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 Oriental migration is closely related to the Siberian. Both have originated within the Asiatic continent, and in many respects a strict line cannot be drawn between them. There can be no doubt that some of the species which we regard as Siberian migrants had their original home in more southern latitudes, and thus may have formed part of the older Oriental migration. The home of that migration I take to be Central and Southern Asia, that is to say, everything south of the Altaï Mountains and the Caucasus. Its members have reached Europe across an old land-connection which united Turkey, Greece, and Syria, while the Siberian animals invaded our continent to the north of the Caspian and Caucasus.
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.
Animals and plants were free to migrate from Central and Southern Asia to Greece by land for untold ages. The vast accumulation of mammalian bones which have been discovered at Pikermi, and so ably described by Gaudry, are probably to a large extent the remains of Asiatic immigrants to Europe. Many of these resemble forms still living in South Africa, which implies that a highway existed also at that time between Asia and Africa. Among these is a giraffe and antelopes closely allied to African species, and other most interesting mammals.
In still earlier European deposits—the Miocene—we find the ancestors of modern Elephants, which are probably of Asiatic origin. The remains of several kinds of monkeys occur, whose nearest relations are now confined to Southern Asia. Altogether the fauna bears a strong Asiatic facies. Many of our European terrestrial invertebrates probably arrived about this time from Asia. The struggle for existence being keener and the facility for migration much greater in the higher vertebrates, they—or at any rate the mammalian faunas—were subjected to more rapid changes than the invertebrates. I have repeatedly expressed my belief that a great number of our familiar insects and mollusca inhabited Europe long before our present mammals came into existence.[2]
Let us now follow one of the miocene Oriental migrants starting from Central Asia on its way to Europe. Very soon after leaving its home, it must have encountered a sea which extended at that time from the Eastern Mediterranean to the borders of Afghanistan. In following a westward course, the emigrant was compelled to keep along the northern shore of it. We do not know the state of the physical geography of the region between the Black Sea and the Tianshan Mountains, but it seems certain that a considerable extent of dry land enabled a wanderer from Central or Southern Asia to reach the Balkan peninsula by skirting the northern shore of that large miocene sea. No miocene deposits occur north of Teheran or of the Upper Euphrates, nor are they known from the islands of the Ægean Sea or the lands surrounding it. From the Balkan peninsula it was possible for our migrant to reach the European Alps, which were then slowly rising as a peninsula out of the western portion of the great miocene sea. What are now the Alps was then hilly ground, which was being raised from the bottom of the sea. It was no doubt connected with the Balkan peninsula, so that an intercourse of species could take place between this newly-formed peninsula and Central Asia. I say peninsula, because the miocene sea almost completely surrounded it. From the Western Mediterranean a wide gulf extended up the Rhone valley into that of the Rhine as far north as Maintz. Then skirting along the northern outliers of the Tyrol, the gulf can be followed as far east as Transylvania. It is quite probable that it extended much farther east still, but there is as yet no geological evidence forthcoming. At any rate, our Asiatic migrant turning northward from the Balkan peninsula found its farther progress barred once more by an arm of the same sea which in its earlier peregrinations had stopped it from going south (cf. Suess, i., p. 406).
In later miocene times the sea does not seem to have surrounded the Alps to the same extent as it did before, but it certainly extended from the Eastern Alps to the shores of the Sea of Asov, so that the direct northward passage was still more or less barred to the Oriental immigrants. At the same time Alpine species were now able to emigrate to the North European provinces. During the last stages of this epoch, the same sea increased its area very considerably in an eastward direction. One continuous expanse of water now stretched from the Alps as far as the Sea of Aral in Central Asia, perhaps even farther.
