294 ON THE ORIGIN OF SPECIES 



blank intervals of vast duration occurred during the 

 periods when the bed of the sea was either station- 

 ary or rising, and likewise when sediment was not 

 thrown down quickly enough to embed and preserve 

 organic remains. During these long and blank inter- 

 vals I suppose that the inhabitants of each region 

 underwent a considerable amount of modification and 

 extinction, and that there was much migration from 

 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 para- 

 graphs, 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 belonging to the same genera, yet 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. 

 Lyell 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 



