CiiAr. IX. IN ANY SINGLE lORMATION. 219 



Although each formation may mark a very long lapse of 

 years, each probably is short compared with the period requi- 

 site to change one sjieoies into another. I am aware that two 

 paleontologists, whose opinions are worthy of much deference, 

 namely, Broim and AVoodward, have concluded that the aver- 

 age duration of each fcjrmation is twice or thrice as long as the 

 average duration of specific forms. But insuperable difficul- 

 ties, as it seems to me, prevent us from coming to an}"^ just 

 conclusion on this head. W^hen we see a species first appear- 

 ing in the middle of any formation, it would be rash in the ex- 

 treme to infer that it had not elsewhere previously existed. 

 So, again, when we find a species disappearing before the last 

 layers have been deposited, it would be equally rash to sup- 

 pose that it then became extinct. We forget how small the 

 area of Europe is compared with the rest of the world ; nor 

 have the several stages of the same formation throughout Eu- 

 rope been correlated Avith perfect accuracy. 



With marine animals of all kinds, we may safelv infer a 

 large amount of migration during climatal and other changes ; 

 and when we see a species first appearing in any formation, the 

 probability is, that it only then first iimnigrated into that area. 

 It is well known, for instance, that several species ajipeared 

 somewhat earlier in the paleozoic beds of North America than 

 in those of Europe; time having apparently been required for 

 their Tnigration from the American to the European seas. In 

 examining the latest deposits in various quai'ters of the world, 

 it has everywhere been noted that some few still existing spe- 

 cies are common in the deposit, but have become extinct in the 

 immediately surrounding sea ; or, conversely, that some arc 

 now abundant in the neighboring sea, but are rare or absent in 

 tliis particular deposit. It is an excellent lesson to reflect on 

 the ascertained amount of migration of the inhabitants of Eu- 

 rope during the glacial epoch, which forms only a part of one 

 whole geolf)gical period ; and likewise to reflect on the changes 

 of level, on the extreme change of climate, and on the great 

 lapse of time — all included within this same glacial period. Yet 

 it may be doubted whether, in any quarter of the world, sedi- 

 mentary deposits, indiidlitf/ fossil rcmahis, have gone on ac- 

 cumulating within the same area during the whole of this 

 period. It is not, for instance, probable that S(Mliment was de- 

 posited during the whole of the glacial jieriod near the mouth 

 of the Mississippi, within that limit of (ie])th at which marine 

 animals can best flourish ; for we know that great geographical 



