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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

SUMMARY OF CHAPTER V.

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

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

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

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

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

CHAPTER VI.
THE ORIENTAL MIGRATION.

The Siberian immigrants into Europe on the whole are not very numerous, but it is different with those from the more southern parts of the Asiatic continent. The members of the Oriental migration form a very large percentage of the European fauna. No other migration has affected our continent so powerfully, because it continued uninterruptedly for a very long time. Hence its results can be traced from one corner of Europe to the other. We have seen that the Siberian migration only commenced after the first portion of the Glacial period had passed away. The Oriental, however, persisted throughout, or at any rate for the greater part of that period. It commenced ages before it, in miocene times, or even earlier. And as the Ægean Sea, which broke up the highway of the Oriental migrants, is only of recent formation, there was a steady westward march for a very considerable time. No doubt the migration was also favoured by the fact that scarcely any formidable barriers had to be crossed.

Many instances might be quoted of the same species forming part of the Oriental and also of the Siberian migration, but as a rule the Siberian migrant belongs to a distinct variety, or has such well-marked racial characters as to be at once detected from its more southern relative. Among the examples of Oriental migrants which I have occasion to bring forward, such instances will be specially dealt with.

In its wild state the Red Deer (Cervus elaphus) is almost extinct in the British Islands, though it still occurs in the moorlands of Devonshire and Somersetshire in England, in the south-west of Ireland, and in some localities in Scotland. Fifty years ago it was also found wild in several other of the Irish western counties; and in the seventeenth century it was common in most of the mountainous districts of Ireland. Its remains have been found fossil in the marls and caves of Ireland, and in the Forest-Bed, as well as in a large number of caves in England. The history of the Red Deer in other countries is very similar. In Scandinavia it flourished as far north as the sixty-eighth degree of latitude, whereas it is now quite extinct on the mainland, though still lingering on in some of the western islands. Denmark and Switzerland know it no more, and it is almost extinct in Belgium. Nearly throughout Europe where it occurs, its numbers are diminishing, greatly owing, perhaps, to the relentless persecution by man, but its gradual disappearance must likewise be partly due to other causes. Formerly it inhabited every country of Europe and all the larger islands. It still exists in Corsica and Sardinia, and at an earlier period it was also met with on the island of Malta. The Red Deer found in Corsica and Sardinia is smaller than that inhabiting Central Europe, and is by some authorities regarded as a distinct species, which has been named Cervus corsicanus. But Sir Victor Brooke has pointed out that the antlers of some of the Scotch Deer agree in every point with those of the Sardinian species. Indeed, the West European Red Deer altogether is a small-antlered form, compared with the Eastern one. This character, however, is only a racial one, and not of specific value. In the pleistocene deposits of Eastern and Central Europe, a very large-antlered race has been discovered, and identified by Professor Nehring with Cervus canadensis—the Canadian Red Deer. Tcherski, the Siberian traveller, believed that Cervus canadensis was identical with, or a variety of, the Asiatic species of Deer, Cervus eustephanus, Cervus xanthopygus, and Cervus maral. Some authorities—and to these belong Mr. Lydekker—think that we ought perhaps to regard the whole number of Red Deer-like forms as local varieties of one widely-spread species. Besides the deer already referred to, the following belong to this same group:—Cervus cashmirianus, Cervus affinis, Cervus Roosvelti, from North America, and the North African Cervus barbarus.

The question now is, where have these varieties originated? Or, if we go to the root of the matter, where is the original home of their ancestors? Considering that so many Cervidæ have been found in French and English pliocene deposits, and that remains of the Red Deer occur not only in the English Forest-Bed, but have been found associated with those of the Pigmy Hippopotamus in Malta, it would only be reasonable to suppose that the genus Cervus had originated in Europe. It might also be argued with equal force that the Red Deer had its birthplace in our continent. But when we carefully study its present range this verdict cannot be accepted. The view of the Asiatic origin of the Red Deer, so ably maintained by Köppen, corresponds far better with its present distribution, especially if we look upon the Asiatic, North American, and North African forms as varieties of the same species.

