Part 10 (1/2)

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 acc.u.mulations 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 ma.s.s of ice, it was thought, must have acc.u.mulated 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 pa.s.sed 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 a.s.sumption. 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 a.s.sisting 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 sh.o.r.es 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, a.s.serts 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. Koppen, 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 a.s.sumed 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 c.u.macea, 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 (_Limnocala.n.u.s 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 a.s.sumed 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. Sjogren 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.

[Ill.u.s.tration: 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 lat.i.tudes 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, Koppen, 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 pa.s.sed 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 sh.o.r.es? 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 a.s.sume 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 lat.i.tudes.

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 a.s.sume then that the retreat of the Northern Sea (Fig. 13, p. 170) opened up a pa.s.sage 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.