312 GEOLOGICAL SUCCESSION. Chap. X. 



only certain classes of organic beings have been largely 

 preserved in a fossil state ; that the number both of 

 specimens and of species, preserved in our museums, is 

 absolutely as nothing compared with the incalculable 

 number of generations which must have passed aAvay 

 even during a single formation; that, owing to sub- 

 sidence being necessaiy for the accumulation of fossili- 

 ferous deposits thick enough to resist future degradation, 

 enormous intervals of time have elapsed between the 

 successive formations ; that there has probably been 

 more extinction during the periods of subsidence, and 

 more variation during the periods of elevation, and 

 during the latter the record will have been least per- 

 fectly kept ; that each single formation has not been 

 continuously deposited ; that . the duration of each 

 formation is, perhaps, short compared with the average 

 duration of specific forms ; that migration has played 

 an important part in the first appearance of new forms 

 in any one area and formation ; that widely ranging 

 species are those which have varied most, and have 

 oftenest given rise to new species ; and that varieties 

 have at first often been local. All these causes taken 

 conjointly, must have tended to make the geological 

 record extremely imperfect, and will to a large extent 

 explain why we do not find interminable varieties, con- 

 necting together all the extinct and existing forms of 

 life by the finest graduated steps. 



He who rejects these views on the nature of the 

 geological record, will rightly reject my whole theory. 

 For he may ask in vain where are the numberless 

 transitional links which must formerly have connected 

 tlie closely allied or representative species, found in 

 the several stages of the same great formation. He 

 may disbelieve in the enormous intervals of time which 

 have elapsed between our consecutive formations ; he 