If the Red Deer were of European origin, it must have come into existence at a time when Malta was part of the mainland, when North Africa and the British Islands were connected with the continent of Europe, and of course before the deposition of the Forest-Bed. Such land-connections existed probably during the Pliocene Epoch. Migrants would have wandered from Europe into Asia. These would have developed into larger races, which again furnished emigrants for North America. The latter crossed by the old land-connection which once joined America and Asia at Behring's Straits. During pleistocene times the large Siberian race would now have re-migrated to the home of its ancestors in Europe, for we find the remains only in Central and Eastern Europe, indicating that an invasion of the Red Deer from Asia must then have taken place.

Against this view of the European origin of the Red Deer, it may be urged that deer are known from Indian as well as from European pliocene deposits, and that a migration could have taken place from the Oriental Region to Europe just as easily as from the latter to Asia. The majority of the species of the genus Cervus (in a wide sense), moreover, are Asiatic, ranging to Borneo, Sumatra, and the Philippine Islands, all of which islands have been separated from the mainland for a considerable time. Finally, the original home of a species, as we have learned, generally corresponds with the centre of its geographical range, and this lies in the case of the Red Deer in Central Asia.

One of the highest authorities on the deer family, Sir Victor Brooke, also was of opinion that the Cervidæ originated in Asia, and from there spread east and west. Of the two divisions into which true deer are divided, viz., the Plesiometacarpalia and the Telemetacarpalia, the former is almost confined to the Old and the latter to the New World. The only North American species belonging to the first division is the Canadian Red Deer, which fact clearly indicates its recent immigration to that continent.

There were probably two distinct migrations of the Red Deer into Europe. An older one coming from Asia Minor into Greece, which stocked Sardinia, Corsica, Malta, and North Africa in the first place, when these were still connected with one another. This same migration likewise affected western continental Europe, the Irish Red Deer being probably the descendant of this very ancient stock. The latter entered the island when it was still part of the Continent. The later migration of a larger form came from Siberia and spread mainly over Eastern and Central Europe, but it appears that it also reached England, although there is no evidence of any of these Siberian deer having ever inhabited Ireland.

The range of this deer, therefore, to some extent corresponds to that of another described on p. 153. We found then that two races of Reindeer had migrated to the British Islands—one from the Arctic Regions, and the other from Siberia, but that only the former had reached Ireland.

The so-called Irish Elk (Cervus giganteus) has been referred to the Oriental migration, but, as stated below, it has some claims to be regarded as a European. Unfortunately it is now extinct; it seems not unlikely, however, that it inhabited Ireland when man had already made his appearance on the island. Although its remains are found in such extraordinary abundance in Ireland, it certainly did not originate there. It lived also in England and Scotland, and in the Isle of Man, in France, Denmark, Germany, Austria, North Italy, and Russia. Its remains have been discovered even in Siberia. It must either have originated in Europe and then migrated to Asia, or have had its birthplace in Asia and wandered to Europe. There is nothing to lead any one to assert positively that either of these two continents was the one in which the original home of the Irish Elk was situated, and we can only be guided in this case by the history of its nearest relatives. These are the Fallow Deer (Cervus dama). There are two very closely allied species, the Persian and the European, but several others have been discovered in the Forest-Bed and the pliocene deposits of the Auvergne. As no remains of the Fallow Deer are known from Asia, it seems probable that it and also the Irish Elk originated in Southern Europe, and only invaded Asia in early pleistocene times.

The Mammoth (Elephas primigenius) is a familiar example among a large number of mammals which have come to us about the same time from Asia by the Asia Minor route. It had a much wider range than the Irish Elk, since its remains have been discovered in a large number of European localities as far west as Ireland, also in Siberia, and even North America. Though we have had Proboscidea in Europe from the Middle Miocene onwards, Mr. Lydekker (d, p. viii.) holds that "our comparatively full knowledge of Lower Miocene and Upper Eocene mammalian faunas of the greater part of Europe and North America, renders it almost certain that neither of those regions was the home of the direct ancestors of the Elephantidæ; and we must therefore look forward to the discovery of mammaliferous Lower Miocene or Upper Eocene strata in some other region of the (probably old) world which may yield these missing forms."

