, are less difficult to explain on the hypothesis of distribution by floating ice than on that of transport by land-ice, because, in the former case, though the drift of winds and currents would be generally in one direction, both might be varied at particular seasons. So far as concerns the distribution and thickness of the glacial deposits, there is not much to choose between either hypothesis; but on that of land-ice it is extremely difficult to explain the intercalation of perfectly stratified sands and gravels and of boulder-clay, as well as the not infrequent signs of bedding in the latter."

Now with regard to the land-ice theory, several serious difficulties present themselves in connection with the origin of the European fauna. In the first place, as the climate renders Northern Siberia almost uninhabitable for mammals at the present day, how much more severe must it have been during the time of the maximum glaciation in Europe. As the then existing fauna was not driven into Europe, where could it possibly have survived? Secondly, how can we reconcile the contemporaneous existence of a great inland sea (the Aralo-Caspian) containing survivals of mild Sarmatic times with an immense glacier almost touching it on its northern shores? How did one of the most characteristic species of that sea, Dreyssensia polymorpha, come to make its appearance in the lower boulder-clay of Prussia and then disappear in the upper? And finally, how are we to explain the sudden appearance of a Siberian fauna after the deposition of the lower boulder-clay, except by the removal of a barrier which had prevented their egress from Siberia?

If we assume that the continental boulder-clay of Russia has been formed in the manner so ably explained by Murchison, de Verneuil, and von Keyserling, viz., by a sea with floating icebergs, the temperature of Siberia might have been higher than at present, and have supported a fauna in more northern latitudes.

The contemporaneousness of the deposits of this sea with those of the Aralo-Caspian is also rendered more intelligible. If we suppose, moreover, the connection between the Aralo-Caspian and the White Sea (Fig. 12, p. 156) to have existed at this time, we possess an explanation of the method of migration of the Arctic marine species into the Southern and of the Caspian species (Dreyssensia) into the Northern Sea.

An inter-glacial phase is believed to have supervened after the deposition of the lower boulder-clay, and it is during this period that the Siberian species first appeared in Central Europe. If we assume then that the retreat of the Northern Sea (Fig. 13, p. 170) opened up a passage for the Siberian fauna, we have in this very fact also an explanation of the extraordinarily large exodus of Asiatic mammals, because the great reduction of the marine area in Northern Europe would have had an important influence in lowering the temperature in Asia. Only a sudden change of climate in Siberia could have brought about the migration of the vast hordes of Asiatic mammals whose remains we find in Central and Western Europe in deposits of that period.

Throughout this work we are made acquainted with facts which bear out the view that the climate during the greater part of the Glacial period was mild rather than intensely arctic in Europe. That a huge ice-sheet could have covered Northern Europe under such conditions appears to me very doubtful. No one can deny, however, that glaciers must have existed during the Glacial period in all the mountainous regions of Central and Northern Europe, though their existence is not incompatible with a mild climate. Tree-ferns and other tropical vegetation grow at the foot of glaciers in New Zealand. We need not even go so far afield, for in Switzerland grapes ripen and an abundant fauna and flora thrive in close proximity to some of the well-known glaciers.

One matter of importance still remains to be considered before concluding this chapter, viz., the fauna contained in the English geological deposit known as the "Forest-Bed." This interesting deposit is exposed at the base of a range of cliffs on the coast of Norfolk. It is composed of beds of estuarine and marine origin. The tree-stumps formerly believed to be the remains of trees in situ have, after more careful examination, proved to be in all cases drifted specimens. A portion of the "Forest-Bed" no doubt was laid down in close proximity to a large river, and subject to being periodically flooded by it. It is not absolutely certain, therefore, that all the mammals whose remains occur in this deposit lived in England or whether only on the banks of the river farther south. Nevertheless, we may take for granted that some of them did. England was at the time connected with France and Belgium, and for our purpose it matters little whether they had crossed the Channel or inhabited those parts of the Continent through which the great river flowed which sent its alluvial detritus as far as the plains of Norfolk. All we have to remember is that certain mammals, which appear to have originated in Siberia, and of which we have some evidence that they crossed Central Europe in their westward course, had now reached the great river just alluded to, which some geologists believe to have been the Rhine.

I have had occasion to refer to a number of British mammals (p. 202)—some of which are now extinct—which I believe to have migrated across the plains of continental Europe direct from Siberia. There were twenty-six species of these Siberian mammals; and no less than ten of these occur in the Forest-Bed. None appear in any older British deposit. It is perfectly clear, therefore, that the Forest-Bed must have been laid down after their immigration into Europe. They probably wandered to Western Europe very soon after crossing the eastern boundaries of our continent; the deposits in which they are found are therefore contemporaneous. But we have learned above (p. 208), that the beds in Eastern Europe in which the Siberian mammal-remains are found are more recent than the lower boulder-clay. As already stated, the Forest-Bed must also be more recent than the lower continental boulder-clay, and should be included in the pleistocene series.