During pliocene times especially, the northern parts of the Balkan peninsula were occupied by a series of freshwater lakes, while Greece was joined to Southern Italy, Sicily, and Tunis. Central and Northern Italy were represented by a long narrow peninsula connected in the north with the Alps. Corsica and Sardinia were joined to Sicily, and the Straits of Gibraltar did not exist. When I first published my views regarding these geographical conditions of the Mediterranean area, Professor Depéret was good enough to send me his criticisms from a purely geological standpoint. He is of opinion that though Sicily and Sardinia might at this time have still been connected with Tunis, the Straits of Messina must already have been formed—in other words, Southern Italy and Sicily could no longer have been connected with one another. This opinion is based upon the fact that in the upper strata of the enormously thick Sicilian pliocene deposits are found a number of arctic or subarctic species of mollusca which are entirely foreign to the Mediterranean fauna. It is generally supposed that these reached the Mediterranean area by the newly opened Straits of Gibraltar in later pliocene times, and that the lower Sicilian deposits must therefore have been laid down earlier. So far the deductions are perfectly correct, if we assume the northern mollusca to have arrived in the Atlantic at the time stated. However, they must have reached the Atlantic much later—not till pleistocene times—if we adopt the above-stated suggestions as to the age of the Forest-Bed (cf. p. [125]). Moreover, the great similarity between the faunas of Southern Spain and North-western Africa indicate that the formation of the Straits of Gibraltar is of very recent date. The northern mollusca, of course, could not have reached Sicily till later. To suppose that the Sicilian deposits have been uplifted 7000 feet since then is no doubt contrary to all our geological teaching, but we must remember that this is altogether an exceptional case. The area in question has probably ever since been in the immediate neighbourhood of an active volcano, and the rate of the uplift has therefore been immeasurably greater than at other localities with which this one might be compared. The disconnection between Tunis, Sicily, and Southern Italy was evidently produced by a subsidence of the tract of land uniting these countries. If we suppose that this happened in early pliocene times, we have either to take for granted that the terrestrial fauna and flora of these countries are of miocene origin, or that they were joined again during the Pleistocene Epoch. The range of a very large number of animals and plants is such as can only be explained by assuming that Tunis, Sicily, Sardinia, Corsica, and Southern Italy were connected with one another. Of such extensive land-connections subsequent to the arrival of the northern marine mollusca we possess, however, no geological evidence whatsoever; and it is extremely improbable that the land-areas which had sunk were once more raised before again subsiding. The many animals whose presence in the Mediterranean Region bears witness to these ancient land-connections could not have arrived there in miocene times—in fact, they could hardly have lived there before the end of the Pliocene Epoch. On the other hand, it seems difficult to believe, once the Straits of Gibraltar were open and the waters of the Atlantic able to enter the Mediterranean, that the sunken parts between Sicily, Italy, and Tunis could have been raised without affecting the entire area of that sea. Nor is it likely that the junction between these countries could have then been brought about by a general lowering of the Mediterranean waters. As it may be asked what evidences we possess at all for the supposition of such land-connections as I have indicated, also that Southern Italy and Greece were connected, a few of the more salient instances of distribution bearing on this problem may be of interest.
I have already referred to the occurrence of the remains of a small race of Red Deer in the caves of Malta, similar to those still living in North-west Africa, Corsica, and Sardinia. The Black-mouthed Weasel (Mustela boccamela) inhabits Persia, Asia Minor, Greece, South Italy, Sicily, and Sardinia, while Mustela africana is found in Malta and Algiers. The European Porcupine inhabits Asia Minor, the island of Rhodos, Greece, Southern Italy, Sicily, North Africa, and Spain. Then we have the Wild Sheep of Asia Minor, Cyprus, Sardinia, and Corsica, all of which are closely allied. The small shrew-like Crocidura etrusca occurs in South France, Italy, Sicily, and North Africa. Many other mammalia might be quoted, but these are sufficient for our purpose.
There are a good many reptiles and amphibians with a similar distribution. The European Chamæleon (Chamæleon vulgaris) has been found in South Spain, North Africa, and Sicily. The Snake Periops hippocrepis is confined to Spain, Sardinia, and Greece. The worm-like Lizard Blanus cinereus inhabits some of the Greek islands, North Africa, and Spain. Another Lizard belonging to the Scincidæ has also been found in some of the Greek islands, Sicily, Sardinia, Southern Spain, and the Canary Islands. Discoglossus pictus—a toad—occurs in Spain, North-west Africa, Malta, Sicily, Sardinia, and Corsica. A variety of the Tree Frog (Hyla arborea Savignyi) is found in Europe only in Corsica, Sardinia, and the Greek Archipelago.
Eight species of Reptiles and Amphibia—some of which I have just referred to—are enumerated by Dr. Forsyth Major as occurring eastward and westward of the Italian peninsula (and almost all also in North Africa) without being known on the mainland of Italy. And in order to show that Sardinia and Corsica are more closely related to North Africa than to Italy, he indicates the general range of the Reptiles and Amphibians found in these islands. Of the twenty-one species, only twelve inhabit Italy, but at least sixteen North Africa and seventeen Spain. Indeed, he shows that Corsica, Sardinia, Sicily, and North-west Africa form a zoogeographical province, from which Italy, with the exception of a few localities on its west coast, is excluded. It is a remarkable fact that there are a few localities on the west coast of Italy which in their fauna and flora exhibit closer relationship with Corsica and Sardinia than with the mainland. Thus Dr. Major pointed out that the Catena Mettalifera, the Monte Argentario, and Monte Circeo all belong to what we may call the former Tyrrhenian continent. They are to be regarded as its eastern limits, which remained standing, while the central portion—now occupied by the Tyrrhenian Sea—subsided, and is at present covered by deep sea. Subsequently these remnants of the old continent became joined with the newly-formed Italian peninsula, but the plants and animals belonging to the older flora and fauna were mostly destroyed by newer and more vigorous immigrants. A few of the more hardy ones survived, and are a standing testimony of the geographical revolutions of that part of Southern Europe.