The genus Elephas makes its first appearance in the Upper Miocene of India. Our European E. antiquus is, according to Professor Zittel, probably identical with E. armeniacus of Asia Minor, while E. meridionalis agrees in all essential characters with the Indian E. hysudricus. The Indian and European species of fossil elephants altogether are very closely related, and the supposition that they all have had their original home in the Oriental Region offers, I think, no serious obstacle. The view of the European origin of the mammoth especially is open to very serious objections. It does not occur in any European pliocene deposits, and could not therefore have originated in our Continent until pleistocene times. That it should then have commenced its travels through Europe and Siberia to the New Siberian Islands and North America seems almost an impossibility. But if we suppose the mammoth to have had its home in India in pliocene times, it could then easily have migrated to all the parts of the world where its remains have been discovered.

Of the Asiatic mammals still living, some have only just crossed the borders of Europe and then died out again. Similar cases have been referred to in discussing the Siberian migration. Thus remains of the camel have been found in Roumania and in Southern Russia in pleistocene deposits. Others have lingered on to the present day. Crocidura etrusca, for instance, still lives in Southern France, Italy, Sicily, and North-western Africa. All its nearest relations are typically Oriental species. In spite of the fact that a Crocidura is known from French and German miocene deposits, the general range of the genus suggests an Oriental origin. In early Tertiary times a section spread into African territory and another eastward as far as the island of Timor. This may possibly have happened in miocene times, when a few species likewise found their way into Europe. Many other mammals have wandered still farther west, and now form an important percentage of the European fauna.

Of Birds, too, a large number might be mentioned which had their home in Asia and have found their way to Europe with the Oriental migrants. A few instances have already been alluded to, and some additional ones may be specified at random, without attempting to give a complete list.

Some of the Wagtails (Motacilla), as I mentioned in the last chapter, have certainly come to us with the Siberian migration; but others seem to be Oriental, such as Motacilla melanope, which is resident in Southern Europe and migratory in the North. M. campestris—the Yellow Wagtail—has a most peculiar discontinuous range. One colony breeds in the British Isles and Western Europe generally, where it is known as a summer visitor, retiring to West Africa during winter; another is found from South-east Russia to Turkestan in summer, and winters in Southern Africa. This fact may possibly be due to two distinct migrations from Asia having taken place: an earlier one from the South-east—that is to say, an Oriental one—and a Siberian one more recently. In this case the members of the two migrations have not become sufficiently differentiated to be regarded as distinct varieties. Though most of the Wagtails have a somewhat northern range, none (except perhaps M. borealis) are truly Arctic; and indeed, as almost all of them pass the winter in southern latitudes, it may be assumed that they are of southern and not of northern origin.

The Dippers (Cinclus) are practically unknown in the Central European plain, but they occur in Western Europe as far north as Scandinavia, also in the Alps, Carpathians, and Southern Europe, including Sicily and Sardinia. Some authorities distinguish three species, others only one. As a matter of fact, the difference between the three forms is very slight, and their nests and eggs are undistinguishable. Eight other species have been recognised, and all these are either Asiatic or American. As one of the American forms is peculiar to Peru and another to Ecuador and Columbia, and since the genus as a whole is a mountain-genus, it probably is an ancient one. Its European range alone, however, implies that it has inhabited our continent for a considerable time and is no new-comer. We may look upon it as of Asiatic origin. The ancestors have spread east and west, the European species having arrived with the earlier Oriental migrants, and wandered along the Mediterranean at a time when the geographical conditions of that sea were vastly different from what they are to-day.

Not quite so ancient as the Dippers, but likewise Asiatic in their origin, are the Bullfinches (Pyrrhula). The closely allied Pine-Grosbeak (Pinicola enucleator) has already been referred to (p. 191) as a member of the Siberian migration. The distribution of the European Bullfinch (P. europæa) is very interesting, as it occurs in two distinct forms, by some authorities regarded as races, by others as species. In all probability these two races owe their origin to two different migrations from the same ancestral stock. We may suppose that P. europæa came to Europe along with the Oriental migration, spreading chiefly over the south and west, while another branch developed in Siberia into the larger and more brilliant race (P. major), which subsequently entered the neighbouring continent with the Siberian fauna. The latter race inhabits, according to Mr. Saunders, Northern and Eastern Europe, and also Siberia. All the other species—there are eight more—except one, are found in Asia. This one species, which inhabits the Azores, appears to be more closely related to one of the Siberian bullfinches than to the European. It stands isolated, and is an extraordinary instance of discontinuous distribution, as no Bullfinch inhabits either Madeira or the Canary Islands. We must assume that the form connecting it with the Asiatic probably lived in Southern Europe, and has become extinct.