That the Forest-Bed is an inter-glacial deposit has been urged long ago by various writers. Professor Geikie regards it as stratigraphically contemporaneous with the peat and freshwater beds below the lower diluvium of Western and Middle Germany, and as having been laid down during the first Inter-glacial Epoch of the great Ice-Age. The fact that no boulder-clay underlies the Forest-Bed seems rather a strong argument against the view of its being an inter-glacial deposit. It lies directly on what is known as the Newer Pliocene Crags. If the Forest-Bed is included in the pleistocene series, as I suggested it should, the crags, or a portion of them, would therefore be equivalent as regards time of deposition to the lower continental boulder-clay. And again, if the lower continental boulder-clay is contemporaneous with the Newer Crags, the latter should also be classed with the pleistocene strata. I can scarcely hope that geologists will be ready to admit such a sweeping change of nomenclature without a protest. I venture, therefore, to explain more fully my reasons for adhering to these unorthodox views.

Let us look once more at the map which I constructed (Fig. 12, p. 156) to elucidate the migration of the Arctic terrestrial species to the British Islands. It will be noticed that one continuous ocean extends from the east coast of England across Holland, Northern Germany, and Russia to the White Sea. At the same time Greenland and Northern Scandinavia, Scotland and Southern Scandinavia, are united by a narrow strip of land, and so are England and France. The waters of the Atlantic and this North European Sea do not therefore intermingle at any point, the two seas being absolutely independent of one another.

Such I assume to have been the geographical condition of Northern Europe during the deposition of the Red Crag. Arctic mollusca were then brought to the east coast of England, and boulders were scattered through the beds laid down on that coast by icebergs which had been cast off by Scandinavian glaciers on reaching the sea. Bedded clays which have yielded arctic shells lie beneath the lower continental boulder-clay on the Baltic coast-lands and on the coast of the White Sea. According to Professor Geikie, marine clays on the same geological horizon reach an elevation of some 230 feet. "It would seem, then," he says, "that before the deposition of the lower boulder-clay of those regions the Baltic Sea had open communication with the German Ocean" (p. 442). All these clays are evidently deposits of the same sea. But apart from the fact that the Red Crag and these Baltic deposits are the oldest of the upper Tertiary beds containing arctic shells, there is no evidence that they are contemporaneous.

Overlying the same Baltic deposits comes the lower boulder-clay, reaching a thickness of several hundred feet in some parts of Germany. It presents, like the upper clay, frequent interstratification with well-bedded deposits of sand and gravel. The scarcity of marine mollusca, the occurrence of striated surfaces, and the occasional presence of so-called giants' kettles, appear to favour the view, which at present is generally adopted by both British and Continental geologists, that the boulder-clay owes its origin to land-ice. I have stated on several occasions that the view of the marine origin of the boulder-clay agrees best with the known facts of distribution, and with the history of the European fauna (pp. 80-86, and p. 129). It may be urged that if the lower boulder-clay were contemporaneous with the British Crags which succeeded the Red Crag, how can we explain the fact that these crags contain plenty of shells, while in the lower continental boulder-clay there are scarcely any?

But as yet our knowledge of the conditions of life of the marine mollusca and of their distribution is extremely scanty. We are apt to imagine that the bottom of the sea is covered by a more or less uniform thick layer of shells; but whenever a careful survey of the nature of the deposits now forming there has been made, such is by no means found to be the case. Some of the best results obtained by that useful body, the Liverpool Marine Biological Committee, have been precisely in this direction. A most interesting account has been published by Professor Herdman and Mr. Lomas on the floor deposits of the Irish Sea, in which the authors state (p. 217), that "a place may be swarming with life and yet leave no trace of anything capable of being preserved in the fossil state, whereas in other places, barren of living things, banks of drifted and dead shells may be found, and remain as a permanent deposit on the ocean floor."

