One of the most important results of recent research in the subject
has been the establishment of the existence of man in interglacial
times. When Lyell wrote, it was not fully recognised that the
glaciation of Europe was not one continuous process, but that it
could be divided into several episodes, glaciations, or advances of
the ice, separated by a warm interglacial period. The monumental
researches of Penck and Bruckner in the Alps have there established
four glaciations with mild interglacial periods, but all of these
cannot be clearly traced in Britain. One very important point also
is the recognition of the affinities of certain types of
Palaeolithic man to the Eskimo, the Australians, and the Bushmen of
South Africa. However, it is impossible to give here a review of
the whole subject. Full details of recent researches will be found
in the works mentioned in the notes at the end of the book.

Another point of great interest and importance, arising directly
from the study of early man is the nature of the events
constituting the glacial period in Britain and elsewhere. This has
been for many years a fertile subject of controversy, and is likely
to continue such. Lyell, in common with most of the geologists of
his day, assumes that during the glacial period the British Isles
were submerged under the sea to a depth of many hundreds of feet,
at any rate as regards the region north of a line drawn from London
to Bristol. Later authors, however, explained the observed
phenomena on the hypothesis of a vast ice-sheet of the Greenland
type, descending from the mountains of Scotland and Scandinavia,
filling up the North Sea and spreading over eastern England. This
explanation is now accepted by the majority, but it must be
recognised that it involves enormous mechanical difficulties. It is