The History of the European Fauna Part 12

Such a state of close and permanent friendship is called "commensalism."

Now it appears as if the members of this family of beetles (_Clavigeridae_) 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 antennae, with which they ungraciously beat their hosts, when they are in want of food.

According to some authorities, they even occasionally gnaw at the pupae and larvae 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 _Clavigeridae_ 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 j.a.pan, and southward to Madagascar and South Africa. The genus _Hopatroides_, belonging to the same family as the so-called Spanish-fly (_Tenebrionidae_), 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 a.s.sume 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 Gra.s.shoppers 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, j.a.pan, 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. Poc.o.c.k, 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 pontic.u.m_) 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 aegean 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 aegean 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 acc.u.mulation 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 sh.o.r.e 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 sh.o.r.e 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 aegean 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 sh.o.r.es of the Sea of Asov, so that the direct northward pa.s.sage 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 Deperet 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 a.s.sume 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 a.s.suming 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 Chamaeleon (_Chamaeleon 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 _Bla.n.u.s cinereus_ inhabits some of the Greek islands, North Africa, and Spain. Another Lizard belonging to the _Scincidae_ 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. Rutimeyer 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 Tunesie, on se croirait sur le meme 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 aegean 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 alt.i.tudes 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 a.n.a.lyse the const.i.tuent 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.

SUMMARY OF CHAPTER VI.

Like the last chapter, this deals with the Asiatic migrants. But while the former described the history of the northern invasion, those animals which entered Europe from the south-east are here more particularly referred to. They originated in Central, Southern, and Western Asia. It is not easy to discriminate in all cases between this Oriental migration and the Siberian. To a certain extent, even an entry of Northern Asiatic species has taken place by the southern route, and _vice versa_. On the other hand, southern species might have come to Europe by the southern route--that is to say, to the south of the Caspian--and also by the northern, which lay to the north of that great inland sea. The Red Deer is a good example. It arrived on our continent by both routes. However, there is a racial difference in the members of the two migrations. The small race now found in Corsica, Sardinia, North-west Africa, and Western Europe, is probably the older of the two, while the larger one--resembling the American Wapiti Deer--arrived very much later from Siberia.

The Mammoth, Wild Boar, Badger, the Dippers and Pheasants, are all Oriental species which have come to us from the south-east; but there are also Reptiles and Amphibians, and a host of Invertebrates. Not all the animals, for instance, which have reached us in England from the south-east are of Asiatic origin. There is an active centre of distribution in South-eastern Europe itself, from which species radiate out in all directions. This fact is well ill.u.s.trated by the genus _Clausilia_. Species from this centre, and also from the Alps, joined the Oriental stream in their northward course.

In reviewing a number of instances of Oriental species in Europe, one is struck by the peculiarity of their having apparently followed two distinct routes. All entered from Asia Minor, which is proved to have been connected with Greece until recent geological times. From here some seem to have proceeded straight west, others northward. Further study reveals the fact that the first route was followed by a much older set of migrants at a time when the Mediterranean area was greatly different from what it is at the present day. Greece was then joined to Southern Italy, Sicily, and Tunis. The latter was also connected with Sardinia and Corsica, and the Straits of Gibraltar did not exist. Under such geographical conditions a direct migration on land from Southern Greece to Spain was not only possible, but was actually undertaken by a very large number of Oriental species.

FOOTNOTES:

[1] Since writing the above account, Mr. Boulenger, in his new work on the Batrachia of Europe, has accepted the specific distinctions between the two fire-toads.

[2] In some cases the accuracy of this view is proved by fossil evidence, _Helix rotundata_, a common and widely spread British species, having been found in miocene strata near Bordeaux.

[3] There are a great many instances of discontinuous distribution among Oriental Invertebrates. Thus the Freshwater Crab (_Thelphusa fluviatilis_) occurs in Southern Italy, Greece, Turkey, Cyprus, and Asia Minor. Another crustacean--a Freshwater Crayfish--(_Hemicaridina Desmaresti_) inhabits Spain, Corsica, Sardinia, Sicily, and Asia Minor.

