276 THE ORIGIN OF SPECIES 



bespeaks its purity. The many cases on record of a formation 

 conformably covered, after an immense interval of time, by an- 

 other and later formation, without the underlying 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 percolation of rain water charged with car- 

 bolic acid. Some of the many kinds of animals which live on the 

 beach between high and low water mark seem to be rarely pre- 

 served. 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 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 existed 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 forma- 

 tion, 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 vegetable 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 degree. For in- 

 stance, until recently not a land-shell was known belonging to 

 either of these vast periods, with the exception of one species dis- 

 covered by Sir C. Lyell and Dr. Dawson in the carboniferous 

 strata of North America; but now land-shells have been found 

 in the lias. In regard to mammiferous remains, a glance at the 

 historical table published in 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 re- 

 member 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 largely results 

 from another and more important cause than any of the fore- 



