Seite - 341 - in The Origin of Species

PALyEONTOLOGICAL COLLECTIONS 341 of time, by another and later formation, without the under- lying bed having suffered in the interval any wear and tear, seem explicable only on the view of the bottom of the sea not rarely lying for ages in an unaltered condition. The remains^ which do become embedded, if in sand or gravel, will, when the beds are upraised, generally be dissolved by the percola- tion of rain-water charged with carbonic acid. Some of the many kinds of animals which live on the beach between high '' and low water mark seem to be rarely preserved. For in- stance, 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 excep- tion of a single Mediterranean species, which inhabits deep water, and this has been found fossil in Sicily, whereas not one other species has hitherto been found in any tertiary formation ; yet it is known that the genus Chthamalus ex- isted during the Chalk period. Lastly, many great deposits requiring a vast length of time for their accumulation, are entirely destitute of organic remains, without our being able to assign any reason: one of the most striking instances is that of the Flysch formation, which consists of shale and sandstone, several thousand, occasionally even six thousand feet in thickness, and extending for at least 300 miles from Vienna to Switzerland; and although this great mass has been most carefully searched, no fossils, except a few vege- table remains, have been found. With respect to the terrestrial productions which lived during the Secondary and Palaeozoic periods, it is superfluous to state that our evidence is fragmentary in an extreme de- gree. For instance, until recently not a land-shell was known belonging to either of these vast periods, with the exception of one species discovered by Sir C. Lycll and Dr. Dawson in the carboniferous strata of North America ; but now land- shells have been found in the lias. In regard to mammifer-'~^ ous remains, a glance at the historical table published in Lyell's Manual wmII 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