on the coasts of France and of the North Sea; and to the facts disclosed
on the tide-washed flats of the latter shores by excavations in Halligs
or inhabited mounds which were probably raised before the era of the
Roman Empire. [Footnote: For a very picturesque description of the
Halligs, see Pliny, N.H., Book xvi, c. 1.] These remains are memorials
of races which have left no written records, which perished at a period
beyond the reach of even historical tradition. The plants and animals
that furnished the relics found in the deposits were certainly
contemporaneous with man; for they are associated with his works, and
have evidently served his uses. In some cases, the animals belonged to
species well ascertained to be now altogether extinct; in some others,
both the animals and the vegetables, though extant elsewhere, have
ceased to inhabit the regions where their remains are discovered. From
the character of the artificial objects, as compared with others
belonging to known dates, or at least to known periods of civilization,
ingenious inferences have been drawn as to their age; and from the
vegetable remains which accompany them, as to the climates of Central
and Northern Europe at the time of their production.

There are, however, sources of error which have not always been
sufficiently guarded against in making these estimates. When a boat,
composed of several pieces of wood fastened together by pins of the same
material, is dug out of a bog, it is inferred that the vessel, and the
skeletons and implements found with it, belong to an age when the use of
iron was not known to the builders. But this conclusion is not warranted
by the simple fact that metals were not employed in its construction;
for the Nubians at this day build boats large enough to carry half a
dozen persons across the Nile, out of small pieces of acacia wood pinned
together entirely with wooden bolts, and large vessels of similar
construction are used by the islanders of the Malay archipelago. Nor is