That the Mediterranean area has undergone such profound geographical changes as I have endeavoured to indicate is no new theory. Many zoologists who have investigated the fauna of that region, and have attempted to explain the faunistic relations, had to acknowledge that the migrations must have taken place under geographical conditions entirely different from those obtaining at present. Rütimeyer long ago remarked that it seemed to him much more probable that Morocco, Algeria, and Tunis were peopled by way of Gibraltar, and perhaps also by Sicily and Malta from Europe, than Southern Europe from Africa. After careful conchological researches in the Western Mediterranean region, Dr. Kobelt came to the conclusion that formerly Southern Spain and Morocco must have been united by a broad land-connection. Sicily and Algeria do not apparently show any very intimate relationship conchologically, but farther west—in the mountains of Tetuan—Dr. Kobelt discovered a colony of Sicilian forms.[3]
"The close relationship," remarks Dr. Major (a, p. 106), "shown in the fauna of Corsica and Sardinia to Africa, permits the supposition that the connection with these islands had persisted to a much more recent date than that with Europe."
Many other authors have pointed out the close similarity existing between the faunas of Southern Europe and North Africa. We need only refer to the writings of Professor Suess, Milne-Edwards, and Boyd Dawkins. Mr. Blanchard went even so far as to say, "a comparer les plantes et les animaux de la Sicile et de la Tunésie, on se croirait sur le même terrain" (p. 1047).
No less than 113 species of phanerogamic plants are enumerated by Professor Engler (p. 53) as occurring in the Mediterranean coast region east and west of Italy without being found in that peninsula, or at least only in the extreme south of it. But he tells us that these species represent only a portion of such plants, which are extremely numerous.
In taking a general survey of these plants, Professor Engler is of opinion that their range implies that a large number of the Mediterranean species have migrated along a line which can be drawn between North Africa, Sicily, Greece, Crete, and Asia Minor, and that from this line the distribution started northward again.
Many of these plants then, and also some of the animals I have referred to, formed part of the older stream of migration which entered Europe from Asia Minor (vide [Fig. 5], p. [117]). There were only two courses open to them as they arrived on our continent during earlier Tertiary times. They could either go straight west towards Greece, or in a more northward direction to the newly-formed Alps. As the latter were raised, some of the immigrants were modified so as to adapt themselves to the new surroundings. Others became extinct; but a great many have persisted in the Alps to the present day and exhibit discontinuous distribution, having meanwhile disappeared in the intermediate tract between the latter and their original home in Asia. The lowlands of Eastern and Central Europe were either occupied by the sea or by large freshwater lakes, so as effectually to prevent a direct migration northward.
When the newer migrants arrived from Asia not only had the Alps risen to a lofty mountain chain acting as an effectual barrier, but Southern Italy and Greece had become disconnected. Some time after, Sicily and Southern Italy also became separated. Meanwhile the stream of migrants which consisted less and less of typically southern forms, emigrants from Central Asia and even Southern Siberia, mingled with the southern forms on their way to Europe, and these now poured across the newly opened plain of Central and Northern Europe. But it was not until some time after this that the Mediterranean Sea broke across the Ægean region, and that the Northern Sea retired from the plains of Eastern Russia to admit the typical Siberian fauna and flora into our continent (vide pp. [189]-[241]).
I cannot close this chapter without referring to the active distributional centre—or I might say, centre of origin—of species situated in South-eastern Europe. No group of animals is more instructive in elucidating the paths of migration from this centre than the terrestrial mollusca. Wherever the original home of the genus Clausilia may have been in early Tertiary times, it is certain that the most active centre of origin is now, and has been for a considerable time past, in South-eastern Europe. One of the earliest migrants from that modern centre of this interesting genus is Clausilia bidentata, which is the only species found in Southern Spain, and one of the two met with in Ireland, and which has been observed in high altitudes in the Alps and in Scandinavia. As we go eastward from Western Europe the number of species of Clausilia, as we have seen, increases until we reach a maximum in the Balkan peninsula and the region of the Caucasus. Limax, Agriolimax, and Amalia, three genera of slugs, likewise appear to have originated in the same region and spread over Europe from there. Some species like Limax maximus and L. marginatus are very ancient, and probably commenced their wanderings in early Tertiary times. In this manner many animals of European origin have joined the Oriental migrants in their westward and also in their later northward travels. In a similar way species of plants and animals of Alpine origin might have joined these migrants in their northward course, and it is only when we come to carefully analyse the constituent parts of all these members which have come to us in England from the south, that we realise the complexity of their origin. Finally, even the Siberian migrants mingled with the later Oriental ones, and in some cases the decision as to whether a certain species belongs to the former or to the latter migration becomes a matter of great difficulty.