One of the most typically Oriental genera of birds is Phasianus, to which our Common Pheasant belongs. Out of twenty species, nineteen are found exclusively in Asia, most of them being confined to the central plateaux of that continent. Only one species passes the confines of Asia into Greece, Turkey, and Southern Russia. This is Phasianus colchicus. Formerly, however, the Pheasant appears to have had a wider range in Europe, for three species are known fossil from France. Altogether, it is not quite certain whether the Pheasant is not really an indigenous bird in the British Islands, having survived from pre-glacial times. It is believed that the Romans brought it to England, but there is no record of an introduction at that time.

Among the older Oriental bird migrants might be mentioned the Fire-crested Wren (Regulus ignicapillus), which has even occasionally visited England. It becomes commoner as we go south-eastward. In Asia Minor it is more abundant than the Gold-crest; and throughout the year it is resident in Southern Europe, where it occurs in Turkey, Greece, Italy, Spain, Sardinia, and Malta. On the opposite shore, in North-west Africa, it again makes its appearance, and its range extends westward to the Canaries (R. teneriffæ) and Madeira (R. maderensis).

The genus to which our common Goldfinch belongs, viz., Carduelis, is also probably of Oriental origin, and may be looked upon as one of the earlier migrants. That species (C. elegans) breeds throughout Europe, except in the extreme north, but it is especially abundant in Southern Europe and North-west Africa. It is also resident in Madeira and the Canaries. Eastward its range extends to Persia. A larger race (C. major) inhabits Western Siberia and crosses the European border into Russia. It interbreeds in Siberia with C. caniceps, an East Siberian form.

A few instances of Reptiles and Amphibia with a similar range will show that the Oriental migration was not confined to the higher vertebrates.

Two species of the genus Eremias (Podarcis) occur in South-eastern Europe. This is a genus of Lizards with rather a wide distribution, ranging from Central Asia to South Africa southward and China eastward. Altogether there are twenty-four species, two of which just enter Europe; and of the rest half are Asiatic and half African. Even if the genus were of African origin, it is extremely unlikely that the Asiatic species came by way of Europe. We may assume, therefore, with a fair degree of probability that the two European species wandered westward along with the Oriental migrants.

The genus Ablepharus belongs to a family of Lizards in which the legs are either very fully developed, or quite absent as in the Slow-worm (Anguis fragilis). It is an ancient genus, having a wide range from Central Asia to Australia on the one hand, and to South Africa on the other. One species of this Scink-like Lizard, viz., Ablepharus pannonicus, enters Europe in the south-east, inhabiting Greece as far north as Southern Hungary. In Asia it is found in Syria and North Arabia. This clearly signifies that the Lizard is an Oriental migrant.

Among the Snakes which participated in the Oriental migration might be mentioned Eryx jaculus, whose home is probably in Western Asia. It is known in Europe from the Greek islands of Tinos and Naxos, from Turkey and Southern Russia. Another, a peculiar worm-like form, lives underground in damp earth and under stones—Typhlops lumbricalis. This species inhabits the mainland of Greece as well as the Greek islands, and Asia Minor as far as the Caucasus.

A most interesting case of distribution is that of the pretty little Toad so well known on the Continent under the name of "fire-toad" (Bombinator igneus). Though some authorities, such as Boulenger, recognise only one form of Bombinator,[1] others are of opinion that two well-marked varieties exist in Europe. These are looked upon by Dr. von Bedriaga as good species, but he acknowledges that they are rather critical and difficult to identify. No other species of Bombinator occur in Europe. Bombinator pachypus, the western race,—or if we choose to call it species,—occurs in France, Germany, Switzerland, Austria, Sicily, and Greece. B. igneus—the eastern race—is found in Southern Sweden, Denmark, Germany, Austria, and Russia. The latter has therefore a more northerly and easterly range. The species is not known from Siberia, but makes its appearance again in China in a form which, according to Dr. von Bedriaga, does not quite agree with either of the two European races.

