182 THE ORIOIN OF SPECIES 



single formation; that, owing 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 extinction 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 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 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 stages, it is 

 probable that the periods, during which each underwent 

 modification, though many and long as measured by 

 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, connecting together all extinct 

 and existing forms by the finest graduated steps. Il 

 should also be constantly borne in mind that any linking 

 variety between two forms which might be found wouk 

 be ranked, unless the whole chain could be perfectly 

 restored, as a new and distinct species; for it is no' 

 pretended that we have any sure criterion by whicl) 

 species and varieties can be discriminated. 





