472 CONCLUSION. 



the nature of long-lost structures. Species and groups of 

 species which are called aberrant, and which may fancifully 

 be called living fossils, will aid us in forming a picture of 

 the ancient forms of life. Embryology will often reveal to 

 us the structure, in some degree obscured, of the prototypes 

 of each great class. 



When we can feel assured that all the individuals of the 

 same species, and all the closely allied species of most 

 genera, have, within a not very remote period, descended 

 from one parent, and have migrated from some one birth- 

 place ; and when we better know the many means of migra- 

 tion, 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 between the inhabitants of the sea on the oppo- 

 site 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 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 recognized as having depended on an 

 unusual occurrence of favorable circumstances, and the 

 blank intervals between the successive stages as having 

 been of vast duration. But we shall be able to guage with 

 some security the duration of these intervals by a compari- 

 son 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 inde- 

 pendent of altered and perhaps suddenly altered physical 

 conditions, namely, the mutual relation of organism to 

 organism — the improvement of one organism entailing the 

 improvement or the extermination of others ; it follows, 

 $hat; the amount of organic change in the fossils of qon 



