328 GEOLOGICAL SUCCESSION. Chap. X. 



other parts of the world. As we have reason to 

 believe that large areas are affected by the same move- 

 ment, it is probable that strictly contemporaneous for- 

 mations have often been accumulated over very wide 

 spaces in the same quarter of the world ; but we are 

 far from having any right to conclude that this has in- 

 variably been the case, and that large areas have invari- 

 ably been affected by the same movements. When two 

 formations have been deposited in two regions during 

 nearly, but not exactly the same period, we should find 

 in both, from the causes explained in the foregoing 

 paragraphs, the same general succession in the forms of 

 life ; but the species would not exactly correspond ; for 

 there will have been a little more time in the one region 

 than in the other for modification, extinction, and im- 

 migration. 



I suspect that cases of this nature have occurred in 

 Euixype. Mr. Prestwich, in his admirable Memoirs on 

 the eocene deposits of England and France, is able to 

 draw a close general parallelism between the successive 

 stages in the two countries; but when he compares 

 certain stages in England with those in France, although 

 he finds in both a curious accordance in the numbers 

 of the species belonging to the same genera, yel the 

 species themselves differ in a manner very difficult to 

 account for, considering the proximity of the two areas, 

 — unless, indeed, it be assumed that an isthmus separated 

 two seas inhabited by distinct, but contemporaneous, 

 faunas. Ly ell has made similar observations on some of 

 the later tertiary formations. Barrande, also, shows that 

 there is a striking general parallelism in the successive 

 Silurian deposits of Bohemia and Scandinavia ; never- 

 theless he finds a surprising amount of difference in 

 the species. If the several formal ions in these re- 

 gions have not been deposited during the same exact 