Owing to the fact of the peculiar geographical position of Scandinavia at this time—an isthmus of land with a high mountain range lying between the warm Atlantic and the cold Arctic Sea—the snowfall must have been excessive, and large glaciers were evidently forming. These produced icebergs as soon as the lower parts had advanced to the Baltic coast-land and deposited their detritus in the sea. Immense masses of mud and stones were thus cast to the bottom of the sea, and under these circumstances no delicate mollusca or other marine life probably could have developed within a considerable distance from the shore. To judge from the direction pursued by the majority of the boulders from their source of origin, the prevailing current during the deposition of the lower boulder-clay was from north-west to south-east. It is possible that little marine life, except free-swimming forms, would have been able to live within the Russian area of this sea. But the free-swimming larvÆ of molluscs and other surface species were not prevented from passing from the White Sea south-westward, and in sheltered localities where little or no mud deposition was going on, these no doubt might have developed into adults on the sea-floor. It is quite conceivable, therefore, that in one portion of the North European Sea, which was fully exposed to the destructive influences of the iceberg action, the fauna was scanty or totally absent, while in another part there lived a fairly abundant one. The unfossiliferous state of the lower continental boulder-clay does not, therefore, offer any serious difficulty to the supposition that some of the so-called Newer Pliocene Crags of the east coast of England were laid down at the same time by the same sea.

This would also explain how the Arctic species come to inhabit the Caspian, as the old Aralo-Caspian Sea could have had some communication (Fig. 12, p. 156) with the North European Sea. And this again offers an explanation of the otherwise mysterious occurrence of the Caspian Dreyssensia polymorpha in the lower continental boulder-clay.

The climatic reasons for the supposition that the boulder-clay is a marine deposit have already been given (p. 66). However, it may be asked what about the glacial flora which has been proved to have existed all over the plains of Northern Europe?—what about the relics of this same flora which still linger on in a few localities to the great delight of the systematic botanist? They have been spoken of as indications of a former Arctic climate in Europe. The presence of an Arctic species such as Dryas octopetala in any of the pleistocene deposits is often looked upon as an absolute proof of a very severe climate having prevailed at the time they were laid down. Professor Geikie tells us that the South of England was clothed with an Arctic flora, when the climate became somewhat less severe than it had been during the climax of the glacial cold (p. 398). Relics of such a flora have been detected at Bovey Tracey, in Devonshire, the Arctic plants found comprising Betula nana and B. alba, Salix cinerea and Arctostaphylos uva-ursi.

Now three of these four species of plants are still natives in the British Islands, and all are forms which probably came to us with the Arctic migration which I described in Chapter IV. They travelled south with the reindeer, or before it, and may have covered large tracts of country at the time. With the increased struggle for existence on the arrival of the Siberian and Oriental migrants, they have probably been evicted by these more powerful rivals. A discovery of their remains does not necessarily indicate that a great change of climate has taken place since they lived in the country. And certainly these Arctic plants cannot be taken as indicating a low temperature, for it has been shown that Alpine plants are mostly intolerant of very low temperatures. "Arctic and Alpine species in the Botanical Gardens at Christiania," says Professor Blytt (p. 19), "endure the strongest summer heat without injury, while they are often destroyed when not sufficiently covered during the winter." Similar observations have been made in other countries. For this reason they have to be generally wintered in frames in the Botanic Gardens at Kew and Dublin, and are thus exposed to higher temperatures than at present obtain in the British Islands. This fact suggests that the Alpine and Arctic plants really did not originate in countries with cold temperatures. They probably made their first appearance long before the Glacial period—perhaps in early Tertiary times—chiefly in the Arctic Regions, which at that time had a mild climate. They have since become adapted to live in cold countries where they flourish, provided they receive sufficient moisture in the summer, and are protected from severe frost in the winter by a covering of snow.

When we carefully examine the present range of Arctic plants in the British Islands, a curious fact presents itself which no doubt has frequently been noted by botanists, viz., that some of the most characteristically Arctic species, and some which are often quoted by glacialists in support of their theories, flourish at the present moment in very mild situations. I have already referred to the fact that the Mountain Avens (Dryas octopetala) abounds in the west of Ireland (County Galway) down to sea-level. Now it is well known that the mean winter temperature of that part of Ireland resembles that of Southern Europe, being no less than 12° F. (=7° Cent.) above freezing point. The plant, of course, is here a native, and not introduced. This instance shows clearly, that as long as more vigorous competitors are absent, and as long as it is not exposed to severe frost or undue dryness, this and allied species do just as well in a mild climate as in their native Arctic home.

