A contribution to our knowledge of the origin of the British flora

A lecture delivered lately by Dr. W. G. N. van der Sleen at a meeting of the Royal Dutch Geographical Society on the so-called Cromer Forest Bed has revived in this country the interest in an old theory, advocated by Prestwitch, Harmer and others, according to which many thousands of years ago, in an age when the North Sea was still land, the Rhine took its course through the east part of England, entering it near Walton on the coast of the county of Essex, to the south of Harwich, and leaving England again after passing in a northerly direction through Essex, Suffolk and Norfolk at Cromer on the north-coast of the last-named county. With regard to this I have asked myself, whether it would not be possible to find in the distribution of certain species of plants in England and on the Continent some arguments in favour of the above-mentioned theory, upheld energetically by Van der Sleen. Any Dutch botanist of experience knows, that the valleys of our big rivers Meuse and Rhine are characterized by certain species of plants, which strictly follow the beds of these rivers, rarely-if ever-occur in other places and the distribution of which is obviously dependent on them. If it could be proved, that these species occur all or partially in England too, and this strictly locally in the counties of Essex, Suffolk and Norfolk, in the vicinity of the hypothetical former river-bed, then doubtless an important argument, if not definite evidence would be hit upon. The reader may judge for himself the value of what I as a botanist have to say hereunder on the problem, which interests us here, basing myself amongst others specially on the information, which Prof. A. G. Tansley of Cambridge, Englands wellknown plant-geographer, has been so kind as to furnish me. In order to make the following quite clear, I intend first of all to say a few words about the geological aspect of the problem. Harmer has found between Walton and Cromer, in a soil which in other respects does not show much variety, a broad and winding strip of river-clay and in its neighbourhood a peculiar kind of small white pebbles, such as are found also in the southern parts of our country. In this way he -became an advocate of the idea, that the Rhine might at one time have flowed through the eastern counties of England. Van der Sleen finds back the river-bed near Walton as well as near Cromer. According to him, its position is fairly high near Walton, which is possible, as the soil may have raised itself in course of time, and it is based on an underground, which, as appears from the fossils it contains, is evidently of late tertiary, late pliocen origin. Near Cromer its position is lower and here especially it is buried under the deposits of the ice-age. Van der Sleen concluded, that doubtless during the transition-period from the tertiary to the ice-age the Rhine took its course through England, being enabled to do so by the circumstance, that the North Sea, which formerly already extended more to the south, was then dry till far to the north.