creatures feed. Protected by this constant element the geographical range
of these animals and plants is as remarkable as their high antiquity.
There are in lake Tanganyika or the rivers of Japan exactly the same kinds
of shells as in the Thames, and the sedges and reeds of the Isis are found
from Cricklade to Kamschatka and beyond Bering Sea to the upper waters of
the Mackenzie and the Mississippi. The Thames, our longest fresh-water
river, and its containing valley form the largest natural feature in this
country. They are an organic whole, in which the river and its tributaries
support a vast and separate life of animals and plants, and modify that of
the hills and valleys by their course. Civil law has recognised the Thames
system as a separate area, and given to it a special government, that of
the Conservators, whose control now extends from the Nore to the remotest
springs in the hamlets in its watershed; and natural law did so long
before, when the valley became one of the migration routes of certain
southward-flying birds. Its course is of such remote antiquity that there
are those who hold that its bed may twice have been sunk beneath the sea,
and twice risen again above the face of the waters.[1] It has ever been a
masterful stream holding its own against the inner forces of the earth;
for where the chalk hills rose, silently, invisibly, in the long line from
the vale of White Horse to the Chilterns the river seems to have worn them
down as they rose at the crossing point at Pangbourne, and kept them
under, so that there was no barring of the Thames, and no subsequent
splitting of the barrier with gorges, cliffs, and falls. Its clear waters
pass from the oolite of the Cotswolds, by the blue lias and its fossils,
the sandstone rock at Clifton Hampden, the gravels of Wittenham, the great
chalk range of the downs, the greensand, the Reading Beds, to the
geological pie of the London Basin, and the beds of drifts and brick earth
in which lie bedded the frames and fragments of its prehistoric beasts. In
and beside its valley are great woods, parks, downs, springs, ancient
mills and fortresses, palaces and villages, and such homes of prehistoric