ies of the globe, and that this fraction is only
imperfectly known. With respect to the fewness of the organisms found in
the Silurian and other Palaeozoic formations, there is less difficulty,
inasmuch as (besides their gradual obliteration) we can expect
formations of this vast antiquity to escape entire denudation, only when
they have been accumulated over a wide area, and have been subsequently
protected by vast superimposed deposits: now this could generally only
hold good with deposits accumulating in a wide and deep ocean, and
therefore unfavourable to the presence of many living things. A mere
narrow and not very thick strip of matter, deposited along a coast where
organisms most abound, would have no chance of escaping denudation and
being preserved to the present time from such immensely distant
ages{316}.

{315} <Pencil insertion by the author.> The parent-forms of Mollusca
would probably differ greatly from all recent,--it is not directly
that any one division of Mollusca would descend from first time
unaltered, whilst others had become metamorphosed from it.

{316} _Origin_, Ed. i. p. 291, vi. p. 426.

If the several known formations are at all nearly consecutive in time,
and preserve a fair record of the organisms which have existed, my
theory must be abandoned. But when we consider the great changes in
mineralogical nature and texture between successive formations, what
vast and entire changes in the geography of the surrounding countries
must generally have been effected, thus wholly to have changed the
nature of the deposits on the same area. What time such changes must
have required! Moreover how often has it not been found, that between
two conformable and apparently immediately successive deposits a vast
pile of water-worn matter is interpolated in an adjoining district. We
have no means of conjecturing in many cases how long a period{317} has
elapsed between successive formations, for the species are often wholly
different: as remarked by Lyell, in some cases probably as long a period
has elapsed between two formations as the whole Tertiary system, itself
broken by wide gaps.

{317} <Note in original.> Reflect on coming in of the Chalk,
extending from Iceland to the Crimea.

Consult the writings of any one who has particularly attended to any one
stage in the Tertiary system (and indeed of every system) and see how
deeply impressed he is with the time required