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 for its accumulation{318}. Reflect on the years elapsed in many cases, since the latest beds containing only living species have been formed;--see what Jordan Smith says of the 20,000 years since the last bed, which is above the boulder formation in Scotland, has been upraised; or of the far longer period since the recent beds of Sweden have been upraised 400 feet, what an enormous period the boulder formation must have required, and yet how insignificant are the records (although there has been plenty of elevation to bring up submarine deposits) of the shells, which we know existed at that time. Think, then, over the entire length of the Tertiary epoch, and think over the probable length of the intervals, separating the Secondary deposits. Of these deposits, moreover, those consisting of sand and pebbles have seldom been favourable, either to the embedment or to the preservation of fossils{319}. {318} _Origin_, Ed. i. p. 282, vi. p. 416. {319} _Origin_, Ed. i. pp. 288, 300, vi. pp. 422, 438.