For instance, the several
species of the Chthamalinae (a sub-family of sessile cirripedes) coat
the rocks all over the world in infinite numbers: they are all
strictly littoral, with the exception of a single Mediterranean
species, which inhabits deep water and has been found fossil in
Sicily, whereas not one other species has hitherto been found in any
tertiary formation: yet it is now known that the genus Chthamalus
existed during the chalk period. The molluscan genus Chiton offers a
partially analogous case.
With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence from fossil remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging to either of these vast
periods, with one exception discovered by Sir C. Lyell in the
carboniferous strata of North America. In regard to mammiferous
remains, a single glance at the historical table published in the
Supplement to Lyell's Manual, will bring home the truth, how
accidental and rare is 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 the imperfection in the geological record mainly results from
another and more important cause than any of the foregoing; namely,
from the several formations being separated from each other by wide
intervals of time.
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