124 SUMMARY OP THE [Chap, XL 



other species and the other old genera having become 

 utterly extinct. In failing orders, with the genera and 

 species decreasing in numbers as is the case with the 

 Edentata of South America, still fewer genera and spe- 

 cies 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 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 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 al- 

 most 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 less perfectly kept; that each single for- 

 mation has not been continuously deposited; that the 

 duration of each formation is probably short compared 

 with the average duration of specific forms; that mi- 

 gration has played an important part in the first ap- 

 pearance 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, 



