PRECEDING AND PRESENT CHAPTERS. 34. 



to subsidence being almost necessary for the accumulation 

 of deposits rich in fossil species of many kinds, and thick 

 enough to outlast future degradation, great intervals of 

 time must have elapsed between most of our successive 

 formations ; that there has probably been more extinc- 

 tion during the periods of subsidence, and more variation 

 during the 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 the 

 duration of each formation is probably short compared 

 with the average duration of specific forms ; that migra- 

 tion has played an important part in the first appear- 

 ance of new forms in any one area and formation ; that 

 widely ranging species are those which have varied most 

 frequently, and have oftenest given rise to new species ; that 

 varieties have at first been local ; and lastly, although each 

 species must have passed through numerous transitional 

 stager it is probable that the periods, during which each 

 undeiv/ent modification, though many and long as meas- 

 ured by years, have been short in comparison with the 

 periods during which each remained in an unchanged con- 

 dition. These causes, taken conjointly, will to a large 

 extent explain why — though we do find many links — we 

 do not find interminable varieties, connecting together all 

 extinct and existing forms by the finest graduated steps. 

 It should also be constantly borne in mind that any linking 

 variety between two forms, which might be found, would 

 be ranked, unless the whole chain could be perfectly 

 restored, as a new and distinct species; for it is not pre- 

 tended that we have any sure criterion by which species 

 and varieties can be discriminated. 



He who rejects this view of the imperfection of the 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 representative species, found in the successive 

 stages of the same great formation ? He may disbelieve in 

 the immense intervals of time which must have elapsed 

 between our consecutive formations ; he may overlook how 

 important a part migration has played, when the forma- 

 tions of any one great region, as those of Europe, are 

 considered ; he may urge the apparent, but often falsely 

 apparent, sudden coming in of whole groups of species. 

 He may ask where are the remains of those infinitely n» 



