The History of the European Fauna

by R. F. Scharff

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

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

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

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

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

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

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

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

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