In his interesting essay on the distribution of the Arctic plants in Europe during the Glacial period, Professor Nathorst adduces the fact that all the localities but one, in which remains of such plants have been discovered, lie either within or close to the limits of the maximum extension of the supposed northern ice-sheet, or within those of the former Alpine glaciers. Whether we look upon the boulder-clay as a marine or a terrestrial product, it is quite conceivable that, in many instances, the remains of the Arctic plants may have been carried by ice to great distances from where they grew. The probability, however, is in favour of most of them having lived where their remains are now found. Now, it is a remarkable fact, that the single instance in Europe of a deposit of Arctic plants having been found far removed from the maximum extension of the northern ice-sheet is the one quoted above, viz., at Bovey Tracey, in Devonshire. Even up to recent times Arctic plants may have persisted at Bovey Tracey just as they do in Galway under the influence of a mild coast climate. Similar circumstances may have led to their survival along the shores of the sea which deposited the North European boulder-clay, while they moved northward from the Alps along with the glaciers, which always supplied them with an abundance of moisture. Alpine plants probably became exterminated in the plain of Central Europe at a much earlier period.

SUMMARY OF CHAPTER V.

What has been spoken of in the earlier parts of this book as the eastern migration, refers in a general way to the animals which have come to England from the east. But these are by no means natives of one country alone. We can trace a number of the British mammals to a Siberian origin, and also some birds; among many of the lower vertebrates and invertebrates, however, there are few species which have reached us from Siberia. They may have had their original homes in the Alps, in Eastern Europe, or in Central and Southern Asia, and have joined in their westward course the later, more quickly travelling mammals. Many instances are given from all the more important groups of animals to show how we may proceed in approximately identifying the home of a species.

The periodical invasion into our continent of Pallas's Sandgrouse and other birds, suggests an explanation as to the cause of the great westward migration in former times of the Siberian mammals. Since a considerable amount of fossil evidence is available to show the path of migration pursued by these mammals, other important problems, such as the time of their arrival in Europe and the geographical conditions surrounding them, may perhaps be approximately ascertained, and thus throw much light on the general features of the European fauna. It has been proved by Professor Nehring that the Siberian mammals arrived in Eastern Europe after the deposition of the lower continental boulder-clay. He believes that the climate of Germany at that time had ameliorated so far, after the maximum cold of the Glacial period, that steppes with a Siberian fauna could exist. Other groups, such as the Mollusca, however, do not support Professor Nehring's theory, and in order to arrive at an independent solution of this and the other problems referred to, a short history is given of the Siberian fauna. Recent geological ages have witnessed the arrival in Southern Europe of mammals now almost confined to the arctic and subarctic regions. In Siberia, on the other hand, many southern species penetrated, apparently about the same time, to the extreme northern limits of that country. The greatest authority on the Siberian fossil fauna, Tcherski, believes that this took place in pliocene times, the gradual retreat occupying the whole of the Glacial period. If this were correct, the retreat from the Arctic Regions would have occurred at the same time when, according to our European authorities, Professors Nehring and Geikie, the much more southern parts of our continent were already uninhabitable. But Siberia could not have supported the large mammals at all at a time when Europe was uninhabitable, as it would be difficult to conceive under what geographical conditions the climate of the latter was arctic and that of the former temperate. If the whole fauna was driven into Southern Asia, how is it that the Siberian invasion of Europe occurred immediately after the deposition of the lower boulder-clay, that is to say, after the earlier part of the Glacial period? The difficulty can be met by the supposition that both Europe and Siberia had a temperate climate at that time. This view is supported by certain evidences, fully described, of a connection between the Caspian and the White Sea, which would have had the effect of influencing the climate. The Siberian fauna would thus have been prevented from spreading westward in Pliocene and early Glacial times. But on the disappearance of the marine connection, a way would have been opened into our continent, which again had an effect on the climate. The latter would have become sensibly colder and thus have reduced the habitable area of the Siberian fauna.

Such geographical conditions would have been incompatible with a great northern mer de glace, and the boulder-clay in Northern Europe could not have represented a ground moraine but is a marine deposit. The sea is supposed to have covered the Northern Russian and German plains, and into it icebergs discharged the detritus which had accumulated on them when they were still Scandinavian glaciers.

As regards the time of the arrival of the Siberian migrants in Europe, the English Forest-Bed gives us an additional clue to its determination. Since Siberian migrants are unknown from earlier deposits than this, it is reasonable to suppose that they arrived in England about the time when it was laid down. But since they appear in Germany in the inter-glacial beds subsequent to the deposition of the lower boulder-clay, the former are probably contemporaneous with the Forest-Bed. Some of the deposits generally regarded as upper pliocene by British geologists would therefore have to be classed with the lower continental boulder-clay as lower pleistocene. In connection with this theory some interesting faunistic data are given which seem to support it.

In conclusion, the former presence of Arctic plants in Central Europe and their bearing on the climatic problems are discussed.