PAL^ONWLOGICAL COLLECTIONS. 305 



the sedimentary beds were removed whicli rest uiicoii- 

 formably on them, and which could not have formed part 

 of the original mantle nnder which they were crystallized. 

 Hence, it is probable that in some parts of the world wh(>le 

 formations have been completely denuded, witli not a wreck 

 left behind. 



One remark is here worth a passing notice. During 

 periods of elevation the area of the laud and of the adjoin- 

 ing shoal parts of the sea will be increased and now t^ta- 

 tions will often be formed — all circumstances favorable, as 

 previously explained, for the formation of new varieties 

 and species; but during such periods there will general Iv 

 be a blank in the geological record. On the other hand', 

 during subsidence, the inhabited area and number of 

 inhabitants will decrease (excepting on the shores of a 

 continent when first broken np into an archipelago), and 

 consequently during subsidence, though there will bo 

 much extinction, few new varieties or species will be 

 formed; and it is during these very periods of subsidence 

 that the deposits which are richest in fossils have been 

 accumulated. 



ON" THE abse:n'ce of numerous intermediate varie- 

 ties IK ANY SII^GLE FORilATIOif. 



From these several considerations it cannot be doubted 

 that the geological record, viewed as a whole, is extremely 

 imperfect; but if we confine our attention to any one 

 formation, it becomes much more difficult to understand 

 why we do not therein find closely graduated varieties 

 between the allied species which lived at its commence- 

 ment and at its close. Several cases are on record of the 

 same species presenting varieties in the upper and lower 

 parts of the same formation. Thus Trautsehold gives a 

 number of instances with Ammonites, and Ililgendorf has 

 described a most curious case of ten graduated forms of 

 Planorbis multiformis in the successive beds of a fresh- 

 water formation in Switzerland. Although each formation 

 has indisputably required a vast number of years for its 

 deposition, several reasons can be given why each should 

 not commonly include a graduated series of links between 

 the species which lived at its commencement and close, but 



