BOTANY fontana, Prunus spinosa, Poterium officinale, Pyrus torminalis (?), Hippuris "vulgaris, Myriophyllum, Cornus sanguinea, Sambucus nigra, Eupatorium cannabinum, Fraxinus excelsior, Menyantbes trifoliata, Lycopus europeeus, Ajuga reptans, Alnus glutinosa, Quercus robur, Ceratopbyllum demersum, Sparganium, Potamogeton crispus (and two other species of the genus), Naias marina, Scirpus lacustris (and one other species), Carex. Several species of Chara also occur. Mr. Reid remarks : ' Such trees as the oak, ash, sloe, cornel, elder, and alder point unmistakably to a temperate climate, and the fauna and flora as a whole suggest climatic conditions not differing greatly from those we now enjoy. . . . The occurrence of Naias marina, now only found in Britain in two of the Norfolk Broads, is singular, though the plant was evidently more common in former times than it is at the present day.' This is the only plant on the list which is not now found in the county, and with this exception the whole of the plants are common or fairly common with us ; more than half the number are of the generally diffused or British type, two (Pyrus torminalis and Naias marina) are exclusively English, one (Cornus sanguinea) is nearly so, and the rest are mainly British but more frequent in England than in the rest of the British Isles. By ' exclusively English ' is meant confined to England in Britain, for all are continental, and all but Naias marina, which is a French and south German plant only, are widely diffused over the continent. There is one point of great interest in this assemblage of plants, corroborating other evidence of the change which has taken place in our climate. All the herbaceous species are hygrophilous or moisture- loving, or actually water-plants, while one at least of the trees, the alder (Alnus glutinosa} , grows only in wet places (on river-banks or in marshes). Here we have an indication of very different conditions from those which now prevail in the neighbourhood of Hitchin ; the lake, the swamp, and the moist woods of this bygone period having given place to the dry gravelly hills and open chalk downs which are so characteristic of the north of Hertfordshire. While climate has by far the greatest influence upon the distribu- tion of plants, that exercised by geological formations is next in import- ance, and it should not be overlooked that geological formations have an influence upon climate. On a damp soil, especially in a well-wooded district, more rain will fall than on a dry soil, which will naturally tend to be a barren one. When the Reading Beds and London Clay extended over the whole of Hertfordshire and perhaps the greater part of the county was forest or swamp, the rainfall would be heavier and the temperature would probably be lower than at a later time, when the greater part of the clays and sands of these formations had been carried away, exposing the chalk beneath them, and when beds of permeable gravel were deposited upon both clay and chalk. The subsoil, originally eugeogenous, that is abrading easily and yielding much detritus, would give place to a subsoil of a dysgeogenous nature, that is disintegrating 45
