SUMMARY 391 



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 ex- 

 tremely imperfect; that only a small portion of the globe has 

 been geologically explored with care; that only certain 

 classes of organic beings have been largely preserved in a 

 fossil state; that the number both of specimens and of spe- 

 cies, preserved in our museums, is absolutely as nothing com- 

 pared with the number of generations which must have 

 passed away even during a 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 ex- 

 tent 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 



