All the British reptiles and amphibia appear to have reached us from the south or east, but among the fishes there are a good many northern forms. The whole salmon family—the Salmonidæ—are typical northern immigrants. The Stickleback (Gasterosteus aculeatus), too, has undoubtedly come to us from the north. The genus Cottus, like Gasterosteus, is certainly Arctic in origin. Originally freshwater forms, many species are now found between tide-marks, and of these a few have migrated southward along the coasts of the great continents. Thus we meet with various species of Cottus as far south as California and Japan, on the American and Asiatic coasts of the Pacific respectively. In Europe, two species, viz., C. scorpio and C. bubalis, range as far south as the French coast. Our freshwater Cottus, the Miller's thumb (Cottus gobio), has migrated to us from the north with the Arctic species. All the freshwater forms, indeed, of this genus are typically Arctic.
A large number of land and freshwater invertebrates too have no doubt reached us from the north. Some of them may have originated in Scandinavia or within the Arctic Circle, but others probably came still farther, either from America or even from Asia, and used the Arctic land-connection viâ Greenland in their migration to Europe. As I shall give a number of additional instances of such migrants in the succeeding chapters, I need not, perhaps, dwell upon them now any longer, except to mention a few of the more typical ones. Vertigo alpestris, a minute snail with an amber-coloured shell, and our freshwater pearl-mussel, Unio (Margaritana) margaritifer, belong to this migration. Then among butterflies we may cite the Marsh-ringlet (Coenonympha typhon), and among beetles, Pelophila borealis and Blethisa multipunctata. There are a number of northern spiders, among which a few certainly indicate an Arctic origin, or at any rate, that they have wandered to Europe across Greenland and the old Arctic land-connections. Bathyphantes nigrinus, Linyphia insignis, and Drapetisca socialis, for instance, are three British species whose range indicates a northern origin, and which also occur, according to Mr. Carpenter, in North America. Mr. Carpenter also tells me that the Collembolan, Isotoma littoralis, is a typical northern migrant. He has recently discovered it in the west of Ireland, its only station in the British Islands.
Among the crustacea, the genus Apus forms an exceedingly interesting illustration of the northern migration, Apus glacialis having been discovered in a Scottish pleistocene freshwater deposit, whilst it is now almost confined to the Arctic regions.
To the same group of animals also belong the three remarkable species of freshwater sponges, Ephydatia crateriformis, Heteromeyenia Ryderi, and Tubella pensylvanica, which Dr. Hanitsch has described from some lakes in Western Ireland. None of these are known from Great Britain or from the continent of Europe. A few North American plants grow wild in the same district. That any of these should owe their existence in Ireland to accidental introduction appears to me exceedingly improbable. In a former contribution to this subject (a, p. 475) I assumed that these American plants and animals had migrated to Europe at the same time as the other northern forms referred to. My friend Mr. Carpenter, however, takes exception to this (p. 383), and I quite recognise the force of his argument. "Their very restricted and discontinuous ranges," he says, "along the extreme western margin of Europe mark them as decidedly older than those northern animals and plants which have a circumpolar distribution." We have indeed quite similar examples in the Oriental migration, of which part is very ancient, surviving here and there and exhibiting discontinuous distribution. We may therefore look upon these American immigrants as among the oldest members of that northern stock which have survived in our islands—probably a mere remnant of a once luxuriant flora and fauna.
In order to show the importance of the Eastern or Siberian element in the English, or, we might say with Dr. Sclater, the Anglo-Scotian mammalian fauna, I herewith give a list of the species of mammals which probably migrated to Great Britain from Siberia. I have marked with an asterisk those which still exist in this country (not in Ireland), or have become extinct within historic times.
We have evidence that most of these twenty-six species of mammals came from Eastern Europe, but there is no reason to suppose that they originated there. On the contrary, it is highly probable, as I said before, that their native home is Siberia, and that they entered Europe to the north of the Caspian. Along with these, vast numbers of other forms of life, and also plants, swarmed into our continent, and as we advance eastward from England we meet with them in increasing numbers to the present day. But not only on the Continent do we find these survivals of the great Siberian migration, which has been so ably described by Professor Nehring; no less than nine species still inhabit Great Britain (if we include the recently extinct Beaver). On the other hand, not more than three have been found fossil in Ireland, and of these only one still survives. This very significant fact will be referred to again more fully on p. [153]. Meanwhile it should be remembered that these three species, viz., Mustela erminea, Equus caballus, and Rangifer tarandus, occur in Ireland in varieties distinct from those found in Central Europe. It is upon this, and many other circumstances, that I founded my belief that Ireland was already separated from England at the time of the arrival of the Siberian emigrants in the latter country. As we shall see, the Irish Stoat, Horse, and Reindeer probably came by a different route from that taken by the English representatives of the same species.
Very few of the lower animals of Siberian origin have reached the British Islands. Most of those which were formerly thought to be Siberian are either of East European or of Central and South Asiatic origin, though they probably joined the Siberian migration on their way to England. The Arctic migration brought a greater variety of species to this country than the Siberian, but neither the one nor the other has contributed more than a small percentage to the British fauna. The bulk of that fauna is derived from the various European centres of dispersal, and especially from Central and Southern Asia.
Those animals which have their home in the latter area, I have named Orientals, though it must be remembered that they need not necessarily have come from what is known among zoologists as the "Oriental Region." The terms "Oriental animals" and "Oriental migration" are used here in a wider sense, and include even those species which reached Central and Northern Europe from South-Eastern Europe. It is astonishing, what a vast number of both vertebrate and invertebrate animals can be traced back to this Oriental migration. Great tracts of Europe were repeatedly submerged beneath the sea during Tertiary times, and on their re-appearance were formed into green fields and pastures new for the rich Asiatic fauna, which was ever ready to flood the neighbouring continent. This went on, and not for a comparatively short space of time, as in the case of the Siberian invasion; the immeasurable ages which passed, whilst several of the Tertiary epochs dawned upon Europe, witnessed an almost constant stream of Asiatic immigrants pouring in upon us. Europe returned her own products in exchange, but they must have been scanty in comparison to the enormous number of species which have undoubtedly originated in Central and Southern Asia. Very many of the widely distributed forms in the British Islands are of Oriental origin. Among these are also the cosmopolitan species, such as the Barn Owl (Strix flammea) and the Painted Lady Butterfly (Vanessa cardui). A great number of our British Mammals, Birds, Butterflies, and Beetles have come to us with the Oriental migration. But, as I shall explain in the special chapter devoted to it, the earlier migrants from the south-east found their northward progress barred by a great sea which stretched through Central Europe from west to east. The Mediterranean was then divided into two smaller basins. On their arrival in Greece, which was then connected with Asia Minor and Southern Italy, the Oriental migrants seem to have turned westward, skirting the shores of the Mediterranean. When they finally reached Spain, many then changed their course northward (see [Fig. 5], p. [117]) and wandered to the British Islands with the Lusitanian animals which came from South-Western Europe.