Now we may enquire what are the limits within which we may speculate as to the age of the Fen Beds.
These Turbiferous deposits all belong to one stage, though it may be one of long duration. They are sharply separated from the Areniferous deposits, i.e. the sands and gravels of the terraces and spurs which always pass under and, in fairly large sections, can always be clearly distinguished from the resorted layers at the base of the Fen Beds.
There is no definite chronological succession which will hold throughout the Fens. The variations observed are geographical--clay, marl, peat, etc., alternating in different order in different localities and subaerial, fluviatile, estuarine, and marine, having only a changing topographical significance.
The Fen Beds crept over an area where the underlying formation had been undergoing vicissitudes due to slow geographical changes--changes which, being at sea level and near the conflict of tides and upland water, produced irregular but often important results.
There is not in the Fens any _continuous_ record of what took place between the age in which the Little Downham Rhinoceros was buried in the gravel and that in which the Neolithic hunters poleaxed the Urus in the peat near Burwell.
Nor do we find any constant succession in the fauna and flora in the sections in the Fens any more than we find a uniform distribution of plants and animals over the surface to-day. The most numerous and largest specimens of the Urus I have obtained from near Isleham: the best preserved Beaver bones from Burwell. Modern changes of conditions have limited the district in which the fen fern (_Thelypteris_) or the swallow-tailed butterfly may now be seen; but nature in old times produced as great changes in local conditions as those now due to human agency.
When we compare the fauna of the Areniferous Series with that of the Turbiferous, although there is not an entire sweeping away of the older vertebrate and invertebrate forms of life and an introduction of newer, there is a marked change in the whole facies.
There is plenty of evidence about Cambridge of the gradual extermination of species still going on. Indeed, I feel inclined to say that there is no such thing as a Holocene age. I remember land shells being common of which it is difficult now to find live specimens, and my wife[6] has shown how the mollusca are being differentiated in isolated ponds left here and there along the ancient river courses above the town.