view of the bottom of the sea not rarely lying for ages in an
unaltered condition." Next, as regards littoral animals, he shows the
difficulty which they must have in becoming fossils, and gives a
striking example in several of the existing species of a sub-family of
cirripedes (_Chthamalinae_), "which coat the rocks all over the world in
infinite numbers," yet, with the exception of one species which inhabits
deep water, no vestige of any of them has been found in any tertiary
formation, although it is known that the genus _Chthamalus_ existed
through the Chalk period. Lastly, "with respect to the terrestrial
productions which lived through the secondary and palaeozoic periods, it
is superfluous to state our evidence is fragmentary in an extreme
degree. For instance, until recently not a land shell was known
belonging to either of these vast periods," with one exception; while,
"in regard to mammiferous remains, a glance at the historical table in
Lyell's Manual will bring home the truth, how accidental and rare has
been their preservation, far better than pages of detail. Nor is their
rarity surprising, when we remember how large a proportion of the bones
of tertiary mammals have been discovered either in caves or in
lacustrine deposits; and that not a cave or true lacustrine bed is known
belonging to the age of our secondary or palaeozoic formations."

But perhaps of even more importance than all these known causes which
prevent the formation of fossils, is the existence of unknown causes
which make for the same result. For example, the Flysch-formation is a
formation of several thousand feet in thickness (as much as 6000 in some
places), and it extends for at least 300 miles from Vienna to
Switzerland; moreover, it consists of shale and sandstone. Therefore,
alike in respect of time, space, and character, it is just such a
formation as we should expect to find highly rich in fossils; yet,
"although this great mass has been most carefully searched, no fossils,
except a few vegetable remains, have been found."

So much then for the difficulty, so to speak, which nature experiences
in the manufacture of fossils. Probably not one per cent. of the species
of animals which have inhabited the earth has left a single individual
as a fossil, whereby to record its past existence.

But of even more importance than this difficulty of making fossils in
the first instance, is the difficulty of preserving them when they are
made