CHAPTER V.

THE SIBERIAN MIGRATION.

In dealing with the British fauna in particular, I have drawn attention

to the fact that it is chiefly in the south of England that we find

fossil remains of eastern species of mammals in recent geological

deposits. We can actually trace the remains of these species and their

course of migration across part of the Continent towards Eastern Europe,

and as none of their bones have been discovered in the southern or

northern parts of our Continent, it must be 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. europaea_) probably came with the

Oriental migrants, or perhaps its ancestors did. But the larger Northern

or Russian Bullfinch (_P. major_) has no doubt entered our Continent

directly from the east. We have in many groups similar instances of

closely allied species or varieties, one of which, originating at a

somewhat later stage than the other, took a different route of migration

from that followed by its near relative.

The Pine-Grosbeak (_Pinicola enucleator_) is only known to British

ornithologists as an exceedingly rare visitor. Its real home lies in the

northern parts of Europe, Asia, and North America, and it is one of the

most typical of the Siberian migrants.

But there are a number of other species of birds, which, though probably

not of Siberian origin, only migrated westward recently, and have

either not yet reached the British Islands, or which lead one to

suppose, from their British range, that they are eastern forms.

Such, for instance, is the Nightingale (_Daulias luscinia_), which is

probably of Oriental origin, but only visits England regularly in

spring. There is no authenticated record of its ever having migrated

either to Scotland or Ireland.

The Bearded 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 caesia_), but neither of

these is probably of Siberian origin.

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

Among our British species, the Common Viper (_Pelias berus_), for

example, is a typically eastern form. It is almost unknown in Southern

Europe proper--that is to say, in Italy, the Balkan peninsula, and the

Mediterranean Islands, but its range extends in the west as far as

Spain, and in the east right across the Asiatic continent to 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 _Iguanidae_, which

includes some very large species. Both of them are Asiatic forms, which

have only just penetrated across the eastern steppes into Europe, where

they inhabit the arid regions between the Caspian and the River Don in

Southern Russia.

The species of Mammals living in Europe at the present day have, with

few exceptions, migrated to our continent from other parts of the world.

With regard to the Birds, it is possible that a somewhat larger number

proportionally may be of European origin. Still, the great majority are,

I think, to be regarded as immigrants. The autochthones are about equal

to the immigrant reptiles, but many of the European Amphibians and the

majority of the Fishes have probably originated on our continent. Some

of the European Amphibia--especially among the tailless forms--appear to

be immigrants from Asia. Thus the distribution of _Rana arvalis_ in

Europe is remarkably like that of a Siberian migrant. This frog occurs

in Siberia, ranging southward as far as Persia and parts of Asia Minor.

Crossing the European border, we find it in Russia, Upper Hungary, North

and Central Germany,--being rarer in the south,--Denmark, and

Scandinavia. According to Bedriaga, it crosses the Rhine only in Alsace,

but occurs no farther west. It only just enters Holland. If we suppose

the species to have originated in Central Europe, we should expect to

find it in Switzerland, France, and perhaps England. If it had its

ancestral home in Eastern Europe, we might expect it to occur on the

Balkan peninsula. It seems to me more probable, therefore, that _Rana

arvalis_ came with the Siberian migration. This need not cause surprise,

as the genus _Rana_ is certainly not European. Out of about 110 species,

only four are peculiar to Europe, the rest are scattered over all parts

of the globe. Moreover, the fact that these four species are confined to

Southern Europe would seem to indicate that the first species entered

from the south, and there either became modified or spread over nearly

the whole continent, as did, for instance, _Rana esculenta_ and _R.

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

esculenta_ ranges right across the Asiatic continent to 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 (_Cicindelidae_). There are over forty

species of the genus _Cicindela_ in Europe, five of which reach the

British Islands. This seems a large number; but there are altogether no

less than 600 species of the genus scattered over the greater part of

the world, many of them being Asiatic. The genus is certainly not of

European origin, for not only are most of the European species confined

to the Caucasus and the south-east generally, but no _Cicindelidae_

whatsoever occur, for example, in Madeira or the Canaries, where we

should expect some to have persisted if the genus had originated on our

continent. Moreover, of the five tribes into which the large family of

_Cicindelidae_ can be sub-divided, only two range to Europe, and one of

them is represented by only a single species on our continent.

Some of the _Cicindelas_ may have come with the Oriental migration. I

think this was the case with the only Irish species of the genus, _C.

campestris_. It occurs all over continental Europe and Northern Asia,

and varieties of the species are known from Corsica, Sicily, Crete, the

Cyclades, Sardinia, Asia Minor, Greece, and Spain. Five species of

_Cicindela_, as I said, are known from England, of which _C. silvatica_

and _C. maritima_ are certainly Siberian migrants, and perhaps _C.

hybrida_ too. Neither of the two first species is found in Southern

Europe or in Spain, where we should expect them to occur had they

originated on our continent. _C. silvatica_ and _maritima_ have no doubt

entered Europe from Siberia in recent geological times, probably soon

after a way was opened up across the Tchornosjem district of Southern

Russia--that is to say, in inter-glacial times. The former spread along

the Central European plain as far west as the south-east of England when

Great Britain still formed part of France. _C. maritima_, which

preferred the proximity of the sea, migrated along the 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 (_Carabidae_) furnish us with equally

interesting and instructive proofs of a migration from Asia. Over 300

species of _Carabus_ are known to science. The number of species

inhabiting Asia and Europe are about equal. But the genus does not

extend its range to Southern Asia or to South America or Australia. Very

few species enter Africa, and only nine North America, of which three

also occur in Siberia. The genus is unknown in Madeira, and only

represented by three species in the Canary Islands. To judge from its

distribution, it has probably originated in Western Asia. Probably some

_Carabi_ of European origin have spread into Asia, but the Asiatic--or

we might say the Siberian--origin and subsequent migration westward of a

number of well-known forms appears to me evident. Such forms as _C.

clathratus_, _C. granulatus_, and _C. cancellatus_ are no doubt of

European origin, and have only in recent geological times extended their

range across Northern Asia, whilst _C. marginalis_, coming from Siberia,

can hardly be said to have invaded Europe, since it has never been met

with farther west than the eastern provinces of Prussia.

Among the _Carabidae_ there are altogether very many examples pointing to

a migration from Asia to Europe, but I do not wish here to give a list

of all such cases, and only refer to a few of the more remarkable ones.

One of the European species of Demetrias (_D. unipunctatus_), known to

English entomologists as a south-eastern form, seems to have arrived

with the Siberian migration, whilst the closely allied _D.

atricapillus_, which has been able to reach Ireland, has a wider range

and came earlier with the Orientals.

Messrs. Speyer state (p. 68) that almost all those species of Central

European 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):--

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

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

_Elephantidae_ 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).

[Illustration: 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 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. Limnaea 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 palaeontologists 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 palaeontology 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. Koeppen, 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. Sjoegren 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.

[Illustration: 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, Koeppen, 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

larvae 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 deg. F. (=7 deg. 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.