The History of the European Fauna

by R. F. Scharff

On page [60] will be found a map indicating the physical geography of that part of the ancient continent on which what are now the British Islands were situated. Only one large river has been marked on that map, namely, that flowing out of a lake which occupied part of the Irish Sea. Another probably discharged its waters into the Atlantic midway between France and England, whilst the Thames may have been a tributary of the Rhine, as it emptied itself into the sea near our south-east coast. I have shown in a previous essay that the former presence of a freshwater lake between England and Ireland is indicated by the distribution of the Charrs and also by the various species of British Coregonus. There are three British species of Coregonus, viz., C. clupeoides, C. vandesius, and C. pollan. These are confined to the lakes of North Wales, North-western England, South-western Scotland, and Ireland. All but the latter communicate at present directly with the Irish Sea. The lakes of the latter country, however, must have done so at a time when the west of Ireland stood at a higher level than it does now. The ancestors of the three Coregonus species, and also those of the Charrs, then lived in the large freshwater lake indicated on the map (p. [60]), and when the sea gradually crept up the river valley and finally converted the lake into a gulf, the freshwater fish took refuge in the rivers which supplied it with water.

Now as for the continuous sea-shore between the coast of Brittany and the south-west of Ireland, zoological distribution again aids us in proving that such must have actually existed at no very distant geological date. Most of our common shore forms of life migrate along the coast exactly as land animals do—step by step. Their eggs are carefully attached to fixed objects, so as not to be carried away by the waves, whilst the young often remain and grow old in some particular little pool, rarely venturing farther than a few yards from the spot where they first saw the light of day. A number of such shore forms are found on the west coast of France, the same species recurring again on the south-west coasts of England and Ireland, thus clearly indicating a former continuity of coast-line between these points, now separated by deep sea. A very familiar example to British zoologists is the purple rock-boring Sea-urchin (Strongylocentrotus lividus), but there are a great many others, such as the semi-marine Beetles Octhebius Lejolisii and Æpophilus Bonnairei, the Crustaceans Achæus Cranchii, Inachus leptochirus, Gonoplax angulata, Thia assidua, Callianassa subterranea, the Fishes Blennius galerita and Lepadogaster Decandollii, and the Molluscs Otina otis, Donax politus, and Amphidesma castaneum.

Before concluding this chapter, a few words as to my views on the conditions prevailing during the Glacial period will not be out of place. They do not differ very much from those held formerly by most geologists; and even at present there are, as I have mentioned before, a few upholders of those older views.

The sea, I think, must have gradually crept across England from the east during, or shortly after, the Forest-Bed period, so as to separate the south from the north, whilst Ireland and Scotland were then still connected with one another. At a later stage, the sea also partially invaded Ireland, and this condition is very roughly represented on the accompanying map. Mr. Kendall kindly drew my attention to the fact that several notable areas on which shelly drift has been observed are here placed upon the land; but it must be remembered that one stage only can be shown on the map, and that the sea covered more ground a little later. Many of the smaller islands in the glacial sea, too, are not shown. The map, in fact, is merely meant to give a general idea of the manner in which the great northern sea moved westward and slowly covered a large portion of the British Islands. These peculiar geographical conditions explain, I think, better than anything, the absence from parts of the Midlands and the north of England of such a number of terrestrial invertebrates which are otherwise widely distributed over the British Islands. In spite of the fact that a large portion of the British Islands became submerged, we possessed at that time an extensive area which has since been claimed by the sea, so that there was ample room for the present fauna to survive the Glacial period. The climate during this period was probably much the same as it is at present, though moister, with cooler summers and milder winters.

Fig. 6.—Map of the British Islands, showing approximately in what manner the sea may have invaded the country from the east during, or shortly after, the Forest-Bed period. The darkly shaded parts indicate the areas covered by water, and the lightly shaded and white portions what was land at that time.

It may be asked what proof we have of such an extensive submergence of England and Ireland. My own views are principally based on the general distribution of the fauna in the British Islands, and the belief that nothing but a mild climate during the Glacial period could have brought it about. On purely geological grounds, however, some geologists, notably Mr. Mellard Reade, have come to a similar conclusion. "The whole of Lancashire and Cheshire," he remarks (a, p. 542), "from sea-level up to about 400 feet, and in places 600 feet, is covered by a continuous mantle of boulder-clay and sands." "These clays, as a rule, contain distributed through them, in a greater or less degree, fragments of shells and some perfect ones. I myself have recorded forty-four species." Again he continues (pp. 545 and 546): "A large part of Ayrshire is covered with similar shelly boulder-clays from sea-level to 1061 feet at Dippal. These Ayrshire high-level shells have, in the majority of cases, been taken, not from sand and gravel beds, but from boulder-clay, and in that respect they are most important and unique. In Moel Tryfan the shells are found in sands and gravels at 982 feet; on the range of hills from Miaera to Llangollen from 1000-1200 feet; also in sands and gravels at Gloppa, near Oswestry, at 1100-1200 feet; and near Macclesfield at a level of about 1200 feet. In Ireland marine shells can be traced almost from sea-level to a height of over 1000 feet."

"Again," continues the same author, "if we look broadly at the distribution of these shelly deposits, we find that they occur all round our maritime coasts in Lancashire, Cheshire, and Wales, in Cumberland and Westmoreland, Wigtonshire and Ayrshire, and along the eastern coast of Ireland. The same is to be said of the eastern coasts of England and Scotland."

That a very considerable change of sea-level has taken place in some parts of the British Islands would appear to a zoologist the most logical conclusion after an examination of these "high-level shelly sands and gravels," but the shells contained in them are now generally supposed to have been carried there frozen in the sole of a glacier or pushed up in front of it. The older view, however, which agrees so much better with the facts of distribution, fortunately has not disappeared among geologists. "When we call up," says Mr. Mellard Reade (b, p. 435), "before our mental vision the simple and well-known facts of nature which suffice to explain the marine drifts on the theory of submergence, it seems unnecessary to resort to the ingenious and artificial system of physics elaborated to explain the phenomena of land-ice."