GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 113 



same quarter of the world; but we are very far from 

 having any right to conclude that this has invariably 

 been the case, and that large areas have invariably 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 immigration. 



I suspect that cases of this nature occur in Europe. 

 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 be- 

 longing to the same genera, yet the species themselves 

 difEer in a manner very difficult to account for, consider- 

 ing the proximity of the two areas — unless, indeed, it be 

 assumed that an isthmus separated two seas inhabited by 

 distinct, but contemporaneous, faunas. Lyell has made 

 similar observations on some of the later tertiary forma- 

 tions. Barrande, also, shows that there is a striking 

 general parallelism in the successive Silurian deposits 

 of Bohemia and Scandinavia; nevertheless he finds a 

 surprising amount of difference in the species. If the 

 several formations in these regions have not been de- 

 posited during the same exact periods — a formation in 

 one region often corresponding with a blank interval 

 in the other — and if in both regions the species have 



