CONCLUSION 813 



descended from one parent, and have migrated from 

 some one birthplace; and when we better know the 

 many means of migration, then, by the light which 

 geology now throws, and will continue to throw, on 

 former changes of climate and of the level of • the land, 

 we shall surely be enabled to trace in an admirable man- 

 ner the former migrations of the inhabitants of the whole 

 world. Even at present, by comparing the differences 

 between the inhabitants of the sea on the opposite sides 

 of a continent, and the nature of the various inhabitants 

 on that continent in relation to their apparent means 

 of immigration, some light can be thrown on ancient 

 geography. 



The noble science of Geology loses glory from the ex- 

 treme imperfection of the record. The crust of the earth 

 •with its imbedded remains must not be looked at as a 

 well-filled museum, but as a poor collection made at 

 hazard and at rare intervals. The accumulation of each 

 great fossiliferous formation will be recognized as having 

 depended on an unusual concurrence of favorable circum- 

 stances, and the blank intervals between the successive 

 stages as having been of vast duration. But we shall be 

 able to gauge with some security the duration of these 

 intervals by a comparison of the preceding and succeed- 

 ing organic forms. We must be cautious in attemptiog 

 to correlate as strictly contemporaneous two formations, 

 which do not include many identical species, by the gen- 

 eral succession of the forms of life. As spe cies are pro- 

 duc ed an d extermin ated by slowly acting and still exist- 

 ing^causes, and nqt_by_niiraculous acts of creation; and 

 as the most important of all causes of organic change _is 

 one which is almost independent of altered and perhaps 



