GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 319 



with the genera and species decreasing in numbers as is the case 

 with the Edentata of South America, still fewer genera and species 

 will leave modified blood-descendants. 



SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS 



I have attempted to show that the geological record is extremely 

 imperfect; that only a small portion of the globe has been geo- 

 logically explored with care; that only certain classes of organic 

 beings have been largely preserved in a fossil state; that the num- 

 ber both of specimens and of species, preserved in our museums, 

 is absolutely as nothing compared with the number of generations 

 which must have passed away even during a single formation; 

 that, owing to subsidence being almost necessary for the accumu- 

 lation 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 de- 

 posited; 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 appearance of new 

 forms in any one af ea 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, dur- 

 ing 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. 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 per- 

 fectly 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 geological 

 record, will rightly reject the whole theory. For he may ask in 



