Chat. XIV. CONCLUSION. • 435 



world. Even at present, by comparing the differences of the 

 inluibitaiits of tlie sea on the opposite sides of a continent, 

 and the nature of the various inhabitants of that continent in 

 relation to tlicir apparent means of immig-ration, 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 Avith 

 its embetlded 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 forma- 

 tion will be recognized as having depended on an nnusual con- 

 currence of favorable circumstances, and the l)lank 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 suc- 

 ceeding organic forms. We must be cautious in attempting to 

 correlate as strictly contemporaneous two formations, wliicli do 

 not include many identical species, by the general succession 

 of the forms of life. As species are produced and extermi- 

 nated by slowly acting and still existing causes, and not hy mi- 

 raculous acts of creation and by catastrophes ; and as the most 

 important of all causes of organic change is one which is al- 

 most independent of altered and perliaps suddenly-altered 

 ])hysical conditions, namely, the mutual relation of organism to 

 organism — the improvement of one organism entailing the im- 

 provement or the extermination of others — it follows that the 

 amount of organic change in the fossils of consecutive forma- 

 tions probably serves as a fair measure of the lapse of actual 

 time. A number of species, however, keeping in a body 

 might remain for a long period unchanged, while within this 

 same period, sevend of these species, by migrating into new 

 countries and coming into competition with foreign associates, 

 might become modilied ; so that we nmst not overrate the ac- 

 curacy of organic change as a measure of time. During earlv 

 periods of the earth's liistory, when the forms of life were proba- 

 i)ly fewer and simpler, the rate of change was ])rol)ably slower; 

 and at the fust dawn of life, when very few forms of the sim- 

 plest structure existed, the rate of change niav have been slow 

 in an extreme degree. The history of tlie wt)rld, as at present 

 known, although of innnense length, will liereaiter be reco""- 

 nized as short, compared with the ages which must have 

 eUipsed since the first organic beings, the progenitors of in- 

 numerable extinct and living descendants, appeared on the 

 stay-e. 



