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,videNordquist.
[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.
[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.
[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.
[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,videNordquist.
[4]For additional species with a similar range,videNordquist.
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 moreprobably 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 (videp.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 aninteresting 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 auritusandAgama sanguinolenta. They belong to the familyIguanidæ, 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 ofRana arvalisin 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, thatRana arvaliscame with the Siberian migration. This need not cause surprise, as the genusRanais 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 esculentaandR. temporaria. Neither of these is by any means confined to Europe.R. esculentaranges 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 thanReptiles 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 genusZonites, 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. ThatHelix pomatiahas 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 thatHelix aspersais 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 fromthe 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 whichSiberia has in common with Europe, seems hopeless. Such forms asArion 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 genusCicindelain 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 noCicindelidæwhatsoever 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 ofCicindelidæcan be sub-divided, only two range to Europe, and one of them is represented by only a single species on our continent.
Some of theCicindelasmay 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 ofCicindela, as I said, are known from England, of whichC. silvaticaandC. maritimaare certainly Siberian migrants, and perhapsC. hybridatoo. 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. silvaticaandmaritimahave 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. litteratahas 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 ofCarabusare 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 someCarabiof 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 asC. clathratus,C. granulatus, andC. cancellatusare no doubt of European origin, and have only in recent geological times extended their range across Northern Asia, whilstC. 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 theCarabidæthere 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 entomologistsas a south-eastern form, seems to have arrived with the Siberian migration, whilst the closely alliedD. 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 theGeographical Distribution of Butterfliesthat 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 ArcticMacro-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-EuropeanLepidopteraare 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 numberof 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 theLepidopteraof 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.*Sorex vulgaris.Lagomys pusillus.*Castor fiber.Spermophilus Eversmanni."erythrogenoides.Cricetus songarus.Myodes lemmus.Cuniculus torquatus.*Mus minutus.*Arvicola agrestis.*"amphibius."arvalis.*"glareolus."gregalis."ratticeps.Equus caballus.Saiga tartarica.Ovibos moschatus.Alces latifrons."machlis.Rangifer tarandus.* Those marked with an asterisk still inhabit Great Britain, or did so within historic times.
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 asSorexandLagomys, 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 theElephantidæresided in Europe, neither the lattercontinent nor North America was the home of the direct ancestor of any of the true Elephants. Similarly, though we have had ourSorexin Europe from the Upper Eocene andLagomysfrom the Middle Miocene, the geographical distribution ofSorex vulgarisandLagomys pusillusdoes 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 nowconfined 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 tothe 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).
Fig. 16.—Map of Europe. The dotted portions represent, approximately, the course of migration of the Siberian mammals. The principal mountain ranges are roughly indicated in black.
In the migrations of to-day we perceive the sametendency 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 appliesalone 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 northand 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 beendiscovered, 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 thesmaller 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:—
Only two of these can be looked upon as typically northern species, viz.,Patula ruderataandHelix tenuilabris, though both of them are still found living locally in Germany. Some of the others are decidedly southern species, likeChondrula tridens,Helix obvoluta,H. rotundata, andH. 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, accordingto 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. Notonly 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 Scandinavianice-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, evenduring 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 onewill 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 duringpart of the Pleistocene Epoch was so cold, that the whole country was buried underneath one immensemer 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 theMammoth 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-calledlater inter-glacial periodsto 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 thatat 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 ofthe 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 thenorthern 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.95I 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 atpresent, 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 recentyears 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.
Whatrelict lakesare, 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 crustaceanMysis relicta,—a shrimp-like creature,—which was formerly believed to inhabit also the Caspian, is of particular interest. More recently, the occurrence of thisMysisin 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 ofMysisto the list of those already known to science. These areM. caspiaandM. micropthalma, and both areclosely related to the arctic marineMysis oculata. According to Professor Sars, the genusMysisas 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 isopodIdotea 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, andSyngnathus 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-sheetin 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-moraineof 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.