124 SXBIMARY OF THE [Chap. XL 



species and tlie 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 species will 

 leave modified blood-descendants. 



Summary of the preceding and present (Jliapters. 



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 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 liave varied most frequently, and have 

 oftenest given rise to new species ; that varieties have 

 at first been local; and lastly, although each species 