CHAPTER VII.

THE LUSITANIAN FAUNA.

Under the Roman Emperor Augustus, the Spanish peninsula was divided into three provinces, one of which--namely Lusitania--occupied a large portion of the present area of Portugal. The term "Lusitanian" is therefore almost synonymous with Portuguese, but it has frequently been applied by zoologists and botanists in a much wider sense, so as to vaguely include the extreme south-west of Europe without any definite limits. Neither do I propose to restrict the term to everything found within the borders of Portugal. For the sake of convenience, we may designate as Lusitanian forms those animals and plants which have migrated to Central, Southern, or Northern Europe from South-western Europe. They may really be North-west African species, or they may have originated on land which lay to the west of Portugal, and which is now mostly buried beneath a deep sea. Nevertheless, we have received them from the extreme south-western portion of our continent--they have come to greater Europe from that direction.

In discussing the component elements of the British fauna and flora in the third chapter, I have already referred to the distinguishing characters of the Lusitanian migrants and to their distribution. I need only repeat, therefore, that these are now princ.i.p.ally confined to the south-western portions of the British Islands. The late Edward Forbes was the first to trace the Lusitanian flora to its native home. In his cla.s.sical memoir on the geological relations of the existing fauna and flora of the British Isles, he laid the foundations of a new method of research. We are as yet only beginning to realise the far-reaching conclusions obtainable by a careful study of the geographical distribution of animals and plants, though the lines of investigation were indicated by him more than fifty years ago. Forbes was of opinion that the Lusitanian element in the British flora was of miocene age, and that it survived the Glacial period on a now sunken land to the south-west of Ireland. Mr. Carpenter and myself agree in so far that we are both inclined to look upon this Lusitanian flora and the accompanying fauna in Ireland as of pre-glacial origin. But I am not quite satisfied that the Lusitanian migration ceased to come north then.

It may have received a temporary check; but the presence, for instance, of the Dartford Warbler (_Melizophilus undatus_) in the south-east of England would seem to indicate that its northward migration took place in very recent times. It is possible also that the very restricted occurrence of the Dartford Warbler may imply that it is gradually withdrawing towards its centre of origin from a former wider range. Such an eventuality, as we have seen, has actually taken place in a great number of instances.

It is not only in the British Islands that we perceive the influence of the Lusitanian element. Scandinavia, Russia--indeed almost every part of Europe--can boast of some migrants which have originated in South-western Europe or on the mysterious lands which lay beyond it. As a rule, however, we notice a marked decrease of Lusitanian species as we travel eastward from Western Europe. Nevertheless, certain forms have travelled far beyond the confines of our continent, and we certainly meet with them in Asia and Northern Africa.

It is remarkable that we are apt to mistake sometimes for Lusitanian migrants species which are of Oriental origin. In a previous paper I cla.s.sed such animals which had apparently originated in South-western Europe, but had really come from Asia by a circuitous southern route, with the Lusitanians. However, there is really no reason why the two should not be kept apart, provided we can discriminate between the pseudo-Lusitanians and the true ones. I have already indicated in the last chapter how these pseudo-Lusitanian migrants originated.

Supposing an Oriental species had left Asia for Europe in miocene times, it would on its arrival in Greece have had to decide between two courses. It could either advance into the newly-formed Alpine peninsula and there remain, or at once push on westward into Southern Italy, Sicily, and Tunis, by means of the old land-connections, and thence into Southern Spain. The Atlantic communicated at that time with the Mediterranean across the valley of the Guadalquivir; but that connection ceased to exist towards the end of the Miocene Epoch, when the Oriental migrants were free to ramble through Spain and the whole of the North European plain. I have indicated on a previous occasion (_a_, p. 484) that the earliest members of the Red Deer migration, which have left their traces in the caves of Malta, and whose descendants still live in Corsica, Sardinia, and North Africa, may have found their way to Northern Europe in this manner. Many other Asiatic mammals probably reached the British Islands in a similar way.