CiiAi". X. PRKCKDING AND PKESENT CHAPTER. 319 



with the number of p^encralions which must have iiasscd away 

 even during a siii<:;le formation; lliat, owinfj;' to subsidence 

 being ahuost necessary for the accumulation of deposits rich in 

 fossil species of very many kinds and thick enough to resist fu- 

 ture degradation, great intervals of time must have elapsed 

 between most of our successive formations; that there has 

 probably been more extinction during the periods of subsi- 

 dence, and more variation during tlie periods of elevation, 

 and during the latter the record will have been least perfectly 

 kept ; that each single formation has not been continuously 

 deposited; that tlH> duration of each formation is, probably, 

 short compared with the average duration of specidc forms ; 

 that migration has played an important part in the first ap- 

 pearance of now forms in any one area and fonnation ; that 

 widely-ranging species are those which have varied most fre- 

 quently, and have oftenest given rise to new species ; that va- 

 rieties have at first been local ; and lastly, although each spe- 

 cies must have j^asscd through numerous transitional stages, it 

 is probable that the periods, during which each underwent 

 modification, though many and long as measured l)y years, 

 have been short in comparison with the periods during which 

 each remained in an unchanged condition. These causes, taken 

 conjointly, will, to a large extent, explain why — though we do 

 find many links — we do not find interminable varieties, con- 

 necting together all extinct and existing forms liy the finest 

 graduated steps. It should also be constantly borne in mind 

 that any linking varieties between two or more forms, which 

 might be found, would be ranked, unless the whole chain could 

 be perfectly restored, as so many new and disthict species ; for 

 it is not pretended that we have any sure criterion by which 

 species and varieties can be discriminated. 



H(! who rejt;cts these views on the imperfection of tlie geo- 

 logical record, will rightly reject the whole theory. For he 

 may ask in vain where are the numberless transitional links 

 which must formerly have connected the closely-allied or rep- 

 resentative species found in the successive stages of the same 

 great formation ? lie may disbelieve in the immense intervals 

 of time which have elapsed between our consecutivt; forma- 

 tions ; he may overlook how important a part migration has 

 tjlaycd, when the fonnations of any one gieat region alone, as 

 that of Europe, are considen-d ; he may urge the apparent, but 

 often falsely appanMit, sudden coming in of whole groups of 

 species. He may ask where are the remains of those infinitely- 



