48 Mr. Henry Walker's Lecture:
very few years of continuously equable climate would have
sufficed for the change of habitat. How near Essex and
Middlesex were to the southern migrants is shown by the
presence of the fluviatile 'Cyrena in the tepid waters of the
Thames.*
Such, then, were the geographical conditions and climatal
fluctuations in Pleistocene Britain. Familiar as we are
with the far different conditions of our country to-day—
with the isolation of Britain from the Continent, and the
ascendancy of man over the animal world—it is difficult
to realise the Britain of this earlier period. Yet this
union of England with the Continent, this overlapping in
Essex and Middlesex of the range of the Arctic and sub-
tropical fauna, exceptional and abnormal as it at first sight
appears, may have lasted for a very considerable length of
time. It will hardly be doubted by geologists that this
continental stage of our country's history far exceeded in
* "When the temperature of the river water was congenial to the
Cyrena above mentioned it was also suited to the hippopotamus."—
Sir Charles Lyell: "Principles of Geology," 10th edit., vol. 1, page
192. In the year 18G3 the author of the "Principles" propounded
that interpretation of the zoological phenomena in question which is
now generally accepted. It is summed up in the last edition of the
"Elements of Geology," for 1871, p. 138 : ". . . The apparently conflict-
ing nature of the evidence may be due to the place of our observations
being near the boundary line of a northern and southern fauna, either
of which may have advanced or receded during comparatively slight
or temporary fluctuations of climate." A valuable collection of
zoological data in support of this view has been collected by Mr. Boyd
Dawkins, and is now published in his "Early Man in Britain,"
1880. The Stratigraphical evidence is a far different matter, involving
more difficult and onerous labours, and when this shall have been
completed, and the exact succession of [geographical events and cli-
matal phases of the period in question be detailed to us, some very-
important lacunae in the history of the northern and southern fauna
of the old Thames Valley will have been filled up.— [In order to show
in a general way the relation of the various beds forming the country
around Epping, I have given (see frontispiece) five lines of sections
which I have taken from a series kindly lent to me by Mr. Searles V.
Wood, Jun. The beds numbered 6 and 8 differ somewhat, inter se,
(in a way which is beyond the scope of this lecture to describe), but
they bear that relation to 7 which their numbers import.]