Chap. XV.] CONCLUSION. 303 



we shall surely be enabled to trace in an admirable 

 manner 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 

 extreme 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 recognised as having 

 depended on an unusual concurrence of favourable cir- 

 cumstances, and the blank intervals between the suc- 

 cessive 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 succeeding organic forms. We must be cautious in 

 attempting to correlate as strictly contemporaneous two 

 formations, which do not include many identical species, 

 by the general succession of the forms of life. As 

 species are produced and exterminated by slowly acting 

 and still existing causes, and not by miraculous acts of 

 creation; and as the most important of all causes of 

 organic change is one which is almost independent of 

 altered and perhaps suddenly altered physical conditions, 

 namely, the mutual relation of organism to organism, — 

 the improvement of one organism entailing the improve- 

 ment or the extermination of others ; it follows, that the 

 amount of organic change in the fossils of consecutive 

 formations probably serves as a fair measure of the 