Now if we supposed Bombinator to have originated in Europe, its absence from the British Islands, most of the Mediterranean islands, and the greater part of Scandinavia would not be easy of explanation, while as an Asiatic migrant the European range is more readily understood. Its apparent absence from Western Asia might quite likely be due to the fact that the zoology of that part of the Continent is only now being investigated. The latter has, moreover, undergone great physical changes in recent geological times. The supposition that one migration of Bombinator from the south-east has taken place, and then another from the east, seems to explain this case of distribution, as other similar ones, in a most satisfactory manner.

The Tree-Frog (Hyla arborea) must be an ancient species, but it is not of European origin. Few genera of Amphibia have a wider distribution than Hyla. There are only three species in Asia, Europe, and Africa, the remaining 129 being confined to America and Australia. Two of the three Old World Tree-frogs are so closely allied that until recently they were regarded as mere varieties of one another. These are Hyla arborea and H. chinensis. The former is found in Asia Minor, Persia, China and Japan, and in most of the Mediterranean islands and Southern Europe generally. It does not occur in the British Islands, Norway, or North Russia, but in South Sweden, Germany, France, and Spain. It is also known from North Africa and from Madeira, the Canaries, and the Salvages. The occurrence of the Tree-Frog on so many of the Mediterranean islands is of particular interest, especially as four well-marked varieties have been distinguished by our leading herpetologists, so that the more minute features of the various forms can be traced from island to island, adding one more proof—if proof were needed—of their former continuity. Of course, that Hyla arborea must be considered an Oriental migrant seems so evident that it scarcely needs further comment.

A number of mollusca might be mentioned whose range indicates that they have migrated to Europe from Asia Minor. Buliminus pupa is one of these. It is known from Asia Minor, Greece, South Italy, Sicily, and Algeria. Buliminus detritus is perhaps better known, being common in some parts of Germany. From there its range spreads east as far as Asia Minor. Many closely allied species inhabit Western Asia, to which they are confined, while others enter on European territory in some of the Greek islands. B. fasciolatus occurs on the islands of Crete, Rhodes, Cyprus, and in Greece and Syria. Most of the species of Buliminus have a very restricted range, but Buliminus obscurus is found almost all over Europe, from Ireland in the west to the Crimea and Transcaucasia in the east.

Whether the sub-genus Pomatia of the genus Helix—to which the so-called Roman Snail belongs—is of Asiatic origin, or whether some of the species have migrated from Europe to Asia, I am not prepared to say; but there can be no doubt that Helix pomatia has reached Western Europe from the east.

