‘I seem to see,’ then, an epoch after those strata were laid down with which geology generally deals; after the Kimmeridge clay, Oxford clay, and Gault clay, which form the impervious bedding of the fens, with their intermediate beds of coral-rag and green sand, had been deposited; after the chalk had been laid on the top of them, at the bottom of some ancient ocean; after (and what a gulf of time is implied in that last ‘after!’) the boulder-clay (coeval probably with the ‘till’ of Scotland) had been spread out in the ‘age of ice’ on top of all; after the whole had been upheaved out of the sea, and stood about the same level as it stands now: but before the great valley of the Cam had been scooped out, and the strata were still continuous, some 200 feet above Cambridge and its colleges, from the top of the Gog-magogs to the top of Madingley Rise. In those ages—while the valleys of the Cam, the Ouse, the Nene, the Welland, the Glen, and the Witham were sawing themselves out by no violent convulsions, but simply, as I believe, by the same slow action of rain and rivers by which they are sawing backward into the land even now—I ‘seem to see’ a time when the Straits of Dover did not exist—a time when a great part of the German Ocean was dry land. Through it, into a great estuary between North Britain and Norway, flowed together all the rivers of north-eastern Europe—Elbe, Weser, Rhine, Scheldt, Seine, Thames, and all the rivers of east England, as far north as the Humber. And if a reason be required for so daring a theory—first started, if I recollect right, by the late lamented Edward Forbes—a sufficient one may be found in one look over a bridge, in any river of the East of England. There we see various species of Cyprinidæ, ‘rough’ or ‘white’ fish—roach, dace, chub, bream, and so forth, and with them their natural attendant and devourer, the pike. Now these fish belong almost exclusively to the same system of rivers—those of north-east Europe. They attain their highest development in the great lakes of Sweden. Westward of the Straits of Dover they are not indigenous. They may be found in the streams of south and western England; but in every case, I believe, they have been introduced either by birds or by men. From some now submerged ‘centre of creation’ (to use poor Edward Forbes’s formula) they must have spread into the rivers where they are now found; and spread by fresh water, and not by salt, which would destroy them in a single tide.