Chap. XIV. CONCLUSION. 487 



scended 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 manner the 

 former migrations of the inhabitants of the whole world. 

 Even at present, by comparing the differences of the 

 inhabitants of the sea on the opposite sides of a conti- 

 nent, and the nature of the various inhabitants of that 

 continent in relation to their apparent means of immigra- 

 tion, 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 embedded 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 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 

 succeeding organic forms. We must be cautious in 

 attempting to correlate as strictly contemporaneous 

 two formations, which include few identical species, 

 by the general succession of their forms of life. As 

 species are produced and exterminated by slowly act- 

 ing and still existing causes, and not by miraculous 

 acts of creation and by catastrophes ; 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 being 

 entailing the improvement or the extermination of 



