CONOLXTSION. S03 



parent, and have migrated from some one birth-place; 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 admir- 

 able 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 fossilifer- 

 ous 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 gauge 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 entailmg the 

 improvement or the extermination of others; it follows, 

 that the amount of organic change in the fossils of con- 

 secutive formations probably serves as a fair measure of 

 the relative, though not actual lapse of time. A number 

 of species, however, keeping in a body might remain for a 

 long period unchanged, while within the same period, 

 several of these species, by migrating into new countnes 

 and coming into competition with foreign associates, might 

 become modified; so that we must not overrate the accuracy 

 of organic change as a measure of time. 