On the whole, the number of mollusca which we might point to as having migrated to Europe is not large, the great majority being indigenous to our continent. However, some of the other groups of invertebrates differ very materially in that respect from the mollusca. I cannot leave the consideration of the mollusca without referring to the fact that there appears to be a very important centre of distribution in South-eastern Europe. It is from this centre that many species have spread north and south, east and west. Take, for example, the genus Clausilia, a small land-shell shaped like a pointed round tower, and abundant on old walls and tree trunks. In England we have four species of Clausilia, in Ireland only two. In the greater part of Spain only our common Cl. bidentata occurs. As we go east the number of species rapidly increases. A maximum is reached in South-eastern Europe, where hundreds of different kinds are found. Towards Northern Europe a similar decrease of species takes place. So far the history of the Clausiliæ seems perfectly simple. An active centre of origin appears to exist in South-eastern Europe, from which the species radiate out in all directions. But when we come to look more closely into the extra-European distribution of the genus, and especially when we examine its past history, we find that its origin is extremely complex, and dates back to a much more remote period than would have been imagined, had we merely taken into account its present range in our own continent. Professor Boettger, who is the highest authority on Clausilia, tells us that the genus is known from the earliest deposits of the Tertiary Era. About 700 species are now known, and these have been sub-divided by Professor Boettger and others into a number of sub-genera. Some of these are extinct, but the great majority are still living. The sub-genus Phædusa occurs in the eocene and oligocene of Southern Europe, but it is extinct as far as our continent is concerned. Close upon a hundred species, however, still inhabit India, the Malayan Islands, China, Ceylon, and Japan. Then again, the sub-genus Laminifera occurs in the oligocene and miocene of Central Europe, and survives in a single species, Cl. Pauli, in South-western France. The groups Garnieria of China, Macroptychia of East Africa, Boettgeria of Madeira, and Nenia of South America, have no fossil representatives. We have here some very remarkable cases of discontinuous distribution which testify to the antiquity of the genus, and this is certainly confirmed by the fossil evidence. However, it is hardly likely that the headquarters, as it were, of Clausilia have always been in South-eastern Europe. Most of that part of the Continent has been submerged since eocene times more than once. The peculiar distribution of the genus might be explained, I think, if we supposed the original home of Clausilia to have been in Southern Asia, that from this centre Southern Europe was colonised, where a new centre developed in oligocene and miocene times, sending colonies off to Madeira and across the old land-connection which united Northern Africa and South America about that time. The most active centre of development then gradually shifted eastward again, while the older centres were perhaps submerged during the physical changes in the distribution of land and water.

I should have mentioned that the species wandering westward and northward from this South-European centre of distribution, would naturally have joined the migrants which came from beyond the borders of our continent. They might thus appear to be true Oriental migrants, and on a previous occasion I grouped all these together under the term of "Southern Fauna," as I assumed the observer to be stationed in the British Islands. All new-comers from the south-east, south, or south-west of Europe would be to him southerners quite irrespective of their original home, which might be in Southern Europe, Asia, or Africa.

The Swallow-tail is well known to all collectors of Butterflies in England, though it has of late years become very rare and is now confined to a few localities in the east of England. The members of the family Papilionidæ, to which it belongs, are mostly large and striking species, and their distribution is therefore more accurately known than that of the smaller and less conspicuous butterflies. Only four different kinds of Swallow-tail Butterflies inhabit Europe, but in Southern Asia and the Malay peninsula they attain their maximum as regards numbers; and there we find a great many species of this genus Papilio. Of the four European species only one, viz., Papilio hospiton, is peculiar to Europe; all the others range into Asia. It would seem, therefore, as if this genus was an Asiatic one and had migrated to Europe, and that the route taken was the one from Asia Minor across to Greece. We have a similar case in the closely allied genus Thais two of the three European species living also in Asia Minor. Thais cerisyi inhabits some of the Greek islands, as well as the mainland of Turkey and Greece.

Another genus of the great family Papilionidæ with which most lepidopterists are well acquainted is Parnassius. What butterfly-hunter has been in Switzerland without hearing of, or seeing, the famous Parnassius Apollo? We have four European species of Parnassius, only one of which is peculiar to our continent, but the locality where it occurs, the Caucasus, is on the borders of Asia. Almost all the other species are Asiatic, none however range to the south. Its headquarters, and I think its original home, are the mountains of Central Asia. From there it has spread—some species to the Himalayas, and a few to Europe and North America. But these migrations are not of very recent date. Parnassius no doubt arrived accompanied by a large number of other Central Asiatic mountain insects and plants. I shall refer to the latter again when dealing with the origin of the Alpine fauna, but meanwhile it might be mentioned that the famous Swiss "Edelweiss" (Leontopodium alpinum), which we are accustomed to regard as a typical Alpine plant, is certainly of Asiatic origin. In some parts of Southern Siberia it is one of the common meadow-flowers, and ranges from there south into Kashmere, but not northward. Like the Apollo, it does not occur in Scandinavia or Northern Siberia. Both plant and insect evidently migrated from Central Asia, directly westward along the southern border of the sea, which extended from that region as far as the European Alps in early Tertiary times. At that time the Caucasus was possibly still connected with the Balkan Mountains, across what is now the Black Sea, and that may have been the highway on which they travelled west.

Some of the Clouded-Yellows—butterflies appertaining to the genus Colias—formed part of the Oriental migration. The genus is undoubtedly of Asiatic origin, and while many of the species have turned northward, ranging across Siberia and North America, others have taken a southern and westward turn and thus reached Europe. We have two Clouded-Yellows in Western Europe, and both of them must have come with this migration.

A very good example of an Oriental migrant is Danais chrysippus, a magnificent butterfly found in Greece and Southern Italy. In Asia it is known from Syria, Persia, and from the whole of the southern portion of the Continent. The genus Danais (in its wide sense) is a large one, and principally occurs in the warmer regions of Asia. Three species are found in North America and only one in Europe.

Among the beetles belonging to this migration, there is one of very considerable interest from a distributional point of view, for all the species of the genus—even the whole family to which the genus belongs—are what is known by zoologists as "Commensalists." These are animals habitually associating and living in close connection with others with which they are not tied by any family relations or kinship. Such a state of close and permanent friendship is called "commensalism." Now it appears as if the members of this family of beetles (Clavigeridæ) had of their own free will formed such a close connection with colonies of ants—sometimes with one species, sometimes another. They are the permanent guests of the ants, and in return they secrete a fluid which is apparently highly prized by them. All of the Clavigers are provided with peculiar club-shaped antennæ, with which they ungraciously beat their hosts, when they are in want of food. According to some authorities, they even occasionally gnaw at the pupæ and larvæ of the ant with which they live.

Such beetles naturally can only have extremely limited means of distribution, and they are comparable in that respect with the woodlice of the genus Platyarthrus, to which I have already had occasion to refer. All the species of Claviger are confined to Europe, chiefly to the south, but one species, Cl. testaceus, has wandered farther north and occurs in the nest of the ant Lasius flavus in the south of England, Ireland, and Scotland. Though none of the Clavigers can be claimed as Oriental migrants, the centre of distribution of the genera belonging to the Clavigeridæ is in Southern Asia, and it is probable that the ancestors of the European Clavigers have spread westward from that region to Europe, eastward to Australia and Japan, and southward to Madagascar and South Africa. The genus Hopatroides, belonging to the same family as the so-called Spanish-fly (Tenebrionidæ), has twelve species in Western Asia and Greece. One only, H. thoracicus—an instance of discontinuous distribution—occurs in Andalusia. Amphicoma is represented in Western Asia and the Balkan peninsula by fifteen species, while three others are met with in North-west Africa and Southern Spain.

A genus of Dragon-fly, Onychogomphus, has in Europe a somewhat similar distribution to Claviger, but it has besides a very extensive foreign range. There are altogether thirty-five species; of these ten are Holarctic, twelve Oriental, five Mascarene, and eight Ethiopian. The centre of distribution is therefore in the Oriental region, and we may assume that in all probability the genus has originated there, the European species having travelled west with the Oriental migration at an early date of the Tertiary Era.

Ryothemis, another genus of Dragon-flies, has originated perhaps somewhat farther east than the last, for no less than thirteen species are found in Australia, a like number in India, five in Madagascar and Africa, and five in the Holarctic region. Both of these genera are entirely absent from America, and they have possibly travelled to Europe together.

Among the European Orthoptera—the group to which our Earwigs and Grasshoppers belong—there are also a good many instances of Oriental migrants. One of the most striking of these is the curious "praying insect" (Mantis religiosa). It occurs all over Southern Europe, and ranges as far north as the north of France. It is also found in Southern Germany and in Austria, and has a vast extra-European range. There are even records of its occurrence from all parts of Southern Asia and Java and a great part of Africa. That it belongs to an extremely ancient genus is testified by the fact of its presence in Mauritius, Japan, Australia, New Zealand, South America, and Madagascar. The genus Bacillus—to which the typical Stick-insects belong—has a somewhat similar geographical distribution. But no less than four species of Bacillus are known from Europe, according to our great authority Mr. Brunner von Wattenwyl—all from the south; and some of these also range into North Africa. There are thirty-two other species distributed over Southern Asia, Africa, Australia, New Zealand, and the Sandwich Islands.

Volumes, indeed, might be filled with lists of species and genera of terrestrial invertebrates of Oriental origin, but I will not weary the reader with further enumeration of such instances. Just two more, however, before concluding, as I have not alluded to the large group of the Arachnida.

Two peculiar spider-like genera, viz., Galeodes and Rhax, are found in Southern Europe. Both occur also in North Africa, and in Western and a portion of Southern Asia. As the whole family altogether has an Asiatic character, I cannot agree with Mr. Pocock, who considers them of European origin and believes that they are migrating eastward.

But not only terrestrial forms migrated to Europe from Western and Southern Asia. Freshwater species also took part in this great Oriental migration. I need only refer to the freshwater Crab (Thelphusa fluviatilis), with which Southern Europeans are familiar. It is the sole representative of a large genus which ranges east as far as Australia and southward to Madagascar and the Cape of Good Hope. The European species is found in Turkey, Cyprus, Greece, Southern Italy, Sicily, North Africa, Southern Spain, Syria, and Persia.

There is a corresponding flora with a range exactly similar to that of some of the animals quoted. Thus the Balkan Rhododendron (Rhododendron ponticum) is again met with in the western Mediterranean region in Southern Spain. The Cedar occurs in local varieties in the Himalayan Mountains, in the Lebanon, and the Atlas Mountains. Both of these are instances of discontinuous distribution, a proof of their antiquity; but a large number of plants have a continuous range between Asia Minor and Spain.

On looking through these few instances of what have been called Oriental migrants, one cannot help being struck by the fact that the species after their entry into Europe evidently did not all follow the same path during their westward advance. We have seen that a good many seem to have travelled either due west or north-west on entering our continent from Asia Minor. They may now perhaps be found in Greece, Southern Italy, Algiers, and Spain, also probably on some of the intervening islands in the Greek Archipelago, in Sicily, Sardinia, and Corsica, or they may have travelled north-east and occur in the Alps. This distribution indicates undoubtedly, as I have already set forth in another memoir (c, p. 459), that land extended from Asia Minor across Greece to Southern Italy, that the latter again was disconnected with Central Italy, but united with Sicily, Sardinia, and Tunis, and that the Straits of Gibraltar did not exist at the time when these species migrated westward. Some species are only to be found as far west as Southern Italy, while others occur in Central and Northern Europe, scarcely in the South, and not at all in the larger Mediterranean islands or in North Africa. This appears to me to indicate that the late comers from the east found that geographical changes had taken place in Southern Europe which prevented them from following the same track as the older immigrants. They were now obliged to turn directly northward and then westward. It may be asked, why should not the earlier migrants have taken the same route? This question will be answered immediately. Meanwhile it should be clearly understood that there probably was an older and a newer migration from the east. The Oriental genera—from whose general range we know that they must be very ancient indeed, such as Mantis and Bacillus—are almost invariably confined to Southern Europe. There they are frequently found on some of the Mediterranean islands. The earlier migrants therefore went westward and the later ones northward.

Let us now inquire a little into the reasons why such different courses were pursued by the migrants—why the Oriental migration divided into two streams, an older and a newer.

During early Tertiary times, and probably throughout the Miocene and Pliocene Epochs, the Ægean Sea did not exist. From the island of Crete to the Peloponnesus, and from Asia Minor to Thessaly and Macedonia, stretched a vast and fertile plain dotted over with numerous freshwater lakes. Gradually the sea encroached upon this land from the south, owing chiefly to extensive subsidences having taken place. Only very recently, says Professor Suess, did the whole of the Ægean continent subside (i., p. 437). Huge cliffs of levantine freshwater deposits now mark the new coast-line, and the Mediterranean advances steadily towards the Black Sea and the Sea of Asov. A new order of things is now established, continues the famous author of Das Antlitz der Erde; where there were high mountains we now behold a deep sea, in some places many thousand feet deep. All this took place quite recently,—geologically speaking,—certainly in post-glacial times; and man may even have witnessed these imposing events. Most geologists admit the correctness of these views. They are, moreover, built upon such solid geological evidence, that even if the science of zoogeography had not yet taught us anything, naturalists would not hesitate in accepting them.